JPH10255837A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery which is excellent in both a charging-discharging cycle characteristic and a high rate discharging characteristic by adding an aluminum compound to nonaqueous electrolyte, or covering the surface of a carbon material particle of a negative electrode with it. SOLUTION: A lithium secondary battery is constituted by using a positive electrode using LiCoO2 or the like as an active material, a negative electrode using a carbon material as a lithium ion storage material and nonaqueous electrolyte by dissolving lithium salt in an ethylene carbonate or the like. In that case, an aluminum compound is preferably added by 0.001 to 0.1 mole/l to the nonaqueous electrolyte. Or a particle surface of the carbon material is covered with a coating film having a thickness of about 1 to 500Å of an aluminum compound. AlI3 , Al(OH)3 , AlF3 , Al(PE6 )3 , Al(ClO4 )3 , Al(BF4 )3 , Al(N(CF3 SO2 )2 )3 or the like are preferable as both aluminum compounds. Therefore, the carbon material is protected by a coating film formed at charging time or the coating film, and the degradation is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a lithium secondary battery.

[0002]

2. Description of the Related Art In recent years,
A lithium secondary battery using a non-aqueous electrolyte has a high energy density and, unlike an alkaline secondary battery, does not require consideration of the decomposition voltage of water. Attention is paid to the reason that voltage design is possible.

As a negative electrode material for a lithium secondary battery, carbon materials such as coke and graphite are different from metallic lithium in that there is no risk of internal short-circuit due to the growth of dendritic lithium. It is widely used.

However, since the carbon material reacts with the non-aqueous electrolyte and deteriorates, the charge / discharge cycle characteristics of the lithium secondary battery using the carbon material for the negative electrode are not so good.

In order to improve the charge / discharge cycle characteristics of a lithium secondary battery using a carbon material for the negative electrode, it has been proposed to coat the negative electrode with a film made of lithium fluoride (see Japanese Patent Application Laid-Open No. 7-302617). ).

[0006] However, as a result of investigations by the present inventors, it has been found that this conventional battery has a problem in high-rate discharge characteristics (load characteristics) because lithium ion conductivity of lithium fluoride is not so good. Was.

Accordingly, it is an object of the present invention to provide a lithium secondary battery having good charge / discharge cycle characteristics and high-rate discharge characteristics.

[0008]

The lithium secondary battery (hereinafter referred to as "battery A") according to the first aspect of the present invention comprises:
In a lithium secondary battery including a positive electrode, a negative electrode using a carbon material as a lithium ion storage material, and a nonaqueous electrolyte,
An aluminum compound is added to the non-aqueous electrolyte.

Further, a lithium secondary battery (hereinafter referred to as "battery B") according to the present invention comprises a positive electrode, a negative electrode using a carbon material as a lithium ion storage material, and a non-aqueous electrolyte. In the lithium secondary battery provided, the surface of the particles of the carbon material is coated with a film made of an aluminum compound. In this specification, the battery A and the battery B may be collectively referred to as a battery of the present invention.

In battery A in which an aluminum compound is added to a nonaqueous electrolyte, a film containing aluminum (such as a Li-Al alloy film) is formed on the surface of the negative electrode during charging. On the other hand, in the battery B, a film made of an aluminum compound is formed on the surface of the particles of the carbon material. These films suppress the deterioration of the carbon material due to the reaction with the non-aqueous electrolyte. Therefore, the battery of the present invention is excellent in charge / discharge cycle characteristics. In addition, all of these coatings are compared with lithium fluoride coating and lithium carbonate coating.
Good lithium ion conductivity. This is because the lithium fluoride film and the lithium carbonate film are so dense that lithium ions are not easily released smoothly, whereas the film containing aluminum in the battery A and the film made of the aluminum compound in the battery B are voids. This is considered to be due to the fact that the release of lithium ions is performed smoothly. Therefore, the battery of the present invention is also excellent in high-rate discharge characteristics.

Specific examples of the aluminum compound used as an additive to the non-aqueous electrolyte or as a coating material for the negative electrode include A
11 ₃ , Al (OH) ₃ , AlF ₃ , Al (P
_{F 6) 3, Al (ClO} 4) 3, Al (BF 4) 3 and _{Al (N (CF 3 SO 2} ) 2) 3 and the like. These aluminum compounds may be used alone or in combination of two or more as necessary.

The preferable addition amount of the aluminum compound to the non-aqueous electrolyte in the battery A is 0.001 to 0.1 mol / liter. The preferable thickness of the film made of the aluminum compound in the battery B is 1 to 500 °.
When the addition amount or the coating amount of the aluminum compound is too small, the film may be peeled off at the time of discharge. On the other hand, if the addition amount or coating amount of the aluminum compound is excessive, the ionic conductivity of the non-aqueous electrolyte decreases due to the increase in viscosity and the discharge capacity decreases, or the lithium ion conductivity increases due to the film being too thick. Or decrease.

Examples of the carbon material in the battery of the present invention include graphite, coke, and a fired organic material. Examples of the non-aqueous electrolyte include ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, sulfolane, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, and these. LiPF ₆ , LiCF ₃ SO
₃ , LiBF ₄ , LiClO ₄ , LiAsF _{6 and the} like, in which 0.5 to 1.5 mol / liter of a lithium salt is dissolved.

The present invention provides a method for charging a negative electrode of a lithium secondary battery using a carbon material as a lithium ion storage material by adding an aluminum compound to a non-aqueous electrolyte or coating a carbon material with an aluminum compound. Discharge cycle characteristics and high-rate discharge characteristics are improved.
Therefore, as other members constituting the battery such as the positive electrode active material and the separator, conventionally known materials can be used without any particular limitation. As the positive electrode active material, LiCoO
₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiFeO ₂
Examples of the separator include a microporous sheet made of polypropylene, polyethylene, and the like, and a nonwoven fabric.

[0015]

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 the present invention may be practiced by appropriately changing the gist of the invention. Is possible.

(Examples 1 to 35) [Preparation of Positive Electrode] 80 parts by weight of LiCoO ₂ as a positive electrode active material, 10 parts by weight of artificial graphite as a conductive agent, and 10 parts by weight of polyvinylidene fluoride as a binder Mix
N-Methyl-2-pyrrolidone is added to the mixture, and the mixture is kneaded to prepare a slurry. The slurry is applied to both sides of an aluminum foil as a positive electrode current collector by a doctor blade method, and vacuum dried at 150 ° C. for 2 hours. Thus, a positive electrode was manufactured.

[Preparation of negative electrode] Natural graphite (lattice plane (00
2) plane spacing of 3.35 °; crystallite size in the c-axis direction larger than 1000 °) 90 parts by weight, and 10 parts by weight of polyvinylidene fluoride as a binder were mixed, and N-methyl- A slurry was prepared by adding and kneading 2-pyrrolidone, and this slurry was applied to both sides of a copper foil as a negative electrode current collector.
It was applied by a doctor blade method and vacuum dried at 150 ° C. for 2 hours to prepare a negative electrode.

[Preparation of Non-Aqueous Electrolyte] In a mixed solvent of ethylene carbonate and diethyl carbonate at a volume ratio of 1: 1, 1 mol / l of LiPF ₆ , AlI ₃ , Al
(OH) ₃ , AlF ₃ , Al (PF ₆ ) ₃ , Al (Cl
O ₄ ) ₃ , Al (BF ₄ ) ₃ or Al (N (CF ₃ SO
₂ ) ₂ ) ₃ is 0.0008 mol / liter, 0.00
1 mol / L, 0.01 mol / L, 0.1 mol / L or 0.15 mol / L
Various non-aqueous electrolytes were prepared.

[Preparation of Batteries] AA-sized lithium secondary batteries (Battery A) A1 to A35 were prepared using the above positive electrode, negative electrode and each nonaqueous electrolyte. In addition, a polypropylene microporous sheet was used as a separator.

Comparative Example 1 Example 1 was repeated except that no aluminum compound was added in the preparation of the non-aqueous electrolyte.
To 35, a lithium secondary battery (comparative battery) C
1 was produced.

Comparative Example 2 A lithium secondary battery (comparative battery) C2 was prepared in the same manner as in Examples 1 to 35 except that 400 ppm of water was added without adding an aluminum compound in the preparation of the nonaqueous electrolyte. Was prepared. This battery corresponds to a conventional battery disclosed in Japanese Patent Application Laid-Open No. 7-302617.

<Discharge Capacity of Each Battery at High Rate Discharge> Each of the batteries produced in Examples 1 to 35 and Comparative Examples 1 and 2 was
After charging to 4.1 V with A, the battery was discharged to 2.75 V at 1000 mA (corresponding to 2 C), and the discharge capacity (high-rate discharge capacity) at high rate discharge of each battery was determined. The results are shown in Table 1, Table 2, FIG. 1 and FIG. 1 and 2 are graphs in which the vertical axis represents the high-rate discharge capacity (mAh) of each battery, and the horizontal axis represents the addition amount (mol / liter) of the aluminum compound to the non-aqueous electrolyte.

[0023]

[Table 1]

[0024]

[Table 2]

As shown in Tables 1 and 2, FIGS. 1 and 2,
The batteries A1 to A35 have a higher high-rate discharge capacity than the comparative batteries C1 and C2. From this fact, it is understood that the high-rate discharge characteristics are improved by adding the aluminum compound to the non-aqueous electrolyte. Further, among the batteries A1 to A35, since the high-rate discharge capacity of the battery in which the addition amount of the aluminum compound is 0.001 to 0.1 mol / liter is particularly large, the addition of the aluminum compound to the non-aqueous electrolyte in the battery A It is understood that the amount is preferably 0.001 to 0.1 mol / liter.

(Examples 36 and 37) Natural graphite (Example 1)
98 parts by weight and AlI ₃
2 parts by weight were mixed in a mortar, and the surface of the natural graphite particles was coated with AlI ₃ (Example 36). Also,
Natural graphite (same as used in Examples 1 to 35) 9
9.5 parts by weight and 0.5 parts by weight of Al (OH) ₃ are mixed in a mortar and mixed to form a natural graphite particle surface with Al (O).
H) ₃ (Example 37). A lithium secondary battery (battery B) was prepared in the same manner as in Examples 1 to 35, except that the above-mentioned natural graphite whose particle surface was coated with the aluminum compound was used for the negative electrode without adding the aluminum compound to the nonaqueous electrolyte. ) B1 and B2 were prepared. The same charge / discharge test was performed on each of these batteries to determine the high-rate discharge capacity of each battery. The high-rate discharge capacity of the battery B1 was 600 mA.
h, and the high-rate discharge capacity of Battery B2 was 615 mAh. These are much larger than those of the comparative batteries C1 and C2. From this fact, it is understood that the high-rate discharge characteristics are improved by coating the particle surface of the carbon material with the aluminum compound.

<Charge / Discharge Cycle Characteristics> Batteries A1, A2,
A3, A4, A5, B1, B2 and comparative batteries C1, C2
After charging to 4.1 V at 200 mA, 10
The charge / discharge cycle characteristics of each battery were examined by performing 200 charge / discharge cycles, each including a step of discharging to 2.75 V at 00 mA. The results are shown in FIGS. 3 and 4, the vertical axis indicates the high rate discharge capacity (mA) of each battery.
h) is a graph in which the horizontal axis represents a charge / discharge cycle.

FIGS. 3 and 4 show that the battery of the present invention has better charge / discharge cycle characteristics than the comparative battery.

(Examples 38 to 41) Natural graphite having the weight ratio shown in Table 3 (same as that used in Examples 1 to 35) and AlF ₃ were mixed in a mortar and mixed to obtain natural graphite. Was coated with AlF ₃ . A lithium secondary battery (battery) was prepared in the same manner as in Examples 1 to 35, except that the above-described natural graphite whose particle surface was coated with AlF ₃ was used for the negative electrode without adding an aluminum compound to the nonaqueous electrolyte. B) B
3, B4, B5, and B6 were produced. The same charge / discharge test was performed for each of these batteries, and the high rate discharge capacity of each battery was determined. Table 3 shows the results.

[0030]

[Table 3]

From Table 3, it can be seen that the thickness of the AlF ₃ coating covering the surface of the natural graphite particles in Battery B is preferably 1 to 500 °. It was separately confirmed that a thickness in this range is preferable also when coating with a film made of another aluminum compound.

[0032]

According to the present invention, a lithium secondary battery having good charge / discharge cycle characteristics and high-rate discharge characteristics is provided.

[Brief description of the drawings]

FIG. 1 shows an aluminum compound [Al
₃ is a graph showing the relationship between the added amount of I ₃ , Al (OH) ₃ or AlF ₃ ] and the high rate discharge capacity.

FIG. 2 shows an aluminum compound [Al (P
5 is a graph showing the relationship between the added amount of F ₆ ) ₃ , Al (ClO ₄ ) ₃ , Al (BF ₄ ) ₃ or Al (N (CF ₃ SO ₂ ) ₂ ) ₃ ] and the high-rate discharge capacity.

FIG. 3 is a graph showing charge / discharge cycle characteristics of the battery of the present invention.

FIG. 4 is a graph showing charge / discharge cycle characteristics of the battery of the present invention and a comparative battery.

 ──────────────────────────────────────────────────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 in Sanyo Electric Co., Ltd.

Claims (6)

[Claims] 1. A lithium secondary battery comprising a positive electrode, a negative electrode using a carbon material as a lithium ion storage material, and a non-aqueous electrolyte, wherein an aluminum compound is added to the non-aqueous electrolyte. Rechargeable lithium battery. 2. The method according to claim 1, wherein said aluminum compound is AlI ₃ , A
l (OH) ₃ , AlF ₃ , Al (PF ₆ ) ₃ , Al (C
10 O ₄ ) ₃ , Al (BF ₄ ) ₃ and Al (N (CF ₃ S
_2. The lithium secondary battery according to claim 1, wherein the lithium secondary battery is at least one selected from the group consisting of O2) ₂ ) ₃ . 3. The lithium secondary battery according to claim 1, wherein the non-aqueous electrolyte contains 0.001 to 0.1 mol / L of the aluminum compound. 4. A lithium secondary battery comprising a positive electrode, a negative electrode using a carbon material as a lithium ion storage material, and a non-aqueous electrolyte, wherein the particle surfaces of the carbon material are coated with a film made of an aluminum compound. A lithium secondary battery, characterized in that: 5. The method according to claim 1, wherein the aluminum compound is AlI ₃ , A
l (OH) ₃ , AlF ₃ , Al (PF ₆ ) ₃ , Al (C
10 O ₄ ) ₃ , Al (BF ₄ ) ₃ and Al (N (CF ₃ S
O _{2) 2)} The lithium secondary battery according to claim 4, wherein at least one selected from the group consisting of _3. 6. The lithium secondary battery according to claim 4, wherein the thickness of the film made of the aluminum compound is 1 to 500 °.