JPH07114940A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To reduce a capacity deterioration rate due to electric charging/discharging of a battery and realize an excellent cycle characteristic by restraining decomposition of a non-aqueous electrolyte at the surface of a carbon material by the effect of a coating film made of triester phosphate produced at the surface of the carbon material. CONSTITUTION:A non-aqueous electrolyte secondary battery comprises a positive electrode, and a negative electrode made of a carbon material having 3.37Angstrom or less of a (d) value (d002) at a grid surface (002), and a non-aqueous electrolyte. If the carbon material is composed of a high capacity material having such high crystallinity as a (d) value less than 3.37Angstrom , it is confirmed that decomposition deterioration of the non-aqueous electrolyte is prominent. The non-aqueous electrolyte includes 0.1-10wt% of triester phosphate expressed by the formula I (wherein R<1>, R<2> and R<3> represent an alkyl or phenyl group having C1-4 individual). If the included quantity is less than 0.1wt.%, a coating film having a sufficient thickness cannot be formed so that decomposition at the surface of the carbon material cannot be restrained. If the quantity exceeds 10wt.%, a coating film becomes excessively thick so that permeability of a lithium ion is deteriorated, thus resulting in an increase in reaction resistance of an electrode plate and reduction cycle characteristic.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to an improvement in a non-aqueous electrolyte for the purpose of improving cycle characteristics.

[0002]

2. Description of the Related Art In recent years,
Carbon materials such as coke and graphite have excellent flexibility,
It has been proposed as a new negative electrode material for a non-aqueous electrolyte secondary battery, which replaces the conventional metal lithium, because there is no possibility of internal short circuit due to the growth of dendritic electrodeposited lithium.

However, when a carbon material having a high crystallinity, that is, a high degree of graphitization, is used as a negative electrode material, the non-aqueous electrolyte solution decomposes on the carbon negative electrode with the generation of gas as the charge / discharge cycle progresses. There was a problem that the battery capacity gradually decreased. That is, a carbon material having a high degree of graphitization has an advantage that it can have a high capacity, but has a drawback that it has poor cycle characteristics because it easily reacts with a non-aqueous electrolyte.

The present invention has been made in view of the above circumstances, and an object thereof is to improve cycle characteristics of a non-aqueous electrolyte secondary battery using a carbon material having a high degree of graphitization as a negative electrode material. To do.

[0005]

A non-aqueous electrolyte secondary battery according to the present invention for achieving the above object (hereinafter referred to as "the present battery") has a positive electrode and a lattice plane (002) plane. A non-aqueous electrolyte secondary battery comprising a negative electrode using a carbon material having a d value (d ₀₀₂ ) of 3.37 Å or less as a negative electrode material and a non-aqueous electrolyte solution, wherein the non-aqueous electrolyte solution is represented by the following chemical formula 2. It is characterized in that 0.1 to 10% by weight of phosphoric acid triester is added.

[0006]

[Chemical 2] [In the formula, R ¹ , R ² and R ³ each independently represent an alkyl group having 1 to 4 carbon atoms or a phenyl group. ]

In the present invention, phosphoric acid triester is added to the non-aqueous electrolyte, and the phosphoric acid triester is adsorbed on the carbon material to form a lithium ion permeable coating (protective film) on the surface of the carbon material. The formed film suppresses the decomposition and deterioration of the non-aqueous electrolyte.

Examples of the phosphoric acid triester include trimethyl phosphate, triethyl phosphate, tripropyl phosphate, tributyl phosphate and triphenyl phosphate.

The d _{002 of the} carbon material is regulated to 3.37 Å or less, because when a high-capacity carbon material with a high crystallinity of d ₀₀₂ of 3.37 Å or less is used, the decomposition deterioration of the non-aqueous electrolyte is caused. Is particularly noticeable. Examples of such a carbon material having high crystallinity include graphite (natural graphite and artificial graphite), and modified coke having a d ₀₀₂ value of 3.37 Å or less by enhancing crystallinity by, for example, high pressure treatment.

The addition amount of phosphoric acid triester in the present invention is regulated to 0.1 to 10% by weight with respect to the total amount of the non-aqueous electrolyte, when the addition amount is less than 0.1% by weight. ,
This is because a film with a sufficient thickness cannot be formed, so that the decomposition of the non-aqueous electrolyte on the surface of the carbon material cannot be sufficiently suppressed and the cycle characteristics cannot be sufficiently improved. If more than 10% by weight, the coating becomes too thick and the lithium ion permeability deteriorates, so the reaction resistance of the electrode plate increases and the cycle characteristics deteriorate.

According to the present invention, decomposition and deterioration of a non-aqueous electrolytic solution, which has been a problem when a highly crystalline carbon material is used as a negative electrode material, is caused by adding a predetermined amount of phosphoric acid triester to the non-aqueous electrolytic solution. By forming a film on the surface of the carbon material, the decomposition of the non-aqueous electrolyte is suppressed, and thus the cycle characteristics are improved. Therefore, as the positive electrode material, the solute of the non-aqueous electrolyte solution, the solvent, and the like, various materials that have been conventionally proposed or put into practical use for the non-aqueous electrolyte secondary battery can be used without particular limitation.

LiCoO is used as the positive electrode material (active material).
_{2, LiNiO 2, LiMnO 2,} LiFeO 2 and the like, and as a non-aqueous electrolyte solution, ethylene carbonate, vinylene carbonate, and an organic solvent such as propylene carbonate, these and dimethyl carbonate, diethyl carbonate, 1,2-dimethoxyethane , 1, 2
In a mixed solvent with a low boiling point solvent such as diethoxyethane or ethoxymethoxyethane, LiPF ₆ , LiClO ₄ ,
A solution in which a solute such as LiCF ₃ SO ₃ is dissolved at a ratio of 0.7 to 1.5 M (mol / liter) is exemplified.

[0013]

[Function] Since the phosphoric acid triester added to the non-aqueous electrolyte is adsorbed on the surface of the carbon material having high crystallinity and an adsorption film (protective film) is formed on the surface of the carbon material, the non-electrolytic property on the surface of the carbon material is reduced. Degradation and deterioration of the water electrolyte is suppressed.

By the way, also in a non-aqueous electrolyte secondary battery using metallic lithium as a negative electrode material, a coating formed of a reaction product (organic substance) by reacting metallic lithium with the non-aqueous electrolyte as the charging / discharging cycle progresses. Is generated on the surface of metallic lithium, and the reaction of the electrode plate increases due to the formation of this coating, which causes a problem of deterioration in cycle characteristics. However, when metallic lithium is the negative electrode material, even if phosphoric acid triester is added to the non-aqueous electrolyte, the adsorption of phosphoric acid triester on the reaction product film of metallic lithium and the non-aqueous electrolyte is adsorbed. Only more coating is formed, and cycle performance is not improved. Therefore, it can be said that the effect of improving the cycle characteristics by adding the phosphoric acid triester to the non-aqueous electrolyte can be recognized only when the negative electrode material is a carbon material having high crystallinity.

[0015]

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.

Example 1 AA type (AA size) non-aqueous electrolyte secondary battery (the battery of the present invention) was produced.

[Positive electrode] LiCoO ₂ as a positive electrode active material
And a mixture of artificial graphite as a conductive agent in a weight ratio of 9: 1 were dispersed in a 5 wt% N-methylpyrrolidone (NMP) solution of polyvinylidene fluoride to prepare a slurry. After applying on both sides of the aluminum foil as the positive electrode current collector by the doctor blade method, 15
Vacuum drying was carried out at 0 ° C. for 2 hours to prepare a positive electrode.

[Negative Electrode] Graphite powder (d ₀₀₂ = 3.354)
Å) 5% by weight of polyvinylidene fluoride as a binder
A slurry was prepared by dispersing the slurry in an NMP solution, and the slurry was applied to both surfaces of a copper foil as a negative electrode current collector by a doctor blade method, and then vacuum dried at 150 ° C. for 2 hours to produce a negative electrode.

[Non-aqueous Electrolyte] LiPF ₆ was added to a mixed solvent of ethylene carbonate and dimethyl carbonate in an equal volume.
Was dissolved at a ratio of 1 M, and then trimethyl phosphate was added and mixed to prepare a non-aqueous electrolytic solution containing 0.1% by weight of trimethyl phosphate.

[Production of Battery] AA-type battery BA1 of the present invention was produced using the positive and negative electrodes and the non-aqueous electrolyte described above. As the separator, a polypropylene microporous film (Hoechst Celanese Co., Ltd., trade name “Celgard”) was used and impregnated with the above non-aqueous electrolyte.

FIG. 1 is a sectional view schematically showing the produced battery BA1 of the present invention. The illustrated battery BA1 of the present invention includes a positive electrode 1, a negative electrode 2, a separator 3 for separating these electrodes, a positive electrode lead 4, and a negative electrode. Lead 5, positive electrode external terminal 6, negative electrode can 7
And so on. The positive electrode 1 and the negative electrode 2 are housed in the negative electrode can 7 in a spirally wound state via the separator 3 in which the non-aqueous electrolyte solution is injected, and the positive electrode 1 is connected to the outside of the positive electrode via the positive electrode lead 4. The terminal 6 and the negative electrode 2 are connected to the negative electrode can 7 via the negative electrode lead 5 so that chemical energy generated inside the battery can be taken out as electric energy to the outside.

Comparative Example 1 A non-aqueous electrolytic solution was prepared in the same manner as in Example 1 except that trimethyl phosphate was not added and mixed. Next, an AA type comparative battery BC1 was produced in the same manner as in Example 1 except that this nonaqueous electrolytic solution was used.

[Cycle Characteristics] The battery BA1 of the present invention and the comparative battery BC1 (the discharge capacity at the beginning of the cycle are both 60
It is 0 mAh. ) Was charged to a final charge voltage of 4.2 V at 200 mA and then discharged to a final discharge voltage of 2.75 V at 200 mA to examine the cycle characteristics of each battery. The results are shown in Figure 2.

FIG. 2 is a graph showing the cycle characteristics of each battery, in which the vertical axis represents the discharge capacity (mAh) and the horizontal axis represents the number of cycles (times). As shown in FIG. The discharge capacity at the 1000th cycle of the battery BA1 is 581.
The discharge capacity at 1000th cycle of the comparative battery BC1 is as small as 482.3 mAh (capacity deterioration rate: 20%), while it is as large as 9 mAh (capacity deterioration rate: 3.5%). From this, it is understood that the decrease in discharge capacity due to the decomposition of the non-aqueous electrolyte during charge / discharge cycles is significantly suppressed by adding trimethyl phosphate to the non-aqueous electrolyte.

<Relationship between Addition Amount of Trimethyl Phosphate and Cycle Characteristics> Example 1 except that the addition amount of trimethyl phosphate was 0.05% by weight, 5% by weight, 10% by weight or 20% by weight. Similarly, a battery of the present invention and a comparative battery were produced. Next, a charge / discharge cycle test was performed under the same conditions as above to determine the capacity deterioration rate at the 1000th cycle of each battery, and the relationship between the amount of trimethyl phosphate added and the cycle characteristics was investigated. The results are shown in Table 1. In addition, in Table 1, 10 of the battery BA1 of the present invention and the comparative battery BC1 shown in FIG.
The capacity deterioration rate at the 00th cycle is also shown for convenience of comparison.

[0026]

[Table 1]

From Table 1, when the amount of trimethyl phosphate added to the non-aqueous electrolyte is 0.1 to 10% by weight, the capacity deterioration rate can be made particularly small and excellent cycle characteristics are exhibited. It can be seen that a water electrolyte secondary battery can be obtained.

In the above embodiments, the case where the present invention is applied to an AA type battery has been described as an example. However, the shape of the battery of the present invention is not particularly limited, and a flat type, a square type, etc. The present invention can be applied to non-aqueous electrolyte secondary batteries of various shapes.

In the above embodiments, the case where trimethyl phosphate is used as the phosphate triester has been described as an example. However, in addition to triethyl phosphate, tributyl phosphate, triphenyl phosphate, and other phosphates. Even when a triester is used, it is possible to obtain a non-aqueous electrolyte secondary battery exhibiting similar excellent cycle characteristics.

[0030]

Since the decomposition of the non-aqueous electrolyte solution on the surface of the carbon material is suppressed by the coating film of phosphoric acid triester formed on the surface of the carbon material, the capacity of the battery of the present invention is deteriorated as the charge / discharge cycle progresses. The present invention has excellent peculiar effects such as a low rate and excellent cycle characteristics.

[Brief description of drawings]

FIG. 1 is a sectional view of an AA battery of the present invention.

FIG. 2 is a graph showing cycle characteristics of a battery of the present invention and a comparative battery.

[Explanation of symbols]

 BA1 Inventive battery 1 Positive electrode 2 Negative electrode 3 Separator

Front page continuation (72) Inventor Koji Nishio 2-18 Keihan Hondori, Moriguchi City, Osaka Sanyo Electric Co., Ltd. (72) Inventor Toshihiko Saito 2-18 Keihan Hondori, Moriguchi, Osaka Sanyo Electric Co., Ltd.

Claims (2)

[Claims] 1. A non-aqueous electrolyte solution comprising a positive electrode, a negative electrode whose negative electrode material is a carbon material having a d value (d ₀₀₂ ) of 3.37 Å or less on a lattice plane (002) plane, and a non-aqueous electrolyte solution. In the secondary battery, a phosphoric acid triester represented by the following chemical formula 1 is added in an amount of 0.1 to 10% by weight to the non-aqueous electrolyte solution. [Chemical 1] [In the formula, R ¹ , R ² and R ³ each independently represent an alkyl group having 1 to 4 carbon atoms or a phenyl group. ] 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the phosphoric acid triester is trimethyl phosphate, triethyl phosphate, tripropyl phosphate, tributyl phosphate or triphenyl phosphate.