JP2001307774A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery having an improved safety during overcharge and less swelling when stored at a high temperature. SOLUTION: The nonaqueous electrolyte secondary battery comprises a positive electrode mix containing a positive active material and lithium carbonate and an electrolyte containing LiPF6 and further at least one kind selected from the groups of LiBF4, imide-based lithium salt and Li(C2F5)nPF6-n (n=1 to 6).

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.

[0002]

2. Description of the Related Art In recent years, with the rapid miniaturization and diversification of portable cellular phones, portable electronic devices and portable information terminals for consumer use, small, lightweight and high energy density batteries have been developed. Further, there is a strong demand for the development of a secondary battery capable of repeatedly charging and discharging for a long period of time. Above all,
Non-aqueous electrolyte secondary batteries such as lithium secondary batteries are the most promising secondary batteries that meet these requirements, compared to lead batteries and nickel cadmium batteries that use aqueous electrolytes. Have been.

[0003] The electrolyte of a non-aqueous electrolyte secondary battery generally includes LiPF _{6 as} a mixed solvent of a high dielectric constant solvent such as ethylene carbonate or propylene carbonate and a low viscosity solvent such as dimethyl carbonate or diethyl carbonate.
And a solution in which a supporting salt such as LiBF ₄ is dissolved is used. Such an electrolytic solution is required to have high electric conductivity and excellent safety, and to be electrically and chemically stable.

It is necessary that an electrolyte for a lithium battery be stable with respect to a lithium anode, but it is said that there is no thermodynamically stable solvent for lithium. At times, the electrolyte is decomposed on the negative electrode, and the reaction product forms an ion-conductive protective film on the lithium surface,
So-called SEI (Solid Electrolyt)
e Interface), which suppresses the reaction between the electrode and the electrolytic solution, and is considered to be stabilized.

[0005]

However, in such a non-aqueous electrolyte secondary battery, when a power supply circuit of an electronic device or a charging device fails and becomes overcharged, abnormal heat generation occurs in the battery. However, in extreme cases, there is a concern that the battery may be damaged or fired.Therefore, it is important to effectively suppress heat generation and ensure battery safety so that the battery does not run away from heat. ing.

[0006] As a measure for preventing the explosion or ignition of a battery at the time of overcharging, a method of controlling a charging voltage by a charger is mainly used. However, at present, the use of protection circuits and protection elements greatly imposes restrictions on miniaturization and cost reduction of battery packs. Therefore, it is desired to ensure safety without protection circuits and protection elements.

In addition, as a measure for preventing rupture and ignition at the time of overcharging other than the protection circuit, by adding lithium carbonate to the positive electrode mixture, 5.0 V (vs. Li ⁺ / L) at the time of overcharging.
i) Lithium carbonate is decomposed in the vicinity, and the generated decomposed gas raises the internal pressure in the battery to promote the operation of safety mechanisms such as current cutoff elements and rupture valves, thereby ensuring safety during overcharge. It has also been devised to do so.

However, lithium carbonate easily reacts with LiPF ₆ contained in the electrolytic solution at a high temperature, and when it reacts, there is a problem that a decomposition product gas is generated, and the pressure inside the battery is increased to cause swelling.

The present invention solves the above problems, and provides a non-aqueous electrolyte secondary battery which is excellent in safety at the time of overcharge and which can suppress the swelling of the battery at the time of high temperature storage.

[0010]

SUMMARY OF THE INVENTION The non-aqueous electrolyte secondary of the present invention
In the battery, the positive electrode mixture includes a positive electrode active material and lithium carbonate,
The electrolyte is LiPF₆And further containing LiBF_Four, Imide type
Lithium salt, Li (C _TwoF_Five)_nPF_6-n(However, n = 1 ~
6) contain at least one member selected from the group consisting of
It is characterized by.

Further, the present invention relates to the above non-aqueous electrolyte secondary battery.
The LiPF contained in the electrolyte ₆Concentration of 0.2 to
0.5 mol / l and contained in the electrolyte
Total salt concentration is 1.0 to 1.5 mol / l
It is characterized by.

Further, the present invention provides the above non-aqueous electrolyte secondary battery, wherein the content of lithium carbonate in the positive electrode mixture is 0.0
It is characterized by being 1 to 10 wt%.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The non-aqueous electrolyte secondary battery of the present invention comprises:
The positive electrode mixture contains a positive electrode active material and lithium carbonate, and the electrolyte solution
LiPF₆And further containing LiBF_Four, Imide lithium
Salt, Li (C _TwoF_Five)_nPF_6-n(However, n = 1 to 6)
At least one selected from the group consisting of:

[0014] By containing lithium carbonate in the positive electrode mixture, when the battery is overcharged, lithium carbonate is decomposed to promote the operation of safety mechanisms such as a current cutoff element and a burst valve. Can generate a sufficient amount of gas.

The electrolyte is LiPF ₆ and LiBF ₄ , an imide-based lithium salt, Li (C ₂ F ₅ ) _n PF _6-n (where n =
By containing at least one selected from the group consisting of 1) to 6), while maintaining the battery performance, the amount of LiPF ₆ contained in the electrolyte solution was reduced, so that it was contained in the positive electrode mixture. The reaction between lithium carbonate and LiPF ₆ is limited, and gas generation before overcharging is suppressed,
Swelling of the battery during high-temperature storage can be suppressed.

Furthermore, in the present invention, the electrolyte solution contains
LiPF₆Concentration of 0.2 to 0.5 mol / l
And the total concentration of lithium salts contained in the electrolyte is 1.
It is preferably from 0 to 1.5 mol / l. For this reason
As LiPF₆Is less than 0.2 mol / l
In the case of the initial charge, dense ionic conduction on the negative electrode
Protective film, so-called SEI (Solid Elecr)
control interface) is not stabilized
Therefore, a sufficient discharge capacity cannot be obtained. Also, the positive electrode
To limit the reaction with lithium carbonate in the mixture,
LiPF during lysis ₆Concentration should be 0.5 mol / l or less.
Need to be

When the total concentration of lithium salt contained in the electrolyte is in the range of 1.0 to 1.5 mol / l, it is possible to obtain an electrolyte having conductivity enough to secure battery performance. . As the total concentration of the lithium salt contained in the electrolytic solution deviates from this range, the conductivity decreases.

In the present invention, the content of lithium carbonate in the positive electrode mixture is preferably 0.01 to 10% by weight. The reason is that if the content is less than 0.01 wt%, the gas is decomposed at the time of overcharge and a sufficient amount of gas that can promote a safety mechanism such as a current cutoff element or a burst valve is not generated,
If the content is more than 10 wt%, the amount of the positive electrode active material is limited, which causes a decrease in the energy density of the battery.

The positive electrode of the non-aqueous electrolyte secondary battery according to the present invention
As the active material, a general formula LixMO _Two(However, M is one
Or more transition metals) alone or
Can be used as a mixture of two or more. In particular, release
From the height of the electric voltage, Co, Ni, Mn as transition metals M
It is desirable to use a transition metal consisting of

As the negative electrode active material in the present invention, cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, carbon fibers, or metallic lithium, lithium alloy, polyacene, etc. are used alone. Alternatively, two or more kinds can be used as a mixture, but it is particularly preferable to use a carbonaceous material from the viewpoint of high safety.

Examples of the solvent for the non-aqueous electrolyte include ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, trifluoropropylene carbonate, γ-butyrolactone, 2-methyl-γ-butyl lactone, acetyl-γ-butyrolactone, and γ-valero. Lactone, sulfolane, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 3-methyl-1,3-dioxolan, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, Dimethyl carbonate, diethyl carbonate,
Ethyl methyl carbonate, dipropyl carbonate,
Methyl propyl carbonate, ethyl isopropyl carbonate, dibutyl carbonate and the like can be used alone or in combination of two or more.

The imide-based lithium salt used in the present invention includes LiN (SO ₂ R ₁ ) (SO ₂ R ₂ ) (where R ₁ ,
As R ₂ , a compound represented by C _n F _{2n + 1} , n = 1 to 8) can be used.

Further, as the electrolyte, a solid ion-conductive polymer electrolyte can be used as an auxiliary. In this case, as a configuration of the non-aqueous electrolyte secondary battery, a combination of a positive electrode, a negative electrode and a separator with an organic or inorganic solid electrolyte and the above non-aqueous electrolyte, or a positive electrode, an organic or inorganic solid electrolyte as a negative electrode and a separator A combination of the membrane and the above-mentioned non-aqueous electrolyte may be used.

When the polymer electrolyte membrane is made of polyethylene oxide, polyacrylonitrile, polyethylene glycol, or a modified product thereof, it is lightweight and flexible, which is advantageous when used for a wound electrode plate. further,
In addition to the polymer electrolyte, an inorganic solid electrolyte or a mixed material of an organic polymer electrolyte and an inorganic solid electrolyte can be used.

The non-aqueous electrolyte secondary battery of the present invention comprises a positive electrode, a negative electrode, and a combination of a separator and a non-aqueous electrolyte.
A porous polymer membrane such as a porous polyethylene membrane or an ion-conducting polymer electrolyte membrane can be used alone or in combination.

Further, the shape of the battery is cylindrical, square,
Various shapes such as a coin type, a button type, and a laminate type can be used.

[0027]

[Example 1] The content of lithium carbonate in the positive electrode mixture was 3 wt%, the electrolyte contained LiPF ₆ , and the electrolyte contained LiBF ₄ , various imide-based lithium salts, L
A prismatic non-aqueous electrolyte secondary battery to which i (C ₂ F ₅ ) _n PF _6-n (where n = 1 to 6) was added was prepared, and its characteristics were compared.

FIG. 1 shows a schematic cross section of the prismatic nonaqueous electrolyte secondary battery used in the present invention. In FIG. 1, 1 is a non-aqueous electrolyte secondary battery, 2 is a wound electrode group, 3 is a positive electrode, 4 is a negative electrode, 5 is a separator, 6 is a battery case, 7 is a battery cover, 8 is a safety valve, and 9 is a positive electrode. Terminal 10 is a positive electrode lead.

The prismatic nonaqueous electrolyte secondary battery comprises a positive electrode 3 formed by coating a positive electrode mixture on an aluminum current collector and a negative electrode 4 formed by coating a negative electrode mixture on a copper current collector. The spirally wound electrode group 2 wound through the separator 5 into which the liquid has been injected,
It is housed in a battery case 6 having a thickness of 7.8 mm. The battery lid 7 provided with the safety valve 8 is attached to the battery case 6 by laser welding, the positive electrode terminal 9 is connected to the positive electrode 3 via the positive electrode lead 10, and the negative electrode 4 is connected to the inner wall of the battery case 6 by contact. ing.

The positive electrode was prepared by mixing 87 parts by weight of LiCoO _{2 as} an active material, 5 parts by weight of acetylene black as a conductive material, 5 parts by weight of polyvinylidene fluoride as a binder, and 3 parts by weight of lithium carbonate. And dispersed in N-methyl-2-pyrrolidone to prepare a slurry. This slurry was uniformly applied to a 20 μm-thick aluminum current collector, dried, and then compression-molded by a roll press,
The positive electrode 3 was produced.

The negative electrode comprises 90 parts by weight of an active carbon material,
10 parts by weight of polyvinylidene fluoride were mixed to prepare a negative electrode mixture, and the mixture was dispersed in N-methyl-2-pyrrolidone to prepare a slurry. The slurry was uniformly applied to a 10 μm-thick copper current collector, dried, and then compression-molded by a roll press to produce a negative electrode 4. For the separator 5, a microporous polyethylene film having a thickness of 25 μm was used.

The electrolytic solution is a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 1: 1. First, LiPF ₆ is dissolved, and LiB
A predetermined amount of F ₄ or an imide-based lithium salt is dissolved to prepare an electrolytic solution.
Batteries of the present invention (Examples 1-1 to 1-9) of 00 mAh were produced.

In the above components and procedures, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 1: 1 as an electrolytic solution, and LiPF ₆ was dissolved at 1.5 mol / l. The electrolyte solution thus prepared was prepared as Comparative Example 1. Further, in the above-described components and procedure, a battery was prepared in which the concentration of LiPF ₆ in the electrolyte was 0.1 mol / l.

Table 1 summarizes the types and concentrations of lithium salts in the electrolyte solution of the prototype batteries.

[0035]

[Table 1]

Using the above-mentioned batteries (Examples 1-1 to 1-9 and Comparative Examples 1 and 2), constant-current and constant-voltage charging was performed at 25 ° C. and a current of 1 C for 3 hours, and then discharged at a current of 1 C. The discharge capacity at that time was determined. Further, overcharging was performed at a current of 25 ° C. and 1 C up to a charging end voltage of 10.0 V, and the state of the battery at that time was observed. In the storage test, the above battery was
The battery was charged at a constant current and a constant voltage up to 4.1 V with a current of C for 3 hours, stored in a charged state at 45 ° C. for one month, and discharged to obtain a battery thickness. Table 2 summarizes the test results.

[0037]

[Table 2]

From Table 2, it can be seen that Examples 1-1 to 1-9 of the present invention were obtained.
In the battery using the mixed electrolyte as described above, Comparative Example 1
The battery did not swell after standing as compared with the single-electrolyte battery using LiPF ₆ shown in FIG. In addition, no abnormalities were found in any of the batteries in the overcharge test results.

Further, when the concentration of LiPF _{6 in} the electrolytic solution is 0.1.
In the battery of Comparative Example 2 at 1 mol / l, a dense ion-conductive protective film, so-called SEI, was not stabilized on the negative electrode at the time of initial charging, so that a sufficient discharge capacity could not be obtained.

[Example 2] Next, the characteristics of the battery when the content of lithium carbonate in the positive electrode mixture was changed were compared. LiPF ₆ 0.5mol / l + LiN as salt of electrolyte
(CF ₃ SO ₂ ) ₂ except that 1.0 mol / l was used.
A rectangular non-aqueous electrolyte secondary battery similar to that of Example 1 was manufactured, and a test similar to that of Example 1 was performed. The test results are summarized in Table 3.

[0041]

[Table 3]

From the results shown in Table 3, it was found that the battery of Comparative Example 3 containing no lithium carbonate in the positive electrode mixture was inferior in safety during overcharge. Further, in the battery of Comparative Example 4 in which the content of lithium carbonate in the positive electrode mixture was 15 wt%, the discharge capacity was smaller than that of the batteries of Examples 2-1 to 2-6 of the present invention.

[0043]

According to the nonaqueous electrolyte secondary battery of the present invention, when overcharged, the lithium carbonate in the positive electrode mixture is decomposed, and the operation of safety mechanisms such as a current cutoff element and a rupture valve is performed. Gas can be generated in a sufficient amount to promote the reaction.

Further, LiPF contained in the electrolyte solution
_The reduced amount of ₆ limits the reaction between lithium carbonate and LiPF ₆ contained in the positive electrode mixture, suppresses gas generation before overcharging, and swells the battery during high-temperature storage. Can be suppressed.

As described above, the configuration of the present invention provides a non-aqueous electrolyte secondary battery which is excellent in safety at the time of overcharging and can suppress swelling of the battery at the time of high-temperature storage. Becomes possible.

[Brief description of the drawings]

FIG. 1 is a schematic cross section of a prismatic nonaqueous electrolyte secondary battery used in Examples and Comparative Examples of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Non-aqueous electrolyte secondary battery 2 Wound electrode group 3 Positive electrode 4 Negative electrode 5 Separator 6 Battery case 7 Battery cover 8 Safety valve 9 Positive electrode terminal 10 Positive electrode lead

 ──────────────────────────────────────────────────続 き Continued on the front page F-term (reference) 5H029 AJ02 AJ04 AJ12 AK03 AL06 AL07 AL12 AL16 AM02 AM03 AM04 AM05 AM07 AM16 BJ02 BJ03 BJ04 BJ12 BJ14 BJ27 DJ08 DJ09 EJ03 HJ01 HJ10 5H050 AA04 AA09 AA10 CB05 HA02

Claims (3)

[Claims] The positive electrode mixture contains a positive electrode active material and lithium carbonate, the electrolyte contains LiPF ₆ , and further includes LiBF ₄ , an imide-based lithium salt, Li (C ₂ F ₅ ) _n PF _6-n (provided that n
= 1 to 6), comprising at least one member selected from the group consisting of: 2. The concentration of LiPF ₆ contained in the electrolytic solution is 0.2 to 0.5 mol / l, and the total concentration of lithium salt contained in the electrolytic solution is 1.0 to 1.5 mol / l. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein: 3. The method according to claim 1, wherein the content of lithium carbonate in the positive electrode mixture is 0.01 to 10 wt%.
Or the non-aqueous electrolyte secondary battery according to 2.