JP2002216850A - Nonaqueous electrolyte secondary cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary cell with excellent cycle property, which can obtain high charging and discharging efficiency and high discharging capacity. SOLUTION: For the nonaqueous electrolyte secondary cell comprising a positive electrode and a negative electrode enabled to absorb and release lithium, a nonaqueous electrolyte composed of a solute and an organic solvent, a separator, and an outer casing, the organic solvent is made to contain ethylene sulfide, and a pulse current is added at the initial charging.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a non-aqueous electrolyte secondary battery. More specifically, the present invention relates to an improvement of a lithium secondary battery using an electrolyte containing ethylene sulfite. Since the battery of the present invention can obtain high charge / discharge efficiency and high discharge capacity at the initial stage and has improved cycle stability,
The performance of the nonaqueous electrolyte secondary battery can be improved.

[0002]

2. Description of the Related Art With the recent reduction in the weight and size of electric products, lithium secondary batteries having a high energy density have attracted attention and various studies have been conducted. In addition, with the expansion of the application field of the lithium secondary battery, improvement in battery characteristics is also demanded. As a solvent for the electrolyte of such a lithium secondary battery, an electrolyte containing ethylene sulfite has been proposed (JP-A-6-302336). Ethylene sulphite is a solvent having a high dielectric constant and a low melting point, and exhibits excellent electric conductivity even at a low temperature, and therefore has excellent properties as a solvent for an electrolytic solution.

[0003]

However, a secondary battery using an electrolyte containing ethylene sulfite has the drawbacks of low initial charge / discharge efficiency, low reversible capacity, and poor cycle stability. The present invention provides a non-aqueous electrolyte secondary battery having high energy density and excellent cycle characteristics, which contains ethylene sulfite as a solvent for the electrolyte of the non-aqueous electrolyte secondary battery.

[0004]

Means for Solving the Problems The present inventors have conducted intensive studies in view of the above circumstances, and as a result, have found that in a nonaqueous lithium secondary battery, an organic solvent containing ethylene sulfite is used.
It has been found that by applying a current pulse at the time of the first charge, the initial discharge capacity is increased and the cycle stability is improved, and the present invention has been completed.

That is, the gist of the present invention is to store and store lithium.
A non-aqueous electrolyte solution comprising a negative electrode and a positive electrode capable of being released, a solute and an organic solvent, and a separator and an outer can, in which ethylene sulfite is wrapped as the organic solvent. A non-aqueous electrolyte secondary battery characterized by applying a current pulse at the time of initial charging.

[0006]

[Function] By applying a current pulse during the first charge,
It is thought that a stable protective film with low resistance by ethylene sulfite is selectively formed on the negative electrode, minimizing decomposition of the electrolyte, and enabling smooth absorption and desorption of lithium to the negative electrode. .

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is characterized in that a lithium secondary battery using a non-aqueous electrolyte uses an organic solvent containing ethylene sulfite and applies a current pulse at the time of initial charging. .

The waveform of the current pulse is not particularly limited. For example, a rectangular wave, a triangular wave, a sine wave, or the like can be used. Further, a waveform in which a current pulse is superimposed on a DC bias current may be used. Among them, a rectangular wave is preferable from the viewpoint of easy current control. The pulse current at the time of pulse charging using the rectangular wave current is generally 0.01 to 100 A / g, preferably 0.1 to 10 A / g, based on the weight of the negative electrode. Pulse current is less than 0.01 A / g or pulse current is 100 A
If it exceeds / g, the effect of applying the current pulse is small, which is not desirable. The pulse width at the time of pulse charging using a rectangular wave current is generally 0.01 to 300 seconds, and is preferably used in a range of 0.1 to 60 seconds. If the pulse width is smaller than 0.01 second, control for applying a current pulse becomes difficult, which is not desirable. If the pulse width exceeds 300 seconds, the effect is close to the case where no pulse current is applied, and the effect is small, which is not desirable. The pulse interval at the time of pulse charging using a rectangular wave current is generally in the range of 0.1 to 300 seconds, preferably 1 to 100 seconds. In general, the number of pulses during pulse charging using a square wave current is 10 to 10.
It is used in a range of 1,000 times, preferably 50 to 500 times. These pulse currents need to be applied at the initial stage of the first charge, but can be charged with a constant current after pulse charge of 5 to 80%, preferably about 10 to 50% of the charge capacity. This is the preferred method.

The proportion of ethylene sulfite in the solvent of the non-aqueous electrolyte is generally 0.1 to 60% by volume, preferably 1 to 50% by volume. The solvent is a second solvent component other than the ethylene sulfite, ethylene carbonate, propylene carbonate,
Cyclic carbonates such as butylene carbonate, chain carbonates such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate, cyclic esters such as γ-butyrolactone and γ-valerolactone, and chain esters such as methyl acetate and methyl propionate , Tetrahydrofuran, 2-methyltetrahydrofuran, cyclic ethers such as tetrahydropyran, dimethoxyethane,
Chain ethers such as dimethoxymethane, sulfolane,
Sulfur-containing organic solvents such as diethyl sulfone can be mixed and used. These solvents may be used as a mixture of two or more kinds. As the solute, LiClO ₄ , LiPF ₆ , L
Inorganic lithium salt such as iBF ₄ or LiCF ₃ SO ₃ , L
iN (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ CF ₂ S
O ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ S
Fluorine-containing organic lithium salts such as O ₂ ) and LiC (CF ₃ SO ₂ ) ₃ can be used. These solutes may be used as a mixture of two or more kinds. The molar concentration of the solute lithium salt in the electrolyte is desirably 0.5 to 2.0 mol / liter. If the amount is less than 0.5 mol / L or exceeds 2.0 mol / L, the electric conductivity of the electrolytic solution is low, and the performance of the battery is undesirably reduced.

The negative electrode active material constituting the battery includes:
The d value of the lattice plane (002 plane) in X-ray diffraction is 0.33
5 to 0.34 nm artificial graphite and purified natural graphite or materials obtained by subjecting these graphites to various surface treatments including pitch, non-graphitic carbon materials such as non-graphitizable carbon or low-temperature calcined carbon, tin oxide, silicon oxide And the like, and further, lithium metal and various lithium alloys can be used.
As the shape of the negative electrode, a sheet electrode which is mixed with a binder and a conductive agent as necessary and then applied to a current collector and a pellet electrode which has been subjected to press molding can be used. As the material of the current collector for the negative electrode, metals such as copper, nickel, and stainless steel are used, and among these, copper foil is preferable from the viewpoint of easy processing into a thin film and cost. As the separator constituting the battery, a porous sheet or a nonwoven fabric made of a polyolefin such as polyethylene or polypropylene can be used.

The positive electrode active material constituting the battery includes:
Materials that can occlude and release lithium, such as lithium transition metal composite oxide materials such as lithium cobalt oxide, lithium nickel oxide, and lithium manganese oxide, can be used. As the shape of the positive electrode, a sheet electrode applied to a current collector after mixing with a binder and a conductive agent, if necessary, and a pellet electrode subjected to press molding can be used. As a material of the positive electrode current collector, a valve metal such as aluminum, titanium, and tantalum or an alloy thereof is used. Among these valve metals, aluminum or its alloy is particularly preferable in terms of energy density because of its light weight. It is not preferable to use a metal material other than the valve metal, for example, stainless steel, because the oxidative decomposition reaction of ethylene sulfite proceeds and the cycle characteristics deteriorate.

As the shape of the battery, a cylinder type in which a sheet electrode and a separator are formed in a spiral shape, a cylinder type having an inside-out structure in which a pellet electrode and a separator are combined, and a coin type in which a pellet electrode and a separator are laminated can be used. As the material of the outer can of the battery, stainless steel is preferably used, but a portion electrically connected to the positive electrode and in contact with the electrolytic solution may be a valve metal such as aluminum to suppress oxidative decomposition of ethylene sulfite. desirable. Further, aluminum or an aluminum alloy may be used as the outer can material. In addition,
The outer can mentioned here includes a lead wire housed inside the battery, a safety valve that operates when the internal pressure inside the battery rises, and the like.

[0013]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples, but the present invention is not limited to these examples unless it exceeds the gist thereof.

(Example 1) LiCoO as a positive electrode active material
₂ (85 parts by weight), carbon black (6 parts by weight) and polyvinylidene fluoride (9 parts by weight) were added and mixed, and dispersed with N-methyl-2-pyrrolidone to form a slurry. A 20 μm-thick aluminum foil was uniformly coated, dried, and punched into a predetermined shape to obtain a positive electrode. Polyvinylidene fluoride (6 parts by weight) was mixed with artificial graphite powder KS-44 (manufactured by Timcal Co., Ltd., trade name) (94 parts by weight) as a negative electrode active material and dispersed in N-methyl-2-pyrrolidone to form a slurry. The resultant was uniformly coated on a copper foil having a thickness of 18 μm as a negative electrode current collector, dried, and punched into a predetermined shape to obtain a negative electrode. As for the electrolytic solution, fully dried lithium hexafluorophosphate (LiPF ₆ ) was used as a solute in a dry argon atmosphere, and ethylene sulfite (ES) and propylene carbonate (P) were used.
C) and a solution having a composition ratio of 5:95 by volume.
The F ₆ was prepared by dissolving at a rate of 1 mole / liter.

Using these positive electrode, negative electrode, and electrolyte, a coin-type nonaqueous electrolyte battery as shown in FIG. 1 was prepared in a dry argon atmosphere. In the following, referring to FIG. 1, the positive electrode 1 and the negative electrode 2 are accommodated in a stainless steel positive electrode can 3 and a sealing plate 4, respectively, and a separator 5 made of a polypropylene microporous film impregnated with each electrolytic solution is formed. At this time, the inside of the positive electrode can 3 was used by previously covering the inside of the positive electrode can 3 with an aluminum foil 6 in order to use a valve metal as a material of the liquid contact portion on the positive electrode side. Subsequently, the positive electrode can 3 and the sealing plate 4 were caulked and sealed via the gasket 7 to complete a coin-type battery. A current pulse having a rectangular waveform was applied to the battery at the time of the first charge as shown in FIG. The applied current pulse has a pulse current of 0.1 A / g per negative electrode active material weight,
The pulse width was 60 seconds, the pulse interval was 300 seconds, and the number of pulses was 60 times. This battery was charged at a constant current of 0.1 A / g per weight of the negative electrode active material at 25 ° C. to a final charge voltage of 4.2.
A charge / discharge test was performed up to 100 cycles at V and a discharge end voltage of 2.5 V.

Comparative Example 1 A coin-type battery was manufactured using the same positive electrode, negative electrode, and electrolyte as in Example 1, and a current pulse was not applied at the time of the first charge.
g at a constant current of 4.2 V and a discharge end voltage of 4.2 V.
The charge / discharge test was performed up to 100 cycles at 5V.

Comparative Example 2 A coin-type battery was manufactured using the same positive electrode, negative electrode, and electrolyte as in Example 1, and a current pulse was not applied at the time of initial charging.
g of constant current charging. Thereafter, a charge / discharge test was performed at a constant current of 0.1 A / g per negative electrode active material weight at a charge end voltage of 4.2 V and at a discharge end voltage of 2.5 V up to 100 cycles.

Example 2 Ethylene sulfite (E
S) and propylene carbonate (PC) in a volume ratio of 1
LiPF ₆ was added to the solution mixed at a composition of 0:90 at 1 mol / mol.
A coin-type battery was prepared using the same positive electrode and negative electrode as in Example 1 using an electrolytic solution dissolved at a liter ratio. A current pulse having a rectangular waveform was applied to the battery at the time of the first charge. The applied current pulse had a pulse current of 0.4 A / g per negative electrode active material weight, a pulse width of 5 seconds, a pulse interval of 25 seconds, and a pulse count of 120 times. Replace this battery with 2
At 5 ° C., at a constant current of 0.1 A / g per weight of the negative electrode active material, a charge end voltage of 4.2 V and a discharge end voltage of 2.5 V, 1
A charge / discharge test was performed up to 00 cycles.

Example 3 A coin-type battery was manufactured using the same positive electrode, negative electrode, and electrolyte as in Example 2, and a current pulse having a rectangular waveform applied at the time of the first charge was applied. The pulse width was 1 A / g, the pulse width was 1 second, the pulse interval was 10 seconds, and the number of pulses was 70 times. This battery was subjected to a charge / discharge test at 25 ° C. at a constant current of 0.1 A / g per weight of the negative electrode active material, a charge end voltage of 4.2 V, and a discharge end voltage of 2.5 V up to 100 cycles.

Example 4 A coin-type battery was prepared using the same positive electrode, negative electrode, and electrolyte as in Example 2, and a current pulse having a rectangular waveform applied at the time of the first charge was applied. 1 A / g, pulse width was 1 second, pulse interval was 20 seconds, and the number of pulses was 250 times. The battery was charged at a constant current of 0.1 A / g per negative electrode active material weight at 25 ° C. at a charge end voltage of 4.2 V and a discharge end voltage of 2.5 V.
And a charge / discharge test was performed up to 100 cycles.

Example 5 A coin-type battery was manufactured using the same positive electrode, negative electrode, and electrolyte as in Example 2, and a current pulse having a rectangular waveform applied at the time of the first charge was applied. 2 A / g, a pulse width of 0.1 second, a pulse interval of 20 seconds, and a pulse count of 120 times. At 25 ° C., the battery was charged at 0.1 A /
g at a constant current of 4.2 V and a discharge end voltage of 4.2 V.
The charge / discharge test was performed up to 100 cycles at 5V.

Example 6 Ethylene Sulfite (E
S) and propylene carbonate (PC) in a volume ratio of 1
LiN (CF ₃ CF ₂₎ was added to the solution mixed with the composition of 0:90.
Using an electrolytic solution in which SO ₂ ) ₂ was dissolved at a ratio of 1 mol / liter, a coin-type battery was prepared using the same positive electrode and negative electrode as in Example 1. A constant current pulse was applied to this battery during the first charge. The current pulse having the added rectangular waveform has a pulse current of 0.4 A / g per negative electrode active material weight, a pulse width of 5 seconds, a pulse interval of 25 seconds, and a pulse number of 12
It was set to 0 times. The battery was charged at a constant current of 0.1 A / g per negative electrode active material weight at 25 ° C. with a charge end voltage of 4.2 V,
A charge / discharge test was performed up to 100 cycles at a discharge end voltage of 2.5 V. In the batteries of Examples 1, 2, 3, 4, 5 and 6, and Comparative Examples 1 and 2, the discharge capacity per 5% cycle negative electrode active material weight and the discharge capacity at the 100th cycle relative to the discharge capacity at the 5th cycle Is shown in Table 1. As is clear from Examples 1, 2, 3, 4, 5, and 6 and Comparative Examples 1 and 2, when a current pulse was applied during the initial charging, the initial capacity was increased and the capacity retention in the cycle test was significantly improved. I have. Example 1 and Comparative Example 1
FIG. 3 shows the charge / discharge efficiency of each cycle up to 20 cycles in the battery of FIG. As is clear from FIG. 3, the charging / discharging efficiency of the battery to which the current pulse was applied at the time of the initial charge is remarkably improved as compared with the battery to which the current pulse was not applied.

[0023]

[Table 1]

[0024]

According to the present invention, when a non-aqueous electrolyte secondary battery contains ethylene sulphite as an organic solvent of the electrolyte and performs current pulse charging at the time of initial charging, a high initial charge / discharge efficiency and a high discharge capacity can be obtained. In addition, a battery having excellent cycle characteristics can be produced, which can contribute to the performance enhancement of a non-aqueous electrolyte secondary battery.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing the structure of each coin-type battery of an example of the present invention and a comparative example.

FIG. 2 is a diagram showing a current pulse charging method of the present invention.

FIG. 3 is a diagram showing the charge / discharge efficiency of each coin-type battery of Example 6 of the present invention and Comparative Example 1 up to 40 cycles.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Positive electrode can 4 Sealing plate 5 Separator 6 Aluminum foil 7 Gasket

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Tomohiro Sato, Investigator, Tsukuba Research Laboratories, Mitsubishi Chemical Co., Ltd. No. 3-1 Mitsubishi Chemical Corporation Tsukuba Research Laboratory F term (reference) 5H029 AJ02 AJ03 AJ05 AK03 AL02 AL06 AL07 AL12 AM01 AM02 AM03 AM04 AM05 AM07 BJ02 BJ03 BJ04 CJ16 DJ02 DJ04 DJ07 EJ01 HJ00 HJ07 HJ17

Claims (5)

[Claims] 1. A non-aqueous electrolyte battery comprising a negative electrode and a positive electrode capable of inserting and extracting lithium, a non-aqueous electrolytic solution comprising a solute and an organic solvent, a separator and an outer can. A non-aqueous electrolyte secondary battery comprising ethylene sulfite as an organic solvent and applying a current pulse at the time of initial charging. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the content of ethylene sulfite in the organic solvent is 0.05 to 60% by volume. 3. The current pulse has a constant current value of 0.01 to 100 A / g per negative electrode active material weight, a pulse width of 0.01 to 300 seconds, a pulse interval of 0.1 to 300 seconds, and the number of times of pulse application. The non-aqueous electrolyte secondary battery according to claim 1, wherein a rectangular wave having a value of 10 to 1000 times is used. 4. The non-aqueous electrolyte secondary solution according to claim 1, wherein a material of a portion of the positive electrode current collector and the cathode side outer can that is in contact with the electrolyte is a valve metal or an alloy thereof. battery. 5. The non-aqueous electrolyte secondary battery according to claim 4, wherein the valve metal or its alloy is aluminum or its alloy.