JPH08329946A - Manufacture of nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE: To prevent deposition of metallic lithium on a negative electrode in low temperature charging by heat treating the negative electrode containing a carbon material capable of absorbing/releasing lithium and a binder or a thickening agent in the non-oxidizing atmosphere, then constituting a battery. CONSTITUTION: A negative electrode 3 containing a carbon material capable of absorbing/releasing lithium and a binder or a thickening agent is heat treated in the vacuum of 1333 pa, in the non-oxidizing atmosphere of a mixed gas of at least two gases selected from Ar, N2 , He, and air at 150-350 deg.C. The negative electrode 2 and a positive electrode 1 comprising lithium-containing composite oxide are spirally wound through a separator 5 to obtain an electrode group. The electrode group is housed in a battery case 8, a nonaqueous electrolyte is poured, then the battery case 8 is sealed with a sealing plate 10.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a non-aqueous electrolyte secondary battery, especially a negative electrode thereof.

[0002]

2. Description of the Related Art In recent years, portable electronic devices for consumer use,
Cordless is advancing rapidly. Along with this, there is an increasing demand for a small-sized, lightweight secondary battery having a high energy density, which serves as a driving power source. From this point of view, non-aqueous electrolyte secondary batteries, especially lithium secondary batteries, have great expectations as batteries having high voltage and high energy density, and their development is urgently needed.

Conventionally, manganese dioxide, vanadium pentoxide, titanium disulfide, etc. have been used as a positive electrode active material of a lithium secondary battery, and an attempt has been made to construct a battery with these positive electrodes, a lithium metal negative electrode and an organic electrolyte. Has been done. However, generally, in a secondary battery using lithium metal for the negative electrode, internal short circuit due to dendritic metal lithium (dendrites) deposited on the negative electrode surface during charging and charge / discharge characteristics due to side reaction between the deposited active lithium and the electrolytic solution Deterioration, further abnormal heat generation, and in extreme cases, ignition has been a major obstacle to practical use.
Furthermore, there are many problems in high-rate charge / discharge characteristics and over-discharge characteristics, and no satisfactory solution has been found.

In place of these negative electrode materials, a carbon material capable of inserting and extracting lithium by charge and discharge has various characteristics necessary for a practical battery such as safety, high rate charge and discharge characteristics, and charge and discharge cycle characteristics. Is attracting attention as.

To obtain high energy density batteries, charcoal
As a positive electrode active material with the use of a raw material for the negative electrode,
LiC, which is a compound having a higher voltage and containing Li
oO ₂, LiNiO₂, LiFeO₂, LiMn₂O_Four,
Some of these Co, Ni, Fe and Mn
It has been proposed to use complex oxides substituted with elements.
It

[0006]

When a carbon material is used for the negative electrode, lithium is occluded during charging and released during discharging, so that the charge / discharge reaction proceeds. However, as the charging current density is higher and the charging temperature is lower, the rate at which lithium is occluded cannot follow up, and metallic lithium is deposited on the negative electrode. Since this metallic lithium is electrochemically inactive, the discharge reaction is difficult to occur, and the charge / discharge efficiency of the battery tends to decrease significantly. Further, when the metallic lithium deposits in a dendritic form, it penetrates the separator and comes into contact with the positive electrode to cause a short circuit, which adversely affects charging and discharging.

As a solution to this problem, a method has been adopted in which the positive and negative electrode plates are made thin and have a large area (length) to reduce the current density flowing per unit area and prevent metallic lithium from being deposited. However, since the thin negative electrode plate as described above cannot be manufactured with carbon powder alone, a binder or thickener is added to carbon powder to form a paste, which is then collected into a foil-shaped collector such as copper or nickel. It is applied onto the body and dried to prepare a sheet-shaped negative electrode plate. A battery using such an electrode plate exhibits a charge / discharge efficiency of almost 100% at room temperature and good cycle characteristics.

However, in charging at a low temperature of 5 ° C. or lower, particularly 0 ° C. or lower, the absorption rate and absorption amount (acceptability) of lithium in the negative electrode carbon are remarkably reduced as compared with those at room temperature, so that particularly at a large current. It was found that metallic lithium was deposited on the negative electrode during charging. Since the metal lithium once deposited does not easily disappear even if the charge / discharge reaction is repeated thereafter, the charge / discharge capacity is not recovered when the temperature is returned to room temperature and the charge / discharge is performed, which causes capacity deterioration. As described above, the deposited lithium has an adverse effect such as an internal short circuit.

It is considered that the main cause of such a deposition phenomenon of metallic lithium during charging is that the binder and the thickener covering the surface of the negative electrode carbon material hinder the smooth charging reaction of the negative electrode. Based on such an idea, the present inventors paid attention to the coating state of the binder and the thickener on the negative electrode carbon material, and conducted various studies, but when the amount of the binder was reduced, it was sufficient. It was not possible to obtain a thin electrode plate required for the present battery because sufficient electrode plate strength could not be obtained and sufficient paste viscosity could not be obtained when the amount of thickener was reduced.

The present invention obtains a negative electrode which does not have the above-mentioned adverse effects of the binder and thickener covering the surface of the negative electrode carbon material and has a sufficient electrode plate strength, so that the negative electrode can remain on the negative electrode even during low temperature charging. To obtain a high capacity, high energy density non-aqueous electrolyte secondary battery with high capacity recovery rate when returned to room temperature without causing precipitation of metallic lithium and excellent in various characteristics such as charge and discharge cycle characteristics. It is intended for.

[0011]

The present invention comprises a positive electrode composed of a lithium-containing composite oxide, a non-aqueous electrolyte, and a negative electrode containing a carbon material capable of inserting and extracting lithium and a binder or a thickener. In order to solve the above-mentioned problems of the provided non-aqueous electrolyte secondary battery, the negative electrode is heat-treated in a non-oxidizing atmosphere at 150 ° C to 350 ° C, and then this is used to form a battery. . More preferably, the oxygen partial pressure of the non-oxidizing atmosphere is 267 Pa or less. The non-oxidizing atmosphere is any one of Ar, N ₂ , He, and decompressed air of 1333 Pa or less, or a mixed gas of two or more kinds of these gases. More preferably, a graphite carbon material is mainly used as the negative electrode, and a cellulosic material is used as a thickener or a binder.

[0012]

The present invention comprises applying a paste containing a negative electrode carbon material, a binder, and a thickener to a metal current collector, and drying the electrode plate to be heat-treated in a non-oxidizing atmosphere. ,
The binder or thickener covering the carbon material is thermally decomposed to be partially eliminated, and the charging reaction of the negative electrode can be smoothly performed to increase the rate of lithium absorption into the negative electrode during charging. Further, it is possible to obtain a negative electrode which maintains a necessary binding force as an electrode plate by the action of the binder or the thickener which remains to an appropriate degree. By using this negative electrode, metal lithium deposition was not observed on the electrode surface even at low temperature charging, and the discharge capacity when the battery was returned to room temperature and charged and discharged was 100% of the discharge capacity before low temperature charging.
It is possible to obtain a non-aqueous electrolyte secondary battery that recovers up to nearly 100%.

[0013]

【Example】

(Example 1) The present invention will be described in detail below with reference to examples.

FIG. 1 is a vertical sectional view of the cylindrical battery used in this embodiment. In FIG. 1, 1 is a positive electrode, 2 is a positive electrode lead drawn from a positive electrode plate, 3 is a negative electrode, 4 is a negative electrode lead drawn from a negative electrode, and the positive electrode 1 and the negative electrode 3 are separators 5.
It is wound in a spiral shape a plurality of times through and is housed in a battery case 8 made of stainless steel. 6 and 7 are lower and upper insulating plates, 9 is a sealing gasket, and 10 is a sealing plate provided with a safety valve. Further, the positive electrode lead 2 is connected to the sealing plate 10 and also serves as the positive electrode terminal 11.

The positive and negative electrodes will be described in detail below. The positive electrode was prepared by mixing Li ₂ CO ₃ and Co ₃ O ₄ and
100 parts by weight of LiCoO ₂ powder synthesized by firing at 00 ° C. for 10 hours, 3 parts by weight of acetylene black and 7 parts by weight of fluororesin binder were mixed and suspended in an aqueous solution of carboxymethyl cellulose (CMC). Made into a paste. This paste is applied to both sides of an aluminum foil having a thickness of 0.03 mm, dried and rolled to a thickness of 0.18 mm,
It was cut into a width of 37 mm and a length of 240 mm, and the positive electrode lead 2 was attached to the positive electrode 1.

For the negative electrode 3, 100 parts by weight of a graphite-based carbon material obtained by firing mesocarbon microbeads (MCMB) having an average particle size of 5.7 μm at 2800 ° C. was mixed with 5 parts by weight of styrene-butadiene rubber (SBR) to prepare CMC. It was suspended in an aqueous solution to form a paste. This paste has a thickness of 0.02
0.2 mm thick copper foil is applied to both sides, dried and rolled to a thickness of 0.2
It was cut into 0 mm, width 39 mm, and length 260 mm, and the negative electrode lead 4 was attached to it to make the negative electrode 3.

After the negative electrode 3 is heat-treated under the conditions described below, the positive electrode 1 and the negative electrode 3 are formed into a thickness of 0.025 mm,
It was spirally wound through a polyethylene separator 5 having a width of 45 mm and a length of 730 mm to form an electrode plate group, which was housed in a battery case 8 having a diameter of 14.0 mm and a height of 50 mm. Ethylene carbonate (EC), diethyl carbonate (DEC), and methyl propionate (MP) were mixed at a volume ratio of 3: 4: 3 in a solvent, and lithium hexafluorophosphate (LiPF ₆ ) was used as a solute at 1 mol / mol. After injecting an electrolyte solution dissolved at a concentration of 1 and then sealing, a battery was constructed.

Regarding the heat treatment temperature of the negative electrode, heat treatment was performed in a reduced pressure air of 10 Pa at each treatment temperature from 25 ° C. (room temperature) to 500 ° C. for 6 hours, and the relationship with the battery characteristics was examined. The result is shown in FIG.

Regarding the air pressure in the heat treatment atmosphere of the negative electrode, the heat treatment temperature is 250 ° C. and the air pressure in the atmosphere is 1
6 under each reduced pressure condition from 33 Pa to 13330 Pa
The heat treatment was performed for a period of time to examine the relationship with the battery characteristics. The result is shown in FIG.

(Example 2) Regarding the heat treatment atmosphere gas of the negative electrode, the heat treatment process was carried out at 250 ° C. under 1 atmosphere of nitrogen (N ₂ ), argon (Ar) and helium (He) atmospheres.
After heat treatment for 6 hours, the relationship between the heat treatment atmosphere gas and the battery characteristics was examined.

The batteries of Examples 1 and 2 were subjected to a charge / discharge cycle test under the following conditions. 4.1V charging
Charging was carried out for 2 hours at a constant current and a constant voltage and a limiting current of 350 mA. The discharge was a constant current discharge of 500 mA, and the discharge end voltage was 3.0V. After 20 cycles of charging and discharging at 20 ° C. under such conditions, the environmental temperature was set to 0 ° C. in the discharged state, and after leaving for 6 hours, 2
Charge / discharge was performed for 0 cycles. Then, after the environmental temperature was returned to 20 ° C. in the discharged state for 6 hours, the battery was charged and discharged for 50 cycles, and the cycle test was completed.

In the evaluation of each battery, the discharge capacity at the initial 10th cycle at 20 ° C was taken as the initial capacity, and the discharge capacity at the 3rd cycle at 0 ° C was taken as the 0 ° C capacity. Then, the discharge capacity at the third cycle after returning to 20 ° C. was taken as the recovery capacity, and the capacity at the 50th cycle was taken as the final capacity. Then, the value of (0 ° C. capacity) / (initial capacity) × 100 is defined as the 0 ° C. maintenance rate,
The value of (recovery capacity) / (initial capacity) × 100 was taken as the recovery rate.

After completion of the cycle test, each battery was disassembled and the surface of the negative electrode plate was observed. As shown in FIG. 2, the test result of each battery constructed by changing the heat treatment temperature in Example 1 shows that the initial capacity is as large as 500 mAh or more in the heat treatment temperature range of 150 to 350 ° C. and the 0 ° C. maintenance rate is good. Yes, the recovery rate was close to 100%. The battery heat-treated in this temperature range showed almost no deterioration in the subsequent charge / discharge cycles. No noticeable change was observed on the negative electrode plate of the battery disassembled after the cycle test, and deposition of metallic lithium was not observed at all.

However, even under a reduced pressure of 10 Pa, 100
In the battery where the negative electrode was heat-treated at a temperature of ℃ or less, since the treatment temperature was low, extra binder and thickener covered the negative electrode surface, so the initial capacity of the battery was slightly low and the 0 ° C maintenance rate was low. The recovery rate was also inadequate. Precipitation of metallic lithium was observed on almost the entire surface of the negative electrode plate of the battery disassembled after the test. This indicates that metallic lithium was deposited on the surface of the negative electrode during charging at low temperature as a result of the reduction in lithium acceptability of the negative electrode. In addition, it is 1 when charging / discharging only at 20 degreeC and not charging / discharging at low temperature in the meantime.
It has already been confirmed that metal lithium is not deposited on the surface of the negative electrode plate even after the lapse of 00 cycles.

In a battery in which the negative electrode plate is heat-treated at a temperature of 400 ° C. or higher under a reduced pressure of 10 Pa, the binder is thermally decomposed due to the high temperature, and the binding force of the mixture becomes insufficient. The phenomenon that the mixture was peeled off from the current collector was noticeable. As a result, the initial capacity was low, it gradually deteriorated even during the charge / discharge cycle at 20 ° C., and the 0 ° C. maintenance rate and the recovery rate were very low. Further, as a result of the observation of decomposition, lithium deposition was not seen on the negative electrode mixture, but the mixture was peeled off, and lithium deposition was seen on the surface of the negative electrode plate where the current collector was exposed.

FIG. 3 shows the test results of the battery prepared in Example 1 by changing the degree of pressure reduction of air, which is one of the atmospheric conditions for heat treatment. In the figure, when the air pressure of the heat treatment atmosphere is 1330 Pa or less, the initial capacity was as large as 500 mAh or more, the 0 ° C. maintenance rate was good, and a recovery rate of nearly 100% was achieved. Almost no deterioration was observed in the subsequent charge / discharge cycle. No noticeable change was observed in the negative electrode of the battery disassembled after the cycle test, and deposition of metallic lithium was not observed at all.

However, the atmospheric pressure during the heat treatment is 300.
At 0 Pa or higher, the binder is oxidatively decomposed and 4
Similar to the case of heat treatment at a high temperature of 00 ° C. or higher, the phenomenon that the mixture was peeled off from the current collector due to the insufficient binding force of the mixture was noticeable. As a result, the initial capacity is low and 2
It gradually deteriorated during the cycle at 0 ° C, and both the 0 ° C maintenance rate and the recovery rate became very low. In addition, as a result of observation by decomposition, lithium deposition was not seen on the negative electrode mixture, but the mixture was peeled off, and lithium deposition was seen on the surface of the negative electrode plate where the current collector was exposed.

In the battery of Example 2, in which atmosphere the negative electrode plate was heat-treated, the initial capacity was as large as 500 mAh or more as in the battery of Example 1 which was heat-treated at 150 to 350 ° C. in the reduced pressure air atmosphere of 10 Pa. The 0 ° C maintenance rate was also good, and a recovery rate of almost 100% was achieved. Almost no subsequent cycle deterioration was observed. No noticeable change was found on the negative electrode plate of the disassembled battery after the cycle test,
No precipitation of metallic lithium was observed.

From the results of the examination of these examples, the heat treatment atmosphere for the negative electrode according to the present invention should be a non-oxidizing atmosphere, particularly in a reduced pressure air of 1333 Pa or less, and Ar,
It was confirmed that the effect was extremely remarkable in He and N ₂ gas. In addition, it is clear from these results that the oxygen partial pressure in the atmosphere is the controlling factor.
Even in the case of heat treatment in an atmosphere in which two or more kinds of gases among r, He, and N ₂ are mixed, the same effect can be obtained if the oxygen partial pressure is regulated to 267 Pa or less corresponding to the oxygen partial pressure in depressurized air of 1333 Pa or less. It turns out that

Furthermore, when an inert gas such as N ₂ , Ar, He, or a mixed gas thereof is used as the non-oxidizing atmosphere gas, the excess binder and thickener in the negative electrode plate are thermally decomposed. It is more preferable because the reaction of (2) does not occur, but the effects of the present invention can be obtained even when H ₂ and CO ₂ which are non-oxidizing gases are used. Further, the firing time may be shorter as the temperature is higher within the range of the present invention, and is usually about 5 minutes to 1 week.

In this embodiment, LiCo is used as the positive electrode active material.
O ₂ was used, but other than this, LiNiO ₂ , LiFe
O ₂ , LiMn ₂ O ₄ , and these Co, Ni, Fe, M
Any lithium-containing composite oxide such as one obtained by substituting a part of n with another transition metal may be used.

In this embodiment, the negative electrode carbon material is 2800.
Although MCMB calcined at ℃ was used, it is not limited by the kind of the negative electrode carbon material, and shows the same effect in any carbon material capable of inserting and extracting lithium.
In addition to MB, when using various graphite-based materials such as natural graphite and artificial graphite, a non-aqueous electrolyte secondary battery having high voltage and high capacity and excellent cycle characteristics can be obtained. .

Further, as the organic solvent of the non-aqueous electrolytic solution, propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 1, 3, which are conventionally known. -Dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane, methylsulfolane, acetonitrile, propionitrile, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methyl formate, ethyl formate, propyl formate,
Methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, etc. may be used alone or in admixture of two or more.

LiC which is conventionally known as a solute
lO ₄ , LiAsF ₆ , LiBF ₄ , LiPF ₆ , LiB
(C ₆ H ₅ ) ₄ , LiCl, LiBr, CH ₃ SO ₃ Li,
CF ₃ SO ₃ Li or the like may be used. As the thickener for the negative electrode, various cellulosic materials other than CMC can be used. For example, methyl cellulose, ethyl cellulose, benzyl cellulose, trityl cellulose, cyanethyl cellulose, carboxyethyl cellulose, aminoethyl cellulose, oxyethyl cellulose and the like are non-oxidized. The effect of the present invention is great because it is thermally decomposed in a volatile atmosphere at a temperature of 350 ° C. or lower. However, the thickener is not limited to cellulose, polyvinyl alcohol,
Water-soluble polymers such as polyethylene oxide and polyacrylic acids can be used as well. As the solvent for these thickeners, glycols such as ethylene glycol and propylene glycol, which have high viscosity at room temperature, and solvents such as cyclohexanol and pyrocarbonate may be used.

As the binder for the negative electrode, in addition to SBR, it is preferable to use a binder material that does not undergo thermal decomposition in a non-oxidizing atmosphere of 150 ° C. or lower. Acrylonitrile butadiene rubber (NBR), butadiene rubber (BR), isoprene Rubbers such as rubber (IR) and polyvinylidene fluoride (PVD)
F), fluorine-based resins such as polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer, resins such as phenol resin and acrylic resin, polyethers such as polybutylene oxide, polyesters The same effect can be obtained even with polyvinyls.

The thickener used here is mainly used to obtain a paste-like negative electrode mixture having good coatability, and the binder obtains the bondability between the carbon particles and with the current collector. Of the thickener materials and binder materials exemplified above, it may be used also as the thickener and binder, and the CMC used in the examples of the present invention may be used. An example is a cellulosic material at the beginning. Therefore, in the description of the present invention, the thickener and the binder are described separately, but in some cases they cannot be substantially distinguished, and the negative electrode according to the present invention is composed of a carbon material, a thickener and a binder. It includes, of course, a negative electrode containing a carbon material and a thickener or a binder.

[0037]

As described above, in the non-aqueous electrolyte secondary battery of the present invention, the negative electrode is heated in a non-oxidizing atmosphere at 150 ° C. to 350 ° C., and then the battery is constructed to cover the surface of the negative electrode carbon material. Since excess binder and thickener are vaporized and do not interfere with lithium intercalation during negative electrode charging, there is no deposition of metallic lithium on the negative electrode surface even at low temperature charging, and 100% when returned to room temperature A battery having a recovery rate can be obtained.

[Brief description of drawings]

FIG. 1 is a vertical sectional view of a cylindrical battery used in this example.

FIG. 2 is a diagram showing a relationship between a heat treatment temperature, a recovery rate after a low temperature cycle, and a battery capacity.

FIG. 3 is a diagram showing a relationship between pressure during heat treatment, recovery rate after low temperature cycle, and battery capacity.

[Explanation of symbols]

 1 Positive electrode 2 Positive electrode lead 3 Negative electrode 4 Negative electrode lead 5 Separator 6 Insulating plate 7 Insulating plate 8 Battery case 9 Sealing gasket 10 Sealing plate 11 Positive electrode terminal

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akikatsu Morita 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (4)

[Claims] 1. A positive electrode comprising a lithium-containing composite oxide,
In a method for producing a non-aqueous electrolyte secondary battery, comprising a non-aqueous electrolyte solution, a carbon material capable of inserting and extracting lithium, and a negative electrode containing a binder or a thickener, the negative electrode is heated to 150 ° C to 350 ° C.
A method for producing a non-aqueous electrolyte secondary battery, which comprises heat-treating in a non-oxidizing atmosphere at ℃, and using the same to construct a battery. 2. The method for producing a non-aqueous electrolyte secondary battery according to claim 1, wherein the oxygen partial pressure of the non-oxidizing atmosphere is 267 Pa or less. 3. A non-oxidizing atmosphere having a reduced pressure of 1333 Pa or less, any one of Ar, N ₂ and He, or air, A.
The method for producing a non-aqueous electrolyte secondary battery according to claim 2, which is a mixed gas of two or more kinds of gases selected from r, N ₂ , and He. 4. The method for producing a non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode is mainly composed of a graphite carbon material and contains a cellulosic thickener or a binder.