JP3191614B2 - Manufacturing method of non-aqueous electrolyte secondary battery - Google Patents

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 a method for producing a negative electrode thereof.

[0002]

2. Description of the Related Art In recent years, portable electronic devices have become more portable.
Cordless technology is rapidly advancing. Accordingly, there has been an increasing demand for a small, lightweight, and high energy density secondary battery that serves as a driving power supply. From such a viewpoint, a nonaqueous electrolyte secondary battery, particularly a lithium secondary battery, is expected to be a battery having a high voltage and a high energy density, and its development is urgently required.

Conventionally, manganese dioxide, vanadium pentoxide, titanium disulfide, and the like have been used as a positive electrode active material of a lithium secondary battery. Attempts to construct a battery with these positive electrodes, a lithium metal negative electrode, and an organic electrolyte have been made. Has been done. However, in general, a secondary battery using lithium metal for the negative electrode has an internal short circuit due to dendritic metal lithium (dendrites) that precipitates on the surface of the negative electrode during charging, and has a poor charge-discharge characteristic due to a side reaction between the precipitated active lithium and the electrolyte. Deterioration, furthermore, abnormal heat generation, and safety problems such as ignition in severe cases have been major obstacles to practical use.
Furthermore, there are many problems in high-rate charge / discharge characteristics and overdischarge characteristics, and no satisfactory solution has been found.

Instead of these negative electrode materials, carbon materials capable of inserting and extracting lithium by charging and discharging are provided with various characteristics necessary for a practical battery, such as safety, high-rate charge / discharge characteristics, and charge / discharge cycle characteristics. As attention has been drawn.

In order to obtain a battery with a high energy density,
With the use of elemental materials for the negative electrode, as a positive electrode active material,
LiC which is a compound having a higher voltage and containing Li
oO _Two, LiNiO_Two, LiFeO_Two, LiMn_TwoO_Four, Update
Some of these Co, Ni, Fe and Mn are
It has been proposed to use complex oxides substituted with elements.
You.

[0006]

When a carbon material is used for the negative electrode, lithium is occluded during charging and released during discharging, whereby the charge / discharge reaction proceeds. However, the higher the charging current density and the lower the charging temperature, the less the speed at which lithium can be occluded, and the more the metallic lithium precipitates on the negative electrode. Since the metallic lithium is electrochemically inactive, a discharge reaction is difficult to be performed, and the tendency of the charge / discharge efficiency of the battery to decrease is remarkable. When the metallic lithium precipitates in a dendritic manner, it penetrates through the separator and contacts the positive electrode to cause a short circuit, which has an adverse effect such as making charging and discharging impossible.

As a solution to this problem, a method has been adopted in which the positive and negative electrode plates are made thinner and larger (longer) to reduce the current density flowing per unit area so that metallic lithium is not deposited. However, since carbon powder alone cannot produce such a thin negative electrode plate as described above, a binder is added to the carbon powder to form a paste, which is then formed into a foil-like current collector such as copper or nickel. It is applied on a body and dried to produce a sheet-shaped negative electrode plate. A battery using such an electrode plate exhibits a charge / discharge efficiency close to 100% at room temperature and exhibits good cycle characteristics.

However, when charged at a low temperature of 5 ° C. or less, particularly 0 ° C. or less, the rate of occlusion of lithium in the negative electrode carbon and the amount of occlusion (acceptability) are significantly lower than those at room temperature. It was found that metallic lithium was deposited on the negative electrode during charging. Since once deposited metallic lithium does not disappear easily even if the charge / discharge reaction is repeated thereafter, the charge / discharge capacity does not recover when the temperature is returned to normal temperature and the charge / discharge is performed, resulting in capacity deterioration. As described above, the deposited lithium has an adverse effect such as an internal short circuit.

[0009] It is considered that the main cause of such a precipitation 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 this idea, the present inventors focused on the state of coating of the binder and the thickener on the negative electrode carbon material, and performed various studies.However, when the amount of the binder was reduced, it was sufficient. When the electrode strength was not obtained and the amount of the thickener was reduced, a sufficient paste viscosity was not obtained, so that a thin electrode plate required for the present battery could not be obtained.

The present invention provides a negative electrode having a sufficient electrode strength without the above-mentioned adverse effects caused by a binder and a thickener covering the surface of the negative electrode carbon material, so that the negative electrode can be maintained on the negative electrode even during low-temperature charging. High-capacity, high-energy-density non-aqueous electrolyte secondary battery with a high capacity recovery rate when returned to normal temperature without causing precipitation of metallic lithium on the surface and excellent in various characteristics such as charge-discharge cycle characteristics It is intended for.

[0011]

The present invention provides a positive electrode comprising a lithium-containing composite oxide, a non-aqueous electrolyte, and a negative electrode comprising 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 nonaqueous electrolyte secondary battery, the negative electrode is heat-treated in a non-oxidizing atmosphere at 150 ° C. to 350 ° C., and then a battery is configured using the heat treatment. . More preferably, the oxygen partial pressure of the non-oxidizing atmosphere is 267 Pa or less. Further, the non-oxidizing atmosphere is any of Ar, N ₂ , He, and reduced-pressure air of 1333 Pa or less, or a mixed gas of two or more of these gases. More preferably, the negative electrode is mainly composed of a graphitic carbon material, and a cellulosic material is used as a thickener or a binder.

[0012]

According to the present invention, a paste containing an appropriate amount of a negative electrode carbon material, a binder, and a thickener is applied to a metal current collector, and the resulting electrode plate is subjected to a heat treatment in a non-oxidizing atmosphere. ,
The binder or the thickener covering the carbon material is thermally decomposed and partially disappeared, and the charge reaction of the negative electrode is smoothly performed, so that the rate of occlusion of lithium into the negative electrode during charging can be increased. Further, it is possible to obtain a negative electrode maintaining a necessary binding force as an electrode plate by the action of a binder or a thickener which has remained moderately. By using this negative electrode, no deposition of metallic lithium was observed on the electrode surface even at the time of low-temperature charging, and the discharge capacity when the battery was returned to normal temperature and charged / discharged was 100% of the discharge capacity before low-temperature charging.
% Can be obtained.

[0013]

【Example】

(Example 1) Hereinafter, the present invention will be described in detail with reference to examples.

FIG. 1 is a longitudinal sectional view of a 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.
, And are housed in a stainless steel battery case 8 by being spirally wound a plurality of times. Reference numerals 6 and 7 denote lower and upper insulating plates, 9 denotes a sealing gasket, and 10 denotes 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 manufacturing method of the positive electrode is to mix Li ₂ CO ₃ and Co ₃ O ₄
100 parts by weight of LiCoO ₂ powder synthesized by baking at 00 ° C. for 10 hours, 3 parts by weight of acetylene black and 7 parts by weight of a fluororesin binder were mixed, and suspended in an aqueous solution of carboxymethyl cellulose (CMC). Into a paste. This paste is applied to both sides of an aluminum foil having a thickness of 0.03 mm, dried, and then rolled to a thickness of 0.18 mm.
It was cut into a width of 37 mm and a length of 240 mm, and a positive electrode lead 2 was attached to obtain a positive electrode 1.

For the negative electrode 3, 5 parts by weight of styrene-butadiene rubber (SBR) was mixed with 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. It was suspended in an aqueous solution to form a paste. This paste has a thickness of 0.02
mm copper foil on both sides, dried and rolled to a thickness of 0.2
The resultant was cut into 0 mm, 39 mm in width, and 260 mm in length.

After heat-treating the negative electrode 3 under the following conditions, the positive electrode 1 and the negative electrode 3 are separated by a thickness of 0.025 mm.
The electrode plate group was spirally wound through a polyethylene separator 5 having a width of 45 mm and a length of 730 mm, and stored in a battery case 8 having a diameter of 14.0 mm and a height of 50 mm. Lithium hexafluorophosphate (LiPF ₆ ) was added as a solute to a solvent in which ethylene carbonate (EC), diethyl carbonate (DEC), and methyl propionate (MP) were mixed at a volume ratio of 3: 4: 3. After the electrolyte dissolved at a concentration of 1 was injected, the battery was sealed and closed.

Regarding the heat treatment temperature of the negative electrode, the heat treatment was performed for 6 hours at a treatment temperature of 25 ° C. (room temperature) to 500 ° C. in a reduced pressure air of 10 Pa 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 for the negative electrode, the heat treatment temperature was set to 250 ° C., and the air pressure in the atmosphere was set to 1
Under each reduced pressure condition from 33 Pa to 13330 Pa, 6
After heat treatment for a long time, the relationship with battery characteristics was examined. The result is shown in FIG.

(Example 2) Regarding the heat treatment atmosphere gas for the negative electrode, the heat treatment step was performed at 250 ° C. under the atmosphere of nitrogen (N ₂ ), argon (Ar) and helium (He) at 1 atm.
Heat treatment was performed for 6 hours to examine the relationship between the heat treatment atmosphere gas and battery characteristics.

Each of the batteries of Examples 1 and 2 was subjected to a charge / discharge cycle test under the following conditions. Charging is 4.1V
, And charging was performed for 2 hours with a limiting current of 350 mA. The discharge was a constant current discharge of 500 mA, and the discharge end voltage was 3.0 V. After performing 20 cycles of charge / discharge at 20 ° C. under such conditions, the ambient temperature was set to 0 ° C. in the discharge state, and after leaving for 6 hours, 2
0 cycles of charging and discharging were performed. Then, in the discharged state, the environmental temperature was returned to 20 ° C. again, and after 6 hours, charging and discharging were performed 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 defined as the initial capacity, and the discharge capacity at the third cycle at 0 ° C. was defined as the 0 ° C capacity. Then, the discharge capacity at the third cycle after returning to 20 ° C. was set as the recovery capacity, and the capacity at the 50th cycle was set 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 defined as the recovery rate.

After the 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 configured by changing the heat treatment temperature in Example 1 has a large initial capacity of 500 mAh or more when the heat treatment temperature is in the range of 150 to 350 ° C., and has a good 0 ° C. maintenance rate. Yes, and showed a recovery rate of nearly 100%. The battery heat-treated in this temperature range showed almost no deterioration even in the subsequent charge / discharge cycle. No noticeable change was observed in the negative electrode plate of the battery decomposed after the cycle test, and no deposition of lithium metal was observed.

However, even under a reduced pressure of 10 Pa, 100
In the battery where the heat treatment of the negative electrode was performed at a temperature of not more than ℃, the initial temperature of the battery was slightly lower and the 0 ° C. retention rate was lower because the excess temperature of the processing temperature was low, so that the extra binder and thickener covered the negative electrode surface. The recovery rate was also poor. Almost all of the surface of the negative electrode plate of the battery decomposed after the test was found to have deposited metallic lithium. This indicates that metallic lithium was deposited on the surface of the negative electrode during charging at a low temperature as a result of a decrease in lithium acceptability of the negative electrode. If only charge / discharge cycles at 20 ° C. and no charge / discharge at low temperature during that cycle, 1
It has already been confirmed that no metal lithium is deposited on the surface of the negative electrode plate even after the lapse of 00 cycles.

Further, in a battery in which the heat treatment of the negative electrode plate is performed at a temperature of 400 ° C. or more under a reduced pressure of 10 Pa, since the temperature is high, the binder is thermally decomposed, and the binding power of the mixture becomes insufficient. A phenomenon in which the mixture was peeled off from the current collector was remarkably observed. As a result, the initial capacity was low, gradually deteriorated even during the charge / discharge cycle at 20 ° C., and both the 0 ° C. maintenance rate and the recovery rate were extremely low. As a result of decomposition observation, although no lithium was found to be deposited on the negative electrode mixture, the mixture was peeled off, and lithium was found to be deposited 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 decompression of air, which is one of the atmospheric conditions of the heat treatment. In the figure, when the air pressure of the heat treatment atmosphere was 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 decomposed after the cycle test, and no deposition of metallic lithium was observed.

However, when the atmospheric pressure during the heat treatment is 300
Above 0 Pa, the binder is oxidatively decomposed and
As in the case of heat treatment at a high temperature of 00 ° C. or higher, a phenomenon in which the mixture peeled off from the current collector due to insufficient binding force of the mixture was remarkably observed. As a result, the initial capacity is low and 2
It deteriorated gradually even during the cycle at 0 ° C., and both the 0 ° C. maintenance rate and the recovery rate became very low. As a result of disassembly observation, no lithium was found to be deposited on the negative electrode mixture, but the mixture was peeled off, and lithium was found to be deposited on the surface of the negative electrode plate where the current collector was exposed.

In any of the batteries of Example 2, 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 a reduced-pressure air atmosphere of 10 Pa under any atmosphere. , 0 ° C maintenance rate was also good, and a recovery rate of almost 100% was achieved. Subsequent cycle deterioration was hardly observed. No noticeable change was seen on the negative electrode plate of the disassembled battery after the cycle test,
No deposition of metallic lithium was observed.

From the examination results of these examples, the heat treatment atmosphere for the negative electrode according to the present invention may be a non-oxidizing atmosphere.
A very remarkable implementation effect in He, N ₂ gas was confirmed. Further, by summing up these results, it is clear that the oxygen partial pressure in the atmosphere is the dominant factor.
Even when heat treatment is performed in an atmosphere in which two or more gases are mixed among r, He, and N ₂ , the same effect can be obtained by regulating the oxygen partial pressure to 267 Pa or less corresponding to the oxygen partial pressure in reduced-pressure air of 1333 Pa or less. Was obtained.

Further, 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 the thickener in the negative electrode plate are thermally decomposed. Although more preferable because reaction does not occur, even with H _2, CO ₂ is a non-oxidizing gas, the effect of the present invention is obtained. Also, 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 LiNiO ₂ , LiFe
O ₂ , LiMn ₂ O ₄ , and Co, Ni, Fe, M
Any lithium-containing composite oxide such as one in which n is partially substituted with another transition metal may be used.

In this embodiment, the carbon material for the negative electrode is 2,800.
Although MCMB calcined at ℃ was used, it is not limited by the type of carbon material of the negative electrode, and shows the same effect in any carbon material capable of inserting and extracting lithium.
When various graphite-based materials such as natural graphite and artificial graphite are used in addition to MB, a non-aqueous electrolyte secondary battery having high voltage, high capacity, and excellent cycle characteristics can be obtained. .

As the organic solvent of the non-aqueous electrolyte, propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 1,3 -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 and the like may be used alone or in combination of two or more.

LiC which has been conventionally known as a solute
10 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 can be used in addition to CMC.For example, methyl cellulose, ethyl cellulose, benzyl cellulose, trityl cellulose, cyanethyl cellulose, carboxyethyl cellulose, aminoethyl cellulose, oxyethyl cellulose, etc. Thermal decomposition at about 350 ° C. or less in a neutral atmosphere has a large effect in the present invention. However, the thickener is not limited to cellulose, polyvinyl alcohol,
Water-soluble polymers such as polyethylene oxide and polyacrylic acids can also be used. Further, as a solvent for these thickeners, glycols such as ethylene glycol and propylene glycol having a high viscosity at room temperature, and solvents such as cyclohexanol and pyrocarbonate may be used.

As the binder for the negative electrode, it is preferable to use a binding material which does not thermally decompose at a temperature of 150 ° C. or lower in addition to SBR, such as acrylonitrile butadiene rubber (NBR), butadiene rubber (BR), isoprene. Rubbers such as rubber (IR) and polyvinylidene fluoride (PVD)
F), fluororesins such as polytetrafluoroethylene (PTFE), tetrafluoroethylene hexafluoropropylene copolymer, resins such as phenolic resin and acrylic resin, polyethers such as polybutylene oxide, and polyesters The same effect can be obtained with polyvinyls.

The thickener used here is mainly used to obtain a paste-like negative electrode mixture having good coatability, and the binder obtains a bond between carbon particles and with a current collector. However, among the above-listed thickener materials and binder materials, the CMC used in the examples of the present invention may be used in combination with the role of a thickener and a binder. First, a cellulosic material is an example. Therefore, in the description of the present invention, the thickener and the binder are distinguished from each other, but may not be substantially distinguished, and the negative electrode according to the present invention is formed from a carbon material, a thickener, and a binder. In addition, the present invention includes a negative electrode containing a carbon material and a thickener or a binder.

[0037]

As described above, in the nonaqueous 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 surface of the negative electrode carbon material is covered by forming the battery. The excess binder and thickener are vaporized and do not inhibit lithium intercalation at the time of charging the negative electrode. Therefore, no metal lithium is deposited on the surface of the negative electrode even at the time of low-temperature charging. A battery having a recovery rate can be obtained.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a cylindrical battery used in the present embodiment.

FIG. 2 is a graph 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 the relationship between pressure during heat treatment, recovery rate after low-temperature cycle, and battery capacity.

[Explanation of symbols]

 Reference Signs List 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 Kazuma Kadoma, Kadoma-shi, Osaka Matsushita Electric Industrial Co., Ltd. (56) References JP-A-4-269466 (JP, A) JP-A-5-205 74462 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H01M 4/02-4/04

Claims (4)

(57) [Claims] 1. A positive electrode comprising a lithium-containing composite oxide,
In a method for producing a non-aqueous electrolyte secondary battery including a non-aqueous electrolyte and a negative electrode containing a carbon material capable of inserting and extracting lithium and a binder or a thickener, the negative electrode may be used at 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 a temperature of ° C. and then using the heat treatment. 2. The method for producing a non-aqueous electrolyte secondary battery according to claim 1, wherein the partial pressure of oxygen in the non-oxidizing atmosphere is 267 Pa or less. 3. A non-oxidizing atmosphere having a reduced pressure of 1333 Pa or less, any of Ar, N ₂ , He, or air, A
_{3. The} method for producing a non-aqueous electrolyte secondary battery according to claim 2, wherein the mixed gas is a mixed gas of two or more of 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 graphitic carbon material and contains a cellulosic thickener or a binder.