JP2011210654A - Method for manufacturing nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery excellent in cycle characteristics.SOLUTION: The method for manufacturing a nonaqueous electrolyte secondary battery comprising a cathode having a spinel type lithium manganate as a cathode active material, an anode, and a nonaqueous electrolyte having a nonaqueous solvent and an electrolyte salt includes a plasma treatment process for performing a plasma treatment on the surface of the cathode by using a non-reactive gas, and a coating layer formation process for forming a coating layer with an organic monomer plasma-polymerized on the surface of the cathode after the plasma-treatment.

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to improvement of a positive electrode for a non-aqueous electrolyte secondary battery.

  Nonaqueous electrolyte secondary batteries have high energy density and high capacity, and are therefore widely used as drive power sources for portable devices.

  Conventionally, lithium cobalt oxide having excellent discharge characteristics has been used as a positive electrode active material for non-aqueous electrolyte secondary batteries, but cobalt is expensive because it has a small reserve and is expensive. Attention has been focused on spinel type lithium manganate.

  However, when spinel type lithium manganate is subjected to a charge / discharge cycle or exposed to a high temperature environment, manganese elutes from the crystal, moves to the negative electrode and precipitates, and has battery characteristics such as cycle characteristics and high temperature storage characteristics. There was a problem of lowering.

  By the way, Patent Documents 1 and 2 propose a technique of providing a coating layer on an electrode in order to improve the characteristics of a nonaqueous electrolyte secondary battery.

Japanese Patent Laid-Open No. 10-241666 JP-A-6-310146

  Patent Document 1 is a technique using a positive electrode whose surface is coated with a thin film made of a carbonaceous material. According to this technique, the side reaction of the electrode-electrolyte solution during the charging process can be suppressed.

  Patent Document 2 is a technique in which an outer surface of a metal support that includes an active material is coated with an organic material having a thickness of 0.1 to 2 μm. According to this technique, it is said that a short circuit based on precipitation due to charge / discharge can be prevented.

  However, these techniques have a problem that it is not possible to suppress the elution of manganese from the spinel type lithium manganate crystal.

  This invention is made | formed in view of the above, Comprising: It aims at suppressing the elution of the manganese of the nonaqueous electrolyte battery which has a spinel type lithium manganate.

The present invention for solving the above problems is configured as follows.
In a method for producing a non-aqueous electrolyte secondary battery comprising: a positive electrode having spinel type lithium manganate as a positive electrode active material; a negative electrode; and a non-aqueous electrolyte having a non-aqueous solvent and an electrolyte salt. A plasma processing step of performing plasma processing using a reactive gas, and a coating layer forming step of forming a coating layer formed by plasma polymerization of an organic monomer on the surface of the positive electrode after the plasma processing, .

  According to this configuration, the coating layer formed by plasma polymerization of the organic monomer acts so as to suppress elution of manganese. Further, by performing plasma treatment using a non-reactive gas, a radical layer is formed on the surface of the positive electrode, and this radical layer improves the adhesion between the coating layer formed by plasma polymerization of an organic monomer and the positive electrode. Therefore, even if the charge / discharge cycle is repeated or stored under high temperature conditions, the coating layer does not peel off from the positive electrode, so that the manganese elution suppression effect by the coating layer can be continuously obtained, and the high temperature storage characteristics and cycle characteristics are Improve dramatically.

  Here, the non-reactive gas preferably includes at least one element selected from the group consisting of He, Ne, Ar, Kr, and Xe.

  The organic monomer preferably includes at least one compound selected from the group consisting of propylene, benzene, and styrene.

  Moreover, as a temperature of the positive electrode in a plasma treatment process, it is preferable that it is 27-727 degreeC (300-1000K).

  If the thickness of the coating layer of the organic monomer plasma polymer is less than 0.01 μm, there is a possibility that a sufficient effect cannot be obtained. In addition, since the conductivity of the plasma polymer of the organic monomer is low, when the thickness of the coating layer is larger than 1 μm, the conductivity of the positive electrode is lowered and the load characteristics may be lowered. Therefore, the thickness of the coating layer of the organic monomer plasma polymer is preferably 0.01 to 1 μm.

  As explained above, according to the present invention, a non-aqueous electrolyte secondary battery having excellent cycle characteristics can be obtained.

  The best mode for carrying out the present invention will be described in detail through the following examples. In addition, this invention is not limited to the following form, In the range which does not change the summary, it can change suitably and can implement.

(Example)
[Example 1]
[Production of positive electrode]
Mixing 85 parts by mass of spinel type lithium manganate (LiMn ₂ O ₄ ), 5 parts by mass of graphite powder, 5 parts by mass of carbon black, 5 parts by mass of polyvinylidene fluoride, and N-methyl-2-pyrrolidone A positive electrode active material slurry was obtained. This positive electrode active material slurry was applied to both sides of an aluminum current collector having a thickness of 20 μm by the doctor blade method, dried, rolled with a roller press, and cut to obtain a positive electrode.

(Plasma treatment process)
After this positive electrode was installed in the plasma generator and the gas atmosphere in the apparatus was replaced with an Ar atmosphere (introduction for 5 minutes three times), the Ar gas pressure in the apparatus was maintained at 0.1 Torr, Plasma was generated by applying high frequency power of 56 MHz. The discharge output was 200 W, the discharge time was 5 minutes, and the temperature was 200 ° C. (473 K). Note that 1 Torr is 133.322 Pa.

(Coating layer forming process)
The gas atmosphere in the plasma generator was replaced with propylene (organic monomer) without exposing the positive electrode subjected to the plasma treatment to the air (introduction for 5 minutes three times). Thereafter, the propylene gas pressure in the apparatus was maintained at 0.1 Torr, high frequency power of 13.56 MHz was applied to both electrodes to generate plasma, and propylene was plasma polymerized to form a coating layer on the positive electrode surface. The discharge output was 200 W, the discharge time was 5 minutes, and the temperature was 200 ° C. (473 K).

(Production of negative electrode)
95 parts by mass of natural graphite powder (Lc value is 150 Å or more and d ₍₀₀₂₎ value is 3.38 Å or less), 5 parts by mass of polyvinylidene fluoride, and N-methyl-2-pyrrolidone are mixed to obtain a negative electrode An active material slurry was obtained. This negative electrode active material slurry was applied to both sides of a 12 μm thick copper current collector by the doctor blade method, dried, rolled with a roller press, and cut to obtain a negative electrode.

(Production of electrode body)
The positive electrode and the negative electrode on which the coating layer was formed were wound through a separator made of a polyethylene microporous film, and a polypropylene-made tape was attached to the outermost periphery to produce a cylindrical electrode body. Then, it pressed and stuck the insulating tape on the can bottom part, and it was set as the flat spiral electrode body.

[Nonaqueous electrolyte adjustment]
Ethylene carbonate and diethyl carbonate were mixed at a mass ratio of 3: 7, and LiPF ₆ as an electrolyte salt was dissolved so as to be 1.0 M (mol / liter) to obtain a nonaqueous electrolyte.

[Assembling the battery]
The flat spiral electrode body was inserted into a rectangular outer can. Thereafter, the opening of the outer can is sealed with a sealing body having a liquid injection port, the nonaqueous electrolyte is injected from the liquid injection port, and then the liquid injection port is sealed, so that the nonaqueous solution according to Example 1 is used. An electrolyte secondary battery was produced.

[Example 2]
A nonaqueous electrolyte secondary battery according to Example 2 was fabricated in the same manner as in Example 1 except that He used instead of Ar as the gas used in the plasma treatment process.

[Example 3]
A nonaqueous electrolyte secondary battery according to Example 3 was fabricated in the same manner as in Example 1 except that the gas used in the plasma treatment step was changed to Ne instead of Ar.

[Example 4]
A nonaqueous electrolyte secondary battery according to Example 4 was produced in the same manner as in Example 1 except that the gas used in the plasma treatment step was changed to Kr instead of Ar.

[Example 5]
A nonaqueous electrolyte secondary battery according to Example 5 was produced in the same manner as in Example 1 except that the gas used in the plasma treatment step was changed to Xe instead of Ar.

[Example 6]
A nonaqueous electrolyte secondary battery according to Example 6 was produced in the same manner as in Example 1 except that benzene was used instead of propylene as the gas (organic monomer) used in the coating layer forming step.

[Example 7]
A nonaqueous electrolyte secondary battery according to Example 7 was produced in the same manner as in Example 1 except that styrene was used instead of propylene in place of the gas (organic monomer) used in the coating layer forming step.

[Comparative Example 1]
A nonaqueous electrolyte secondary battery according to Comparative Example 1 was produced in the same manner as in Example 1 except that the plasma treatment step and the coating layer forming step were not performed.

[Comparative Example 2]
A nonaqueous electrolyte secondary battery according to Comparative Example 2 was produced in the same manner as in Example 1 except that the coating layer forming step was not performed.

[Comparative Example 3]
A nonaqueous electrolyte secondary battery according to Comparative Example 2 was produced in the same manner as in Example 3 except that the plasma treatment step was not performed.

[Comparative Example 4]
A nonaqueous electrolyte secondary battery according to Comparative Example 4 was produced in the same manner as in Example 1 except that N ₂ was used instead of Ar as the gas used in the plasma treatment process.

[Comparative Example 5]
A nonaqueous electrolyte secondary battery according to Comparative Example 5 was fabricated in the same manner as in Example 1 except that CH ₄ was used instead of Ar as the gas used in the plasma treatment process.

[Comparative Example 6]
A nonaqueous electrolyte secondary battery according to Comparative Example 6 was fabricated in the same manner as in Example 1 except that O ₂ was used instead of Ar as the gas used in the plasma treatment process.

[Comparative Example 7]
A nonaqueous electrolyte secondary battery according to Comparative Example 7 was fabricated in the same manner as in Example 1 except that H ₂ was used instead of Ar as the gas used in the plasma treatment process.

<Cross-cut test>
A positive electrode was produced in the same manner as in Examples 1 to 5 and Comparative Examples 3 to 7, and in order to confirm the adhesion between the positive electrode and the coating layer, a cross-cut tape peeling test was performed in accordance with JIS D0202-1988. . Cellophane tape (“CT24”, manufactured by Nichiban Co., Ltd.) was used as the tape, and the tape was peeled off after the tape was brought into close contact with the positive electrode with the belly of the finger. The results are shown in Table 1 below. In addition, the presence or absence of peeling was confirmed by analyzing the composition of the tape deposit using an X-ray photoelectron spectrometer.

<High temperature cycle characteristics test>
Batteries were produced in the same manner as in Examples 1, 6, 7 and Comparative Examples 1 to 3, and charged at a constant current of 1.20 A until the voltage reached 4.20 V. Thereafter, the current was applied at a constant voltage of 4.20 V. The battery was charged until it reached 0.030A. Thereafter, the battery was rested for 10 minutes and discharged at a constant current of 1.20 A until the voltage reached 2.75V. This charge / discharge cycle was performed 300 times, and the cycle characteristics were calculated by the following formula. All the charge / discharge cycles were performed in a constant temperature bath at 60 ° C. Further, the battery after the charge / discharge cycle was disassembled, and the amount of manganese deposited on the negative electrode was analyzed by an ICP (inductively coupled plasma) analyzer. These results are shown in Table 1 below. The amount of manganese is shown as a percentage of the theoretical amount of manganese contained in the positive electrode.

  Capacity maintenance ratio (%) = 300th cycle discharge capacity / first cycle discharge capacity × 100

<High temperature storage characteristics test>
Batteries were respectively produced under the same conditions as in Examples 1, 6, 7 and Comparative Examples 1 to 3, and charged at a constant current of 1.20 A until the voltage reached 4.20 V, and then at a constant voltage of 4.20 V. The battery was charged until the current reached 0.030A. Thereafter, the battery was rested for 10 minutes and discharged at a constant current of 1.20 A until the voltage reached 2.75 V, and this discharge capacity was taken as the initial capacity. Thereafter, the battery was charged at a constant current of 1.20 A until the voltage reached 4.20 V, and then charged at a constant voltage of 4.20 V until the current reached 0.030 A. The battery after charging was left in a constant temperature bath at 60 ° C. for 20 days. The battery after standing was discharged at a constant current of 1.20 A until the voltage reached 2.75 V, and this discharge capacity was defined as the remaining capacity. Thereafter, charging / discharging was performed again under the above conditions, and the discharge capacity was taken as the return capacity. And the remaining capacity rate and the return capacity rate were calculated by the following formula. Further, the battery after the test was disassembled, and the amount of manganese deposited on the negative electrode was analyzed with an ICP (inductively coupled plasma) analyzer. These results are shown in Table 1 below.

Remaining capacity ratio (%) = remaining capacity / initial capacity x 100
Recovery capacity ratio (%) = Recovery capacity / Initial capacity x 100

From Table 1 above, Examples 1-5 in which a plasma treatment was performed with a non-reactive gas (Ar, He, Ne, Kr, Xe), and then a polypropylene was plasma-polymerized to form a coating layer were 100 Peeling was not confirmed in 18 to 80 squares in the square, but plasma treatment was performed using Comparative Example 3 in which plasma treatment was not performed, and reactive gases (N ₂ , CH ₄ , O ₂ , H ₂ ). In Comparative Examples 4 to 7, it can be seen that peeling was confirmed in all the squares (the cross-cut test results are all 0/100).

This is considered as follows. Radical layer with excellent adhesion to the positive electrode surface without losing active sites present on the positive electrode surface by plasma treatment using non-reactive gas (He, Ne, Ar, Kr, Xe) Can be formed. This radical layer improves the adhesion between the coating layer (propylene plasma polymer) formed after the plasma treatment step and the positive electrode, making it difficult for the coating layer to peel off from the positive electrode. In contrast, when a reactive gas (N ₂ , CH ₄ , O ₂ , H ₂ ) is used for the plasma treatment, the active sites present on the surface of the positive electrode disappear due to the plasma treatment, and functional groups based on the reactive gas are used instead. (N ₂ is a nitrogen functional group, CH ₄ is a methyl group, O ₂ is an oxygen functional group, and H ₂ is a hydrogen functional group), but these functional groups have no effect of increasing the adhesion of the coating layer. The adhesion between the positive electrode and the coating layer does not improve (Comparative Examples 4 to 7). In addition, in the case where the plasma treatment is not performed, the active site does not disappear, but a radical layer having excellent adhesion with the coating layer cannot be formed on the surface of the positive electrode. The property does not improve (Comparative Example 3).

  From Table 1 above, among the non-reactive gases, Example 1 using Ar had a cross-cut test result of 80/100, and other non-reactive gases (He, Ne, Kr, Xe) were used. It turns out that it is superior to 18 / 100-65 / 100 of Examples 2-5.

  This is considered as follows. He and Ne have a small molecular weight, a collision energy of plasma particles is small, and it is difficult to form a radical layer unless a high gas pressure is applied. On the other hand, since Kr and Xe have a large molecular weight, the excitation energy increases, and it is necessary to apply higher high-frequency power to generate plasma, which makes it difficult to form a radical layer. Since Ar has a moderate molecular weight, such a problem does not occur.

  Further, from Table 1 above, the cross-cut test result of Example 6 using benzene as the organic monomer is 84/100, Example 7 using styrene as the organic monomer is 78/100, and polypropylene is used as the organic monomer. It can be seen that an effect equivalent to 80/100 of Example 1 was obtained.

  Further, from Table 1 above, Examples 1, 6, and 7 in which a coating layer was formed by performing plasma treatment using Ar and plasma-polymerizing organic monomers (propylene, benzene, styrene) are as follows. Maintenance rate is 75 to 82%, Mn elution amount is 2.53 to 2.61%, Remaining volume ratio after high temperature storage is 76.3 to 80.1%, Recovery capacity ratio is 89.4 to 95.3% The capacity after high-temperature cycle test of Comparative Examples 1 to 3 in which the elution amount of Mn is 0.53 to 0.57% and one or both of the plasma treatment using Ar and the coating layer formation by plasma polymerization are not performed. Maintenance rate 58 to 62%, Mn elution amount 3.54 to 3.62%, Remaining volume ratio after high temperature storage 69.3 to 70.5%, Restoration capacity ratio 80.5 to 83.0%, Mn elution amount It can be seen that it is superior to 1.20 to 1.54%.

  This is considered as follows. The coating layer formed by plasma polymerization of the organic monomer acts to suppress elution of manganese from the spinel type lithium manganate crystal. For this reason, the fall of the discharge characteristic by the eluting manganese moving to a negative electrode and depositing can be suppressed. For this reason, the amount of manganese elution is smaller in Examples 1, 6 and 7 than in Comparative Examples 2 and 3 in which no coating layer is provided, and the discharge characteristics (capacity maintenance rate, remaining capacity rate, recovery capacity rate) are improved. . However, when plasma treatment is not performed before the formation of the coating layer (Comparative Example 3), the adhesion between the positive electrode and the coating layer is low, and thus the coating layer is easily peeled off from the positive electrode by charge / discharge cycles and high-temperature storage. Since elution cannot be suppressed, the effect of improving the discharge characteristics as in Examples 1, 6, and 7 cannot be obtained.

(extra content)
The positive electrode active material used in the present invention must contain spinel type lithium manganate, but other known active material materials (lithium cobaltate, lithium nickelate, cobalt nickel lithium manganate having a layered structure, olivine Type lithium iron phosphate). In addition, other elements such as Co, Ni, Mg, Zr, Al, Ti, and Sn may be added to the spinel type lithium manganate and other active materials.

  Moreover, since the present invention can produce an effect that elution of manganese can be suppressed if it is a positive electrode containing spinel type lithium manganate, the positive electrode mainly composed of spinel type lithium manganate (50 mass% or more). As a matter of course, the present invention can be applied to a positive electrode mainly composed of other active material (the spinel type lithium manganate is less than 50% by mass).

  Moreover, it is preferable that a positive electrode is not exposed to air | atmosphere between a plasma treatment process and a coating layer formation process, Therefore, it is preferable to perform both processes continuously. In both steps, the gas pressure in the apparatus is 0.005 to 1 Torr, the frequency of the current applied to the electrode is 2 to 500 MHz, the power is 2 to 500 kW, and the temperature of the positive electrode during processing is 27 to 727 ° C. (300 to 300 ° C.). 1000K), more preferably 170 ° C to 220 ° C (443K to 493K), and the treatment time is preferably 5 seconds to 30 minutes.

  Moreover, there exists a possibility that a sufficient effect may not be acquired as the coating layer by the plasma polymer of an organic monomer is less than 0.01 micrometer. In addition, since the conductivity of the plasma polymer of the organic monomer is low, when it is larger than 1 μm, there is a possibility that the load characteristic is lowered due to the decrease in the conductivity of the positive electrode. Therefore, it is preferable that the coating layer by the plasma polymer of an organic monomer is 0.01-1 micrometer.

Nonaqueous electrolyte solvents include cyclic carbonates typified by propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, lactones typified by γ-butyrolactone, γ-valerolactone, diethyl carbonate, dimethyl carbonate, ethylmethyl. Chain carbonate typified by carbonate, tetrahydrofuran, 1,2-dimethoxyethane, diethylene glycol dimethyl ether, 1,3-dioxolane, 2-methoxytetrahydrofuran, ether typified by diethyl ether, etc. alone or in combination of two or more. Can be used. As the electrolyte salt in the nonaqueous _electrolyte, it is possible to use _{LiPF 6, LiAsF 6, LiClO 4} , LiBF 4, LiCF 3 SO 3, LiN (CF 3 SO 2) 2 and the like.

  As described above, according to the present invention, elution of manganese under a high temperature condition of a nonaqueous electrolyte secondary battery having an inexpensive spinel type lithium manganate as a positive electrode active material can be suppressed. Therefore, industrial applicability is great.

Claims (4)

In a method for producing a non-aqueous electrolyte secondary battery comprising a positive electrode having spinel type lithium manganate as a positive electrode active material, a negative electrode, and a non-aqueous electrolyte having a non-aqueous solvent and an electrolyte salt,
A plasma treatment step of performing a plasma treatment on the positive electrode surface using a non-reactive gas;
A coating layer forming step of forming a coating layer formed by plasma polymerization of an organic monomer on the surface of the positive electrode after the plasma treatment;
The manufacturing method of the nonaqueous electrolyte secondary battery characterized by the above-mentioned. In the manufacturing method of the nonaqueous electrolyte secondary battery according to claim 1,
The non-reactive gas includes at least one element selected from the group consisting of He, Ne, Ar, Kr, and Xe.
A method for producing a non-aqueous electrolyte secondary battery. In the manufacturing method of the nonaqueous electrolyte secondary battery according to claim 1 or 2,
The organic monomer includes at least one compound selected from the group consisting of propylene, benzene, and styrene.
A method for producing a non-aqueous electrolyte secondary battery. In the manufacturing method of the nonaqueous electrolyte secondary battery according to claim 1, 2, or 3,
The temperature of the positive electrode in the plasma treatment step is 27 to 727 ° C.
A method for producing a non-aqueous electrolyte secondary battery.