JP4798952B2 - Method for manufacturing lithium secondary battery - Google Patents

Description

  The present invention relates to a method for manufacturing a lithium secondary battery, and more specifically, a lithium secondary battery using a negative electrode in which a silicon amorphous thin film or an amorphous thin film mainly composed of silicon is deposited on a current collector. It is related with the manufacturing method.

  In recent years, a lithium secondary battery that uses a non-aqueous electrolyte and charges and discharges by moving lithium ions between a positive electrode and a negative electrode has been used as one of the new secondary batteries with high output and high energy density. Yes.

  As such a negative electrode for a lithium secondary battery, a material using a material alloyed with lithium as a negative electrode active material has been studied. As a material alloyed with lithium, for example, silicon has been studied. However, materials that alloy with lithium, such as silicon, expand and contract the volume of the active material when occluding and releasing lithium, so that the active material is pulverized with charge and discharge, or the active material is a current collector Detach from. For this reason, there existed a problem that the current collection property in an electrode fell and charging / discharging cycling characteristics worsened.

  The present applicant uses silicon as an active material, and as an electrode for a lithium secondary battery exhibiting good charge / discharge cycle characteristics, by a thin film formation method such as sputtering, chemical vapor deposition (CVD), and vapor deposition, An electrode in which an amorphous thin film of silicon is formed on a current collector has been proposed (Patent Document 1). Further, an electrode for a lithium secondary battery in which another element such as cobalt is added to silicon has been proposed (Patent Document 2). On the other hand, in a lithium secondary battery using a carbon material or metallic lithium as a negative electrode active material, it has been proposed to dissolve carbon dioxide in a nonaqueous electrolyte (for example, Patent Documents 3 to 13).

  The lithium secondary battery proposed by the present applicant is a battery having a large charge / discharge capacity and excellent cycle characteristics, but the active material layer becomes porous due to repeated charge / discharge, and the thickness of the active material layer increases. There was a problem to do.

In order to solve the above problem, the present applicant has proposed to use a non-aqueous electrolyte in which carbon dioxide is dissolved (Japanese Patent Application No. 2003-163692). By dissolving carbon dioxide in the non-aqueous electrolyte, it is possible to prevent the active material layer from becoming porous due to the charge / discharge reaction. Therefore, an increase in the thickness of the active material layer due to charge / discharge can be reduced, the volume energy density of the lithium secondary battery can be increased, and charge / discharge cycle characteristics can be improved. However, since the amount of carbon dioxide that can be dissolved in the non-aqueous electrolyte is temperature-dependent, a non-aqueous electrolyte in which carbon dioxide is dissolved under normal temperature and normal pressure has a low carbon dioxide solubility at high temperatures. Carbon dioxide dissolved in the gas may be generated as a gas. When such a gas is generated between the positive electrode and the negative electrode, the portion of the electrode that is in contact with the gas is not involved in charge / discharge, and the charge / discharge capacity is significantly reduced. In addition, there is a problem that the battery is filled with gas and the thickness of the battery increases. In order to reduce such problems, it is conceivable to reduce the amount of carbon dioxide dissolved in the non-aqueous electrolyte, but the effect of improving the charge / discharge cycle characteristics is proportional to the amount of dissolved carbon dioxide. For this reason, if the amount of carbon dioxide dissolved is reduced, the charge / discharge cycle characteristics are reduced accordingly.
International Publication No. 01/29913 Pamphlet International Publication No. 02/071512 Pamphlet U.S. Pat. No. 4,853,304 JP-A-6-150975 JP-A-6-124700 JP 7-176323 A Japanese Patent Laid-Open No. 7-249431 JP-A-8-64246 Japanese Patent Laid-Open No. 9-63649 Japanese Patent Laid-Open No. 10-40958 JP 2001-307771 A JP 2002-329502 A JP 2003-86243 A

  An object of the present invention is to provide a method capable of producing a lithium secondary battery exhibiting good charge / discharge cycle characteristics even when the amount of carbon dioxide dissolved in a non-aqueous electrolyte is small.

  The present invention is a method for producing a lithium secondary battery comprising a negative electrode obtained by depositing a silicon amorphous thin film or an amorphous thin film mainly composed of silicon on a current collector, a positive electrode, and a non-aqueous electrolyte. A step of charging and discharging at least one cycle by bringing the negative electrode and the positive electrode into contact with the first nonaqueous electrolyte in which carbon is dissolved, and the amount of carbon dioxide dissolved after the charge and discharge is less than that of the first nonaqueous electrolyte And a step of assembling a final battery using the second non-aqueous electrolyte.

  In the present invention, the first non-aqueous electrolyte and the second non-aqueous electrolyte are used. First, at least one cycle of charging / discharging in the first non-aqueous electrolyte having a relatively large amount of dissolved carbon dioxide. Do. Thereafter, a final battery is assembled using a second nonaqueous electrolyte in which the amount of dissolved carbon dioxide is relatively smaller than that of the first nonaqueous electrolyte. Thus, even if the electrolyte in the final battery has a small amount of dissolved carbon dioxide, the final battery is charged and discharged in a non-aqueous electrolyte with a large amount of dissolved carbon dioxide, so that the final The effect of improving the charge / discharge cycle characteristics comparable to that using a non-aqueous electrolyte with a large amount of carbon dioxide dissolved in the battery can be obtained. The reason for this is considered as follows.

  That is, by performing at least one cycle of charge / discharge in the first non-aqueous electrolyte in which the amount of carbon dioxide dissolved is relatively large, a stable coating with good lithium ion conductivity is formed on the thin film surface of the negative electrode. Therefore, it is considered that good charge / discharge cycle characteristics can be exhibited even if the amount of carbon dioxide dissolved in the nonaqueous electrolyte is small in the subsequent charge / discharge cycle.

  Therefore, it can be seen that the amount of carbon dioxide dissolved in the nonaqueous electrolyte is important in the initial charge / discharge cycle. In the present invention, since the amount of carbon dioxide dissolved in the non-aqueous electrolyte in the final battery is reduced, problems such as a decrease in charge / discharge capacity due to the generation of carbon dioxide gas and an increase in battery thickness have been problems. Can be prevented.

  In the present invention, a second nonaqueous electrolyte having a smaller amount of carbon dioxide than that of the first nonaqueous electrolyte is used. Such a second non-aqueous electrolyte can be prepared, for example, by replacing at least a part of the first non-aqueous electrolyte with a non-aqueous electrolyte that dissolves less carbon dioxide than the first non-aqueous electrolyte. .

  Further, after charge / discharge using the first non-aqueous electrolyte, the pressure may be reduced to release carbon dioxide in the first non-aqueous electrolyte, and the concentration of carbon dioxide may be lowered to form the second non-aqueous electrolyte. . When the components in the non-aqueous electrolyte are evaporated and reduced by this pressure reduction, components such as a non-aqueous electrolyte with a small amount of dissolved carbon dioxide or an evaporated solvent may be replenished.

  Further, after charging / discharging using the first non-aqueous electrolyte, the temperature of the non-aqueous electrolyte may be raised to release dissolved carbon dioxide, thereby reducing the amount of dissolution. In addition, after charging / discharging using the first non-aqueous electrolyte, nitrogen gas, inert gas, or the like is blown into the first non-aqueous electrolyte, so that the carbon dioxide dissolved in the first non-aqueous electrolyte is discharged. The amount may be reduced to form the second nonaqueous electrolyte.

  In the present invention, the amount of carbon dioxide dissolved in the first electrolyte is preferably 0.01% by weight or more, more preferably 0.1% by weight or more. Usually, it is preferable to dissolve carbon dioxide until saturation.

  Here, the amount of carbon dioxide dissolved does not include carbon dioxide inevitably dissolved in the nonaqueous electrolyte. That is, carbon dioxide that dissolves in the non-aqueous electrolyte in a normal manufacturing process is not included. Therefore, the amount of carbon dioxide dissolved can be determined, for example, by measuring the weight of the non-aqueous electrolyte after dissolving carbon dioxide and the weight of the non-aqueous electrolyte before dissolving carbon dioxide. Specifically, it can be obtained by the following equation.

Dissolved amount of carbon dioxide in non-aqueous electrolyte (% by weight) = [(weight of non-aqueous electrolyte after dissolving carbon dioxide) − (weight of non-aqueous electrolyte before dissolving carbon dioxide)] / (dioxide Weight of non-aqueous electrolyte after dissolving carbon) × 100
In the present invention, the amount of carbon dioxide dissolved in the second non-aqueous electrolyte should be less than that of the first non-aqueous electrolyte, but preferably the non-aqueous electrolyte in which carbon dioxide is not dissolved is used in the first non-aqueous electrolyte. 2 as a non-aqueous electrolyte.

  In the present invention, examples of the method for dissolving carbon dioxide in the non-aqueous electrolyte include a method for dissolving carbon dioxide by bringing carbon dioxide into contact with the non-aqueous electrolyte. As such a method, a method of blowing gaseous carbon dioxide into the non-aqueous electrolyte can be mentioned. By this method, a non-aqueous electrolyte in which carbon dioxide is dissolved can be obtained efficiently and easily. Other methods include a method of stirring the non-aqueous electrolyte in carbon dioxide and a method of bringing high-pressure carbon dioxide into contact with the non-aqueous electrolyte. Further, carbon dioxide may be dissolved in the non-aqueous electrolyte by adding a substance that generates carbon dioxide to the non-aqueous electrolyte. Examples of the substance that generates carbon dioxide include bicarbonate and carbonate. Also, dry ice or the like may be used.

  In the present invention, charging / discharging performed in the first nonaqueous electrolyte is performed in a state where the negative electrode and the positive electrode are in contact with the first nonaqueous electrolyte. Since the battery manufactured at this stage is not a final battery, it is preferable to carry out the battery with a part of the battery outer body opened. After charging / discharging in such a state, the nonaqueous electrolyte in the exterior body is made the second nonaqueous electrolyte by a method such as replacing at least a part of the first nonaqueous electrolyte in the exterior body, and then the exterior body. As a result, the final battery is assembled.

  In the present invention, a negative electrode in which a silicon amorphous thin film or an amorphous thin film mainly composed of silicon is deposited on a current collector is used. In the present invention, non-crystalline means amorphous and fine crystals having a crystallite size of 100 nm or less. The determination of whether or not it is amorphous and the measurement of the crystallite size in the microcrystalline thin film can be performed by applying the presence or absence of a peak in the X-ray diffraction spectrum and the half-width of the peak to the Scherrer equation. it can. As is clear from the above definition of amorphous, the amorphous thin film in the present invention does not include a single crystal thin film and a polycrystalline thin film.

  The amorphous thin film mainly composed of silicon is an amorphous alloy thin film containing 50 atomic% or more of silicon. Specific examples include Si—Co alloy thin films, Si—Fe alloy thin films, Si—Zn alloy thin films, Si—Zr alloy thin films, and the like.

  In the present invention, as a method of forming the amorphous thin film on the current collector, a method of depositing the amorphous thin film by supplying the raw material from the gas phase is preferably used. Examples of such a method include a sputtering method, a CVD method, and a vapor deposition method.

  In the present invention, the arithmetic average roughness Ra of the current collector surface on which the thin film is deposited is preferably 0.1 μm or more. The arithmetic average roughness Ra is defined in Japanese Industrial Standard (JIS B 0601-1994). The arithmetic average roughness Ra can be measured by, for example, a stylus type surface roughness meter. By depositing a thin film on the current collector having such large unevenness, unevenness corresponding to the unevenness of the current collector surface can be formed on the surface of the thin film. When charging / discharging using an amorphous thin film with large irregularities on the surface as an active material, the stress accompanying expansion and contraction of the thin film concentrates on the valleys of the irregularities of the thin film, and a cut is formed in the film thickness direction as described above. The thin film is separated into columns. As a result, stress generated by charging / discharging is dispersed, and reversible structural change of the amorphous thin film is facilitated.

  However, when the thin film is separated into columns, the contact area between the thin film and the nonaqueous electrolyte is dramatically increased. As described above, in the conventional electrode, it is known that the active material is altered from the surface of the thin film that is in contact with the nonaqueous electrolyte, and the thin film becomes porous. According to the present invention, such porous formation can be suppressed, charge / discharge cycle characteristics can be improved, and an increase in the thickness of the thin film can be suppressed to improve the volume energy density in the battery. Can do.

  The upper limit value of the arithmetic mean roughness Ra of the current collector surface is not particularly limited, but it is preferable that the current collector has a thickness in the range of 10 to 100 μm. The upper limit of the thickness Ra is preferably substantially 10 μm or less.

  In the present invention, it is preferable to use a heat-resistant copper alloy foil as the current collector. Here, the heat resistant copper alloy means a copper alloy having a tensile strength of 300 MPa or more after annealing at 200 ° C. for 1 hour. As such a heat-resistant copper alloy, for example, those listed in Table 1 can be used.

  In the production of the negative electrode in the present invention, the mechanical strength of the current collector may be reduced due to a temperature change when forming a thin film on the current collector, which may make it difficult to process the battery. By using a heat-resistant copper alloy thin film as the current collector, it is possible to prevent a decrease in mechanical strength due to a temperature change, and to ensure sufficient conductivity.

  As described above, the current collector used in the present invention preferably has large irregularities on the surface thereof. For this reason, when arithmetic mean roughness Ra of heat-resistant copper alloy foil is not large enough, you may provide a large unevenness | corrugation in the surface by providing electrolytic copper or an electrolytic copper alloy in the foil surface. The electrolytic copper layer and the electrolytic copper alloy layer can be formed by an electrolytic method.

The solvent of the non-aqueous electrolyte used in the lithium secondary battery of the present invention is not particularly limited, but cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl A mixed solvent with a chain carbonate such as carbonate is exemplified. Further, mixed solvents of the above cyclic carbonate and ether solvents such as 1,2-dimethoxyethane and 1,2-diethoxyethane are also exemplified. The solutes of the nonaqueous electrolyte include LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C _{4 F 9 SO 2), LiC} (CF 3 SO 2) 3, LiC (C 2 F 5 SO 2) 3, LiAsF 6, LiClO 4, Li 2 B 10 Cl 10, Li 2 B 12 Cl 12 and the like and their Mixtures are exemplified. In particular, LiXF _y (wherein X is P, As, Sb, B, Bi, Al, Ga, or In, y is 6 when X is P, As, or Sb, and X is Bi, Al, Ga or y when in is 4), lithium perfluoroalkyl sulfonic acid imide _{LiN (C m F 2m + 1} SO 2) (C n F 2n + 1 SO 2) ( wherein, m and n are each independently, to an integer of 1 to 4) or lithium perfluoroalkyl sulfonic acid methide _{LiN (C p F 2p + 1} SO 2) (C q F 2q + 1 SO 2) (C r F 2r + 1 SO 2) ( wherein Among them, solutes such as p, q and r are each independently an integer of 1 to 4 are preferably used. Among these, LiPF ₆ is particularly preferably used. Further, examples of the electrolyte include gel polymer electrolytes in which a polymer electrolyte such as polyethylene oxide and polyacrylonitrile is impregnated with an electrolytic solution, and inorganic solid electrolytes such as LiI and Li ₃ N. The electrolyte of the lithium secondary battery of the present invention is not limited as long as the lithium compound as a solute that exhibits ionic conductivity and the solvent that dissolves and retains the lithium compound do not decompose at the time of battery charging, discharging, or storage. Can be used.

Examples of the positive electrode material of the lithium secondary battery of the present invention include LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiMnO ₂ , LiCo _0.5 Ni _0.5 O ₂ , LiNi _0.7 Co _0.2 Mn _0.1 O ₂ and other lithium-containing transition metal oxides. Examples thereof include metal oxides that do not contain lithium, such as MnO ₂ . In addition, any substance that electrochemically inserts and desorbs lithium can be used without limitation.

  According to the present invention, even if the amount of carbon dioxide dissolved in the nonaqueous electrolyte is small, a lithium secondary battery exhibiting good charge / discharge cycle characteristics can be obtained.

  Hereinafter, the present invention will be described in more detail based on examples. However, the present invention is not limited to the following examples, and can be implemented with appropriate modifications within a range not changing the gist thereof. Is.

(Experiment 1)
(Production of negative electrode)
Heat-resistant copper alloy foil (arithmetic mean roughness) roughened by depositing copper on the surface of a heat-resistant copper alloy rolled foil made of zirconium copper alloy (zirconium content 0.03% by weight) by electrolytic method Ra 0.25 μm, thickness 26 μm) was used as a current collector. An amorphous silicon thin film was deposited on the current collector using the sputtering apparatus shown in FIG.

  As shown in FIG. 1, a rotatable cylindrical substrate holder 2 is provided in the chamber 1, and a current collector is attached to the surface of the substrate holder 2. A Si sputtering source 3 is provided in the chamber 1, and a DC pulse power source 4 is connected to the Si sputtering source 3. In the chamber 1, a gas introduction port 6 for introducing Ar gas is provided, and an exhaust port 7 for exhausting the inside of the chamber 1 is provided.

After exhausting the inside of the chamber to 1 × 10 ⁻⁴ Pa by evacuating from the exhaust port 7, Ar gas was introduced into the chamber 1 from the gas introduction port 6 to stabilize the gas pressure, and the gas pressure was stabilized. In this state, a DC pulse was applied to the Si sputtering source 3 from the DC pulse power source 4 to generate plasma 5, and an amorphous silicon thin film was deposited on the current collector attached to the surface of the substrate holder 2. Specific thin film deposition conditions are as shown in Table 2.

  After depositing the thin film to a thickness of 5 μm, the current collector is removed from the substrate holder 2, the thin film and the current collector are both cut into a size of 2.5 cm × 2.5 cm, and a negative electrode tab is attached thereto. A negative electrode was produced.

[Production of positive electrode]
90 parts by weight of LiCoO ₂ powder and 5 parts by weight of artificial graphite powder as a conductive agent are mixed with a 5% by weight N-methylpyrrolidone aqueous solution containing 5 parts by weight of polyvinylidene fluoride as a binder, and a positive electrode mixture slurry It was. This slurry was applied on a 2 cm × 2 cm region of an aluminum foil (thickness: 18 μm) as a positive electrode current collector by a doctor blade method and then dried to form a positive electrode active material layer. A positive electrode tab was attached on the region of the aluminum foil where the positive electrode active material layer was not applied, and a positive electrode was produced.

[Production of non-aqueous electrolyte]
A solution prepared by dissolving LiPF ₆ in a solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 3: 7 so as to be 1 mol / liter was prepared, and this was used as the nonaqueous electrolyte b1.

  Carbon dioxide was blown into the nonaqueous electrolyte b1 at a temperature of 25 ° C. for 30 minutes to dissolve the carbon dioxide until the saturation amount was reached, and this was designated as the nonaqueous electrolyte a1. The weight before dissolving carbon dioxide and the weight after dissolving carbon dioxide were measured, and the amount of carbon dioxide dissolved was determined to be 0.37% by weight.

The nonaqueous electrolytes a1 and b1 are as follows.
Nonaqueous electrolyte a1: Nonaqueous electrolyte in which CO ₂ is dissolved Nonaqueous electrolyte b1: Nonaqueous electrolyte in which CO ₂ is not dissolved

[Production of battery]
In a carbon dioxide atmosphere, after attaching a positive electrode current collector tab and a negative electrode current collector tab to the positive electrode and the negative electrode, respectively, a separator made of porous polyethylene is sandwiched between the positive electrode and the negative electrode to form an electrode group. The group was inserted into an exterior body made of aluminum laminate. Thereafter, 600 μl of the non-aqueous electrolyte a1 was injected to produce a battery A1 and a battery B1.

[Charge / discharge cycle test]
The lithium secondary batteries A1 and B1 produced as described above were subjected to a charge / discharge cycle test. The charging / discharging conditions are as follows: At 25 ° C., the battery is charged with a charging current of 13 mA until the charging end voltage is 4.2 V, and then discharged with a discharging current of 13 mA until the discharging end voltage is 2.75 V. It was.

  After one cycle of charge and discharge, the nonaqueous electrolyte of each battery was replaced under the conditions shown in Table 3, and the cycle test was continued under the above charge and discharge conditions.

  Table 4 shows the maximum discharge capacity, discharge capacity and capacity retention rate at the 100th cycle for each battery. Note that the maximum discharge capacity is the maximum discharge capacity in all the cycles, and the capacity maintenance rate is a value obtained by setting the maximum discharge capacity to 100%.

  As shown in Table 4, the battery A1 replaced with the nonaqueous electrolyte b1 in which carbon dioxide was not dissolved after the first cycle charge / discharge was a nonaqueous electrolyte in which carbon dioxide was dissolved after the first cycle charge / discharge. Charge / discharge cycle characteristics equivalent to those of the battery B1 replaced with a1 were shown. This is probably because a good film was formed on the thin film surface of the electrode during the first charge / discharge cycle, and therefore good cycle characteristics were maintained even after the nonaqueous electrolyte was replaced. Note that the capacity maintenance rate at the 100th cycle of the battery using the nonaqueous electrolyte b1 not dissolving carbon dioxide from the beginning is about 18%. Therefore, by performing only the charge / discharge of the first cycle in the nonaqueous electrolyte in which carbon dioxide is dissolved, the charge / discharge cycle characteristics can be remarkably improved as described above.

  As described above, according to the present invention, since a non-aqueous electrolyte with a small amount of carbon dioxide dissolved can be used in the final battery, lithium that generates little carbon dioxide from the non-aqueous electrolyte during high-temperature storage can be used. It can be set as a secondary battery. Therefore, a lithium secondary battery that does not cause problems such as a decrease in charge / discharge capacity due to gas generation and an increase in battery thickness can be obtained.

  In the above embodiment, when replacing the nonaqueous electrolyte, all the nonaqueous electrolytes are replaced. However, the present invention is not limited to this, and only a part of the nonaqueous electrolyte may be replaced.

The schematic diagram which shows the sputtering device used in the Example according to this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Chamber 2 ... Substrate holder 3 ... Si sputter source 4 ... DC pulse power supply 5 ... Plasma 6 ... Gas introduction port 7 ... Gas exhaust port

Claims (5)

A method for producing a lithium secondary battery comprising a negative electrode obtained by depositing a silicon amorphous thin film or an amorphous thin film mainly composed of silicon on a current collector, a positive electrode, and a nonaqueous electrolyte,
Charging and discharging at least one cycle by bringing the negative electrode and the positive electrode into contact with a first non-aqueous electrolyte in which carbon dioxide is dissolved;
And a step of assembling a final battery using the second non-aqueous electrolyte in which the amount of carbon dioxide dissolved is smaller than that of the first non-aqueous electrolyte after the charge / discharge. Manufacturing method.   The second non-aqueous electrolyte is obtained by replacing at least a part of the first non-aqueous electrolyte with a non-aqueous electrolyte that dissolves less carbon dioxide than the first non-aqueous electrolyte. The method for producing a lithium secondary battery according to claim 1. After the step of performing at least one cycle of charging and discharging, performing a step of reducing the pressure to release carbon dioxide in the first nonaqueous electrolyte and lower the concentration of carbon dioxide in the first nonaqueous electrolyte. ,
2. The method of manufacturing a lithium secondary battery according to claim 1, wherein the first non-aqueous electrolyte having a reduced carbon dioxide concentration is used as the second non-aqueous electrolyte. After the step of performing at least one cycle of charging / discharging, in order to release carbon dioxide in the first nonaqueous electrolyte and reduce the carbon dioxide concentration in the first nonaqueous electrolyte, the first a non-aqueous electrolyte performs addition or heat process,
2. The method of manufacturing a lithium secondary battery according to claim 1, wherein the first non-aqueous electrolyte having a reduced carbon dioxide concentration is used as the second non-aqueous electrolyte. After the step of performing at least one cycle of charging / discharging, nitrogen gas and / or inert gas is blown into the first non-aqueous electrolyte in order to reduce the carbon dioxide concentration in the first non-aqueous electrolyte. Perform the process,
2. The method of manufacturing a lithium secondary battery according to claim 1, wherein the first non-aqueous electrolyte having a reduced carbon dioxide concentration is used as the second non-aqueous electrolyte.

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