JP2006185829A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a non-aqueous electrolyte secondary battery in which a decomposition of the electrolytic solution and degradation of active material are suppressed to improve charge and discharge cycle characteristic. <P>SOLUTION: The non-aqueous electrolyte secondary battery includes: a negative electrode having negative electrode active material occluding or discharging lithium; a positive electrode having positive electrode active material occluding or discharging lithium; and non-aqueous electrolytic solution. The negative electrode active material is a thin film formed by being deposited on a collector with CVD process, sputtering process, vacuum deposition process, spraying process or plating process, the thin film is separated into pillar-shaped parts by cut line formed in the thickness direction, and the pillar-shaped parts have the bottom adhering closely to the collector. The non-aqueous electrolytic solution contains phosphazen derivative expressed by the general fomula ( NPR<SB>1</SB>R<SB>2</SB>)<SB>n</SB>(1)wherein, R<SB>1</SB>and R<SB>2</SB>indicate a monovalent substituent or halogen element, and n=3-14. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a non-aqueous electrolyte secondary battery such as a lithium secondary battery.

  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 a silicon thin film 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 phosphazene in a non-aqueous electrolyte (for example, Patent Document 3).

The lithium secondary battery using the above electrode proposed by the present applicant is a battery having a very large charge / discharge capacity, but it causes deterioration of the active material and decomposition of the electrolyte due to repeated charge / discharge, resulting in cycle characteristics. May be insufficient.
International Publication No. 01/29913 Pamphlet International Publication No. 02/071512 Pamphlet International Publication No. 02/021629 Pamphlet

  An object of the present invention is to provide a non-aqueous electrolyte secondary battery capable of suppressing the decomposition of the electrolyte and the deterioration of the active material and improving the cycle characteristics.

  The non-aqueous electrolyte of the present invention includes a negative electrode that occludes and releases lithium, a positive electrode that includes a positive electrode active material that occludes and releases lithium, and a non-aqueous electrolyte, and the negative electrode active material includes a CVD method and a sputtering method. A thin film deposited on a current collector by a vapor deposition method, a thermal spraying method, or a plating method, and the thin film is separated into columns by a cut formed in the thickness direction, and the bottom of the columnar portion is It is in close contact with the current collector, and is characterized in that the phosphazene derivative represented by the general formula (1) is contained in the nonaqueous electrolytic solution.

(Here, R ₁ and R ₂ represent a monovalent substituent or a halogen element, and n = 3-14.)
In the present invention, a thin film deposited on a current collector by a CVD method, a sputtering method, a vapor deposition method, a thermal spraying method, or a plating method is used as the negative electrode active material. Further, the thin film is separated into a columnar shape by a cut formed in the thickness direction, and the bottom of the columnar portion is in close contact with the current collector. Such a thin film can be formed by using silicon, silicon alloy, tin, tin alloy, or the like, deposited on a current collector by CVD or plating, and has a cut formed in the thickness direction. Can be formed by expanding and contracting the thin film by a charge / discharge reaction or the like. The thin film is separated into columnar shapes by the cuts, and voids are formed around the columnar portions. For this reason, when the negative electrode active material absorbs lithium and expands, the volume expansion can be received by the voids, and the generation of stress due to the volume expansion can be suppressed. In addition, since the bottom of the columnar part is in close contact with the current collector, even if the negative electrode active material expands or contracts due to charge / discharge, it does not peel off from the current collector, and thus good cycle characteristics can be obtained. .

  However, as described above, since the active material is deteriorated and the electrolytic solution is decomposed by repeated charge and discharge, the cycle characteristics may be deteriorated. That is, it is considered that when the negative electrode active material expands / shrinks due to charging / discharging, a portion that reacts with the electrolytic solution and easily changes in quality is generated particularly on the side surface of the columnar portion of the negative electrode active material thin film. When such a phenomenon is repeated by the charge / discharge cycle, the active material gradually changes in quality, the ability to reversibly store and release lithium is lost, and the capacity deteriorates. By containing the phosphazene derivative represented by the general formula (1) in the electrolytic solution according to the present invention, decomposition of the electrolytic solution and deterioration of the active material can be suppressed, and cycle characteristics can be improved.

  In the present invention, the phosphazene derivative is preferably a cyclic phosphazene derivative in consideration of reactivity with the negative electrode.

Of R ₁ and R ₂ in the general formula (1), the proportion of halogen elements in the total number of substituents directly bonded to phosphorus is preferably 80% or more and less than 100%. Examples of the halogen element include fluorine, chlorine, bromine and the like. In particular, fluorine is preferable.

In the general formula (1), when all of R ₁ and R ₂ are fluorine, the phosphazene derivative has a low boiling point and is difficult to handle, and the melting point is high at about 15 to 20 ° C. Is not preferred because it tends to decrease. In particular, when n is 3, it is not preferred that all of R ₁ and R ₂ are fluorine, since the boiling point is as low as 52 ° C. and volatilization tends to occur. Examples of the monovalent substituent in R ₁ and R ₂ include an alkoxy group, an alkyl group, an acyl group, an aryl group, and a carboxyl group. Examples of the alkoxy group include a methoxy group, an ethoxy group, and a phenoxy group. Further, it may be an alkoxy group-substituted alkoxy group such as a methoxyethoxy group. Of R ₁ and R ₂ , at least one is preferably a phenoxy group.

If the ratio of these monovalent substituents is too large, the viscosity of the electrolytic solution will increase, and there will be an adverse effect on battery performance such that the capacity cannot be stably obtained, and the reactivity with the negative electrode active material will increase. . Therefore, as described above, it is preferable that 80% or more and less than 100% of R ₁ and R ₂ are halogen elements.

  In the present invention, one phosphazene derivative may be used alone, or two or more phosphazene derivatives may be used in combination.

  In the present invention, the phosphazene derivative is preferably contained in an amount of 30% by volume or less, more preferably 0.01 to 30% by volume, and further preferably 20% by volume or less in the solvent component in the non-aqueous electrolyte. And more preferably 0.1 to 20% by volume. If the content of the phosphazene derivative is too large, the discharge capacity may be reduced and the battery performance may become unstable.

  The negative electrode active material of the present invention is an active material that occludes and releases lithium. Preferably, an active material that occludes lithium by being alloyed with lithium is preferably used. Examples of such a negative electrode active material include silicon, a silicon alloy, tin, and a tin alloy. Examples of the silicon alloy include an alloy containing silicon and at least one selected from cobalt, iron, zinc, and zirconium. Specifically, Si—Co alloy, Si—Fe alloy, Si—Zn alloy, Si—Zr alloy, and the like can be given.

  The non-aqueous electrolyte of the present invention is a non-aqueous electrolyte secondary comprising a negative electrode containing a negative electrode active material that occludes and releases lithium, a positive electrode containing a positive electrode active material that occludes and releases lithium, and a non-aqueous electrolyte. A non-aqueous electrolyte used in a battery, in which a negative electrode active material is deposited on a current collector by a CVD method, a sputtering method, a vapor deposition method, a thermal spraying method, or a plating method. The columnar portions are separated by a cut formed in the thickness direction, the bottom of the columnar portion is in close contact with the current collector, and the phosphazene derivative represented by the general formula (1) is contained in the nonaqueous electrolytic solution. It is characterized by that.

  In the present invention, the negative electrode active material is particularly preferably a silicon amorphous thin film or an amorphous thin film containing silicon as a main component. Amorphous means an amorphous crystallite 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 does not include a single crystal thin film and a polycrystalline thin film.

  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 a thin film with large irregularities on the surface as an active material, 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. Are separated into columns. As a result, the stress generated by charging / discharging is dispersed, and the reversible structural change of the thin film is facilitated.

  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.

  In the present invention, the negative electrode is produced by depositing a thin film on a current collector. As a method of depositing and forming a thin film, a method of depositing a thin film by supplying a raw material from a gas phase is preferably used. According to this method, since the thin film is deposited almost uniformly on the unevenness of the current collector surface, the unevenness corresponding to the uneven shape of the current collector surface can be formed on the surface of the thin film. Examples of a method for depositing a thin film by supplying a raw material from the vapor phase include a sputtering method, a chemical vapor deposition method (CVD method), and a vapor deposition method. An example of a method for depositing a thin film by supplying a raw material from a liquid phase is a plating method.

  The non-aqueous electrolyte in the present invention contains a phosphazene derivative. Examples of solvents other than the phosphazene derivative include cyclic carbonates, chain carbonates, lactone compounds (cyclic carboxylic acid esters), chain carboxylic acid esters, and cyclic ethers. , Chain ether, sulfur-containing organic solvent and the like. These solvents may be used alone or in combination with a phosphazene derivative, or two or more kinds may be used in combination with a phosphazene derivative. Among these solvents, cyclic carbonates having 3 to 9 carbon atoms, lactone compounds, chain carbonates, chain carboxylic acid esters, and chain ethers are preferably used. Since these non-aqueous solvents are excellent in lithium ion conductivity and stability, battery characteristics when used in batteries are good.

  Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, vinyl ethylene carbonate, and the like. Examples of the lactone compound include γ-butyllactone, γ-valerolactone, and δ-valerolactone. Examples of the chain carbonate include dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Examples of the chain carboxylic acid ester include ethyl acetate, methyl propionate, and ethyl propionate. Examples of the chain ether include dimethoxyethane and diethoxyethane.

As the solute of the non-aqueous electrolyte in the present invention, a lithium salt is generally used. The lithium salt is not particularly limited as long as it can be used as a solute in a lithium secondary battery. Specific examples include, for example, LiXF _y such as LiPF ₆ , LiBF ₄ , LiAsF ₆ (wherein X is P, As, Sb, B, Bi, Al, Ga, or In, and X is P, As or Y is 6 when Sb, and y is 4 when X is Bi, Al, Ga, or In); an inorganic lithium salt such as Li ₂ B ₁₀ Cl ₁₀ , Li ₂ B ₁₂ Cl ₁₂ Lithium salt; 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 and other lithium perfluoroalkyl sulfonic acid imide LiN (C _m F _{2m + 1} SO ₂ ) (C _n F _{2n + 1} SO ₂ ) (where m and n Are each independently an integer of 1 to 4) and lithium perfluoroal Kirusuruhon acid methide _{LiC (C p F 2p + 1} SO 2) (C q F 2q + 1 SO 2) (C r F 2r + 1 SO 2) ( wherein, p, q and r 1 independently 4); LiPF ₃ (CF ₃ ) ₃ , LiPF ₂ (C ₂ F ₅ ) ₄ , LiPF ₃ (C ₂ F ₅ ) ₃ , LiB (CF ₃ ) ₄ , LiBF ( CF ₃ ) ₃ , LiBF ₂ (CF ₃ ) ₂ , LiBF ₃ (CF ₃ ), LiB (C ₂ F ₅ ) ₄ , LiBF (C ₂ F ₅ ) ₃ , LiBF ₂ (C ₂ F ₅ ) ₂ , LiBF ₃ Examples thereof include fluorine-containing organic lithium salts such as inorganic fluoride salts in which a part of fluorine atoms are substituted with perfluoroalkyl groups, such as (C ₂ F ₅ ). These solutes may be used as a mixture.

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 in the present invention can be used without limitation as long as the lithium compound as a solute that exhibits ionic conductivity and the solvent that dissolves and retains the lithium compound are not decomposed at the time of battery charging, discharging, or storage.

Examples of the positive electrode active material in the present invention include lithium-containing transition metal oxides such as 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 MnO _2. Examples thereof include metal oxides not containing lithium. In addition, any substance that electrochemically inserts and desorbs lithium can be used without limitation.

  ADVANTAGE OF THE INVENTION According to this invention, decomposition | disassembly of electrolyte solution and deterioration of an active material can be suppressed, and cycling characteristics can be improved.

  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 having a roughened surface by depositing copper on the surface of a heat-resistant copper alloy rolled foil made of zirconium copper alloy (zirconium content 0.015 to 0.03% by weight) ( Arithmetic average roughness Ra 0.25 μm, thickness 31 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. 2, a rotatable cylindrical substrate holder 12 is provided in the chamber 11, and a current collector is attached to the surface of the substrate holder 12. A Si sputtering source 13 is provided in the chamber 11, and a DC pulse power source 14 is connected to the Si sputtering source 13. In the chamber 11, a gas introduction port 16 for introducing Ar gas is provided, and an exhaust port 17 for exhausting the inside of the chamber 11 is provided.

After evacuating the chamber to 1 × 10 ⁻⁴ Pa by evacuating from the exhaust port 17, Ar gas was introduced into the chamber 11 from the gas inlet 16 to stabilize the gas pressure, and the gas pressure was stabilized. In this state, a DC pulse was applied from the DC pulse power source 14 to the Si sputtering source 13 to generate plasma 15, and an amorphous silicon thin film was deposited on the current collector attached to the surface of the substrate holder 12. Specific thin film deposition conditions are as shown in Table 2.

  After the thin film is deposited to a thickness of 3.5 μm, the current collector is removed from the substrate holder 12, and 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. Thus, 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 in a 5% by weight N-methylpyrrolidone aqueous solution containing 5 parts by weight of polytetrafluoroethylene as a binder, and a positive electrode mixture A slurry was obtained. 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.

[Preparation of non-aqueous electrolyte]
LiPF ₆ was dissolved in a solvent in which propylene carbonate and a cyclic phosphazene derivative having the structure shown below were mixed at a volume ratio of 9: 1 and 8: 2 so as to be 1 mol / liter, and non-aqueous electrolysis was performed. Liquids a1 and a2 were prepared.

Further, LiPF ₆ was dissolved in a solvent in which propylene carbonate and diethyl carbonate were mixed at a volume ratio of 9: 1 so as to be 1 mol / liter to prepare a non-aqueous electrolyte b1.

[Production of lithium secondary battery]
A lithium secondary battery shown in FIGS. 3 and 4 was produced using the above negative electrode, positive electrode, and non-aqueous electrolyte. FIG. 3 is a front view, and FIG. 4 is a cross-sectional view showing the structure of the electrode group in the battery. As shown in FIG. 4, a negative electrode active material layer 3 a is provided on the negative electrode current collector 3 b made of copper of the negative electrode 3. Positive electrodes 1 are arranged on both sides of the negative electrode 3 with separators 2 interposed therebetween. A positive electrode active material layer 1 a is provided on the positive electrode current collector 1 b made of aluminum of the positive electrode 1, and the positive electrode active material layer 1 a and the negative electrode active material layer 3 a face each other with the separator 2 interposed therebetween. . As the separator 2, porous polyethylene is provided.

  A negative electrode tab 3c made of nickel is attached to the negative electrode current collector 3b. A positive electrode tab 1c made of aluminum is attached to the positive electrode current collector 1b.

  As shown in FIG. 3, in the lithium secondary battery, the electrode group shown in FIG. 4 is inserted into the outer package 4 made of an aluminum laminate, 500 μl of the nonaqueous electrolyte is injected, and then the outer package 4 is sealed. The portion 4a was sealed to produce a lithium secondary battery with a design capacity of 10 mAh. The positive electrode tab 1 c and the negative electrode tab 3 c are provided so as to come out of the exterior body 4. The batteries corresponding to the nonaqueous electrolytes a1, a2, and b1 were designated as batteries A1, A2, and B1, respectively.

[Evaluation of cycle characteristics]
Charge / discharge cycle characteristics of the batteries A1, A2, and B1 were evaluated. Each battery was charged to 4.2 V at a current value of 10 mA at 25 ° C., and then discharged to 2.75 V at a current value of 10 mA. The discharge capacity in each cycle is shown in FIG.

  As is apparent from the results shown in FIG. 1, the batteries A1 and A2 containing the cyclic phosphazene derivative in the non-aqueous electrolyte according to the present invention have better cycle characteristics than the comparative battery B1 containing no cyclic phosphazene derivative. I understand that.

  Further, the battery A2 using the electrolytic solution a2 containing 20% by volume of the cyclic phosphazene derivative has a capacity after 100 cycles as compared with the battery A1 using the electrolytic solution a1 containing 10% by volume of the cyclic phosphazene derivative. However, the capacity of the first cycle is low, and it can be seen that it is not stably obtained.

The figure which shows the cycle characteristic evaluation result of the lithium secondary battery of the Example according to this invention. The schematic diagram which shows the sputtering device used for preparation of a negative electrode in the Example according to this invention. The front view which shows the lithium secondary battery of the Example according to this invention. Sectional drawing which shows the electrode group used for the lithium secondary battery shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Positive electrode 1a ... Positive electrode active material layer 1b ... Positive electrode collector 1c ... Positive electrode tab 2 ... Separator 3 ... Negative electrode 3a ... Negative electrode active material layer 3b ... Negative electrode collector 3c ... Negative electrode tab 4 ... Exterior body 4a ... Sealing part

Claims (7)

A non-aqueous electrolyte secondary battery comprising a negative electrode containing a negative electrode active material that occludes / releases lithium, a positive electrode containing a positive electrode active material that occludes / releases lithium, and a non-aqueous electrolyte solution,
The negative electrode active material is a thin film formed by depositing on a current collector by a CVD method, a sputtering method, a vapor deposition method, a thermal spraying method, or a plating method, and the thin film is formed into a columnar shape by a cut formed in its thickness direction. Non-aqueous electrolysis characterized in that the bottom of the columnar part is in close contact with the current collector, and the phosphazene derivative represented by the general formula (1) is contained in the non-aqueous electrolyte. Liquid secondary battery.

(Here, R ₁ and R ₂ represent a monovalent substituent or a halogen element, and n = 3-14.)   The non-aqueous electrolyte secondary battery according to claim 1, wherein the phosphazene derivative is a cyclic phosphazene derivative. _3. The ratio of the halogen element in the total number of substituents directly bonded to phosphorus among R ₁ and R ₂ in the general formula (1) is 80% or more and less than 100%. Non-aqueous electrolyte secondary battery. 4. The nonaqueous electrolyte secondary battery according to claim 1, wherein at least one of R ₁ and R ₂ in the general formula (1) is a phenoxy group.   5. The non-aqueous electrolyte secondary battery according to claim 1, wherein a ratio of the phosphazene derivative in a solvent component in the non-aqueous electrolyte is 20% by volume or less.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode active material is silicon, a silicon alloy, tin, or a tin alloy. A non-aqueous electrolyte used in a non-aqueous electrolyte secondary battery comprising a negative electrode including a negative electrode active material that absorbs and releases lithium, a positive electrode including a positive electrode active material that stores and releases lithium, and a non-aqueous electrolyte. There,
The negative electrode active material is a thin film formed by depositing on a current collector by a CVD method, a sputtering method, a vapor deposition method, a thermal spraying method, or a plating method, and the thin film is formed into a columnar shape by a cut formed in its thickness direction. Non-aqueous electrolysis characterized in that the bottom of the columnar part is in close contact with the current collector, and the phosphazene derivative represented by the general formula (1) is contained in the non-aqueous electrolyte. liquid.

(Here, R ₁ and R ₂ represent a monovalent substituent or a halogen element, and n = 3-14.)

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