JP4318405B2 - Lithium secondary battery - Google Patents

Description

【００４４】
表１に示す結果から明らかなように、本発明に従いフッ素置換芳香族系ホウ酸リチウムを溶質として含有した実施例１は、比較例１よりも高い容量維持率を示しており、充放電サイクル特性に優れていることがわかる。
【００４５】
上記実施例においては、一般式ＬｉＢＦ（Ｃ₆Ｈ_5-pＦ_p）₃で表されるフッ素置換芳香族系ホウ酸リチウムを用いているが、一般式ＬｉＢ（Ｃ₆Ｈ_5-qＦ_q）₄及びＬｉＢ〔Ｃ₆Ｈ_5-r(ＣＦ₃）_r〕₄で表されるフッ素置換芳香族系ホウ酸リチウムについても同様の効果が得られることを確認している。
【００４６】
【発明の効果】
本発明によれば、リチウム二次電池の充放電サイクル特性をサイクル向上させることができる。
【図面の簡単な説明】
【図１】本発明における電極表面を模式的に示す断面図。
【図２】本発明の実施例において作製したリチウム二次電池を示す平面図。
【図３】図２に示すリチウム二次電池における電極の組み合わせ構造を示す断面図。
【符号の説明】
１…正極
１ａ…正極活物質層
１ｂ…正極集電体
１ｃ…正極タブ
２…セパレータ
３…負極
３ａ…負極活物質層
３ｂ…負極集電体
３ｃ…負極タブ
４…外装体
４ａ…外装体の封止部
１０…集電体
１０ａ…集電体の表面
１０ｂ…集電体表面の凹凸の谷部
１１…活物質薄膜の柱状部分
１２…切れ目（空隙）
１３…被膜[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a lithium secondary battery, and more particularly to improvement of a nonaqueous electrolyte in a lithium secondary battery using an electrode formed by depositing an active material thin film that absorbs and releases lithium on a current collector. It is.
[0002]
[Prior art and problems to be solved by the invention]
In recent years, lithium secondary batteries have been actively developed. Lithium secondary batteries are greatly affected by battery characteristics such as charge / discharge voltage, charge / discharge cycle life characteristics, and storage characteristics, depending on the electrode active material used.
[0003]
The present applicant has shown that an electrode formed by depositing an active material thin film that absorbs and releases lithium, such as an amorphous silicon thin film and a microcrystalline silicon thin film, on a current collector exhibits a high charge / discharge capacity and is excellent. It has been found that it exhibits charge / discharge cycle characteristics. In such an electrode, the active material thin film is separated into a columnar shape by a cut formed in the thickness direction, and the bottom of the columnar portion has a structure in close contact with the current collector. In the electrode having such a structure, a gap is formed around the columnar portion, so that the stress due to expansion and contraction of the thin film accompanying the charge / discharge cycle is relieved by this gap, and the active material thin film is separated from the current collector Therefore, excellent charge / discharge cycle characteristics can be obtained.
[0004]
However, in a lithium secondary battery using such an electrode, the relationship between the nonaqueous electrolyte and the charge / discharge cycle characteristics has not been sufficiently studied.
An object of the present invention is to provide a lithium secondary battery using an electrode formed by depositing an active material thin film that occludes / releases lithium on a current collector, wherein the charge / discharge cycle characteristics are further improved. It is to provide.
[0005]
[Means for Solving the Problems]
The lithium secondary battery of the present invention is an electrode that includes a positive electrode, a negative electrode, and a nonaqueous electrolyte, and the positive electrode or the negative electrode is formed by depositing an active material thin film that absorbs and releases lithium on a current collector. and are separated in a columnar shape by cuts active material thin film is formed in the thickness direction, an electrode in which the bottom of the columnar portion is in close contact with the current collector, the active material thin film is silicon down thin film And the current collector contains copper as a current collector component, and the non-aqueous electrolyte is a general formula LiBF (C ₆ H _5−p F _p ) ₃ (wherein p is an integer of 1 to 5), LiB ( _{C 6 H 5-q F q} ) 4 ( wherein, q is an integer of from 1 to 5), and LiB _{[C 6 H 5-r (CF} 3) r ] ₄ (wherein, r is an integer of 1 to 5 ) Is included as a solute.
[0006]
In the present invention, the non-aqueous electrolyte contains a fluorine-substituted aromatic lithium borate represented by the above general formula, whereby a film containing fluorine and boron is formed on the side surface of the columnar portion of the active material thin film. Selectively formed. It is considered that the columnar structure of the active material thin film is stabilized by the coating formed in this manner, and deterioration and collapse of the columnar part are suppressed. By suppressing the deterioration and collapse of the columnar part, it is considered that the close contact state with the current collector at the bottom of the columnar part is maintained well, and the charge / discharge cycle characteristics can be improved.
[0007]
The active material thin film in the present invention is preferably formed by a method of depositing a thin film from a gas phase or a liquid phase. Examples of the method for depositing a thin film from the gas phase include a CVD method, a sputtering method, a vapor deposition method, and a thermal spraying method. Among these, the CVD method, the sputtering method, and the vapor deposition method are particularly preferably used. Examples of the method for depositing a thin film from the liquid phase include plating methods such as an electrolytic plating method and an electroless plating method.
[0008]
The thin film formed by the above-described thin film forming method is generally formed as a continuous thin film. When this thin film occludes lithium, the volume expands, and when the occluded lithium is released, the volume shrinks. Due to the expansion and contraction of the volume, a cut is formed in the active material thin film.
[0009]
In the present invention, such a cut is formed in the thickness direction of the thin film, and the thin film is separated into columns. By separating the thin film into a columnar shape, even if the volume of the thin film expands and contracts due to charge and discharge, the expansion and contraction of the volume can be absorbed by the voids present around the columnar part. Generation of stress can be suppressed. For this reason, it can prevent that a thin film pulverizes or a thin film peels from a collector, the adhesiveness of a collector and a thin film is maintained, and a charge / discharge cycle characteristic can be improved.
[0010]
In the present invention, since the film from the solute is further formed on the side surface of such a columnar portion, the structure of the columnar portion is stabilized, and charge / discharge cycle characteristics can be further improved.
[0011]
The break due to the expansion and contraction of the volume of the active material thin film is preferably formed by charge and discharge after the first time. As will be described later, when an active material thin film is formed on a current collector having irregularities on the surface by a thin film formation method, a low density region is formed upward from the irregular valleys on the current collector surface. There is a case. The cut may be formed along a low density region extending in the thickness direction of such an active material thin film.
[0013]
In the above aspect, the active material thin film expands and contracts due to charge and discharge after the first time, and a cut is formed in the thin film. The fluorine-substituted aromatic lithium borate represented by the above general formula reacts with the surface of the cut of the thin film thus formed, and a film containing fluorine and boron is formed on the surface. By forming such a film, the active material thin film is prevented from being pulverized, and charge / discharge cycle characteristics can be improved. Moreover, it is preferable that such a cut | interruption is formed in the thickness direction of an active material thin film, and an active material thin film is isolate | separated into columnar shape.
[0014]
In the present invention, among the above-mentioned solutes, in particular (in the formula, p is an integer of 1 to 5) Formula _{LiBF (C 6 H 5-p} F p) 3 preferably contains a compound represented by the solute.
[0015]
A method for producing a fluorine-substituted aromatic lithium borate used as a solute in the present invention is described, for example, in a paper by Lambert et al. Organometallics, 13, 2430 (1994).
[0016]
In the present invention, in the nonaqueous electrolyte, as a solute, the general formula LiXF _y (wherein X is P, As, Sb, B, Bi, Al, Ga, or In, and X is P, Y is 6 when As or Sb, and y is 4 when X is B, Bi, Al, Ga, or In. Therefore, in the present invention, a mixed solute of fluorine-substituted aromatic lithium borate and LiXF _y is preferable. The mixing ratio is preferably in the range of 1: 0.01 to 1: 1 in terms of a molar ratio of fluorine-substituted aromatic lithium borate: LiXF _y . If the content of the fluorine-substituted aromatic lithium borate is less than this range, the effect of the present invention that the charge / discharge cycle characteristics are improved may not be sufficiently obtained. If the content exceeds this range, the conductivity of the non-aqueous electrolyte may decrease. Among the compounds represented by LiXF _y , LiPF ₆ and LiBF ₄ are particularly preferably used.
[0017]
The solvent in the non-aqueous electrolyte used in the present invention is not particularly limited as long as it is a solvent used in a lithium secondary battery. For example, cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, and dimethyl Examples include chain carbonates such as carbonate, diethyl carbonate, and methyl ethyl carbonate. Preferably, a mixed solvent of a cyclic carbonate and a chain carbonate is used. Such a mixed solvent preferably contains ethylene carbonate as a cyclic carbonate.
[0018]
In the present invention, the nonaqueous electrolyte may be a gel polymer electrolyte obtained by impregnating a polymer electrolyte such as polyethylene oxide, polyacrylonitrile, or polyvinylidene fluoride with an electrolytic solution, or an inorganic solid electrolyte such as LiI or Li ₃ N. .
[0019]
In the present invention, the positive electrode active _material, and _{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 lithium-containing transition metal oxides such as O _2, Examples include metal oxides such as MnO ₂ that do not contain lithium. In addition, any substance that electrochemically inserts and desorbs lithium can be used without limitation.
[0020]
Thin film of active material in the present invention is divorced thin.
[0021]
In the present invention, the active material thin film is preferably an amorphous thin film or a microcrystalline thin film. Therefore, an amorphous silicon thin film or a microcrystalline silicon thin film is particularly preferable.
[0022]
The current collector used in the present invention contains copper as a current collector component.
[0023]
The current collector is preferably thin, and is preferably a metal foil. The current collector is preferably formed of a material that does not alloy with lithium, and a particularly preferable material is copper. The current collector is preferably a copper foil, and is preferably a copper foil having a roughened surface. Examples of such copper foil include electrolytic copper foil. For example, an electrolytic copper foil is prepared by immersing a metal drum in an electrolytic solution in which copper ions are dissolved, and flowing current while rotating the copper drum, thereby depositing copper on the surface of the drum and peeling it off. It is the obtained copper foil. One side or both sides of the electrolytic copper foil may be subjected to roughening treatment or surface treatment.
[0024]
Moreover, copper foil which roughened the surface by depositing copper on the surface (one side or both sides) of the rolled copper foil by an electrolytic method may be used.
Further, an intermediate layer may be formed on the current collector, and an active material thin film may be formed on the intermediate layer. In this case, the intermediate layer preferably includes a component that easily diffuses into the active material thin film, and for example, a copper layer is preferable. For example, a current collector in which a copper layer is formed on a nickel foil (such as an electrolytic nickel foil) whose surface is roughened may be used. Moreover, you may use the nickel foil which precipitated copper on the nickel foil by the electrolytic method and was roughened by this.
[0025]
In the present invention, the surface of the current collector is preferably roughened as described above. The surface roughness Ra of the current collector is preferably 0.01 μm or more, and more preferably 0.01 to 1 μm. The surface roughness Ra is defined in Japanese Industrial Standard (JIS B 0601-1994), and can be measured by, for example, a surface roughness meter.
[0026]
By depositing and forming an active material thin film on a current collector having irregularities on the surface, it is possible to form irregularities corresponding to the irregularities of the current collector surface, which is the underlayer, on the surface of the active material thin film. it can. As described above, the low density region is easily formed in the region connecting the uneven valley of the active material thin film and the uneven valley of the current collector surface. The cut is formed along such a region, and the active material thin film is separated into columns. As described above, the fluorine-substituted aromatic lithium borate represented by the above general formula acts on the side surface of the columnar portion thus formed, and a film is formed.
[0027]
In the present invention, it is preferable that components of the current collector are diffused in the active material thin film. Due to the diffusion of the components of the current collector, good adhesion between the active material thin film and the current collector can be maintained. As a component of the current collector, by copper element which is not alloyed with lithium is diffused, since the alloying with lithium is suppressed in diffusion region, it is possible to suppress the expansion and contraction of the film due to charge and discharge reaction Further, it is possible to suppress the generation of stress that causes peeling of the active material thin film from the current collector.
[0028]
The diffused current collector component preferably forms a solid solution in the active material thin film without forming an intermetallic compound with the active material thin film component . When the current collector component is copper (Cu) and the active material thin film component is silicon (Si), copper and silicon do not form an intermetallic compound in the active material thin film, and form a solid solution. It is preferable. Here, the intermetallic compound refers to a compound having a specific crystal structure in which metals are combined at a specific ratio. The active material thin film component and the current collector component form a solid solution rather than an intermetallic compound in the thin film, so that the adhesion state between the active material thin film and the current collector is improved, and the better Charge / discharge cycle characteristics can be obtained.
[0029]
Moreover, lithium may be occluded or added to the active material thin film in the present invention in advance. Lithium may be added when forming the active material thin film. That is, lithium may be added to the active material thin film by forming an active material thin film containing lithium. Further, after forming the active material thin film, lithium may be occluded or added to the active material thin film. Examples of a method for inserting or adding lithium into the active material thin film include a method for electrochemically inserting or adding lithium.
[0030]
In the present invention, an intermediate layer may be provided between the current collector and the active material thin film for the purpose of improving the adhesion between the current collector and the active material thin film.
FIG. 1 is a schematic cross-sectional view showing the state of the electrode surface in the present invention. As shown in FIG. 1, an active material thin film 11 is formed on the surface 10 a of the current collector 10. Concavities and convexities are formed on the surface 10 a of the current collector 10, and the active material thin film 11 is separated into columns by the cuts 12 formed above the concave and convex valley portions 10 b. Therefore, voids are formed around the columnar portions of the active material thin film 11, and volume expansion and contraction associated with charge / discharge of the active material thin film 11 can be absorbed by the voids. A film 13 is formed on the side surface of the columnar portion of the active material thin film 11. The coating 13 is a coating formed by containing the fluorine-substituted aromatic lithium borate as a solute in the nonaqueous electrolyte in the present invention, and is a coating containing fluorine and boron.
[0031]
By forming such a coating 13 on the side surface of the columnar portion 11, the columnar structure is stabilized, deterioration and collapse of the columnar portion 11 are suppressed, and the close contact state with the current collector 10 at the bottom of the columnar portion 11 is maintained. It can be kept very good. For this reason, charge / discharge cycle characteristics can be improved.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in more detail on the basis of examples. However, the present invention is not limited to the following examples, and can be implemented with appropriate modifications without departing from the scope of the present invention. It is.
[0033]
(Production of negative electrode)
An electrolytic copper foil (thickness 18 μm, surface roughness Ra = 0.188 μm) was used as a current collector, and a silicon thin film was formed on the electrolytic copper foil by RF sputtering. The sputtering conditions were a sputtering gas (Ar) flow rate: 100 sccm, a substrate temperature: room temperature (no heating), a reaction pressure: 0.133 Pa (1.0 × 10 ⁻³ Torr), and a high-frequency power: 200 W. The silicon thin film was deposited until its thickness was about 5 μm. When the obtained silicon thin film was subjected to Raman spectroscopic analysis, a peak in the vicinity of 480 cm ⁻¹ was detected, but a peak in the vicinity of 520 cm ⁻¹ was not detected. This shows that the obtained silicon thin film is an amorphous silicon thin film.
The electrolytic copper foil on which this amorphous silicon thin film was formed was cut into a size of 2.5 cm × 2.5 cm and dried under vacuum at 100 ° C. for 2 hours to obtain a negative electrode.
[0034]
[Production of positive electrode]
85% by weight of LiCoO ₂ powder having an average particle diameter of 10 μm, 10% by weight of carbon powder as a conductive agent, and 5% by weight of polyvinylidene fluoride powder as a binder were mixed, and N-methylpyrrolidone was added to the resulting mixture. And kneaded to prepare a slurry. This slurry was applied to one side of a current collector made of an aluminum foil having a thickness of 20 μm by a doctor blade method. This was dried under vacuum at 100 ° C. for 2 hours and then cut into a size of 2.0 cm × 2.0 cm to form a positive electrode.
[0035]
[Preparation of Electrolytic Solution A]
LiF and B (C ₆ F ₅ ) ₃ are each 0.1 mol / liter, and LiPF ₆ is 1 mol / liter with respect to a solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio of 3: 7. 1 liter was dissolved. By dissolving LiF and B (C ₆ F ₅ ) ₃ , LiBF (C ₆ F ₅ ) ₃ was synthesized by 0.1 mol / liter and contained in the solvent. The method for synthesizing the solute LiBF (C ₆ F ₅ ) ₃ used here is described in J. McBreen et al., Journal of Power Sourses 89 (2000) 163-167.
As described above, an electrolytic solution A containing 0.1 mol / liter (0.1 M) of LiBF (C ₆ F ₅ ) ₃ and 1 mol / liter (1.0 M) of LiBF ₆ was produced.
[0036]
[Preparation of Electrolytic Solution B]
An electrolyte solution B was prepared by dissolving 1 mol / liter of LiPF ₆ in a solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 3: 7.
[0037]
[Production of battery]
The positive electrode and the negative electrode were bonded together through a polyethylene microporous film in a glove box under an argon gas atmosphere, and inserted into an outer package made of an aluminum laminate material. Into this, 500 μl of the electrolyte A or B was injected to produce a lithium secondary battery. The design capacity of the battery is 14 mAh.
[0038]
FIG. 2 is a plan view showing the manufactured lithium secondary battery. As shown in FIG. 2, a positive electrode 1 and a negative electrode 3 are combined and inserted into an exterior body 4 through a separator 2 made of a polyethylene microporous membrane. After being inserted into the outer package 4, an electrolytic solution is injected to seal the sealing portion 4 a of the outer package 4, thereby producing a lithium secondary battery.
[0039]
FIG. 3 is a cross-sectional view for showing a combination state of the batteries inside the battery. As shown in FIG. 3, the positive electrode 1 and the negative electrode 3 are combined so as to face each other with the separator 2 interposed therebetween. In the positive electrode 1, a positive electrode active material layer 1 a is provided on a positive electrode current collector 1 b made of aluminum, and the positive electrode active material layer 1 a is in contact with the separator 2. In the negative electrode 3, a negative electrode active material layer 3 a is provided on a negative electrode current collector 3 b made of copper, and the negative electrode active material layer 3 a is in contact with the separator 2.
[0040]
As shown in FIG. 3, a positive electrode tab 1c made of aluminum for external extraction is attached to the positive electrode current collector 1b. The negative electrode current collector 3b is also provided with a negative electrode tab 3c made of nickel for external extraction.
[0041]
[Measurement of charge / discharge cycle characteristics]
The charge / discharge cycle characteristics of each battery of Example 1 and Comparative Example 1 using the electrolytic solutions A and B were evaluated. Charging was performed at a constant current of 14 mA up to 4.20 V, and constant voltage charging at a cycle of 4.20 V was performed up to 0.7 mA. Discharging was performed at a constant current of 14 mA up to 2.75 V, which was one cycle. The capacity retention rate after 70 cycles was determined from the following formula. The results are shown in Table 1. The measurement was performed at 25 ° C.
[0042]
Capacity maintenance ratio (%) = (discharge capacity at the 70th cycle / discharge capacity at the first cycle) × 100
[0043]
[Table 1]

[0044]
As is apparent from the results shown in Table 1, Example 1 containing a fluorine-substituted aromatic lithium borate as a solute according to the present invention shows a higher capacity retention rate than Comparative Example 1, and charge / discharge cycle characteristics. It turns out that it is excellent in.
[0045]
In the above embodiment, fluorine-substituted aromatic lithium borate represented by the general formula LiBF (C ₆ H _5-p F _p ) ₃ is used, but the general formula LiB (C ₆ H _5-q F _{q is used.} ) It has been confirmed that the same effect can be obtained with respect to the fluorine-substituted aromatic lithium borate represented by ₄ and LiB [C ₆ H _5-r (CF ₃ ) _r ] ₄ .
[0046]
【The invention's effect】
According to the present invention, the charge / discharge cycle characteristics of a lithium secondary battery can be improved.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view schematically showing an electrode surface in the present invention.
FIG. 2 is a plan view showing a lithium secondary battery manufactured in an example of the present invention.
3 is a cross-sectional view showing a combined structure of electrodes in the lithium secondary battery shown in FIG. 2;
[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 portion 10 ... current collector 10a ... current collector surface 10b ... uneven surface 11 of the current collector surface ... active material thin film columnar portion 12 ... cut (void)
13 ... coating

Claims (15)

A positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode is an electrode formed by depositing an active material thin film that absorbs and releases lithium on a current collector, and the active material thin film is formed in a thickness direction thereof In the lithium secondary battery, which is an electrode that is separated into a columnar shape by a cut formed in the columnar portion and the bottom of the columnar portion is in close contact with the current collector,
The active material thin film is a silicon down thin film, wherein the current collector comprises copper as a current collector components,
The nonaqueous electrolyte is, (wherein, p is an integer of 1 to 5) Formula _{LiBF (C 6 H 5-p} F p) 3, LiB (C 6 H 5-q F q) 4 ( wherein, q Is an integer of 1 to 5), and LiB [C ₆ H _5-r (CF ₃ ) _r ] ₄ (wherein r is an integer of 1 to 5) A lithium secondary battery comprising:   The lithium secondary battery according to claim 1, wherein the active material thin film is a thin film formed by a CVD method, a sputtering method, a vapor deposition method, a thermal spraying method, or a plating method.   The lithium secondary battery according to claim 1, wherein the cut is formed by charge and discharge after the first time.   The lithium secondary battery according to claim 1, wherein the cut is formed along a low density region extending in a thickness direction of the active material thin film. Compound contained as the solute, the general formula _{LiBF (C 6 H 5-p} F p) 3 ( where, p is an integer of 1 to 5) any one of the preceding claims, characterized in that a 1 The lithium secondary battery according to item. In the non-aqueous electrolyte, as a solute, the general formula LiXF _y (wherein X is P, As, Sb, B, Bi, Al, Ga, or In, and when X is P, As, or Sb, y 6 is included, and when X is B, Bi, Al, Ga, or In, y is 4. The compound represented by any one of claims 1 to 5 is included. The lithium secondary battery according to item. The lithium secondary battery according to any one of claims 1 to 6, wherein the thin film of active material is amorphous silicon thin film or a microcrystalline silicon thin film. The lithium secondary battery according to any one of claims 1 to 7, the surface roughness Ra of the current collector is characterized in that it is a 0.01 to 1 [mu] m. The lithium secondary battery according to any one of claims 1 to 8, wherein the current collector is a copper foil. The lithium secondary battery according to claim 9 , wherein the copper foil is a copper foil having a roughened surface. The lithium secondary battery according to claim 9 , wherein the copper foil is an electrolytic copper foil. The lithium secondary battery according to any one of claims 1 to 1 1, components of the current collector to the active material thin film is characterized by being diffused. Diffuse component of the current collector, wherein the active material thin film, according possible to Claim 1 2, characterized in that to form a solid solution without forming component and intermetallic compounds of the thin film of active material Lithium secondary battery. The lithium secondary battery according to any one of claims 1 to 1 3, characterized in that it comprises a mixed solvent in which the non-aqueous electrolyte is composed of two or more solvents. The mixed solvent is a mixed solvent containing a cyclic carbonate and a chain carbonate, a lithium secondary battery according to claim 1 4, characterized in that it contains ethylene carbonate as the cyclic carbonate.

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