JP4526806B2 - Method for producing lithium ion secondary battery - Google Patents

Description

The present invention relates to a method for manufacturing a lithium ion secondary battery .

Lithium ion secondary battery, the negative electrode current collector, the negative electrode active material, the electrolyte, the separator, the positive electrode active material, the main component of the positive electrode current collector. The lithium ion secondary battery plays a major role as an energy source for mobile communication devices and various types of AV equipment. Compact high-performance device coupled with miniaturization of the lithium ion secondary battery has higher energy density is advanced, much effort on improving the elements constituting the battery have been made.

  For example, Patent Document 1 discloses that a high energy density can be realized by using, as a positive electrode active material, an amorphous oxide obtained by heating, melting, and rapidly cooling a mixed powder of a specific transition metal oxide. It is disclosed.

  In Patent Document 2, a transition metal oxide containing lithium is used as a positive electrode active material, a compound containing a silicon atom is used as a negative electrode active material, and the weight of the positive electrode active material is larger than the weight of the negative electrode active material. Can increase battery capacity and cycle life.

Further, Patent Document 3 discloses using an amorphous silicon thin film as a negative electrode active material. As a result, a larger amount of lithium can be occluded than when carbon is used, and it is expected that the capacity can be increased.
JP-A-8-78002 Japanese Unexamined Patent Publication No. 2000-12092 JP 2002-83594 A

  The amorphous silicon thin film can be formed by a vacuum film forming method such as a sputtering method or a vapor deposition method. The use of this amorphous silicon thin film as a negative electrode active material is promising for increasing the capacity, but the silicon thin film has many problems such as expansion / contraction during charge / discharge due to its large amount of occlusion of lithium. For this purpose, ensuring the cycle characteristics is an issue. In order to ensure cycle characteristics, it is necessary to clarify the chemical problem as well as the physical problem such as expansion / contraction and take countermeasures. However, a chemical approach to the cycle characteristics of the negative electrode active material is still not sufficient.

An object of this invention is to provide the manufacturing method of the lithium ion secondary battery with improved cycling characteristics.

In the method for producing a lithium ion secondary battery of the present invention, an amorphous silicon thin film containing silicon as a main component as a negative electrode active material thin film is formed on a current collector directly or via an underlayer by a vacuum film forming method . In the method of manufacturing a lithium ion secondary battery having a step, the step includes firstly depositing first evaporated particles mainly from a first vapor deposition source made of silicon, and then gradually from the second vapor deposition source. A step of increasing a ratio of the second evaporation particles and finally mixing and depositing the first evaporation particles and the second evaporation particles, and the second evaporation source includes Ti, Zr, La, Ce, sc, and wherein the Y is either.

According to the present invention, a lithium ion secondary battery with improved cycle characteristics can be provided.

The first lithium ion secondary battery of the present invention is a lithium ion secondary battery in which a negative electrode active material thin film containing silicon as a main component is formed on a current collector directly or via an underlayer, The peak value of the oxygen concentration in the vicinity of the surface of the negative electrode active material thin film is expressed as O _P1 , the depth in the negative electrode active material thin film excluding the vicinity of the interface between the current collector or the underlayer and the negative electrode active material thin film, and the vicinity of the surface. oxygen concentration O _C in the portion where the direction of the oxygen concentration distribution can be regarded as substantially constant, the oxygen concentration in the vicinity of the surface _{{(O P1 -O C) /} 2} + O position D ₁ which is _C, the position D ₁ T ₁ ≦ 20 nm where T ₁ is the depth from the surface of the negative electrode active material thin film.

Since T ₁ ≦ 20 nm, that is, the thickness of the oxide layer in the vicinity of the surface of the negative electrode active material thin film is thin, lithium ions can be easily moved and cycle characteristics are improved.

As a first manufacturing method for thinning the oxide layer in the vicinity of the surface of the negative electrode active material thin film, when forming a negative electrode active material thin film containing silicon as a main component directly on the current collector or through an underlayer, It is effective to provide a material containing at least one selected from Ti, Zr, La, Ce, Sc, and Y as a main component on the surface of the negative electrode active material thin film.

These are, it is because it is effective to reduce the thickness of the oxide layer.

The material is preferably applied by a vacuum film forming method. This is because the material can be efficiently applied.

  As a second manufacturing method for thinning the oxide layer near the surface of the negative electrode active material thin film, a negative electrode active material thin film containing silicon as a main component in a reducing atmosphere is formed directly on the current collector or through an underlayer. It is effective to do.

  Hydrogen gas or the like can be used to realize a reducing atmosphere.

The second lithium ion secondary battery of the present invention is a lithium ion secondary battery in which a negative electrode active material thin film containing silicon as a main component is formed on a current collector, wherein the current collector and the negative electrode active material The peak value of the oxygen concentration in the vicinity of the interface with the thin film is O _P2 , the depth in the negative electrode active material thin film excluding the vicinity of the interface between the current collector and the negative electrode active material thin film and the vicinity of the surface of the negative electrode active material thin film. The oxygen concentration at a portion where the oxygen concentration distribution in the direction can be regarded as substantially constant is O _C , and the position D ₂ is sandwiched in the depth direction in the vicinity of the position D ₂ where the oxygen concentration takes the peak value _{OP 2. O P2 -O C) / 2}} + O C a is position D _3, D _4, and the distance in the depth direction of the position D ₃ and the position D ₄ was T _2, is T ₂ ≦ 5 nm .

Since T ₂ ≦ 5 nm, that is, the thickness of the oxide layer in the vicinity of the interface between the current collector and the negative electrode active material thin film is thin, the movement of lithium ions and the movement of electrons (current collection) are facilitated, and the cycle characteristics are improved. In addition, since the thickness of the oxide layer in the vicinity of the interface is thin, the brittleness in the vicinity of the interface against the expansion / contraction of the negative electrode active material thin film due to charge / discharge is improved, so that the cycle characteristics are improved.

  As a manufacturing method of thinning the oxide layer in the vicinity of the interface between the current collector and the negative electrode active material thin film, the surface of the current collector is subjected to plasma treatment in a reducing atmosphere, and silicon is then applied to the surface of the current collector subjected to the plasma treatment. It is effective to form a negative electrode active material thin film containing as a main component.

The third lithium ion secondary battery of the present invention is a lithium ion secondary battery in which a negative electrode active material thin film containing silicon as a main component is formed on a current collector via a base layer, the base layer The negative active material thin film excluding the peak value of the oxygen concentration in the vicinity of the interface between the negative electrode active material thin film and O _P2 , the vicinity of the interface between the underlayer and the negative active material thin film, and the vicinity of the surface of the negative active material thin film The oxygen concentration in the portion where the oxygen concentration distribution in the depth direction is regarded as substantially constant is O _C , and the position D ₂ is sandwiched in the depth direction in the vicinity of the position D ₂ where the oxygen concentration takes the peak value _{OP 2.} when the concentration is _{{(O P2 -O C) /} 2} + O C a is position D _3, D _4, the distance in the depth direction of the position D ₃ and the position D ₄ was T _2, T ₂ ≦ 5 nm.

Since T ₂ ≦ 5 nm, that is, the thickness of the oxide layer in the vicinity of the interface between the underlayer and the negative electrode active material thin film is thin, the movement of lithium ions and the movement of electrons (current collection) are facilitated, and the cycle characteristics are improved. In addition, since the thickness of the oxide layer in the vicinity of the interface is thin, the brittleness in the vicinity of the interface against the expansion / contraction of the negative electrode active material thin film due to charge / discharge is improved, so that the cycle characteristics are improved.

  As a manufacturing method for thinning the oxide layer in the vicinity of the interface between the underlayer and the negative electrode active material thin film, the surface of the underlayer of the current collector having the underlayer formed on the surface is subjected to plasma treatment in a reducing atmosphere, and then the plasma It is effective to form a negative electrode active material thin film containing silicon as a main component on the surface of the treated underlayer.

A fourth lithium ion secondary battery of the present invention is a lithium ion secondary battery in which a negative electrode active material thin film containing silicon as a main component is formed on a current collector directly or through an underlayer, In the negative electrode active material thin film except for the vicinity of the interface between the current collector or the base layer and the negative electrode active material thin film and the vicinity of the surface of the negative electrode active material thin film, the oxygen concentration distribution in the depth direction can be regarded as substantially constant oxygen concentration O _C, when the silicon concentration distribution in the depth direction is the silicon concentration in the portion which can be regarded as approximately constant and S _C, a _{(O C / S C) ×} 100 ≦ 0.5%.

(O _C / S _C ) × 100 ≦ 0.5%, that is, oxygen concentration in the negative electrode active material thin film excluding the vicinity of the interface between the current collector or the underlayer and the negative electrode active material thin film and the surface of the negative electrode active material thin film Therefore, the movement of lithium ions and electrons is facilitated, and the cycle characteristics are improved.

  As a manufacturing method for reducing the oxygen concentration in the negative electrode active material thin film excluding the vicinity of the interface between the current collector or the base layer and the negative electrode active material thin film and the vicinity of the surface of the negative electrode active material thin film, Thus, it is effective to form a negative electrode active material thin film containing silicon as a main component in a reducing atmosphere.

  In the present invention, the negative electrode active material thin film contains silicon as a main component. “Containing silicon as a main component” means that the silicon content is 50 at% or more, desirably 70 at% or more, more desirably 80 at% or more, and most desirably 90 at% or more. The higher the silicon content, the better the battery capacity.

Although the manufacturing method of a negative electrode active material thin film does not have a restriction | limiting in particular, A vacuum film-forming method is preferable. The “vacuum film forming method” includes various vacuum thin film manufacturing processes such as a vapor deposition method, a sputtering method, a CVD method, an ion plating method, and a laser ablation method. An optimum film formation method can be selected according to the type of the thin film. A thin negative electrode active material thin film can be efficiently produced by a vacuum film formation method. As a result, a small and thin lithium ion secondary battery can be obtained.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

A lithium ion secondary battery according to an embodiment of the present invention includes a positive electrode current collector having a positive electrode active material formed on both sides, a separator, and a negative electrode current collector having a negative electrode active material formed on both sides. A cylindrical roll wound with a separator interposed between the current collector and the negative electrode current collector is housed in a battery can, and the battery can is filled with an electrolytic solution.

  As the positive electrode current collector, Al, Cu, Ni, stainless steel foil or net having a thickness of 10 to 80 μm can be used. Alternatively, a polymer substrate such as polyethylene terephthalate or polyethylene naphthalate having a metal thin film formed on the surface can be used.

  The positive electrode active material needs to be able to enter and exit lithium ions, and includes lithium-containing transition metal oxides including transition metals such as Co, Ni, Mo, Ti, Mn, and V, and conductivity aids such as acetylene black. It is also possible to use a mixed paste in which an agent and a binder such as nitrile rubber, butyl rubber, polytetrafluoroethylene, and polyvinylidene fluoride are mixed.

  As the negative electrode current collector, a foil, net, or the like of Cu, Ni, stainless steel having a thickness of 10 to 80 μm can be used. Alternatively, a polymer substrate such as polyethylene terephthalate or polyethylene naphthalate having a metal thin film formed on the surface can be used.

  An underlayer may be formed (surface treatment) on the surface of the negative electrode current collector. The underlayer may be, for example, a layer for the purpose of strengthening the adhesion between the current collector and the negative electrode active material thin film or for preventing rust, and specifically, for example, a silicon-copper thin film or a chromate. A treatment layer or the like can be used. Alternatively, when copper foil is used as the negative electrode current collector, zinc plating, tin, copper, nickel, or alloy plating of cobalt and zinc, a coating layer using an azole derivative such as benzotriazole, chromic acid or dichrome A chromium-containing film by a solution containing an acid salt or a combination thereof can be used. As the negative electrode current collector, instead of copper foil, it is also possible to use a surface of another base material coated with copper. In this case, the above underlayer is formed on the surface of the copper coating. Also good.

  The separator is preferably excellent in mechanical strength and ion permeability, and polyethylene, polypropylene, polyvinylidene fluoride, and the like can be used. The pore diameter of the separator is, for example, 0.01 to 10 μm, and the thickness thereof is, for example, 5 to 200 μm.

As an electrolytic solution, a solution in which an electrolyte salt such as LiPF ₆ , LiBF ₄ , or LiClO ₄ is dissolved in a solvent such as ethylene carbonate, propylene carbonate, methyl ethyl carbonate, hexafluoromethyl acetate, or tetrohydrofuran is used. I can do it.

  As the battery can, a metal material such as stainless steel, iron, aluminum, or nickel-plated steel can be used, but a plastic material can also be used depending on the battery application.

  The negative electrode active material is a silicon thin film containing silicon as a main component. The silicon thin film is preferably amorphous or microcrystalline, and can be formed by a vacuum film formation method such as sputtering, vapor deposition, or CVD.

[ Reference Examples 1 to 8, Comparative Example 1]
First, a method for manufacturing a positive electrode will be described. Li ₂ CO ₃ and CoCO ₃ were mixed at a predetermined molar ratio and synthesized by heating at 900 ° C. in the atmosphere to obtain LiCoO ₂ . This was classified to 100 mesh or less to obtain a positive electrode active material. 100 g of this positive electrode active material, 12 g of carbon powder as a conductive agent, 10 g of polytetrafluoroethylene dispersion as a binder, and pure water were mixed to form a paste. This positive electrode active material-containing paste was applied to both sides of a strip-shaped aluminum foil having a thickness of 25 μm as a positive electrode current collector and dried to obtain a positive electrode.

  A strip-shaped copper foil having a thickness of 30 μm was used as the negative electrode current collector, and a silicon thin film was formed as a negative electrode active material on both surfaces by sputtering. Details will be described later.

  As the separator, a belt-shaped porous polyethylene having a thickness of 35 μm and wider than the positive electrode current collector and the negative electrode current collector was used.

  A positive electrode lead made of the same material was attached to the positive electrode current collector by spot welding. Further, a negative electrode lead made of the same material as the negative electrode current collector was attached by spot welding.

These were overlapped so that a separator was interposed between the positive electrode and the negative electrode obtained as described above, and wound in a spiral shape. Polypropylene insulation plates are arranged on the upper and lower surfaces of this cylindrical wound product, respectively, and housed in a bottomed cylindrical battery can, and a step is formed in the vicinity of the opening of the battery can. Then, an equal volume mixed solution of ethylene carbonate and diethyl carbonate in which LiPF ₆ was dissolved at a concentration of 1 × 10 ³ mol / m ³ was poured into a battery can, and the opening was sealed with a sealing plate to obtain a lithium ion secondary battery. It was.

  A method for forming a silicon thin film as a negative electrode active material will be described.

Prior to the formation of the silicon thin film, the negative electrode current collector made of a strip-shaped copper foil was subjected to plasma processing using the plasma processing apparatus 50 shown in FIG. In the vacuum chamber 51 depressurized by the vacuum pump 59, the negative electrode current collector 5 is unwound from the unwinding roll 52, transported by the plurality of transporting rolls 54, and wound around the winding roll 53. In this process, the negative electrode current collector 5 passes through the plasma generator 55. The plasma generator 55 generates high frequency plasma of 13.56 MHz, and argon gas and hydrogen gas are introduced into the high frequency plasma by a gas introduction pipe 56. When the negative electrode current collector 5 passes through the plasma generator 55, the negative electrode current collector 5 is subjected to reduction plasma treatment by being irradiated with plasma in a reducing gas atmosphere. The amount of argon gas introduced into the plasma generator 55 is 0.07 Pa · m ³ / s, and the amount of hydrogen gas as the reducing gas is 0 Pa · m ³ / s (no introduction), 0.009 Pa · m ³ / s. s, 0.09 Pa · m ³ / s.

  A silicon thin film as a negative electrode current collector was formed by sputtering on the plasma-treated negative electrode current collector 5 using the vacuum film forming apparatus 10 of FIG.

  A vacuum film forming apparatus 10 shown in FIG. 2 includes a vacuum chamber 1 that is partitioned vertically by a partition wall 1a. In a room (conveying chamber) 1b above the partition wall 1a, a separating roll 11, a cylindrical can roll 13, a separating roll 14, and conveying rolls 12a and 12b are arranged. A sputter deposition source 21 and gas introduction nozzles 23a and 23b are disposed in a room (thin film formation chamber) 1c below the partition wall 1a. A mask 4 is provided at the center of the partition wall 1a, and the lower surface of the can roll 13 is exposed to the thin film formation chamber 1c through the opening of the mask 4. The inside of the vacuum chamber 1 is maintained at a predetermined degree of vacuum by a vacuum pump 16.

  The strip-shaped negative electrode current collector 5 squeezed out from the scooping roll 11 is sequentially transported by the transporting roll 12 a, the can roll 13, and the transporting roll 12 b, and scraped off by the scooping roll 14. In this process, particles such as atoms, molecules, or clusters (hereinafter referred to as “sputtered particles”) generated from the sputter deposition source 21 pass through the mask 4 of the partition wall 1 a and travel on the can roll 13. A thin film 6 is formed by adhering to the surface of the negative electrode current collector 5. At this time, the reducing gas is introduced from the gas introduction nozzles 23a and 23b toward the area where the sputter particles of the negative electrode current collector 5 are attached.

Using such an apparatus, silicon was sputtered by argon ions in the sputter deposition source 21 to form a 4 μm thick silicon thin film on the copper foil as the negative electrode current collector 5. The deposition rate of the silicon thin film was set to approximately 2 nm / s. A direct current magnetron sputtering was used as the sputtering film forming source 21. The degree of vacuum before introducing the argon gas was 3 × 10 ⁻⁴ Pa, and the degree of vacuum after introducing the argon gas was 4 × 10 ⁻² Pa. Gas injection nozzle 23a, the introduction amount of 0Pa · m ³ / s (without the introduction) of hydrogen gas as a reducing gas from 23b, a three ways ^{0.003Pa · m 3 /s,0.009Pa · m 3} / s did.

As described above, the amount of hydrogen gas introduced in the plasma treatment for the negative electrode current collector 5 is changed to three ways, and the amount of hydrogen gas introduced during the formation of the silicon thin film is changed to three ways. It produced ( Reference Examples 1-8, Comparative Example 1).

The resulting lithium ion secondary battery was subjected to a charge / discharge cycle test at a test temperature of 20 ° C., a charge / discharge current of 3 mA / cm ² , and a charge / discharge voltage range of 4.2 V to 2.5 V. The ratio of the discharge capacity after 200 cycles to the initial discharge capacity was determined as the battery capacity retention rate (cycle characteristics). The results are shown in Table 1.

シリコン薄膜（負極活物質薄膜）が形成された負極集電体５のオージェデプスプロファイルを測定し、プラズマ処理やシリコン薄膜の成膜中の還元ガス導入がシリコン薄膜に及ぼす化学的影響を調べた。オージェデプスプロファイルは、フィリップス社製のＳＡＭ６７０を用いて測定した。電子銃の加速電圧を１０ｋＶ、照射電流１０ｎＡとし、エッチング用のイオンガンの加速電圧３ｋＶ、スパッタレート０．１７ｎｍ／ｓにて測定した。

The Auger depth profile of the negative electrode current collector 5 on which a silicon thin film (negative electrode active material thin film) was formed was measured, and the chemical effect of plasma treatment and introduction of a reducing gas during the formation of the silicon thin film on the silicon thin film was investigated. The Auger depth profile was measured using a SAM670 manufactured by Philips. The acceleration voltage of the electron gun was 10 kV, the irradiation current was 10 nA, and the measurement was performed with the acceleration voltage of the ion gun for etching being 3 kV and the sputtering rate of 0.17 nm / s.

  FIG. 3 is a schematic diagram showing an example of an Auger depth profile. The “depth from the film surface” on the horizontal axis in the figure is the same as the sample, using the sputter rate obtained by measuring the step formed by sputter etching of the same Si film and Cu film with the step meter. The sputter etching time was obtained by converting into the etching depth in the thickness direction. The “Auger signal intensity” on the vertical axis is normalized and displayed with a maximum value of 1 for each element.

  As shown in FIG. 3, the oxygen concentration distribution in the depth direction in the silicon thin film (negative electrode active material thin film) has a peak near the surface of the silicon thin film and near the interface between the silicon thin film and the negative electrode current collector (copper foil). In this part, it takes a substantially constant value less than this. The peak value of the oxygen concentration near the surface of the silicon thin film is larger than the peak value of the oxygen concentration near the interface between the silicon thin film and the negative electrode current collector. Although not shown, when the above-described underlayer is formed on the negative electrode current collector, the oxygen concentration is the same as in FIG. 3 except that the oxygen distribution has a peak near the interface between the silicon thin film and the underlayer. The distribution was confirmed.

In order to express such an oxygen distribution quantitatively, as shown in FIG. 3, indices T ₁ , T ₂ , and R are defined as follows.

The peak value of the oxygen concentration in the vicinity of the surface of the silicon thin film is O _P1 , and the peak value of the oxygen concentration in the vicinity of the interface between the negative electrode current collector 5 (the base layer when the base layer is formed) and the silicon thin film is O _P2 , The position where the oxygen concentration takes the peak value O _P2 is D ₂ , and the silicon thin film excluding the vicinity of the interface between the negative electrode current collector 5 (underlayer when a base layer is formed) and the silicon thin film and the vicinity of the surface of the silicon thin film Let O _C be the oxygen concentration in a portion where the oxygen concentration distribution in the depth direction in the inside can be regarded as substantially constant.

In the vicinity of the surface of the silicon thin film, the position where the oxygen concentration becomes {(O _P1 −O _C ) / 2} + O _C in the process of decreasing the oxygen concentration from the peak value O _P1 to the constant value O _C is D ₁ , The depth from the surface (depth = 0) of the silicon thin film at position D ₁ is T ₁ . This T ₁ is an index indicating the thickness of the oxide layer in the vicinity of the surface of the silicon thin film.

Further, in the process in which the oxygen concentration decreases from the peak value O _P2 on both sides in the depth direction with respect to the position D ₂ , the position where the oxygen concentration becomes {(O _P2 −O _C ) / 2} + O _C is defined as D ₃ , D ₄ , and the distance between the position D ₃ and the position D ₄ in the depth direction is T ₂ . This T ₂ is an index indicating the thickness of the oxide layer in the vicinity of the interface between the silicon thin film and the negative electrode current collector 5 (underlayer when the underlayer is formed).

Further, when the silicon concentration in the portion of the silicon thin film where the silicon concentration distribution in the depth direction can be regarded as substantially constant is S _C , (O _C / S _C ) × 100 is R (%). This R is an index indicating the degree of oxidation inside the silicon thin film.

The silicon thin film in the above Reference Examples 1 to 8 and Comparative Example 1, before the cycle test described above to measure T _2, R. In the measurement, in order to prevent variation due to an analytical instrument, the measurement result was corrected so that O _C / S _C = 2 when SiO ₂ was analyzed with an Auger depth profile. The measurement results are shown in Table 2 together with the film formation conditions.

表２から分かるように、シリコン薄膜の形成に先立って還元雰囲気で負極集電体５に対してプラズマ処理を行うことによりＴ₂すなわちシリコン薄膜と負極集電体５との界面近傍での酸化層が薄くなっていることが分かる。また、還元ガスを導入しながらシリコン薄膜を成膜することによりＲすなわちシリコン薄膜内部の酸素含有量が低下していることが分かる。

As can be seen from Table 2, by performing plasma treatment on the negative electrode current collector 5 in a reducing atmosphere prior to formation of the silicon thin film, T _2, that is, an oxide layer in the vicinity of the interface between the silicon thin film and the negative electrode current collector 5 is obtained. Can be seen to be thinner. In addition, it can be seen that R, that is, the oxygen content inside the silicon thin film is lowered by forming the silicon thin film while introducing the reducing gas.

From Table 1 and Table 2, when the battery capacity retention rate (cycle characteristic) satisfies one of T ₂ ≦ 5 nm and R ≦ 0.5%, good results are obtained, and both are satisfied. In this case, a better result is obtained. This seems to be due to the following reasons.

When T _2, which is an index of the thickness of the oxide layer in the vicinity of the interface between the silicon thin film and the negative electrode current collector 5, is small, the movement of lithium ions and the movement of electrons (current collection) are facilitated, so the cycle characteristics are improved. It is thought that. Further, since the resistance to expansion / contraction of silicon particles during charge / discharge is improved (brittleness in the vicinity of the interface is improved), the adhesion strength of both layers at the interface is improved, and thus it is considered that the cycle characteristics are improved.

  When R, which is an index indicating the degree of oxidation inside the silicon thin film, is small, that is, when oxygen (oxide) in the silicon thin film is small, the movement of lithium ions and electrons is facilitated. Conceivable.

[Examples 1, 2, Reference Examples 9-11 , Comparative Examples 2-3]
First, a method for manufacturing a positive electrode will be described. Li ₂ CO ₃ and CoCO ₃ were mixed at a predetermined molar ratio and synthesized by heating at 900 ° C. in the atmosphere to obtain LiCoO ₂ . This was classified to 100 mesh or less to obtain a positive electrode active material. 100 g of this positive electrode active material, 12 g of carbon powder as a conductive agent, 10 g of polytetrafluoroethylene dispersion as a binder, and pure water were mixed to form a paste. This positive electrode active material-containing paste was applied to both sides of a strip-shaped aluminum foil having a thickness of 25 μm as a positive electrode current collector and dried to obtain a positive electrode.

  A strip-shaped copper foil having a thickness of 30 μm was used as a negative electrode current collector, and a silicon thin film was formed as a negative electrode active material on both surfaces thereof by vapor deposition. Details will be described later. In some examples, a chromate treatment layer was formed as an underlayer on both surfaces of the copper foil.

  As the separator, a belt-shaped porous polyethylene having a thickness of 35 μm and wider than the positive electrode current collector and the negative electrode current collector was used.

  A positive electrode lead made of the same material was attached to the positive electrode current collector by spot welding. Further, a negative electrode lead made of the same material as the negative electrode current collector was attached by spot welding.

These were overlapped so that a separator was interposed between the positive electrode and the negative electrode obtained as described above, and wound in a spiral shape. Polypropylene insulation plates are arranged on the upper and lower surfaces of this cylindrical wound product, respectively, and housed in a bottomed cylindrical battery can, and a step is formed in the vicinity of the opening of the battery can. Then, an equal volume mixed solution of ethylene carbonate and diethyl carbonate in which LiPF ₆ was dissolved at a concentration of 1 × 10 ³ mol / m ³ was poured into a battery can, and the opening was sealed with a sealing plate to obtain a lithium ion secondary battery. It was.

  A method for forming a silicon thin film as a negative electrode active material will be described.

Prior to the formation of the silicon thin film, the negative electrode current collector made of a strip-shaped copper foil was subjected to plasma processing using the plasma processing apparatus 50 shown in FIG. A high frequency plasma of 100 kHz was generated by the plasma generator 55, and argon gas and hydrogen gas were introduced into the plasma through a gas introduction pipe 56. The negative electrode current collector 5 passing through this is subjected to a reduction plasma treatment by receiving plasma irradiation in a reducing gas atmosphere. The amount of argon gas introduced into the plasma generator 55 is 0.14 Pa · m ³ / s, and the amount of hydrogen gas as a reducing gas is 0 Pa · m ³ / s (no introduction), 0.07 Pa · m ³ / s. Two types of s were used. Plasma treatment was also performed on the surface of the negative electrode current collector 5 on which the chromate treatment layer was formed.

  A silicon thin film serving as a negative electrode current collector was formed by vapor deposition on the plasma-treated negative electrode current collector 5 using the vacuum film forming apparatus 10 shown in FIG.

  The apparatus of FIG. 4 differs from the apparatus of FIG. 2 in that a deposition source 31, an auxiliary deposition source 32, and a shielding plate 35 are disposed in the thin film formation chamber 1c instead of the sputter deposition source 21. 4, the same components as those in FIG. 2 are denoted by the same reference numerals, and description thereof is omitted.

  The vapor deposition source 31, the shielding plate 35, and the auxiliary vapor deposition source 32 are arranged in this order from the upstream side to the downstream side along the traveling direction of the negative electrode current collector 5 conveyed along the can roll 13. Yes.

  The strip-shaped negative electrode current collector 5 squeezed out from the scooping roll 11 is sequentially transported by the transporting roll 12 a, the can roll 13, and the transporting roll 12 b, and scraped off by the scooping roll 14. In this process, particles such as atoms, molecules, or clusters (hereinafter referred to as “evaporated particles”) generated from the vapor deposition source 31 and the auxiliary vapor deposition source 32 pass through the mask 4 of the partition wall 1 a and pass over the can roll 13. A thin film 6 is formed by adhering to the surface of the traveling negative electrode current collector 5. The vapor deposition source 31 and the auxiliary vapor deposition source 32 are arranged facing the negative electrode current collector 5 from the upstream side to the downstream side in the transport direction. The can roll 13 of the shielding plate 35 is mixed so that a part of the evaporated particles from the deposition source 31 and a part of the evaporated particles from the auxiliary deposition source 32 are mixed in the vicinity of the outer peripheral surface of the can roll 13. The distance from the outer peripheral surface is adjusted. Accordingly, the evaporation particles are primarily deposited from the evaporation source 31 first on the surface of the negative electrode current collector 5, and thereafter, the ratio of the evaporation particles from the auxiliary evaporation source 32 gradually increases, and finally, the evaporation source 31. The evaporated particles from the auxiliary vapor source and the evaporated particles from the auxiliary vapor deposition source 32 are mixed and deposited. At this time, the reducing gas is introduced from the gas introduction nozzles 23 a and 23 b arranged so as to sandwich the evaporation source 31 toward the evaporated particle adhesion region of the negative electrode current collector 5.

Using such an apparatus, silicon was evaporated from the evaporation source 31 to form a silicon thin film having a thickness of 8 μm on the copper foil as the negative electrode current collector 5. The deposition rate of the silicon thin film was set to approximately 0.15 μm / s. The degree of vacuum before gas introduction was 5 × 10 ⁻³ Pa. The amount of hydrogen gas introduced as the reducing gas from the gas introduction nozzles 23a and 23b was set to two types of 0 Pa · m ³ / s (no introduction) and 0.09 Pa · m ³ / s. In some examples, titanium was deposited from the auxiliary evaporation source 32 during silicon deposition. An electron beam evaporation source was used as the evaporation source 31 and the auxiliary evaporation source 32.

  Hydrogen gas was introduced at the time of plasma processing for the negative electrode current collector 5, and hydrogen gas introduction at the time of silicon thin film formation was changed to the presence / absence of the hydrogen gas introduction at the time of plasma treatment and silicon thin film formation. The lithium ion secondary battery was fabricated by changing the titanium vapor deposition to two with or without the mixed deposition. In addition, a lithium ion secondary battery was manufactured by changing to the case where the chromate treatment layer of the negative electrode current collector 5 was present or not with respect to the case where the hydrogen gas was not introduced when forming the silicon thin film. As a comparative example, lithium ion was changed to one with or without a chromate treatment layer for the case where hydrogen gas was not introduced both during the plasma treatment and during the formation of the silicon thin film and no titanium mixed vapor deposition was performed. A secondary battery was produced.

About each produced lithium ion secondary battery, the charging / discharging cycle test was done by the test temperature of 20 degreeC, charging / discharging electric current of 3 mA / cm < ² >, and the charging / discharging voltage range 4.2V-2.5V. The ratio of the discharge capacity after 100 cycles to the initial discharge capacity was determined as the battery capacity retention rate (cycle characteristics). Table 3 shows the results together with the production conditions.

表３から分かるように、負極活物質薄膜として蒸着法でシリコン薄膜を形成する場合にも、シリコン薄膜の形成に先立って還元雰囲気で負極集電体５に対してプラズマ処理を行うことや、還元ガスを導入しながらシリコン薄膜を成膜することがサイクル特性に有効であることが分かった。

As can be seen from Table 3, when a silicon thin film is formed as a negative electrode active material thin film by vapor deposition, plasma treatment is performed on the negative electrode current collector 5 in a reducing atmosphere prior to the formation of the silicon thin film, It was found that forming a silicon thin film while introducing gas is effective for cycle characteristics.

Furthermore, as shown in Example 1 and Example 2 , it was found that the cycle characteristics were improved by depositing titanium evaporated particles on the end side of the film formation period of the silicon thin film. In Example 1 and Example 2 , titanium was used as a material to be vapor-deposited with silicon, but the same effect was obtained when Zr, La, Ce, Sc, Y was used instead of titanium.

Auger depth profile of the silicon thin film (negative electrode active material thin film) anode current collector 5 which is formed is measured, the reducing gas introduced during film formation of the plasma treatment or a silicon thin film, of material such as titanium on the surface of the silicon thin film The chemical effect of application on silicon thin film was investigated.

Even when the silicon thin film is formed by the vapor deposition method, the plasma treatment for the negative electrode current collector 5 in the reducing atmosphere prior to the formation of the silicon thin film or the film formation of the silicon thin film is performed as in the results of Reference Examples 1 to 8. It was confirmed that the oxygen concentration in the vicinity of the interface between the silicon thin film and the negative electrode current collector and in the silicon thin film was reduced by introducing the reducing gas therein.

5 Reference Example 11, FIG 6 is a schematic diagram of the Auger depth profile in the vicinity of the surface of the silicon thin film of Example 2. In Example 2 , it can be seen that titanium was distributed in the vicinity of the surface of the silicon thin film by performing titanium mixed vapor deposition at the end of the formation period of the silicon thin film.

When T ₁ , which is an index indicating the thickness of the oxide layer in the vicinity of the surface of the silicon thin film, was measured, T ₁ = 40 nm in Comparative Example 2 and Reference Example 9 , whereas T _{1 in} Reference Example 11 was measured. = 20 nm. In Example 2 , T ₁ = 12 nm. From this result, T _1, which is an index of the thickness of the oxide layer near the surface of the silicon thin film, is reduced by introducing a reducing gas during film formation of the silicon thin film and applying a material such as titanium to the surface of the silicon thin film. I understood it. The cycle characteristics are improved as T ₁ is smaller. The reason for this is that when T _1, which is an index of the thickness of the oxide layer near the surface of the silicon thin film, is small (that is, oxygen (oxide) is small), lithium ions can be easily transferred, so that cycle characteristics are improved. It is thought that.

In the above examples and reference examples , an example in which a negative electrode active material thin film (silicon thin film) is formed by a sputtering method or a vapor deposition method is shown, but the present invention is not limited to this, and other vacuums such as a CVD method are used. A film forming method may be used, and even in this case, the same effect can be obtained.

FIELD lithium ion secondary battery of the present invention is not particularly limited, for example, can be used thin, as a secondary battery of lightweight small portable devices.

It is sectional drawing which showed schematic structure of the plasma processing apparatus with respect to a negative electrode collector. It is sectional drawing which showed schematic structure of an example of the apparatus used for manufacture of the lithium ion secondary battery of this invention. It is the figure which showed an example of the Auger depth profile of the negative electrode electrical power collector in which the silicon thin film was formed. It is sectional drawing which showed schematic structure of one Embodiment of the apparatus used for manufacture of the lithium ion secondary battery of this invention. In the lithium ion secondary battery which concerns on the reference example 11 of this invention, it is the element distribution map of the thickness direction in the surface vicinity of the silicon thin film. In the lithium ion secondary battery which concerns on Example 2 of this invention, it is an element distribution map of the thickness direction in the surface vicinity of the silicon thin film.

DESCRIPTION OF SYMBOLS 1 ... Vacuum chamber 1a ... Partition 1b ... Transfer chamber 1c ... Thin film formation chamber 4 ... Mask 5 ... Negative electrode collector 6 ... Thin film 10 ... Vacuum Deposition apparatus 11 ... Unwinding rolls 12a, 12b ... Conveying roll 13 ... Can roll 14 ... Winding roll 16 ... Vacuum pump 21 ... Sputter film forming sources 23a, 23b ... Gas introduction nozzle 31 ... deposition source 32 ... auxiliary deposition source 35 ... shielding plate 50 ... plasma treatment device 51 ... vacuum tank 52 ... unwinding roll 53 ... winding roll 54 ... Transport roll 55 ... Plasma generator 56 ... Gas introduction pipe 59 ... Vacuum pump

Claims (4)

In a method of manufacturing a lithium ion secondary battery, which has a step of forming an amorphous silicon thin film containing silicon as a main component as a negative electrode active material thin film by a vacuum film forming method directly on a current collector or through an underlayer, The process includes
First evaporating particles from a first evaporation source consisting of silicon primarily deposit,
Thereafter, gradually increase the ratio of the second evaporation particles from the second deposition source,
Finally, a step of mixing and depositing the first vaporized particles and the second vaporized particles,
The method for manufacturing a lithium ion secondary battery, wherein the second evaporation source is any one of Ti, Zr, La, Ce, Sc, and Y. Wherein The method for manufacturing a lithium ion secondary battery according to claim 1 to form the anode active material thin film in a reducing atmosphere. 2. The method of manufacturing a lithium ion secondary battery according to claim 1, wherein the negative electrode active material thin film is formed on the surface of the plasma-treated current collector after the surface of the current collector is plasma-treated in a reducing atmosphere. The surface of the underlying layer of the current collector base layer formed on the surface after the plasma treatment in a reducing atmosphere, according to claim 1 to form the anode active material thin film to the plasma treated surface of the underlying layer Method for producing a lithium ion secondary battery.

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