JP2005135856A - Electrode for lithium secondary battery, manufacturing method of the same, and the lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode for a lithium secondary battery which is unlikely to generate pulverization or fall-off from a current collector caused by swelling or contraction of an electrode activator accompanying repeated charging and discharging, and to provide a manufacturing method of the lithium secondary battery which uses this electrode, and which has superior charge/discharge cycle characteristics. <P>SOLUTION: For the electrode for the lithium secondary battery having the current collector on which a film of activator storing and releasing lithium is formed, and the manufacturing method of the same, the current collector having a elongation rate of 13% or higher is used. If the current collector having a large elongation rate of 13% or higher is used, by having the current collector expand and contract so as to sufficiently follow the swelling and contraction of the activator accompanying the storing and releasing of lithium at the repeated charge and discharge, the activator is restrained from pulverization and drop out. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an electrode for a lithium secondary battery, a method for producing the same, and a lithium secondary battery using the electrode for a lithium secondary battery.

  Lithium secondary batteries are widely used as secondary batteries that are ideal for portable electronic devices such as mobile phones and laptop computers because of their high voltage, small size, and light weight. Development is underway to meet that demand.

  In order to increase the capacity of such a lithium secondary battery, an electrode active material that absorbs and releases more lithium is required.

  Conventionally, as the negative electrode active material, a graphitized carbon material having a theoretical discharge capacity of 372 mAh / g per mass and 850 mAh / cc per volume has been used. Since the improvement has approached the limit, materials having a high theoretical capacity, such as silicon and tin, have been proposed (Patent Documents 1 to 6).

  Of these, silicon is promising because it exhibits a large discharge capacity exceeding 1000 mAh / g, but the silicon active material layer has a significant volume expansion / contraction caused by charging / discharging. In addition, there is a drawback in that it easily falls off the current collector as a substrate and the charge / discharge cycle characteristics are poor.

Under these circumstances, in Patent Document 7, the volume of the active material layer such as silicon that is alloyed with lithium during repetition of charge and discharge using a punched or expanded structure metal body having pores as the current collector of the negative electrode It is described that the change is absorbed by deforming the current collector, the current collector is prevented from being broken, and the capacity reduction is suppressed.
JP-A-7-29602 JP2000-12088A JP 2000-12089 A JP 2003-36840 A WO01 / 29912 JP 2001-266851 A JP-A-11-233116

  However, since the technique of Patent Document 7 needs to process the current collector into a structure having holes in advance, it requires labor and cost for special processing, compared with a normal flat current collector, This is disadvantageous because of increased costs.

  Therefore, an object of the present invention is to improve the charge / discharge cycle characteristics of a high voltage / high capacity lithium secondary battery, that is, to pulverize or collect current due to the expansion / contraction of the electrode active material during repeated charge / discharge. An object of the present invention is to provide an electrode for a lithium secondary battery that is unlikely to fall off from the body, a method for producing the same, and a lithium secondary battery that uses this electrode and has excellent charge / discharge cycle characteristics.

  An electrode for a lithium secondary battery according to the present invention is an electrode for a lithium secondary battery in which an active material thin film that absorbs and releases lithium is formed on a current collector, and the elongation percentage of the current collector is 13% or more. It is characterized by being.

  The method for producing an electrode for a lithium secondary battery according to the present invention comprises the step of producing an electrode for a lithium secondary battery by forming an active material thin film that absorbs and releases lithium on the current collector. A material having an elongation of 13% or more is used.

  In other words, the inventors focused on an essential control parameter relating to the degree of deformation, that is, the amount of deformation of the current collector, and as a result of intensive studies, the current activity of the electrode was improved by using a current collector having a specific elongation rate. The inventors have found that pulverization and dropout from a current collector accompanying expansion and contraction of a substance can be suppressed, and the present invention has been completed.

  Although the details of the reason why it is possible to suppress pulverization associated with expansion / contraction of the electrode active material and dropping from the current collector during repeated charge / discharge according to the present invention are not clear, the elongation is 13% or more. When a current collector with a large elongation rate is used, the current material expands and contracts sufficiently to follow the expansion and contraction of the active material accompanying the insertion and extraction of lithium, so that the active material is pulverized and dropped off. It seems to be difficult to get up.

  In the present invention, the term “elongation rate” refers to the rate of change in length when a flat test piece is applied in the length direction within the flat plate surface and a force of a magnitude until the test piece breaks. In the present invention, the elongation percentage of the current collector is measured as follows.

  A test piece having a width of 12.5 mm, a parallel portion in the length direction of 30 mm, and a distance between measuring points of 25 mm is prepared by a tensile tester (universal tester manufactured by Instron). 4505 type) and a tensile test is performed. The test temperature is room temperature, and the tensile speed is constant at 0.5 mm / min. The elongation rate is determined by the amount of change of 25 mm between the gauge points.

  In the present invention, the current collector is a copper foil, preferably a metal foil such as an electrolytic copper foil, which is preheated prior to the formation of the active material thin film. What adjusted to% or more is preferable. This heat treatment is preferably performed in a vacuum or non-oxidizing atmosphere within a heating temperature range that increases the elongation of the metal foil. Further, it is preferable to etch the surface of the current collector prior to the formation of the active material thin film.

  Moreover, it is preferable that the average surface roughness (Ra) before the active material thin film formation of a collector is 0.05-1.5 micrometers.

  On the other hand, as the active material, those containing a group IVB element (excluding carbon) as a constituent element, particularly those containing silicon alone and / or a compound thereof are preferable. In particular, it is preferably formed by a combination of sputtering and vacuum deposition.

  The lithium secondary battery of the present invention is a lithium secondary battery including a positive electrode, a negative electrode, and an electrolyte. As the negative electrode, the electrode for the lithium secondary battery of the present invention or the lithium secondary battery of the present invention is used. It is formed by using an electrode for a lithium secondary battery manufactured by an electrode manufacturing method, and is excellent in charge / discharge cycle characteristics.

  If it is a lithium secondary battery using the electrode for lithium secondary battery of the present invention, it becomes difficult to cause pulverization or dropout from the current collector due to expansion / contraction of the electrode active material during repeated charge / discharge. The charge / discharge cycle characteristics of the lithium secondary battery are improved.

  Although embodiments of the present invention will be described below, the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the gist of the present invention.

<Electrode for lithium secondary battery and method for producing the same>
The electrode for a secondary battery of the present invention is an electrode for a lithium secondary battery in which an active material thin film that absorbs and releases lithium is formed on a current collector, and the elongation percentage of the current collector is 13% or more. It is characterized by that.

First, the current collector used in the present invention will be described.
The current collector used in the present invention has an elongation of 13% or more, preferably 15% or more, particularly preferably 20% or more.

  If the elongation percentage of the current collector is less than 13%, the current collector does not sufficiently follow the expansion and contraction of the active material film during charge and discharge, and the active material film easily falls off the current collector. The elongation rate of the current collector can be large as long as the mechanical strength is not insufficient.

  The elongation rate of the current collector can be controlled by applying a heat treatment to the metal foil and changing the temperature and time of the heat treatment as will be described later.

  Examples of the material for the current collector include copper, nickel, and stainless steel. Among them, copper that is easy to process into a thin film and inexpensive is preferable. The copper foil includes a rolled copper foil by a rolling method and an electrolytic copper foil by an electrolytic method, both of which can be used as a current collector.

  A current collector made of a copper foil produced by a rolling method is suitable for use in a small cylindrical battery because the copper crystals are arranged in the rolling direction so that the negative electrode is hard to crack even if it is rounded sharply or rounded at an acute angle. be able to. 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. Although obtained, copper may be deposited on the surface of the rolled copper foil by an electrolytic method. One side or both sides of the copper foil may be subjected to roughening treatment or surface treatment.

  Electrolytic copper foil has the advantages of superior mass productivity and relatively low manufacturing costs compared to rolled copper foil. Further, the electrolytic method is easy to make thin and is advantageous in terms of weight reduction, and is suitable for use as a current collector.

  The thinner the current collector made of copper foil, the thinner the negative electrode can be produced, and it is preferable in that a negative electrode with a larger surface area can be packed in a battery container with the same storage volume, but if it is too thin, the strength is insufficient And since there exists a possibility that a crack may arise during charging / discharging, it is preferable that it is a thickness of about 10-70 micrometers. When forming the active material thin film on both sides of the copper foil, the copper foil should be thinner, but from the viewpoint of avoiding the cracking of the current collector due to expansion / contraction of the active material due to charging / discharging, A more preferred thickness is 8 to 35 μm.

  In addition, when using metal foil other than copper foil as a collector, the thing of suitable thickness can be used according to each metal foil.

The following physical properties are desired for the current collector.
The average surface roughness (Ra) of the active material thin film forming surface of the current collector defined by the method described in JIS B0601-1994 is usually 0.05 μm or more, preferably 0.1 μm or more, particularly preferably 0.15 μm or more. In general, it is 1.5 μm or less, preferably 1.3 μm or less, particularly preferably 1.0 μm or less.

  By setting the average surface roughness (Ra) of the current collector within the range between the lower limit and the upper limit, good charge / discharge cycle characteristics can be obtained. That is, by setting it to the above lower limit or more, the area of the interface with the active material thin film is increased, and the adhesion with the active material thin film is improved. In addition, the upper limit value of the average surface roughness (Ra) is not particularly limited, but those having an average surface roughness (Ra) exceeding 1.5 μm are generally obtained as foil having a practical thickness as a battery. Since it is difficult, the thing of 1.5 micrometers or less is preferable.

The tensile strength of the current collector is usually 100 N / mm ² or more, preferably 150 N / mm ² or more, particularly preferably 200 N / mm ² or more.

  The tensile strength is obtained by dividing the maximum tensile force required until the test piece breaks by the cross-sectional area of the test piece. The tensile strength in the present invention is measured by the same apparatus and method as the elongation percentage. When the tensile strength is high, cracking of the current collector due to expansion / contraction of the active material accompanying charging / discharging can be suppressed.

0.2% proof stress of the current collector, normally 30 N / mm ² or more, preferably 50 N / mm ² or more, particularly preferably 100 N / mm ² or more.

  The 0.2% proof stress is the magnitude of the load necessary to give a plastic (permanent) strain of 0.2%. It means that The 0.2% proof stress in the present invention is measured by the same apparatus and method as the elongation rate. When the 0.2% proof stress is high, plastic deformation of the current collector due to expansion / contraction of the active material accompanying charging / discharging can be suppressed, and good cycle characteristics can be obtained.

  The electrode for a lithium secondary battery of the present invention is obtained by forming an active material thin film on such a current collector using a method described later.

  As the active material thin film material, a group IVB element (excluding carbon), particularly silicon simple substance and / or a compound thereof, which can obtain a high capacity when formed into a battery is preferable. The group IVB element is preferably used in an amorphous or microcrystalline state.

  The thickness of the active material thin film is usually 1 to 50 μm, preferably 3 to 10 μm.

  The lithium secondary battery electrode of the present invention and the method for manufacturing the same will be described in more detail below according to the method for manufacturing a lithium secondary battery electrode of the present invention.

  As the current collector, a metal foil made of nickel, stainless steel, or the like can be preferably used in addition to the copper foil described above, and heat treatment is performed on such a metal foil to obtain the above-described electrode collector for a lithium secondary battery. A current collector having an elongation percentage suitable as a current collector can be obtained. At this time, the elongation rate of the current collector can be controlled by changing the temperature and time of the heat treatment. Among them, the method of changing the temperature is preferable because it is easy to control.

  As an example, in the case of no treatment, an electrolytic copper foil current collector having an elongation of 8% was treated in a reducing atmosphere of argon with 3% hydrogen added in a vacuum, the treatment time was 10 minutes, and 300 ° C. When processed at a temperature of about 20%, the elongation is about 25% when processed at 400 ° C.

  When the elongation rate of the current collector is adjusted by heat treatment, the elongation rate before heat treatment of the metal foil of the current collector material is usually 5% or more, preferably 8% or more, and usually 12% or less. If the elongation percentage of the metal foil before the heat treatment is too low, there is a possibility that the current collector cannot sufficiently follow the expansion / contraction of the active material film during charge / discharge after the heat treatment. On the other hand, as long as the mechanical strength is not insufficient, there is no problem even if the elongation rate is large.

  The magnification of the elongation change before and after the heat treatment is usually 1.3 times or more, preferably 1.5 times or more, particularly preferably 2 times or more. If the rate of elongation change is too low, the current collector may not follow the expansion / contraction of the active material film during charge / discharge, and the active material film may easily fall off the current collector.

  The ultimate temperature of the heat treatment varies depending on the material of the current collector to be treated and the desired rate of change in elongation, but is usually 150 ° C or higher, preferably 200 ° C or higher, particularly preferably 250 ° C or higher, usually 600 ° C or lower, Preferably it is 500 degrees C or less, Most preferably, it is 400 degrees C or less. If the heat treatment temperature is too low, there is a problem that desired elongation cannot be obtained, and if it is too high, the mechanical strength may be lowered. The heat treatment time is usually 5 minutes to 30 minutes, preferably 10 minutes to 20 minutes, although it varies depending on the heat treatment temperature and the desired rate of change in elongation. If the heat treatment time is too short, the desired elongation may not be obtained, and if it is too long, the productivity may decrease.

  The atmosphere during the heat treatment is preferably in a non-oxidizing atmosphere gas, in a reducing atmosphere gas, or in a vacuum for the purpose of preventing the surface oxidation of the metal foil. Examples of the non-oxidizing atmosphere gas include inert argon and krypton. In particular, argon is inexpensive and is generally used. In order to obtain a reducing atmosphere, it is preferable to perform the treatment in an argon gas containing 2 to 10%, particularly 3% hydrogen, or in a vacuum.

  Further, after such a heat treatment, an etching treatment may be performed for the purpose of removing an oxide film on the surface of the current collector or removing organic contamination. In particular, the etching treatment is preferably performed in the film forming system prior to film formation of the active material thin film by sputtering or vacuum evaporation described later.

  When a negative electrode for a lithium secondary battery is manufactured by depositing an active material thin film on the current collector thus obtained, the active material thin film is formed by a vapor deposition method, specifically, sputtering. Method, vacuum deposition method, CVD method and the like. Alternatively, the sputtering method and the vacuum evaporation method may be combined to form a film.

Below, the formation method of an active material thin film is demonstrated.
(Sputtering method)
In the sputtering method, a thin film is formed by colliding and depositing an active material emitted from a target made of an active material using plasma on a current collector under reduced pressure. According to the sputtering method, the interface state between the formed active material thin film and the current collector is good, and the adhesion of the thin film to the current collector is also high.

  As a method for applying the sputtering voltage to the target, either a DC voltage or an AC voltage can be used. At that time, a substantially negative bias voltage is applied to the current collector to reduce the collision energy of ions from the plasma. It is also possible to control.

  The ultimate vacuum in the chamber before starting the thin film formation is usually 0.1 Pa or less in order to prevent impurities from being mixed.

  As the sputtering gas, a gas that does not react with a target material such as silicon is preferable, and an inert gas such as Ne, Ar, Kr, or Xe is used. Among these, argon gas is preferably used in terms of sputtering efficiency.

  The sputtering target is made of a group IVB element (excluding carbon), which is a thin film material, and may be single crystal or polycrystalline. The target purity is preferably as high as possible in order to increase the purity of the thin film to be formed, and usually 99% or more is used.

  The temperature of the current collector for forming a thin film by sputtering can be controlled by a heater or the like.

  The film formation speed in forming the active material thin film by the sputtering method is usually 0.01 to 0.5 μm / min.

  Note that the surface of the current collector can be etched by pre-treatment such as reverse sputtering or other plasma treatment before forming the thin film. The pretreatment is effective in removing contaminants and oxide films on the copper foil surface and improving the adhesion of the active material thin film.

(Vacuum deposition method)
In the vacuum evaporation method, a material to be an active material is melted and evaporated and deposited on a substrate. In general, it is a method having an advantage that a thin film can be formed at a higher film formation rate than the sputtering method. The vacuum deposition method can be advantageously used in terms of manufacturing cost from the viewpoint of shortening the formation time of the active material thin film having a predetermined thickness compared with the sputtering method. As a specific method, the resistance heating method is used. And electron beam evaporation. In the resistance heating method, the vapor deposition material is heated and melted and evaporated by an energized heating current such as a vapor deposition boat, and in electron beam vapor deposition by an electron beam.

  The ultimate vacuum in the chamber before starting the thin film formation is usually 0.1 Pa or less in order to prevent impurities from being mixed.

  In order to increase the purity of the thin film to be formed, the higher the purity of the vapor deposition material made of a group IVB element (however, excluding carbon), which is a thin film material, the better, and usually 99% or more is used.

  The temperature of the current collector when forming a thin film by a vacuum deposition method can be controlled by a heater or the like.

  The film formation rate in forming the active material thin film by the vacuum deposition method is usually 0.1 to 50 μm / min.

  Similarly to the case of the sputtering method, before the active material thin film is deposited on the current collector serving as the substrate, the surface of the current collector may be etched by ion irradiation with an ion gun or the like. By such etching treatment, the adhesion between the substrate and an active material thin film such as silicon can be further enhanced. Moreover, the adhesion of the silicon thin film to the current collector can be further improved by causing ions to collide with the current collector during the formation of the thin film.

(Combination of sputtering and vacuum deposition)
Utilizing the advantage of the high deposition rate of the vacuum deposition method and the advantage of strong deposition adhesion to the current collector of the sputtering method, for example, the first thin film layer is formed by the sputtering method, and then the vacuum deposition method is used. By forming the second thin film layer at a high speed, it is possible to form an interface region where the adhesion to the current collector is good and to form an active material thin film at a high film formation rate. By such a hybrid combination method of film formation methods, an electrode for a lithium secondary battery having a high charge / discharge capacity and excellent charge / discharge cycle characteristics can be efficiently produced.

  Forming the active material thin film by combining the sputtering method and the vacuum evaporation method is preferably performed continuously while maintaining a reduced pressure atmosphere. This is because contamination of impurities can be prevented by continuously forming the first thin film layer and the second thin film layer without being exposed to the atmosphere. For example, it is preferable to use a thin film forming apparatus that sequentially performs sputtering and vapor deposition while moving the current collector in the same vacuum environment.

  In the present invention, when the active material thin film is formed on both sides of the current collector, the active material thin film layer on one side of the current collector (a combination of the first thin film layer and the second thin film layer may be used). .) And the formation of the active material thin film layer (which may be a combination of the first thin film layer and the second thin film layer) with respect to the other surface of the current collector maintained a reduced-pressure atmosphere. It is preferable to carry out continuously.

<Lithium secondary battery>
Next, the lithium secondary battery of the present invention will be described.
The electrode for a lithium secondary battery of the present invention is extremely useful as a negative electrode material for a non-aqueous secondary battery such as a lithium secondary battery. For example, a non-aqueous secondary battery constructed by combining an electrode manufactured according to the above method as a negative electrode and combining a commonly used positive electrode for a lithium ion battery and an organic electrolyte mainly composed of a carbonate solvent has cycle characteristics. Is excellent.

  There is no particular limitation on the selection of members necessary for the battery configuration such as the positive electrode and the electrolytic solution constituting such a non-aqueous secondary battery.

  Although the material of the member which comprises a non-aqueous secondary battery etc. is illustrated below, the material which can be used is not limited to these specific examples.

Examples of the positive electrode constituting the lithium secondary battery include lithium transition metal composite oxide materials such as lithium cobalt oxide, lithium nickel oxide, and lithium manganese oxide; transition metal oxide materials such as manganese dioxide; A material that can occlude and release lithium, such as a carbonaceous material, can be used. Specifically, LiFeO ₂ , LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ and their non-stoichiometric compounds, MnO ₂ , TiS ₂ , FeS ₂ , Nb ₃ S ₄ , Mo ₃ S ₄ , CoS ₂ , V ₂ One or more of O ₅ , P ₂ O ₅ , CrO ₃ , V ₃ O ₃ , TeO ₂ , GeO _{2 and the} like can be used. The manufacturing method in particular of a positive electrode is not restrict | limited, It can manufacture by the method similar to the manufacturing method of said electrode.

  For the positive electrode current collector, it is preferable to use a valve metal or an alloy thereof that forms a passive film on the surface by anodic oxidation in an electrolytic solution. Examples of the valve metal include metals belonging to IIIa, IVa, and Va groups (3B, 4B, and 5B groups) and alloys thereof. Specifically, Al, Ti, Zr, Hf, Nb, Ta and alloys containing one or more of these metals can be exemplified. Al, Ti, Ta and alloys containing these metals Can be preferably used. In particular, Al and its alloys are advantageous because of their light weight and high energy density.

As an electrolytic solution used for a non-aqueous secondary battery, a solution obtained by dissolving a solute (electrolyte) in a non-aqueous solvent can be used. As the solute, an alkali metal salt, a quaternary ammonium salt, or the like can be used. Specifically, LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ CF ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F _It is preferable to use one or more compounds selected from the group consisting of ₉ SO ₂ ) and LiC (CF ₃ SO ₂ ) ₃ .

  Non-aqueous solvents include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate and vinylene carbonate, cyclic ester compounds such as γ-butyrolactone; chain ethers such as 1,2-dimethoxyethane; crown ether, 2-methyltetrahydrofuran , 1,2-dimethyltetrahydrofuran, 1,3-dioxolane, tetrahydrofuran and other cyclic ethers; diethyl carbonate, ethyl methyl carbonate, dimethyl carbonate and other chain carbonates.

  One kind of solute and solvent may be selected and used, or two or more kinds may be mixed and used. Among these, the non-aqueous solvent preferably contains a cyclic carbonate and a chain carbonate.

  The material and shape of the separator used for the non-aqueous secondary battery are not particularly limited. The separator separates the positive electrode and the negative electrode so that they do not come into physical contact, and preferably has a high ion permeability and a low electric resistance. The separator is preferably selected from materials that are stable with respect to the electrolytic solution and have excellent liquid retention. Specifically, the electrolyte solution can be impregnated using a porous sheet or non-woven fabric made of a polyolefin such as polyethylene or polypropylene as a separator.

  In addition to the non-aqueous electrolyte solution, the negative electrode, and the positive electrode, an outer can, a separator, a gasket, a sealing plate, a cell case, and the like can be used for the non-aqueous electrolyte secondary battery as necessary. A method for producing a non-aqueous electrolyte secondary battery having at least a non-aqueous electrolyte, a negative electrode, and a positive electrode is not particularly limited, and can be appropriately selected from commonly employed methods. For example, a negative electrode can be placed on an outer can, an electrolytic solution and a separator can be provided thereon, and a positive electrode can be placed so as to face the negative electrode, and can be caulked together with a gasket and a sealing plate to form a battery.

  The shape of the lithium secondary battery of the present invention is not particularly limited, a cylinder type in which a sheet electrode and a separator are spiral, a cylinder type having an inside-out structure in which a pellet electrode and a separator are combined, a coin type in which a pellet electrode and a separator are stacked Etc. can be adopted.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. However, the present invention is not limited to the following examples unless it exceeds the gist.

Example 1
As a current collector, an electrolytic copper foil (elongation rate 7.8%) having an average surface roughness (Ra) of 0.19 μm and a thickness of 18 μm was charged with 3% hydrogen at 350 ° C. for 10 minutes in an infrared image furnace. It heat-processed in the argon gas atmosphere containing, and was used as a collector.

Elongation 24.0 percent of the current collector after the heat treatment, the tensile strength 217N ^/ mm 2, 0.2% proof stress was 128N / mm ^2.

On the electrolytic copper foil current collector, a silicon active material was formed into a thin film having a thickness of about 5 μm under the following film forming conditions using a direct current sputtering apparatus. The electrolytic copper foil was attached to a water-cooled substrate holder of the film forming apparatus, and the chamber was previously evacuated to 2.7 × 10 ⁻⁴ Pa. Thereafter, 90 sccm of high purity argon gas was flowed into the chamber, and the opening of the main valve was adjusted to create an atmosphere of 5.3 Pa.

  Before the thin film was formed, the substrate surface was etched by reverse sputtering for the purpose of removing the oxide film on the surface of the electrolytic copper foil.

The silicon target used had a diameter of 5 inches, and the power density input was 4.76 W / cm ² . The deposition rate was about 2 nm / sec, and it took about 42 minutes to form a 5 μm silicon film.

The current collector with the silicon thin film thus obtained was used as a negative electrode, and for battery evaluation, a half cell consisting of 2016 cells incorporating metal lithium as a counter electrode was assembled together with an electrolyte and a separator by punching into a disk shape with a diameter of 10 mm. It was. As the electrolytic solution, a 3: 7 (volume ratio) solution of ethylene carbonate and ethyl methyl carbonate containing 1 mol / L of LiPF ₆ was used.

The charge / discharge test was performed under the following conditions.
Charging to dope the lithium electrode was treated with constant current charge at 1.23mA / cm ^2, until the amount of current becomes 0.123mA / cm ² upon reaching a 10mV, in CCCV charge of performing constant-voltage charging at 10mV I did it. The discharge for dedoping lithium from the electrode was performed by a CC discharge discharging to 1200 mV at a constant current of 1.23 mA / cm ² .

  This charge / discharge cycle was repeated 50 times, and the battery performance was evaluated by the decrease in discharge capacity after a predetermined cycle. The results are shown in Table 1.

Example 2
As the current collector, an electrolytic copper foil (elongation rate 10.8%) having an average surface roughness of 0.32 μm and a thickness of 10 μm was heat-treated in the same manner as in Example 1.

Elongation 19.4% of the current collector after the heat treatment, the tensile strength 172N ^/ mm 2, 0.2% proof stress was 70N / mm ^2.

  Using this electrolytic copper foil current collector, a silicon active material thin film was formed in the same manner as in Example 1, a battery was produced in the same manner, a similar battery evaluation was performed, and the results are shown in Table 1.

Comparative Example 1
As the current collector, the same electrolytic copper foil as used in Example 1 was used as it was without heat treatment.

The electrolytic copper foil current collector that was not heat-treated had an elongation of 7.8%, a tensile strength of 277 N / mm ² , and a 0.2% proof stress of 219 N / mm ² .

  Using this electrolytic copper foil current collector, a silicon active material thin film was formed in the same manner as in Example 1, a battery was produced in the same manner, a similar battery evaluation was performed, and the results are shown in Table 1.

Comparative Example 2
The same electrolytic copper foil as that used in Example 2 was used as a current collector without any heat treatment.

The elongation percentage of the electrolytic copper foil current collector that was not heat-treated was 10.8%, the tensile strength was 315 N / mm ² , and the 0.2% proof stress was 204 N / mm ² .

  Using this electrolytic copper foil current collector, a silicon active material thin film was formed in the same manner as in Example 1, a battery was produced in the same manner, a similar battery evaluation was performed, and the results are shown in Table 1.

  As is clear from Table 1, a battery made of an electrode using a current collector whose elongation was increased by heat treatment had good charge / discharge cycle characteristics, but was made of an electrode using a current collector that was not heat-treated. The battery has a low discharge capacity at an early stage and is inferior in charge / discharge cycle characteristics.

  According to the electrode for a lithium secondary battery of the present invention, it is possible to stably and efficiently manufacture a lithium secondary battery having good battery cycle characteristics.

Claims (19)

  An electrode for a lithium secondary battery in which an active material thin film that absorbs and releases lithium is formed on a current collector, wherein the current collector has an elongation percentage of 13% or more. electrode.   The electrode for a lithium secondary battery according to claim 1, wherein the current collector is heat-treated in advance before forming the active material thin film.   The electrode for a lithium secondary battery according to claim 1, wherein the current collector is a copper foil.   The electrode for a lithium secondary battery according to claim 3, wherein the current collector is an electrolytic copper foil.   5. The lithium secondary according to claim 1, wherein an average surface roughness (Ra) of the active material thin film forming surface of the current collector is 0.05 to 1.5 μm. Battery electrode.   6. The electrode for a lithium secondary battery according to claim 1, wherein the active material contains a group IVB element (except carbon) as a constituent element.   The lithium secondary battery electrode according to claim 6, wherein the active material contains silicon alone and / or a compound thereof.   The electrode for a lithium secondary battery according to claim 1, wherein the active material thin film is a thin film formed by a vapor deposition method.   The electrode for a lithium secondary battery according to claim 8, wherein the active material thin film is a thin film formed by a combination of a sputtering method and a vacuum deposition method.   In a method for producing an electrode for a lithium secondary battery by forming an active material thin film that occludes and releases lithium on a current collector, the current collector has an elongation of 13% or more. A method for producing an electrode for a lithium secondary battery.   The method for producing an electrode for a lithium secondary battery according to claim 10, wherein the current collector is a metal foil, and the metal foil is preheated prior to the formation of the active material thin film.   The method for producing an electrode for a lithium secondary battery according to claim 11, wherein the elongation percentage of the metal foil before the heat treatment is 8 to 12%.   13. The method for manufacturing an electrode for a lithium secondary battery according to claim 11, wherein the heat treatment is performed in a heating temperature range in which the elongation percentage of the metal foil is increased in a vacuum or a non-oxidizing atmosphere.   14. The method of manufacturing an electrode for a lithium secondary battery according to claim 10, wherein the surface of the current collector is etched prior to the formation of the active material thin film.   The method for producing an electrode for a lithium secondary battery according to claim 10, wherein the active material thin film is formed by a vapor deposition method.   The method for producing an electrode for a lithium secondary battery according to claim 15, wherein the active material thin film is formed by a combination of a sputtering method and a vacuum deposition method.   The method for producing an electrode for a lithium secondary battery according to any one of claims 10 to 16, wherein the active material contains a group IVB element (excluding carbon) as a constituent element.   The lithium secondary battery provided with the positive electrode, the negative electrode, and the electrolyte, The lithium secondary battery which uses the electrode for lithium secondary batteries of any one of Claim 1 thru | or 9 as a negative electrode.   The lithium secondary battery provided with the positive electrode, the negative electrode, and the electrolyte, The electrode for lithium secondary batteries manufactured by the manufacturing method of the electrode for lithium secondary batteries of any one of Claim 10 thru | or 17 as a negative electrode Rechargeable lithium battery using