JP6076802B2 - Method for producing negative electrode for lithium ion secondary battery - Google Patents

Description

  The present invention relates to a method for producing a negative electrode for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery.

  2. Description of the Related Art Lithium ion secondary batteries that charge and discharge by moving lithium ions between a positive electrode and a negative electrode are used as power sources for mobile terminals such as mobile phones and electric vehicles. In recent years, with an increase in performance of mobile phones and the like, there has been a strong demand for a large capacity lithium ion secondary battery.

  Conventionally, carbon-based materials such as natural graphite, artificial graphite, and coke have been used as negative electrode active materials for lithium ion secondary batteries. However, the theoretical capacity of these carbon-based materials is 372 mAh / g, and it has become difficult to meet the demand for a large capacity lithium ion secondary battery. Therefore, development of a negative electrode active material that can achieve a large capacity of a lithium ion secondary battery instead of a carbon-based material has been actively performed.

  Silicon (theoretical capacity: 4298 mAh / g) having a theoretical capacity approximately 12 times that of the carbon-based material has attracted attention as a negative electrode active material replacing the carbon-based material. By using silicon as the negative electrode active material, the capacity of the negative electrode can be significantly increased, so that the storage capacity of the lithium ion secondary battery can be increased.

  As a method for producing a negative electrode using such silicon as a negative electrode active material, a method of preparing a slurry containing silicon particles and a binder and applying this slurry onto a current collector (slurry method) is known. (See Patent Document 1).

International Publication No. 02/21616 Pamphlet

  Silicon as a negative electrode active material of a lithium ion secondary battery occludes lithium ions during charging and is alloyed, and its volume expands to about 3 to 4 times. As a result, the negative electrode repeats expansion and contraction during the charge / discharge cycle.

  As described above, in the negative electrode manufactured by applying the slurry containing silicon particles on the current collector, the silicon particles are adhered to the current collector with almost no gap. Therefore, there is no gap that can absorb the volume expansion of silicon particles due to occlusion of lithium ions, and by repeated expansion and contraction during charging and discharging, collapse due to silicon pulverization and peeling from the current collector occur, The capacity of the negative electrode is reduced, and the cycle characteristics are reduced. As a result, there is a problem that the life of the lithium ion secondary battery is extremely shortened.

  In addition, silicon has poor electrical conductivity compared to carbon-based materials, and the efficient movement of electrons associated with charge / discharge is limited. Therefore, when silicon is used as a negative electrode active material, the conductivity of carbon-based materials and the like is limited. It may be used in combination with a conductive additive that supplements the nature. Even in this case, since the mixture of the silicon particles and the carbon-based material particles is adhered to the current collector without any gap, the same problem as described above occurs.

  Furthermore, for the purpose of holding the silicon particles and optionally the conductive additive on the current collector, a binder is included in the slurry, but a binder that does not directly participate in the electrode reaction is included. As a result, it may be difficult to increase the unit area capacity of the negative electrode.

  In view of such a problem, the present invention makes it difficult for the negative electrode active material particles to be broken or peeled off from the current collector even when the negative electrode active material particles repeatedly expand and contract during charging and discharging, and the cycle characteristics are greatly improved. A method for producing a negative electrode for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery produced by the method, and a lithium ion secondary battery using the negative electrode for the lithium ion secondary battery Objective.

  In order to solve the above-described problems, the present invention provides a method for producing a negative electrode for a lithium ion secondary battery, wherein a metal film is formed by plating a metal on a current collector to form a metal film, and the metal A slurry application step of applying a slurry containing negative electrode active material particles and combustible particles on the film, a heating step of heating the current collector coated with the slurry, and cooling the current collector after the heating A cooling step, wherein the combustible particles have a combustion temperature lower than the combustion temperature of the negative electrode active material particles, and in the heating step, a temperature not lower than the combustion temperature of the combustible particles and not lower than the melting point of the metal. The current collector is heated to combust the combustible particles, the metal is melted, and the molten metal is solidified in the cooling step, so that the negative electrode active material particles are converted into the current collector. Tie on top To provide a method for manufacturing a negative electrode for a lithium ion secondary battery, characterized by (invention 1).

  According to the above invention (Invention 1), the combustible particles applied together with the negative electrode active material particles on the current collector burn, so that the volume of the negative electrode active material particles is between the negative electrode active material particles on the current collector. A void that can absorb expansion is formed. Thereafter, the molten metal is solidified, and can be bound on the current collector while forming voids between the negative electrode active material particles. Therefore, even if the negative electrode active material particles are repeatedly expanded and contracted during charge and discharge, the negative electrode active material particles are not easily broken or peeled off from the current collector, and the lithium ion secondary battery has greatly improved cycle characteristics. A negative electrode can be manufactured.

  Moreover, according to the said invention (invention 1), since the melted metal plays a role as a conductive aid for assisting conductivity when silicon particles or the like are used as the negative electrode active material particles, the negative electrode active material particles are included. The slurry does not need to contain a conductive additive such as a carbon-based material. Therefore, the capacity per unit area of the negative electrode can be increased, and a large capacity negative electrode for a lithium ion secondary battery can be manufactured.

  Furthermore, in the said invention (invention 1), the metal which comprises a metal film plays the role as a binder for binding negative electrode active material particle on a collector. Therefore, since it is not necessary to contain a large amount of the binder that does not directly participate in the electrode reaction in the slurry, the capacity per unit area of the negative electrode can be increased.

  In the above invention (Invention 1), it is preferable to use silicon particles as the negative electrode active material particles (Invention 2). In the above inventions (Inventions 1 and 2), it is preferable to use zinc as the metal (Invention 3). In the said invention (invention 1-3), it is preferable to use a graphite particle as said combustible particle (invention 4).

  In the said invention (invention 1-4), it is preferable to further include the copper plating process of plating copper on the said collector cooled by the said cooling process, and forming a copper thin film (invention 5), this invention (invention) 5) In the copper plating step, it is preferable to form the copper thin film so that the film thickness becomes 0.001 to 0.1 μm (Invention 6).

  According to the present invention, even when the negative electrode active material particles repeatedly expand and contract during charging and discharging, the negative electrode active material particles are hardly broken or peeled off from the current collector, and the cycle characteristics are greatly improved. A method for producing a negative electrode for an ion secondary battery, a negative electrode for a lithium ion secondary battery produced by the method, and a lithium ion secondary battery using the negative electrode for a lithium ion secondary battery can be provided.

FIG. 1 is a process flow diagram schematically showing each process in a method for producing a negative electrode for a lithium ion secondary battery according to an embodiment of the present invention. FIG. 2 is a cross-sectional view schematically showing a schematic configuration of a lithium ion secondary battery in one embodiment of the present invention. FIG. 3 is a cross-sectional view schematically showing a schematic configuration of a lithium ion secondary battery according to another embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[Method for producing negative electrode for lithium ion secondary battery]
FIG. 1 is a process flow diagram schematically showing each process in a method for producing a negative electrode for a lithium ion secondary battery according to an embodiment of the present invention.

  In the method for manufacturing a negative electrode for a lithium ion secondary battery according to this embodiment, first, a metal film 3 is formed on one surface 2a of the current collector 2 by plating (metal film forming step, see FIG. 1A). ).

  As the current collector 2, for example, a foil or a plate made of a metal such as copper, stainless steel, or nickel can be used.

  As the metal constituting the metal film 3 formed by plating on the one surface 2a of the current collector 2, a metal that hardly causes an alloying reaction with lithium ions can be used. Those having a melting point MT lower than the combustion temperature BTn can be used.

  As the metal, for example, zinc (melting point MT: about 420 ° C.) or the like can be used. In particular, when graphite particles (combustion temperature BTc: about 400 ° C.) are used as the combustible particles 5, Zinc is preferably used.

  The thickness of the metal film 3 formed on the one surface 2a of the current collector 2 is such that a sufficient amount of the negative electrode active material particles 4 can be bound onto the current collector 2 by melting and solidifying the metal film 3. The thickness is not particularly limited as long as it is a thickness, and can be, for example, about 0.001 to 1 μm. If the thickness of the metal film 3 exceeds 1 μm, the negative electrode active material particles 4 may be buried in the melted and solidified metal film 3, depending on the average particle diameter of primary particles of the negative electrode active material particles 4. Because it is not desirable.

  Next, a slurry containing at least the negative electrode active material particles 4 and the combustible particles 5 is applied onto the metal film 3 formed on the one surface 2a of the current collector 2 using a coating machine such as a roll coater. And heated to about 80 ° C. and dried (slurry coating step, see FIG. 1B). In addition, you may perform the rolling process by a flat plate press, a calender roll, etc. as needed after application | coating of a slurry.

  As the negative electrode active material particles 4, particles having a melting point MT of the metal constituting the metal film 3 and a combustion temperature BTn higher than the combustion temperature BTc of the combustible particles 5 can be used, for example, silicon, silicon oxide Silicon-based particles (combustion temperature BTn: about 1000 ° C.) such as (silicon monoxide and the like), a compound of silicon and metal (metal silicide such as titanium silicide), and the like can be used.

  The average particle diameter of the primary particles of the negative electrode active material particles 4 is preferably about 0.001 to 1 μm. In addition, the average particle diameter of the primary particle of the negative electrode active material particle 4 can be measured using an electron microscope (SEM) etc., for example.

  In addition, silicon-based particles as the negative electrode active material particles 4 and having a diameter of about 0.01 μm or less include, for example, silicon-based fine particles having an average particle size of about several hundred nm in a hydrofluoric acid aqueous solution. It can manufacture by dispersing (for example, Unexamined-Japanese-Patent No. 2012-229146 etc.).

  In the present embodiment, the combustible particles 5 are included in the slurry together with the negative electrode active material particles 4, so that the combustible particles 5 are combusted by heating in a heating step described later, and voids are formed between the negative electrode active material particles 4. It is formed. Further, the metal film 3 in the melting and cooling process of the metal constituting the metal film 3 in the heating process described later is configured in a state where voids are formed between the negative electrode active material particles 4 by the combustion of the combustible particles 5. The negative electrode active material particles 4 are bound to the current collector 2 (metal film 3) through solidification of the metal. Therefore, as long as the combustion temperature BTc of the combustible particles 5 is lower than the combustion temperature BTn of the negative electrode active material particles 4, the relationship with the melting point MT of the metal constituting the metal film 3 is not particularly limited.

  The combustible particles 5 are not particularly limited as long as the combustion temperature BTc is lower than the combustion temperature BTn of the negative electrode active material particles 4, and graphite particles and organic particles (powder cellulose, etc.) can be used. . For example, when zinc is used as the metal constituting the metal film 3, it is preferable to use graphite particles (combustion temperature BTc: about 400 ° C.) as the combustible particles 5.

  The average particle size of the primary particles of the combustible particles 5 is preferably 0.02 to 1 μm. In addition, the average particle diameter of the primary particle of the combustible particle 5 can be measured using an electron microscope (SEM) etc., for example.

  In consideration of the volume expansion of the negative electrode active material particles 4 in the negative electrode 1 for a lithium ion secondary battery, the average particle size of the primary particles of the combustible particles 5 is larger than the average particle size of the primary particles of the negative electrode active material particles 4. Larger is preferred. In particular, when silicon-based particles are used as the negative electrode active material particles 4, the average particle size of the primary particles of the combustible particles 5 is about 3 to 4 times the average particle size of the primary particles of the negative electrode active material particles 4. preferable.

  Although the content ratio (in terms of solid content) of the negative electrode active material particles 4 and the combustible particles 5 in the slurry is not particularly limited, a void is effectively formed between the negative electrode active material particles 4 by a heating process described later. In addition, a sufficient amount of the negative electrode active material particles 4 that can achieve a large capacity of the negative electrode 1 for a lithium ion secondary battery can be appropriately set within a range that can be bound on the current collector 2.

  The slurry may contain a binder such as polyvinylidene fluoride (PVdF), a thickener such as a cellulose derivative such as carboxymethyl cellulose, if desired. In the present embodiment, the metal film 3 serves as a binder for binding the negative electrode active material particles 4 to the current collector 2. Therefore, when a binder such as PVdF is included in the slurry, the binder content in the slurry is such that the negative electrode active material particles 4 and the combustible particles 5 can be temporarily bonded onto the metal film 3 (the current collector coated with the slurry). It is desirable to set it to a very small amount) that allows temporary bonding so that the body 2 can be easily handled.

  The slurry can be prepared, for example, by adding a negative electrode active material particle 4, a combustible particle 5, a solvent / dispersion medium such as water, and a binder, a thickener, and the like to a mixer as desired, followed by stirring.

  Subsequently, the current collector 2 coated with the slurry is heated to a temperature equal to or higher than the melting point MT of the metal constituting the metal film 3 and equal to or higher than the combustion temperature BTc of the combustible particles 5 in the presence of oxygen (heating). Process). For example, when zinc (melting point MT: about 420 ° C.) is used as the metal constituting the metal film 3 and graphite particles (combustion temperature BTc: about 400 ° C.) are used as the combustible particles 5, heating to about 450 ° C. To do. Thereby, the combustible particles 5 are burned to form voids between the negative electrode active material particles 4 and the metal constituting the metal film 3 is melted.

  The heating temperature in the heating step may be not less than the melting point MT of the metal constituting the metal film 3 and not less than the combustion temperature BTc of the combustible particles 5 and less than the combustion temperature BTn of the negative electrode active material particles 4. By heating at such a temperature, voids can be formed between the negative electrode active material particles 4 by the combustion of the combustible particles 5, and the negative electrode is formed by solidification of the metal while leaving the voids in the subsequent cooling step. The active material particles 4 can be bound to the current collector 2. As a result, among the negative electrode active material particles 4 that can be laminated on the metal film 3 so as to overlap a plurality of layers by application of the slurry, the negative electrode active material particles 4 (negative electrode active material) that are in direct contact with the metal film 3 Particles located in the lowermost layer among the substance particles 4) are bound to at least the current collector 2.

  When graphite particles are used as the combustible particles 5, all the combustible particles 5 may be burned in the heating step. However, as long as sufficient voids can be formed between the negative electrode active material particles 4, heating is possible. In the current collector 2 after the process, some of the combustible particles 5 may remain slightly. Even if the graphite particles as the flammable particles 5 remain, since they can also serve as a conductive auxiliary agent, there is no possibility of deteriorating the characteristics of the obtained negative electrode 1 for lithium ion secondary batteries.

  Then, the current collector 2 after the heating process is cooled to at least less than the melting point MT of the metal constituting the metal film 3 (cooling process). As a result, the metal that has entered a part of the void formed by the heating process is solidified to form a void between the negative electrode active material particles 4 and bind the negative electrode active material particles 4 onto the current collector 2. (See FIG. 1C).

  Finally, a copper thin film 6 is formed by electroless copper plating on the current collector 2 obtained by fixing the negative electrode active material particles 4 as described above (see a copper plating process, FIG. 1 (d)). ). By forming the copper thin film 6 in this manner, it is possible to more effectively prevent the negative electrode active material particles 4 from being separated from the negative electrode 1 for a lithium ion secondary battery. Further, the negative electrode active material particles 4 that are in direct contact with the metal film 3 through the heating step and the cooling step are bound to at least the current collector 2, but above the negative electrode active material particles 4. The negative electrode active material particles 4 to be stacked are not necessarily bound to the current collector 2. However, by forming the copper thin film 6 as described above, the negative electrode active material particles 4 on the negative electrode active material particles 4 bound to the current collector 2 by melting and solidification of the metal film 3 are also collected by the current collector. 2 can be fixed.

  The thickness of the copper thin film 6 formed in the copper plating step is preferably 0.001 to 0.1 μm, and more preferably 0.01 to 0.05 μm. If the thickness of the copper thin film 6 is less than 0.001 μm, it may be difficult to effectively prevent the negative electrode active material particles 4 from peeling from the current collector 2. In addition, when the thickness of the copper thin film 6 exceeds 0.1 μm, in the lithium ion secondary battery, the negative electrode active material particles 4 effectively prevent lithium ions from being brought into contact with the negative electrode active material particles 4. It cannot be occluded, and it may be difficult to increase the capacity of the lithium ion secondary battery.

  The negative electrode 1 for a lithium ion secondary battery can be obtained by the above-described method for manufacturing a negative electrode for a lithium ion secondary battery according to this embodiment (see FIG. 1D). The negative electrode 1 for a lithium ion secondary battery obtained in this way has sufficient voids between the negative electrode active material particles 4, and the negative electrode active material particles 4 are collected through melting and solidification of the metal constituting the metal film 3. It is firmly bound on the electric body 2. Therefore, according to the present embodiment, particle destruction due to expansion / contraction of the negative electrode active material particles 4 during charge / discharge and separation from the current collector 2 can be effectively suppressed, and the cycle characteristics are greatly improved. The negative electrode for lithium ion secondary batteries which can be manufactured can be manufactured.

  Further, the negative electrode 1 for a lithium ion secondary battery obtained as described above has the negative electrode active material particles 4 bound on the current collector 2 by melting and solidification of the metal constituting the metal film 3. The amount of binder used to bind the active material particles 4 onto the current collector 2 can be greatly reduced. The ability to significantly reduce the amount of binder used that does not directly contribute to the electrode reaction can increase the capacity per unit area and produce a negative electrode for lithium ion secondary batteries with excellent electrode characteristics. Can do.

  Further, since the metal for binding the negative electrode active material particles 4 on the current collector 2 also serves as a conductive aid, a conductive aid such as a carbon-based material is added to the slurry containing the negative electrode active material particles 4. It is not necessary to contain. Therefore, the negative electrode 1 for lithium ion secondary batteries which increases the capacity per unit area can be manufactured.

[Lithium ion secondary battery]
Next, the lithium ion secondary battery in this embodiment will be described. FIG. 2 is a cross-sectional view schematically showing a schematic configuration of the lithium ion secondary battery in the present embodiment.

  As shown in FIG. 2, the lithium ion secondary battery 10 in this embodiment includes a negative electrode 1 for a lithium ion secondary battery obtained by the method for manufacturing a negative electrode for a lithium ion secondary battery according to this embodiment; A positive electrode 11 for a lithium ion secondary battery disposed opposite to the negative electrode 1 for a secondary battery; a separator 13 sandwiched between the positive electrode 11 and the negative electrode 1; a coin-type case 14 for housing the positive electrode 11, the negative electrode 1 and the separator 13; A sealing plate 15, and a gasket 16, and the inside of the coin-type case 14, the sealing plate 15 and the gasket 16 is hermetically sealed. The positive electrode 11 includes a positive electrode active material layer 11a and a positive electrode current collector 11b. The positive electrode 11 and the negative electrode 1 are disposed so that the positive electrode active material layer 11 a and the negative electrode active material layer 4 a including the negative electrode active material particles 4 are in contact with the separator 13. An electrode group composed of the positive electrode 11, the negative electrode 1, and the separator 13 is impregnated with the electrolytic solution 12.

A conventionally well-known thing can be used as the positive electrode 11 for lithium ion secondary batteries. For example, positive electrode active material particles such as lithium-containing transition metal oxides such as lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), and lithium manganate (LiMnO ₂ ) are used as metal foil or metal such as aluminum and stainless steel. One having a configuration in which it is bound to one surface of the positive electrode current collector 11b composed of a plate or the like can be mentioned. In addition, the positive electrode 11 for lithium ion secondary batteries may have a conductive support agent (powder which consists of electroconductive substances, such as carbon, a metal (copper, tin, zinc, nickel, silver, etc.)) if desired.

  As the electrolytic solution 12, for example, an organic solvent-based nonaqueous electrolytic solution obtained by dissolving a lithium salt in an organic solvent can be used. The organic solvent contained in the electrolytic solution is not particularly limited. For example, chain esters such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and methyl propyl carbonate; ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate And the like, and mixed solvents of chain esters and cyclic esters.

The lithium salt contained in the electrolytic solution 12 is not particularly limited, and is an inorganic lithium salt such as LiPF ₆ , LiBF ₄ , LiClO ₄ , LiI, LiCl, LiBr; LiB [OCOCF ₃ ] ₄ , LiB [OCOCF ₂ Examples include organic lithium salts such as CF ₃ ] ₄ , LiPF ₄ (CF ₃ ) ₂ , LiN (SO ₂ CF ₃ ) ₂ , and LiN (SO ₂ CF ₂ CF ₃ ) ₂ .

  As the separator 13, a separator used in a conventionally known lithium ion secondary battery can be used, and examples thereof include a microporous film made of polyolefin such as polyethylene and polypropylene, a nonwoven fabric, cloth, and the like.

  In the lithium ion secondary battery 10 according to the present embodiment having the above-described configuration, expansion at the time of charge / discharge when silicon-based particles are used as the negative electrode active material particles 4 constituting the negative electrode 1 for a lithium ion secondary battery. -Problems due to shrinkage (particle collapse, separation from the current collector, etc.) have been resolved. Therefore, according to the present embodiment, it is possible to provide a lithium ion secondary battery that has a larger capacity than the conventional one and is excellent in cycle characteristics.

  The embodiment described above is described for facilitating understanding of the present invention, and is not described for limiting the present invention. Therefore, each element disclosed in the above embodiment is intended to include all design changes and equivalents belonging to the technical scope of the present invention.

  In the above embodiment, the method for producing a negative electrode for a lithium ion secondary battery includes a copper plating step of forming a copper thin film 6 on the current collector 2 that is cooled and bound with the negative electrode active material particles 4. The present invention is not limited to such an embodiment, and may not include this copper plating step. That is, the obtained negative electrode for a lithium ion secondary battery may have the structure shown in FIG.

  Although the coin-type lithium ion secondary battery is exemplified as the lithium ion secondary battery in the above embodiment, the present invention is not limited to such an embodiment. For example, the positive electrode for lithium ion secondary battery and lithium ion Winding electrode plate group formed by winding a negative electrode for a secondary battery and a separator in a spiral shape; a positive electrode for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a separator as necessary A cylindrical lithium ion secondary battery in which an electrode plate group such as a stacked electrode plate group and an electrolytic solution are enclosed in a battery container may be used.

  For example, as shown in FIG. 3, in the lithium ion secondary battery 20, the positive electrode 11 and the negative electrode 1 are stacked in the order of separator-negative electrode-separator-positive electrode via the separator 23, and the positive electrode 11 is positioned inside. Thus, a winding type electrode plate group is formed and inserted into the battery case 24. The positive electrode 11 is connected to the positive electrode terminal 26 via the positive electrode lead 25, and the negative electrode 1 is connected to the battery container 24 via the negative electrode lead 27, and chemical energy generated inside the lithium ion secondary battery 20 is taken out as electric energy to the outside. Configured to get. In the battery container 24, the wound electrode plate group is impregnated with the electrolytic solution 12.

DESCRIPTION OF SYMBOLS 1 ... Negative electrode 2 for lithium ion secondary batteries ... Current collector 3 ... Metal film 4 ... Negative electrode active material particle 5 ... Combustible particle 6 ... Copper thin film 10, 20 ... Lithium ion secondary battery

Claims (6)

A method for producing a negative electrode for a lithium ion secondary battery, comprising:
A metal film forming step of plating a metal on a current collector to form a metal film;
A slurry application step of applying a slurry containing negative electrode active material particles and combustible particles on the metal film,
A heating step of heating the current collector coated with the slurry;
Cooling the current collector after heating, and
The combustible particles have a combustion temperature lower than the combustion temperature of the negative electrode active material particles,
In the heating step, the current collector is heated to a temperature not lower than the combustion temperature of the combustible particles and not lower than the melting point of the metal, thereby burning the combustible particles and melting the metal,
In the cooling step, the molten metal is solidified, and the negative electrode active material particles are bound onto the current collector. A method for producing a negative electrode for a lithium ion secondary battery, comprising:   The method for producing a negative electrode for a lithium ion secondary battery according to claim 1, wherein the negative electrode active material particles are silicon particles.   The method for producing a negative electrode for a lithium ion secondary battery according to claim 1, wherein the metal is zinc.   The method for producing a negative electrode for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the combustible particles are graphite particles.   5. The lithium ion secondary battery according to claim 1, further comprising a copper plating step of plating copper on the current collector cooled in the cooling step to form a copper thin film. Manufacturing method of negative electrode.   The method for producing a negative electrode for a lithium ion secondary battery according to claim 5, wherein in the copper plating step, the copper thin film is formed so as to have a film thickness of 0.001 to 0.1 μm.

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