JP6378875B2 - Negative electrode for secondary battery and method for producing the same - Google Patents

Description

The present invention is a negative electrode for a secondary battery using the iron oxide as an active material, about the negative electrode, and a manufacturing method thereof for a secondary battery, especially involving convergence di ® emission electrode reaction.

In recent years, mobile electronic devices such as mobile phones, digital cameras, multi-function tablet terminals, and notebook computers have played an important role in the information society. These electronic devices are required to be driven for a long time, and it has been desired to reduce the weight and increase the energy density of the lithium ion secondary battery, which is a driving power source. In addition, demand for vehicles such as hybrid vehicles (HEV), electric vehicles (EV), electric motorcycles, and medium-sized and large-sized secondary batteries used in harsh outdoor environments such as construction machinery is rapidly increasing. As a power source for these devices, a high-performance secondary battery that is inexpensive and excellent in durability is required.

A lithium ion secondary battery has a structure in which an electrolytic solution in which a lithium salt is dissolved in a nonaqueous solvent or a lithium solid electrolyte is sandwiched between a negative electrode active material and a positive electrode active material. Lithium ions move back and forth between the materials, and the lithium ions intercalate with the active material applied to the current collector base material of the negative electrode, so that charging / discharging becomes possible.

As the negative electrode active material for the lithium ion secondary battery, amorphous carbon having relatively low crystallinity was used at the beginning of commercialization of the lithium ion battery, but at present, the specific gravity is large and high energy density is easily obtained. Artificial graphite materials are mainly used. Usually, highly crystalline graphite particles or carbon nanotubes are mixed with a binder and applied to a current collector substrate.

However, in a negative electrode using graphite, the number of lithium atoms that can be intercalated between graphite layers is one for every six carbon atoms, and the maximum charge / discharge capacity is theoretically limited to 372 mAh / g. Therefore, research and development for increasing the capacity of a lithium ion secondary battery using a negative electrode material that can theoretically provide a charge / discharge capacity that is greater than that of a carbon-based negative electrode material is underway.

It has been reported that when a transition metal oxide nanoparticle is used as such a negative electrode material, a large capacity can be obtained (Non-patent Document 1). In the case of using an oxide of CoO, NiO, CuO, FeO or the like with a nanoparticle of about 1 to 5 nm as an electrode, for example, the reaction of 2CoO + Li⇔Li ₂ O + 2Co proceeds reversibly and exceeds 700 mAh / h. Large capacity can be obtained. The reversible reaction is also referred to as convergence di tio emission electrode reaction, unlike the formation reaction of insertion reactions and Li alloys Li in the anode, the redox of the metal particles of the anode material is a reaction which proceeds reversibly. A secondary battery using this reversible reaction is also referred to as convergence di ® emission type (decomposition and regeneration type) battery.

Iron oxide (Fe ₂ O ₃ and Fe ₃ O ₄₎ is a low cost, naturally abundant, the expected (Non-patent Document as an electrode material for converting di ® emission type high capacity battery having an advantage environmentally friendly 2). As a negative electrode material of convergence di ® emission type battery, the _{α-Fe 2 O 3, Fe} 2 O 3 + 6Li → 3Li 2 theoretical capacity by the reaction of O + 2Fe is 1008mAh / g, and the approximately 3 of a conventional carbon material is a negative electrode material The theoretical capacity is twice as high.

As a method for forming the negative electrode active material layer convergence di ® emission type battery, Fe ₂ O ₃ powder mixed with a conductive aid and a binder such as a thin coating to a current collector surface of the copper foil choice, and vacuum heating Thereafter, a method of producing a negative electrode by drying and pressing is common. However, a battery based on a conversion reaction usually has a very large irreversible capacity, and the volume expansion of about twice occurs with the electrode reaction. Therefore, in the electrode produced by the coating method, the interface between the current collector and the active material layer Since the active material layer is peeled off, there is a problem that the durability is poor.

In order to prevent such peeling, an invention related to a negative electrode using α-Fe ₂ O ₃ particles as an active material, a binder component made of polyamic acid and a part thereof imidized, and a method for producing the same (patent document) 1) has been applied for a patent.

Also, not a secondary battery in which an active material layer is formed by a general coating method, but SnO ₂ , TiO ₂ , Fe ₂ O ₃ , Fe ₃ O ₄ , CoO, Co ₃ O ₄ , CaO, MgO, CuO, ZnO , In ₂ O ₃ , NiO, MoO ₃ , WO ₃ , Li ₄ Ti ₅ O ₁₂ , SnSiO _3, and mixtures thereof nanoparticles (aggregates of nanoparticles have an average particle size of 200 nm to 2 μm, The invention relates to a secondary battery in which a porous active material layer is formed by spraying a dispersion liquid of particles having an average particle diameter of 2 nm to 200 nm on a current collector (Patent Document 2), physical vapor deposition or chemical vapor deposition. An invention (Patent Document 3) related to a lithium secondary battery in which Fe ₂ O ₃ particles are evaporated by a method and an α-Fe ₂ O ₃ film is deposited on a current collector having a rough surface with Rz of 3 μm or more has been filed. Yes.

As such a converter di tio emission type lithium secondary battery using the iron oxide-based material as an active material of a battery, those using a mixture of Fe ₂ O ₃ and LiFe ₅ O ₈ on one of the positive electrode or the negative electrode ( Patent Document 4), using negative electrode containing catalyst such as Fe ₂ O ₃ powder and perovskite oxide (Patent Document 5), negative electrode active material containing Fe ₂ O ₃ powder on carbon sheet as current collector The thing (patent document 6) provided with the layer is mentioned.

Furthermore, in recent years, sodium ion secondary batteries that use sodium ions instead of lithium have attracted attention as next-generation batteries that greatly exceed the performance of lithium ion batteries. As a negative electrode of a sodium ion secondary battery, compounds containing silicon (Patent Document 7) and hard carbon (Patent Document 8) are mainly studied, but eco-friendly iron oxides such as Fe ₃ O ₄ are it has been reported that converting is available as a di-tio emission type negative electrode (non-patent document 3).

P. Poizot et al. Trascon, Nature, 407, 496, (2000) J. Cabana et al., Adv. Mater., 2010 Sep. 15; 22 (35): E170-92.doi: 10.1002 / adma.201000717 S. Hariharan et al. Physical Chemistry Chemical Physics 15 (2013) 2945-2953

WO2011 / 058981 JP 2010-97945 A WO2010 / 092689 WO2011 / 125202 JP 2012-28248 A JP 2012-84345 A JP 2005-32733 A Table 2010/109889

In particular α-Fe ₂ O ₃ Among the iron oxides, have been attracting attention as an active material for converting di ® emission type battery, α-Fe ₂ O ₃ is usually low reversibility of charge and discharge, irreversible capacity There is a problem that is large. In addition, α-Fe ₂ O ₃ nanoparticles need to be applied to the current collector using a binder and additives, but complicated steps such as a mixing process, a coating process, and a drying process of the nanoparticles and the binder are required. In addition, in an electrode in which an active material layer is formed using a binder, if the adhesive force between the current collector and the active material layer is not large, the electrode is activated from the interface between the current collector and the active material layer by repeated charge and discharge. Since the material layer peels off, there is a problem that the durability is poor.

Further, as shown in Patent Document 3, in the method of depositing a film made of α-Fe ₂ O ₃ crystal by physical vapor deposition or chemical vapor deposition, the surface is used to improve the adhesion to the current collector surface. It is necessary to use a copper foil having a roughness Rz of 3 μm or more. In the case of the vapor deposition method, a nanoparticle film cannot be formed.

Furthermore, as a method for producing metal oxide nanoparticles, a solid phase method (mechanical pulverization method), a gas phase method, and a liquid phase method are generally known, but fine particles having a particle diameter of 1 μm or less are efficiently obtained by the mechanical pulverization method. It is difficult to manufacture well, and the chemical vapor deposition method (CVD method) using a metal halide and an oxidizing gas deteriorates the performance of the nanoparticles because the nanoparticles are mixed with the halide. In the liquid phase method such as the coprecipitation method, there is a problem that the generated nanoparticles grow in the heating process, and the cost using the electrode using the metal oxide nanoparticles is high.

The present invention does not use a coating step using a binder or an additive or a step of depositing an active material layer using a target material, and a conversion type secondary material having excellent durability using nano-oxide particles as a negative electrode active material. It is an object of the present invention to provide a battery and a method for producing the secondary battery by a simple method and at a low price. Another object is to provide an efficient and stable convergence di tio down the reaction can realize a negative electrode structure with a thin active material.

In order to achieve the above object, the present invention comprises a negative electrode , a positive electrode using a lithium compound or a sodium compound as an active material, an electrolyte solution disposed between the positive and negative electrodes, and a separator that separates the positive and negative electrodes. In a negative electrode for a conversion type secondary battery, the surface is formed of an iron oxide nanoparticle film that is formed by oxygen plasma treatment and adhered to the surface of the base material on a base material made of iron or an iron-based alloy. providing a negative electrode for converting di tio emission type secondary battery characterized by having an active material layer.

The present invention is also the iron oxide nanoparticles provide amorphous and / or negative electrode convergence di tio emission type secondary battery which is a Fe ₃ O ₄ fine crystals.

The present invention also provides a convergence di ® emission type secondary battery negative electrode size of the particles of the iron oxide nanoparticles film is characterized in that distributed in the range of 10 to 30 nm.

The present invention also provides a convergence di ® emission type secondary battery negative electrode, wherein the iron oxide layer on the lower layer of the iron oxide nanoparticles film is formed.

The present invention also provides a convergence di ® emission type secondary battery negative electrode, wherein the negative electrode for a lithium secondary battery, wherein the iron oxide of the iron oxide layer is Fe ₃ O ₄ crystalline To do.

The present invention also provides a method for producing a negative electrode having an active material layer made of a metal oxide on the surface of the substrate, wherein the surface of the substrate made of iron or an alloy containing iron as a main component is subjected to oxygen plasma treatment. it the surface oxidation, to provide the convergence di tio emission type secondary method for producing a negative electrode for a battery and forming iron oxide nanoparticles film at the outermost surface of the substrate.

In the present invention, the process pressure of the oxygen plasma treatment is 10 Pa to 1 atm, the flow rate of oxygen gas is 10 to 100 ccm, the power density is 0.5 W / cm ^{2 to} 15 W / cm ² , and the treatment time is 5 minutes to 2 to provide a method of manufacturing the convergence di tio emission type secondary battery negative electrode, characterized in that the time.

The present invention further, a negative electrode, wherein the positive electrode and the electrolyte, the negative electrode to provide a convertible di ® emission type secondary battery, which is a negative electrode for a secondary battery.

The negative electrode for a convertible di tio emission type secondary battery of the present invention is different from the structure of the active material is provided by a coating method conventionally used as a general method, an alloy surface is mainly composed of iron or iron An active material layer made of an iron oxide nanoparticle film that is formed by surface oxidation treatment that irradiates oxygen plasma and adheres to the outermost surface of the substrate.

The nanoparticle is generally an ultrafine particle having a particle size, that is, a particle diameter of about 1 to 100 nanometers among fine particles (1000 nm = 1 μm or less). The term “nanoparticle” follows this definition. In the present invention, the average particle size of the iron oxide nanoparticles is more preferably 10 to 30 nm. The average particle size was measured using the photograph taken with a transmission electron microscope (TEM), and the length of the longest part (major axis) of each particle was measured for 10 arbitrarily selected particles. The calculated value.

Iron oxide convergence di ® emission type secondary battery negative electrode surface is convertible di tio emissions react as Li ions and below. Fe ₂ O ₃ + 6Li ⁺ + 6e ⁻ ⇔2Fe ⁰ + 3Li ₂ O Fe ₃ O ₄ + 8Li ⁺ + 8e ⁻ ⇔3Fe ⁰ + 4Li ₂ O That is, iron oxide is reduced to Fe by charging to produce Li ₂ O, and Fe is generated by discharging. Oxidized to produce iron oxide.

In the conventional coating method for producing a negative electrode, primary particles and aggregates of iron oxide fine particles and nanoparticles produced by a chemical method or a physical method are applied to a current collector using a binder. In the present invention, fine particles or nanoparticles of iron oxide produced in advance as ultrafine particles are not used.

The present invention provides a convertible di tio emission type secondary battery using nanoparticles as an active material, without using a high-quality nanoparticles expensive prepared in advance as ultrafine particles, also applied using a special binder and additives A secondary battery with excellent durability, which can form a dense active material layer with strong adhesion between the negative electrode substrate surface and the active material layer without the need for a post-treatment process or a post-treatment process of the coated film The negative electrode can be manufactured at a low price.

In the secondary battery of the present invention, the thickness of the negative electrode active material layer is on the order of several hundred nm, and the negative electrode thickness is reduced, thereby contributing to space saving by effectively utilizing the remaining space. Therefore, the battery capacity per weight and per volume can be increased.

FIG. 1 (left) shows a TEM image (magnification 200,000 times) of an iron oxide layer formed on the surface of a pure iron foil of a negative electrode substrate and an iron oxide nanoparticle film formed in a state of adhering to the outermost surface of this layer. It is a drawing substitute photograph shown in (). FIG. 1 (right) is a drawing-substituting photograph showing an enlarged TEM image (magnification of 800,000 times) of a part of the iron oxide nanoparticle film shown in FIG. 1 (left). FIG. 2 is a drawing-substituting photograph showing a TEM image (magnification 10,000 times) of the substrate surface after oxygen plasma irradiation in Example 1. FIG. 3 is an electron diffraction pattern (μ-Diff03) of nanoparticles in Example 1. FIG. 4 is a graph showing a charge / discharge curve of a half cell in Example 1. FIG. 5 is a graph showing a charge / discharge curve of a full cell in Example 1. FIG. 6 is a graph showing a charge / discharge curve of a half cell in Example 2.

In the present invention, as the negative electrode base material, pure iron foil, steel foil, stainless steel foil or the like is used. In addition, the negative electrode base material is used as a current collector for lithium ion secondary batteries, and is applied to a commonly used conductive metal such as aluminum, copper, titanium, and stainless steel by a coating method such as vapor deposition or organic compound reduction method. The surface layer which consists of may be formed.

To manufacture the negative electrode for a secondary battery, a negative electrode substrate made of iron or an alloy containing iron as a main component serving as a current collector, or a conductive metal such as copper or aluminum as a current collector Oxygen plasma treatment is performed on the surface of the negative electrode substrate coated with iron or an iron-based alloy to oxidize the iron component on the surface to produce an iron oxide nanoparticle film layer attached to the outermost surface of the substrate. Let

As described above, when oxygen plasma treatment is performed on the surface of the base material, one or two layers of amorphous iron oxide nanoparticle films are formed on the outermost surface via the dense iron oxide layer on the base material surface. It can be seen by TEM observation. These iron oxide nanoparticles are amorphous or microcrystalline, or a mixture of amorphous and microcrystalline. And although it changes with oxygen plasma process conditions, an iron oxide film about 100-300 nm in thickness is formed between an iron oxide nanoparticle film | membrane and a base-material surface. The main component of this iron oxide film is crystalline Fe ₃ O ₄ , but the presence of a small amount of crystalline α-Fe ₂ O ₃ , amorphous Fe ₃ O ₄ or α-Fe ₂ O ₃ is X Recognized by line diffraction results.

FIG. 1 (left) shows a dense iron oxide layer having a crystallinity of about 200 nm formed on the surface of the Fe base material of the negative electrode, and average grains generated in a state of being attached to the outermost surface of the iron oxide layer. An amorphous or microcrystalline iron oxide nanoparticle film having a diameter of about 20 nm is shown as a TEM image. FIG. 1 (right) is an enlarged TEM image of the iron oxide nanoparticles.

In the oxygen plasma treatment of the present invention, it is preferable to use high frequency plasma (RF plasma). The oxygen plasma processing apparatus may be any apparatus provided with a plasma generation mechanism such as an atmospheric pressure plasma apparatus, a vacuum plasma CVD apparatus, and a plasma sputtering apparatus. Industrially, 13.56 MHz, 27.12 MHz, 40.68 MHz, etc. are used as the high frequency, but 13.56 MHz is common.

High-frequency plasma can be formed relatively stably in a wider pressure range (pressure 2 Pa to 100 kPa (atmospheric pressure)) than microwave plasma, and the plasma density is about 10 ⁹ to 10 ¹¹ / cm ^3. Although somewhat lower than the above, for example, atmospheric pressure high-frequency plasma irradiation processing using an atmospheric pressure atmosphere has the advantage of simplifying the device structure and reducing the device cost.

In the plasma irradiation process, since the plasma electron temperature is high (about several eV), the substrate is heated. The heating temperature depends on the plasma irradiation process conditions (distance (irradiation distance) between the substrate and the plasma generation unit, the processing time, It varies greatly from 100 to several hundred degrees C. depending on the input energy.

As the oxygen plasma treatment in the present invention, for example, a negative electrode substrate is set in a chamber of a plasma CVD apparatus, oxygen is used as a flow gas, and a gas flow rate is set to about 10 to 100 sccm, preferably about 10 to 60 sccm. Temperature is 25 ° C. to 300 ° C., process pressure is 10 Pa to 1 atm (101.325 kPa), preferably about 10 to 100 kPa, power density is 0.5 W / cm ^{2 to} 15 W / cm ² , and processing time is 5 minutes to 2 hours. The degree, preferably about 5 to 100 minutes. The frequency of the applied high frequency may be 13.56 MHz.

The active material layer of the negative electrode of the present invention has a structure composed of a layer of an iron oxide nanoparticle film that is generated in a state in which it is firmly attached to the outermost surface of the negative electrode base material without particularly requiring a binder. . For this reason, the active material layer has a strong adhesive force with the substrate and has high mechanical and electrical stability. Furthermore, in order to further increase the packing density of the iron oxide nanoparticles in the iron oxide nanoparticle film, the iron oxide nanoparticles produced by a general manufacturing method after the oxygen plasma treatment are applied by a coating method, a spray method, an electrostatic deposition method, etc. The layer of iron oxide nanoparticle film may be replenished.

The negative electrode of the present invention can be used as a convertible di tio emission type element of a lithium secondary battery and a sodium secondary battery. That is, a negative electrode of the present invention, a positive electrode for a lithium compound or sodium compound as an active material, and an electrolytic solution disposed between the positive and negative electrodes, convertible di tio emission type secondary from a separator, which isolate the positive and negative electrodes A battery can be formed. There are no particular restrictions on the organic solvent and electrolyte of the electrolytic solution, the positive electrode, the separator, and the structure and size of the outer container constituting the secondary battery, and conventionally known ones can be used. Since the negative electrode of the present invention can also serve as a negative electrode current collector, it is not necessary to use a separate current collector in that case.

The positive electrode current collector may be, for example, aluminum, nickel, or stainless steel. The positive electrode active material may be lithium or sodium oxide, a composite oxide containing lithium or sodium and a transition metal, lithium or sodium sulfide, an intercalation compound containing lithium or sodium, or lithium or sodium phosphate compound.

The separator may be a porous film made of polyolefin such as polypropylene (PP) or polyethylene (PE), or a porous film made of ceramic.

As the non-aqueous organic solvent, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate are suitable. Fluoroether may be used to improve the flame retardancy of the electrolytic solution. The non-aqueous organic solvent may contain an additive such as an organosilicon compound.

As the electrolyte salt, in the case of a lithium secondary battery, for example, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiCl, LiBr and the like. In the case of a sodium secondary battery, for example, NaPF ₆ , NaBF ₄ , NaClO ₄ , NaTiF ₄ , NaVF ₅ , NaAsF, NaSbF ₆ , NaCF ₃ SO ₃ , Na (C ₂ F ₅ SO ₂ ) ₂ N, NaB (C ₂ O ₄ ) ₂ , N
aB ₁₀ Cl ₁₀ , NaB ₁₂ Cl ₁₂ , NaCF ₃ COO, Na ₂ S ₂ O ₄ , NaNO ₃ , Na ₂ SO ₄ , NaPF ₃ (C ₂ F ₅ ) ₃ , NaB (C ₆ F ₅ ) ₄ , Na ( CF ₃ SO ₂ ) ₃ C and the like. In addition, 1 type may be used independently among the said salts, and 2 or more types may be combined. Further, an ionic liquid may be used. A gel electrolyte may be used.

<Production of Negative Electrode> As a base material, pure iron foil (manufactured by Niraco Co., Ltd.) having a thickness of 50 μm, a length of 50 mm, and a width of 10 mm was set in a chamber of a plasma-CVD apparatus. Plasma irradiation was performed at a substrate temperature of 400 ° C., a chamber pressure of 16 Pa, an oxygen gas flow rate of 60 ccm, and an output density of 6 W / cm ² for 40 minutes.

As shown in FIG. 1, a dense Fe ₃ O ₄ layer having a thickness of about 200 nm on the surface of a substrate and amorphous iron oxide nanoparticles having an average particle size of about 20 nm attached to the outermost surface of this layer It can be seen that 1-2 layers of film are formed. In FIG. 2, a rough surface due to the unevenness of the iron oxide nanoparticles is observed as shown in a TEM image of the surface of the substrate after plasma irradiation. In addition, as shown in FIG. 3, in the electron diffraction pattern of the nanoparticles, an annular pattern is observed, which is understood to be amorphous, microcrystalline, or a mixture thereof.

<Production of Half Cell> A 2032 coin cell was produced using the negative electrode as an evaluation cell and the counter electrode being lithium metal. The iron foil weight of the negative electrode was 0.094 g. The electrolyte was EC: DMC = 1: 2 (vol%), and the electrolyte was LiPF ₆ : 1 mol / l. Celgard was used as a separator.

<Evaluation by half cell> 5 cycles of charge / discharge were performed with CC charge / discharge and a charge / discharge current of 20 μA. The charge / discharge capacity is shown in Table 1 and FIG. The maximum discharge capacity was 163 μAh (second cycle). As shown in FIG. 4, it was confirmed distinct plateau at around specific voltage 1.1V weak to convergence di tio down the negative electrode of the iron oxide.

<Production of Full Cell> A 2032 coin cell was produced using the above negative electrode and the counter electrode being LiCoO (1.5 mAh / cm ² ). <Evaluation by Full Cell> 5 cycles of charge and discharge were performed with CC charge and discharge and a charge and discharge current of 50 μA. The charge / discharge capacity is shown in Table 2 and FIG. The maximum discharge capacity was 1059 μAh (fourth cycle).

<Production of Negative Electrode> As a substrate, SUS304 stainless steel having a thickness of 0.8 mm and a diameter of 16 mm was set in a chamber of a plasma-CVD apparatus. Plasma was irradiated for 90 minutes at a power density of 6 W / cm ² at a substrate temperature of 200 ° C., a chamber pressure of 16 Pa, an oxygen gas flow rate of 60 ccm. Similar to Example 1, a dense Fe ₃ O ₄ layer having a thickness of about 200 nm on the surface of the substrate and amorphous iron oxide nanoparticles having an average particle size of about 20 nm attached to the outermost surface of this layer One or two layers of the film were formed.

<Production of Half Cell> The evaluation cell was produced in the same manner as in Example 1.

<Evaluation with a half cell> 3 cycles of charge / discharge were performed with CC charge / discharge and a charge / discharge current of 10 μA. The charge / discharge capacity is shown in Table 3 and FIG.

The maximum discharge capacity was 93 μAh (second and third cycles). A plateau was confirmed in the first cycle.

Instead such a lithium ion secondary battery using the conventional carbon as a negative electrode active material, use of a high capacity using the iron oxide nanoparticles as a negative electrode active material, and as an inexpensive and safe convergence di tio emission type secondary battery There is expected.

Claims (5)

In a negative electrode for a conversion type secondary battery comprising a negative electrode, a positive electrode using a lithium compound or a sodium compound as an active material, an electrolytic solution disposed between the positive and negative electrodes, and a separator separating the positive and negative electrodes, on iron or iron an alloy mainly composed of base material, formed by surface oxidation by oxygen plasma treatment have a active material layer consisting of iron oxide nanoparticles layer adhering to the outermost surface of the substrate, Conversion di tio emission type secondary battery negative electrode, wherein the main component in the lower layer of iron oxide nanoparticles film is crystalline Fe ₃ O ₄ in which the iron oxide layer is formed. The negative electrode for a secondary battery according to claim 1, wherein the iron oxide nanoparticles are amorphous and / or microcrystalline Fe ₃ O ₄ . The negative electrode for a secondary battery according to claim 1 or 2, wherein the particle diameter of the iron oxide nanoparticle film is distributed in a range of 10 to 30 nm. In the method for producing a negative electrode having an active material layer made of a metal oxide on the surface of a base material, the surface is oxidized by subjecting the base material made of iron or an iron-based alloy as a main component to oxygen plasma treatment, and the base material The method for producing a negative electrode for a secondary battery according to any one of claims 1 to 3, wherein an iron oxide nanoparticle film is formed on the outermost surface of the secondary battery. The oxygen plasma treatment has a process pressure of 10 Pa to 1 atm, an oxygen gas flow rate of 10 to 100 ccm, a power density of 0.5 W / cm ^{2 to} 15 W / cm ² , and a treatment time of 5 minutes to 2 hours. The manufacturing method of the negative electrode for secondary batteries of Claim 4.