JP7500517B2 - Method for manufacturing negative electrode material for secondary batteries - Google Patents

Description

The present invention relates to a negative electrode material for a secondary battery and a method for manufacturing the negative electrode material for a secondary battery.

Secondary batteries, which can be repeatedly charged and discharged, are widely used as so-called portable power sources for personal computers and mobile terminals, and as power sources for driving vehicles. Among secondary batteries, lithium-ion secondary batteries, which are particularly lightweight and have high energy density, are ideally used as high-output power sources for driving vehicles such as electric vehicles, hybrid vehicles, and plug-in hybrid vehicles.

A lithium-ion secondary battery is a secondary battery that can be charged and discharged by the movement of lithium ions in an electrolyte between a positive electrode and a negative electrode that absorb and release lithium ions. The positive and negative electrode materials used in lithium-ion secondary batteries each include a conductive current collector foil and a composite layer that contains an active material, etc., held on the current collector foil.

For example, a carbon material such as graphite is used as the active material contained in the negative electrode material. Furthermore, in order to improve the output characteristics during charging and discharging of the battery, it is possible to coat the surface of the carbon material with a metal oxide.

Patent Document 1 discloses a lithium ion secondary battery having a positive electrode plate with a positive electrode mixture layer and a negative electrode plate with a negative electrode mixture layer, in which the negative electrode mixture layer contains a graphite-based material, a metal oxide, and a conductive assistant, and the conductive assistant is made of a carbon material that does not dope or dedope lithium ions, and is 0.4% by weight or more and less than 1.2% by weight of the weight of the negative electrode mixture layer.

JP 2016-51655 A

However, in the lithium-ion secondary battery described in Patent Document 1, the metal oxide contained in the negative electrode mixture layer itself has low conductivity, so a conductive material (conductive assistant) is mixed into the negative electrode mixture layer to improve conductivity. This suppresses an increase in electrical resistance in the negative electrode and improves output characteristics. On the other hand, the total amount of active material contained in the negative electrode mixture layer is reduced by the amount of the conductive material, resulting in a problem of a decrease in the energy density of the negative electrode.

The present invention has been made to solve these problems, and aims to provide a negative electrode material for a secondary battery that has a high energy density while suppressing an increase in electrical resistance, and a method for manufacturing the negative electrode material for a secondary battery.

The negative electrode material of the secondary battery according to one embodiment has a negative electrode current collector foil and a negative electrode composite layer formed on the negative electrode current collector foil and containing a composite material, and the composite material has a sea-island structure in which the surface of a carbon material containing amorphous carbon at least in the surface layer is covered in islands with metal generated from a metal oxide.

In addition, the method for manufacturing a negative electrode plate of a secondary battery according to one embodiment includes a mixing step of mixing a carbon material containing amorphous carbon at least in the surface layer with a metal oxide to obtain a mixture, a firing step of firing the mixture at a heating temperature at which an oxidation-reduction reaction between the amorphous carbon and the metal oxide proceeds in a non-oxidizing atmosphere to form a composite material, and a composite layer formation step of forming a negative electrode composite layer containing the composite material on the negative electrode current collector foil.

The present invention provides a negative electrode material for a secondary battery that has a high energy density while suppressing an increase in electrical resistance, and a method for manufacturing the negative electrode material for a secondary battery.

1 is a schematic diagram showing a part of a cross section of a negative electrode material according to a first embodiment. FIG. 2 is a flowchart showing a method for producing the negative electrode material shown in FIG. 1 . 1 is a graph showing an example of an Ellingham diagram. 1 is a graph showing the IV resistance of an evaluation battery cell.

First embodiment
Hereinafter, an embodiment of the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiment. In addition, in order to clarify the description, the following description and drawings are appropriately simplified. What is shown in the drawings is only a part of the whole, and in reality, many other configurations that are not shown are included. Furthermore, in the following description, the same or equivalent elements are given the same reference numerals, and duplicated descriptions are omitted.

First, referring to FIG. 1, an overview of the negative electrode material of the secondary battery according to this embodiment (hereinafter, sometimes simply referred to as "negative electrode material 1") will be described. As one preferred embodiment of the negative electrode material 1 according to this embodiment, the negative electrode material 1 used in a lithium ion secondary battery will be specifically described. FIG. 1 is a schematic diagram showing a part of a cross section of the negative electrode material according to embodiment 1.

The negative electrode material 1 shown in Figure 1 is used together with a positive electrode material as an electrode of a lithium-ion secondary battery. A lithium-ion secondary battery is a secondary battery that uses lithium ions as a charge carrier and realizes charging and discharging by the transfer of charge between the electrodes, the positive electrode and the negative electrode.

The negative electrode material 1 has a negative electrode current collector foil 10 and a negative electrode composite layer 20 formed on the negative electrode current collector foil 10. The negative electrode current collector foil 10 is formed in a plate or foil shape and is made of a metal with good electrical conductivity. Examples of metals that make up the negative electrode current collector foil 10 include copper, copper alloys, nickel, titanium, stainless steel, etc.

The negative electrode composite layer 20 is formed on at least one surface of the negative electrode current collector foil 10, except for an edge portion along one edge in the width direction of the negative electrode current collector foil 10. In addition, the negative electrode material 1 has an exposed portion at the edge portion of the negative electrode current collector foil 10 where the negative electrode composite layer 20 is not formed and the negative electrode current collector foil 10 is exposed.

(Negative electrode mixture layer 20)
The negative electrode mixture layer 20 includes at least a composite material 30 and is held by the negative electrode current collector foil 10. The composite material 30 is mainly composed of an active material. In addition to the composite material 30, the negative electrode mixture layer 20 may include additives such as a binder and a thickener as necessary. In this embodiment, since the negative electrode mixture layer 20 contains the composite material 30, the conductivity between the active materials and the conductivity between the active material and the negative electrode current collector foil 10 can be ensured, and therefore a conductive material is not required. However, when a conductive material is used, for example, various carbon blacks, various graphites, carbon nanotubes, carbon nanohorns, etc. can be used.

The density of the negative electrode mixture layer 20 is, for example, 1.09 g/cm ³ to 1.31 g/cm ^3. The thickness of the negative electrode mixture layer 20 is, for example, 38 μm to 44 μm.

The proportion of the composite material 30 in the entire negative electrode mixture layer 20 is, for example, 70% by mass to 99% by mass, and preferably 90% by mass to 98% by mass, from the viewpoint of realizing high output characteristics and high energy density. The proportion of the additive in the entire negative electrode mixture layer 20 is, for example, 1% by mass to 30% by mass, and preferably 2% by mass to 10% by mass.

(Composite Material 30)
The composite material 30 has a form in which the surface of a carbon material 31 as an active material is covered with a metal 32 in an island shape. The carbon material 31 has electrical conductivity and can absorb and release lithium ions. As the carbon material 31, for example, a carbon material containing amorphous carbon at least in a surface layer such as graphite, hard carbon, soft carbon, activated carbon, etc., whose surface is coated with amorphous carbon, can be used. The carbon material 31 may contain crystalline carbon as a whole, but from the viewpoint of electrical conductivity and redox reactivity described later, it is preferable that the carbon material 31 has a surface layer composed of amorphous carbon that does not contain crystalline carbon. Note that amorphous carbon has many edge faces exposed on its surface, and has high lithium ion acceptance.

The shape of the carbon material 31 is, for example, a particulate form such as a sphere or a plate. The particle size of the carbon material 31 is preferably, for example, an average particle size of 5 μm to 15 μm, and more preferably 7 μm to 10 μm. Here, the average particle size is the average particle size of the primary particles of the carbon material 31, and can be measured using a SEM (Scanning Electron Microscope) or particle size distribution measurement.

The metal 32 that covers the surface of the carbon material 31 is produced by reduction of the metal oxide through an oxidation-reduction reaction between the metal oxide mixed with the carbon material 31 and the amorphous carbon present on the surface layer of the carbon material 31.

The metal oxide mixed with the carbon material 31 contains a metal capable of absorbing and releasing lithium ions and has reducing properties. The metal oxide can be selected based on the Ellingham diagram (e.g., FIG. 3) showing its reactivity with oxygen.

Examples of metal oxides include iron oxide ( _{Fe2O3 , Fe3O4} , FeO), tin oxide (SnO, _SnO2 ), titanium oxide ( _Ti2O3 , _Ti2O , _Ti3O5 , _{TiO2 )} , cobalt oxide (CoO, _Co3O4 ), manganese oxide ( _MnO2 , _Mn2O3 , MnO), copper oxide (CuO, _Cu2O ), nickel oxide (NiO ₎ , niobium oxide ( _Nb2O3 , _NbO2 , NbO _{) ,} vanadium oxide ( _{V2O5 , VO2} , _V2O3 , VO ₎ , chromium oxide ( _{CrO2 , Cr2O3} ), _and tungsten oxide (WO3 _{, WO2 )} .

The particle size of the metal oxide is smaller than that of the carbon material 31. The particle size of the metal oxide is preferably, for example, an average particle size of 10 nm to 100 nm, and more preferably 30 nm to 80 nm. Here, the average particle size is the average particle size of the primary particles of the metal oxide, and can be measured by SEM or particle size distribution measurement.

The metal 32 generated from the metal oxide is capable of absorbing and releasing lithium ions, and is, for example, Fe, Sn, Ti, Co, Mn, Cu, Ni, Nb, V, Cr, W, etc.

The coverage of the metal 32 on the surface of the carbon material 31 is preferably 0.1% to 0.5% by volume. With a coverage in this range, a composite material 30 can be formed that is capable of achieving both high electrical conductivity and high lithium ion acceptance. If the coverage of the metal 32 is less than 0.1% by volume, the electrical conductivity of the composite material 30 tends to decrease. If the coverage of the metal 32 exceeds 0.5% by volume, the lithium ion acceptance of the composite material 30 tends to decrease.

The coverage of the metal 32 can be adjusted, for example, by the amount of metal oxide added to the carbon material 31. The coverage of the metal 32 can be measured from a binarized image in a mapping image obtained by SEM image or Raman spectroscopy analysis.

In this way, the surface of the composite material 30 has a predetermined ratio of areas covered with the metal 32 and areas where the surface layer (amorphous carbon) of the carbon material 31 is exposed. The composite material 30 has a sea-island structure in which the highly conductive metal 32 is scattered like islands on the surface, and amorphous carbon is present like a sea on the surface.

With this structure, the metal 32 scattered on the surface of the composite material 30 ensures conductivity between the particles of the carbon material 31 and between the carbon material 31 and the negative electrode current collector foil 10, suppressing an increase in electrical resistance in the negative electrode material 1. In addition, because amorphous carbon is exposed on the surface of the composite material 30, a reaction area is secured for the electrochemical reaction that occurs at the interface between the carbon material 31 and the electrolyte. This suppresses an increase in interface resistance, enabling high-speed charging and discharging of the battery.

The overall shape of the composite material 30 is, for example, a particulate form such as a sphere or a plate. The particle size of the composite material 30 is preferably, for example, an average particle size of 5 μm to 15 μm, and more preferably 7 μm to 10 μm. Here, the average particle size is the average particle size of the primary particles of the composite material 30, and can be measured by SEM or particle size distribution measurement.

(Additive)
The binder is not particularly limited as long as it is a material that dissolves or disperses in the solvent described below. For example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyethylene oxide (PEO), styrene butadiene rubber (SBR), etc. can be used as the binder. For example, carboxymethyl cellulose (CMC) can be used as the thickener.

Next, the method for manufacturing the negative electrode will be described with reference to FIG. 2. FIG. 2 is a flow chart showing the method for manufacturing the negative electrode material for secondary batteries shown in FIG. 1. As shown in FIG. 2, the method for manufacturing the negative electrode according to this embodiment includes steps S1 to S3.

Step S1 is a mixing step in which a carbon material 31 containing amorphous carbon at least in the surface layer and a metal oxide are mixed to obtain a mixture. Step S2 is a firing step in which the mixture is fired in a non-oxidizing atmosphere at a heating temperature at which an oxidation-reduction reaction between the amorphous carbon and the metal oxide proceeds to form a composite material 30. Step S3 is a composite layer formation step in which a negative electrode composite layer 20 containing the composite material 30 is formed on the negative electrode current collector foil 10.

The details of each of the above steps will be specifically described as necessary in the case where natural graphite is used as the carbon material ₃₁ and _Fe2O3 is used as the metal oxide. Graphite-based materials such as natural _graphite are suitable for this embodiment because they have a large capacity per unit volume, a small volume expansion, and are low cost. Iron oxides such as _Fe2O3 are suitable for this embodiment because they have a large capacity per unit volume and are low cost.

Furthermore, the electrical resistivity of C (carbon) is 1.64×10 ⁻⁵ Ω·m, whereas Fe (iron) has an electrical resistivity of 1.00×10 ⁻⁷ Ω·m, and is therefore characterized by having a lower electrical resistivity than C.

First, the method of mixing the carbon material 31 and the metal oxide in the mixing step is not particularly limited as long as the metal oxide can be attached to the surface of the carbon material 31 and the surface of the carbon material 31 can be covered with the metal oxide at a predetermined ratio. For example, a mortar or a ball mill can be used as the mixing method.

The amount of metal oxide added when mixed with the carbon material 31 is preferably less than 1.0 mass% relative to the carbon material 31, and more preferably 0.1 mass% to 0.5 mass%. By adding an amount within this range, a coverage rate of the metal 32 can be achieved in the range of 0.1 volume% to 0.5 volume%.

The subsequent firing step is carried out in a non-oxidizing atmosphere that can suppress oxidation of the metal oxide by oxygen during the reaction. A non-oxidizing atmosphere is one in which the oxygen partial pressure is lower than the oxygen partial pressure of the atmosphere, and examples of such non-oxidizing atmospheres include an inert atmosphere, a vacuum, and a reducing atmosphere such as a hydrogen gas atmosphere. In particular, an inert atmosphere such as a helium gas atmosphere, an argon gas atmosphere, or a nitrogen gas atmosphere is preferable.

The heating temperature in the firing process can be determined based on the Ellingham diagram shown in FIG. 3. FIG. 3 is a graph showing an example of an Ellingham diagram. The Ellingham diagram is a diagram showing the relationship between the standard reaction Gibbs energy of the oxide of each element and temperature. The standard reaction Gibbs energy of the oxide of each element is the Gibbs free energy of formation ΔG of each element with respect to an oxygen molecule (1 mol). From the Ellingham diagram, it is possible to determine the heating temperature required to reduce the metal oxide to metal 32 by the action of amorphous carbon, which is a reducing agent.

For example, line L1 in Figure 3 shows the change in ΔG with temperature for an oxygen molecule of C (1 mol), and line L2 shows the change in ΔG with temperature for an oxygen molecule of Fe (1 mol).

According to the Ellingham diagram, the larger ΔG, the easier it is to reduce. In the firing process, C reduces _Fe2O3 to Fe under heating temperature conditions where the oxidation-reduction reactions shown in the following formulas (1) and (2) according to the Ellingham diagram occur and in a non _- oxidizing atmosphere. Note that atmospheric gas such as hydrogen may also act as a reducing agent that reduces _Fe2O3 . On the other hand, _Fe2O3 oxidizes C to _CO2 under heating temperature conditions where the oxidation-reduction reactions shown in the following formulas (1) and ( ₂ ) occur and in a _non -oxidizing atmosphere.
2/ _3Fe2O3 →4/3Fe+ _{O2 ...} Formula (1)
C + O ₂ → CO ₂ ... formula (2)

The heating temperature during firing at which the oxidation-reduction reactions shown in formulas (1) and (2) occur is within a range that satisfies ΔG of Fe to oxygen molecules (1 mol) > ΔG of C to oxygen molecules (1 mol). According to the Ellingham diagram, in order to reduce Fe ₂ O ₃ to Fe by C, the heating temperature is preferably in the range of 1100 K to 1300 K. The heating time in the firing process is set appropriately.

The method for forming the negative electrode composite layer 20 in the subsequent composite layer formation process is not particularly limited, but for example, a negative electrode paste containing at least the composite material 30 obtained in the firing process and a solvent is prepared. The negative electrode paste is obtained by adding a solvent capable of uniformly dissolving or dispersing the binder to a powder containing the composite material 30 and, if necessary, additives such as a binder and a thickener, and kneading them. The prepared negative electrode paste is then applied to at least one surface of the negative electrode current collector foil 10 and dried. In this way, a negative electrode material 1 in which the negative electrode composite layer 20 is formed on the negative electrode current collector foil 10 can be manufactured.

The present invention will be specifically explained below with reference to examples. Note that the present invention is not limited to these examples.

(Example)
[Preparation of negative electrode]
In forming the composite material 30, in the mixing step, natural graphite (5 g) whose surface was coated with amorphous carbon and Fe ₂ O ₃ powder (0.025 g) were weighed, and the natural graphite and Fe ₂ O ₃ powder were mixed in a mortar for 10 minutes to obtain a mixture. That is, in the example, the mixing step was performed at a ratio corresponding to an Fe ₂ O ₃ addition amount of 0.5 mass% relative to the natural graphite. The average particle size of the natural graphite was 10 μm, and the average particle size of the Fe ₂ O ₃ powder was 50 nm. Next, in the firing step, the mixture was fired in a helium gas atmosphere under the conditions of a heating temperature of 1100 K and a heating time of 3.5 hours. This formed the composite material 30.

Next, the composite material 30, CMC, and SBR were weighed out so that the mass ratio was 98:1:1, and a required amount of solvent (ion-exchanged water) was added to these electrode materials and kneaded to prepare a negative electrode paste. The negative electrode paste was then applied to one side of a copper foil having a thickness of 10 μm as a negative electrode current collector 10, and then dried to produce a negative electrode material 1. The amount of the negative electrode paste applied was 3.9 mg/ ^cm2 .

Furthermore, negative electrode material 1 was punched out using a punching machine to have a diameter of 15 mm, and then pressed using a pressing machine so that negative electrode mixture layer 20 had a density of 1.17 g/cm ^3. The thickness of negative electrode mixture layer 20 after pressing was 42 μm.

[Preparation of electrolyte solution]
An electrolyte solution was prepared by dissolving a supporting electrolyte LiPF ₆ at a concentration of 1 mol/L in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1:1:1.

[Construction of evaluation battery cell]
A laminate obtained by sandwiching a polypropylene separator between the negative electrode composite layer 20 side of the pressed negative electrode material 1 and a lithium metal foil (lithium metal counter electrode) punched to 18 mmφ was placed in a battery case, and an electrolyte was added and the case was sealed to prepare a battery cell (half cell) for evaluation.

Next, a description will be given of the evaluation battery cells of the Comparative Example and Reference Examples 1 to 3. In the evaluation battery cells of the Comparative Example and Reference Examples 1 to 3, Fe ₂ O ₃ powder was mixed with natural graphite in various ratios when the negative electrode material was manufactured.

Comparative Example
Except for the fact that in the mixing step, _{no Fe2O3 powder} was added, and only natural graphite (5 g) was mixed in a mortar, and the firing step was omitted, an evaluation battery cell was produced in the same manner as in Example 1. That is, in the comparative example, the mixing step was performed at a ratio equivalent to an _Fe2O3 addition amount of _0.0 mass % relative to the natural graphite.

(Reference Example 1)
Except for mixing natural graphite (5 g) and _Fe2O3 powder (0.05 _g ) in the mixing step, an evaluation battery cell was produced in the same manner as in Example 1. That is, in Reference Example 1, the mixing step was carried out at a ratio corresponding to an _Fe2O3 addition amount of _1.0 mass% relative to the natural graphite.

(Reference Example 2)
Except for mixing natural graphite (5 g) and Fe ₂ O ₃ powder (0.25 g) in the mixing step, an evaluation battery cell was produced in the same manner as in Example 1. That is, in Reference Example 1, the mixing step was carried out at a ratio equivalent to 5.0 mass% Fe ₂ O ₃ added to natural graphite.

(Reference Example 3)
Except for mixing natural graphite (5 g) and _Fe2O3 powder (0.5 _g ) in the mixing step, an evaluation battery cell was produced in the same manner as in Example 1. That is, in Reference Example 1, the mixing step was carried out at a ratio corresponding to an _Fe2O3 addition amount of _10.0 mass% relative to the natural graphite.

The results of measuring the IV resistance of each of the evaluation battery cells of the above-mentioned Examples, Comparative Examples, and Reference Examples 1 to 3 are shown in FIG. 4. FIG. 4 is a graph showing the IV resistance of the evaluation battery cells. The horizontal axis of FIG. 4 shows the amount (mass%) of Fe ₂ O ₃ powder added. The vertical axis of FIG. 4 shows the IV resistance of each evaluation battery cell as a relative value, with the resistance value of the evaluation battery cell of the Comparative Example (0.0 mass% Fe ₂ O ₃ added) being taken as 100%.

In measuring the IV resistance, first, in an environment of 25°C, each evaluation battery cell (Example, Comparative Example, Reference Examples 1 to 3) was adjusted so that the SOC (State of Charge) was 50%. Next, the IV resistance was calculated as the voltage drop after constant current discharge for 10 seconds at a rate of 1C divided by the discharge current value. It is considered that the smaller the relative value of the IV resistance, the lower the internal resistance of the battery.

4, when the IV resistance of the evaluation battery cell of the comparative example (0.0 mass% _Fe2O3 added) was taken as ₁₀₀ %, the IV resistance of the evaluation battery cell of the embodiment (0.5 mass% _Fe2O3 added) was 98%. Similarly, the IV resistance of the evaluation battery cell of Reference Example 1 (1.0 mass% _{Fe2O3 added} ) was 101%, the IV resistance of the evaluation battery cell of Reference Example 2 (5.0 mass% _{Fe2O3 added} ) was 104%, and the IV resistance of the evaluation battery cell _of Reference Example 3 (10.0 mass% _{Fe2O3 added} ) was 108%.

From these results, it was found that the amount of Fe ₂ O ₃ powder added required to reduce the internal resistance of the battery is preferably less than 1.0 mass % relative to the natural graphite, and more preferably 0.1 mass % to 0.5 mass %.

On the other hand, when the amount of _{Fe2O3 powder} added is _1.0 mass% or more, it is considered that _Fe2O3 is excessive with respect to the amorphous carbon forming the surface layer of natural graphite. When _Fe2O3 powder is present in excess, the _amorphous carbon is consumed by the oxidation-reduction reaction _{with Fe2O3} , and as a result, no _{more Fe2O3} can be reduced, and _Fe2O3 with low conductivity remains in the negative electrode mixture layer ₂₀ , which is presumably the reason for the increase in the internal resistance of the battery.

As described above, the negative electrode material 1 according to this embodiment has a negative electrode current collector foil 10 and a negative electrode composite layer 20 formed on the negative electrode current collector foil 10 and containing a composite material 30. The composite material 30 has a sea-island structure in which the surface of a carbon material 31 containing amorphous carbon at least in the surface layer is covered in islands with a metal 32 generated from a metal oxide.

Here, for example, it is difficult to ensure good conductivity in the negative electrode composite layer by simply mixing and attaching a metal oxide with low conductivity to the carbon material, so it is necessary to mix in a conductive material. When a conductive material is used, the total amount of carbon material as an active material is relatively reduced, which causes a problem of a decrease in the energy density of the negative electrode in the secondary battery.

For example, even if a metal is mixed and attached to a carbon material instead of a metal oxide, the metals will also aggregate, making it difficult to ensure good electrical conductivity in the negative electrode composite layer.

On the other hand, in the negative electrode material 1 according to this embodiment, the metal 32, which has a lower electrical resistance than carbon, is attached to the surface of the carbon material 31 to form the composite material 30, and as electrons pass through the metal 32, an increase in electrical resistance in the negative electrode material 1 can be suppressed without using a conductive material. When no conductive material is used in the negative electrode material 1, the total amount of the carbon material 31 as the active material can be relatively increased, thereby achieving a high energy density.

In addition, since the metal 32 exists in the form of islands on the surface of the carbon material 31, the amorphous carbon is exposed on the surface of the composite material 30, and the reaction area of the composite material 30 can be secured. As a result, the internal resistance of the secondary battery using the negative electrode material 1 is reduced, and the output characteristics are improved.

Furthermore, in this embodiment, the metal 32 is coated on the surface of the carbon material 31 at a coverage rate of 0.1 volume % to 0.5 volume %, thereby ensuring a sufficient reaction area for the composite material 30. With this configuration, the internal resistance of the secondary battery using the negative electrode material 1 is reduced, and the output characteristics are improved.

Furthermore, in this embodiment, by using Fe ₂ O ₃ as the metal oxide, the negative electrode material 1 that enables a secondary battery with a high capacity can be produced at low cost.

The method for producing the above-mentioned negative electrode material 1 includes a mixing step of mixing a carbon material 31 containing amorphous carbon at least in the surface layer with a metal oxide to obtain a mixture, a firing step of firing the mixture at a heating temperature at which an oxidation-reduction reaction between the amorphous carbon and the metal oxide proceeds in a non-oxidizing atmosphere to form a composite material 30, and a composite layer formation step of forming a negative electrode composite layer 20 containing the composite material 30 on the negative electrode current collector foil 10.

In the manufacturing method of the negative electrode material 1 according to this embodiment, a composite material 30 is formed in which a metal 32 having a lower electrical resistance than carbon is attached to the surface of a carbon material 31, and electrons pass through the metal 32, making it possible to suppress an increase in electrical resistance in the negative electrode material 1 without using a conductive material. When no conductive material is used in the negative electrode material 1, the total amount of the carbon material 31 as an active material can be relatively increased, thereby achieving a high energy density.

The metal 32 that covers the carbon material 31 has had its surface oxide film removed by heating in the firing process, so it has a higher effect of improving electrical conductivity than a metal that has not been fired and has an oxide film formed on it.

Furthermore, in this embodiment, the metal 32 is coated on the surface of the carbon material 31 at a coverage rate of 0.1 volume % to 0.5 volume %, thereby ensuring a sufficient reaction area for the composite material 30. With this configuration, the internal resistance of the secondary battery using the negative electrode material 1 is reduced, and the output characteristics are improved.

Furthermore, in this embodiment, by using Fe ₂ O ₃ as the metal oxide, the negative electrode material 1 that enables a secondary battery with a high capacity can be produced at low cost.

Furthermore, in the mixing step in this embodiment, it is preferable to mix less than 1.0 mass% of metal oxide with the carbon material 31. This makes it possible to form a composite material 30 in which the metal 32 is coated on the carbon material 31 at a coverage rate of 0.5 volume% or less.

Furthermore, in the mixing step of this embodiment, it is particularly preferable to mix 0.1% by mass to 0.5% by mass of metal oxide with the carbon material 31. This makes it possible to form a composite material 30 in which the metal 32 is coated with the carbon material 31 at a coverage rate of 0.1% by volume to 0.5% by volume.

Furthermore, in the firing step of this embodiment, it is preferable to fire the mixture at a heating temperature of 1100K to 1300K in an inert atmosphere. This configuration further enhances the effect of suppressing the oxidation of the metal oxide by oxygen during the reaction, and ensures that the oxidation-reduction reaction between the amorphous carbon and the metal oxide proceeds. As a result, the metal 32 can be produced efficiently.

The above-mentioned negative electrode material 1 can be suitably used for, for example, large batteries that require high energy density, and secondary batteries in the form of assembled batteries. In particular, it can be used as a negative electrode material for secondary batteries that serve as power sources for vehicles that require high output, such as electric vehicles, hybrid vehicles, and plug-in hybrid vehicles.

1 Negative electrode material 10 Negative electrode current collector foil 20 Negative electrode mixture layer 30 Composite material 31 Carbon material 32 Metal

Claims (6)

a mixing step of mixing a carbon material containing amorphous carbon at least in a surface layer with a metal oxide to obtain a mixture;
a calcination step of calcining the mixture in a non-oxidizing atmosphere at a heating temperature at which an oxidation-reduction reaction between the amorphous carbon and the metal oxide proceeds to form a composite material;
a composite layer forming step of forming a negative electrode composite layer containing the composite material on a negative electrode current collector foil;
having
The composite material comprises:
The method for producing a negative electrode material for a secondary battery has a sea-island structure in which the surface of the carbon material is covered in islands with the metal generated from the metal oxide . 2. The method for producing a negative electrode material for a secondary battery according to claim 1 , wherein the coverage of the surface of the carbon material with the metal generated from the metal oxide is 0.1% by volume to 0.5% by volume. The method for producing a negative electrode material for a secondary battery according to claim 1 or ₂ , wherein the metal oxide is _Fe2O3 . In the mixing step,
The method for producing a negative electrode material for a secondary battery according to claim 3 , wherein the metal oxide is mixed in an amount of less than 1.0 mass % relative to the carbon material. In the mixing step,
The method for producing a negative electrode material for a secondary battery according to claim 3 or 4 , wherein the metal oxide is mixed in an amount of 0.1% by mass to 0.5% by mass with respect to the carbon material. In the firing step,
The method for producing a negative electrode material for a secondary battery according to any one of claims 3 to 5 , wherein the mixture is fired at a heating temperature of 1100K to 1300K in an inert atmosphere.

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