JP4625926B2 - ELECTRODE MATERIAL FOR LITHIUM ION SECONDARY BATTERY, ITS MANUFACTURING METHOD, AND SECONDARY BATTERY - Google Patents

Description

  The present invention relates to an electrode material for a high capacity and high power lithium ion secondary battery using nanocrystals such as core-shell type Ni / NiO, a manufacturing method thereof, and a secondary battery.

  The fabrication process of nanostructured materials has progressed through bottom-up processes such as CVD and chemical solution deposition (CBD). The high surface area and high porosity of the materials produced by these methods have resulted in a smooth increase in surface reaction efficiency and electrified transport, and the characteristics of electrochemical devices can be greatly improved.

In general, a typical CBD method using water as a solvent does not require the use of a special device called a vacuum system, and can produce metal oxides and the like at a low temperature. ing.
Furthermore, a film can be uniformly deposited on a substrate having a complicated form in a solution by the CBD method. However, it is difficult to produce a nanoparticle and mesoporous metal oxide film by a general CBD method in which a metal oxide is directly deposited. This is because the metal oxide easily grows in the solution according to the growth mode of the single crystal.

The present inventors have reported a self-template method in which a metal hydroxide, not a metal oxide, is deposited and converted into a metal oxide by heat treatment.
Metal hydroxide films can also be deposited on complex shaped substrates by heterogeneous nucleation in solution, which can be easily converted into nanoparticles and mesoporous metal oxides by pyrolysis, and The metal hydroxide nanostructure is maintained as a self-template without destruction.

  In recent years, Li storage devices for electric vehicles have been studied by many researchers for solving energy and environmental problems. For application to electric vehicles, it is necessary to develop an electrode having a high capacity (high energy density) and a high current density (high power density) at the same time.

Poizot et al. Reported that a transition metal oxide nanomaterial such as NiO has characteristics as a negative electrode material for a Li-ion secondary battery having a high energy density of 700 mAh / g (see Non-Patent Document 1). .
Subsequent to this, many researchers have been studying negative electrode materials using transition metal oxides, but there have been no reports of transition metal oxide electrodes having both high capacity and high output. It has been pointed out that there are four problems that need to be solved. 1) Reduction of particle size to reduce the diffusion length of Li in the active material. 2) Increase of specific surface area to reduce unit surface area current density in rapid charge / discharge process, 3) Improvement of cycle characteristics in rapid charge / discharge process. 4) Improvement of the electron conductivity of the electrode material.
P.Poizot and 4 other authors “Nano-sized transition-metal oxides asnegative-electrode materials for lithium-ion batteries” Nature, 2000, 407,496-499

  The present invention solves the above problems, that is, 1) reduction of the particle diameter to reduce the diffusion length of Li in the active material, and 2) reduction of the unit surface area current density in the rapid charge / discharge process. 3) Improvement of cycle characteristics during rapid charge / discharge process 4) Electrode material for lithium ion secondary battery, method for producing the same, and secondary battery capable of improving electronic conductivity of electrode material I will provide a.

In view of the above problems, the present invention provides:
1) An electrode material for a lithium ion secondary battery characterized by having a transition metal oxide coating film made of mesoporous nanocrystals on a nickel mesh. 2) The nickel mesh has 50 to 400 mesh. 3. The lithium ion secondary battery electrode material as described in 1 above, wherein the gap between the nickel meshes has a function as a diffusion path for electrolyte and ions. 4. Electrode material for battery 4) Electrode material for lithium ion secondary battery according to any one of 1 to 3 above, characterized in that it is composed of 2 to 50 nm nanoparticles and 2 to 50 nm mesopores. 5. The electrode material for a lithium ion secondary battery as described in any one of 1 to 4 above, which has a BET specific surface area of 300 m ² / g.

The present invention also provides:
6) A mesoporous nanocrystalline transition metal oxide film is formed on the nickel mesh by directly depositing a transition metal hydroxide on a 50-400 mesh nickel mesh and thermally decomposing it. Method for producing electrode material for lithium ion secondary battery 7) A nickel hydroxide is directly deposited on a 50-400 mesh nickel mesh and thermally decomposed to form a mesoporous nanocrystalline nickel oxide coating on the nickel mesh. 8) Ni (CH ₃ COO) ₂ · 4H ₂ O is dissolved in a water / methanol mixed solution, and a nickel mesh is immersed in the solution. Forming a layered nickel hydroxide acetate as a template film on the nickel mesh and further heat-treating the layered nickel hydroxide acetate. A method for producing an electrode material for a lithium ion secondary battery, characterized in that a nanocrystalline nickel oxide coating is formed on the nickel mesh. 9) Cobalt hydroxide is directly deposited on a 50 to 400 mesh nickel mesh. A mesoporous nanocrystalline cobalt oxide film is formed on the nickel mesh by thermal decomposition of the electrode material. 10) Co (CH ₃ COO) ₂ · 4H ₂ O Is dissolved in a water / methanol mixed solution, and nickel mesh is immersed in the solution to form a layered cobalt hydroxide acetate salt as a template film on the nickel mesh. An electrode material for a lithium ion secondary battery, characterized in that a cobalt coating is formed on the nickel mesh. Production Method 11) Iron (III) hydroxide or iron (III) oxide hydroxide (FeOOH) is directly deposited on a 50-400 mesh nickel mesh and thermally decomposed to obtain mesoporous nanocrystalline iron oxide. (III) A method for producing an electrode material for a lithium ion secondary battery, characterized in that a film is formed on the nickel mesh 12) Fe (NO3) 3 · 9H2O is mixed with water / methanol mixed solution with urea added The nickel mesh is immersed in this, and iron (III) hydroxide or iron (III) oxide hydroxide (FeOOH) as a template film is formed on the nickel mesh. A porous nanocrystalline iron oxide (Fe ₂ O ₃ ) is formed on the nickel mesh by further heat treatment at 300 ° C. for 10 minutes, and an electrode material for a lithium ion secondary battery, 13) A secondary battery using the electrode material according to any one of 1) to 5) above, wherein the charge or discharge energy density is 10A / g. Alternatively, in the secondary battery using the electrode material described in any one of 1) to 5) above, the secondary battery 14) is maintained at 50% or more when the discharge rate is 0.1 A / g. The secondary battery is characterized in that the energy density of charging or discharging when the charging or discharging rate is 10 A / g is maintained at 70% or more when the charging or discharging rate is 0.1 A / g. provide.

  The electrode material for a lithium ion secondary battery of the present invention reduces the particle size for reducing the diffusion length of Li in the active material by forming a mesoporous nanocrystalline nickel oxide film on the nickel mesh, It has an excellent effect that the unit area current density in the rapid charge / discharge process can be reduced, the cycle characteristics in the rapid charge / discharge process can be improved, and the electron conductivity of the electrode material can be improved.

  The features of the present invention will be specifically described below with reference to examples and drawings. In addition, the following description is for making an understanding of this invention easy, and is not restrict | limited to these. That is, it should be understood that all modifications, embodiments, and other examples based on the technical idea of the present invention are included in the present invention.

The negative electrode material for lithium ion secondary batteries is extremely useful as an electrode material for Li ion secondary batteries having both high capacity and high output density. Basically, a transition metal oxide film consisting of mesoporous nanocrystals is formed on a nickel mesh. Formed. Specific examples of the coating include a nickel oxide coating, a cobalt oxide coating, and an iron oxide coating.
As a result, the above problems are solved, that is, the particle size for reducing the diffusion length of Li in the active material is decreased, the unit area current density in the rapid charge / discharge process is decreased, and the rapid charge / discharge process is reduced. Has succeeded in improving the cycle characteristics of the electrode material, and also obtaining remarkable characteristics that the electron conductivity of the electrode material can be improved.

The nickel mesh preferably has a size of 50 to 400 mesh (mesh). However, it should be understood that other meshes can be used and that other sizes are not excluded. Usually, a mesh produced by knitting micrometer-sized nickel wire is used. However, if it is a mesh shape, there is no restriction | limiting in particular in the manufacturing method of a nickel mesh.
A nickel mesh produced by knitting a nickel wire has a mesh-like gap, and this gap functions as an electrolyte and ion diffusion path. This is useful for improving the function as a negative electrode material for a lithium ion secondary battery. The 50-400 mesh (mesh) size is a suitable numerical value for improving this function.
Nickel wire has low resistance and serves as an electronic transmission path. The nickel oxide film surface having a high surface area reduces the diffusion length and unit area current density in the active material.

Furthermore, the nickel oxide film having a mesoporous nanocrystal structure of the present invention, and the transition metal oxide film such as cobalt oxide and iron oxide are characterized by being composed of 2 to 50 nm nanoparticles and 2 to 50 nm mesopores. It is. This enlarges the surface area and greatly affects the characteristics as an electrode material for a lithium ion secondary battery.
This has a BET specific surface area of about 20-300 m ² / g. It should be understood that this number is a preferred example of a transition metal oxide coating made of mesoporous nanocrystals formed according to the present invention and does not exclude surface areas other than this surface area.

The method for producing an electrode material for a lithium ion secondary battery according to the present invention comprises, for example, directly depositing nickel hydroxide on a nickel mesh of 50 to 400 mesh and thermally decomposing it to thereby form a mesoporous nanocrystalline nickel oxide coating. Is formed on the nickel mesh, which is one of the major features of the present invention.
Furthermore, in the case of nickel oxide, Ni (CH ₃ COO) ₂ · 4H ₂ O is dissolved in a water / methanol mixed solution, nickel mesh is immersed in this, and the layered nickel hydroxide acetate that is the template film This is further subjected to heat treatment to form a mesoporous nanocrystalline oxide film on the nickel mesh.
As a result, a mesoporous nanocrystalline nickel oxide coating can be easily obtained on the nickel mesh, and many of the characteristics described above can be obtained.
In the case of cobalt oxide and iron oxide, a mesoporous nanocrystalline oxide film is similarly formed on the nickel mesh. Incidentally, a NiO and Co ₃ O ₄ anode material, Fe ₂ O ₃ from it is a cathode material, in any of the positive and negative electrodes of a lithium secondary battery, it can be seen that can be applied the method of the present invention.

  Hereafter, it demonstrates concretely using the Example of this invention. The following description is intended to facilitate understanding of the present invention and is not limited thereto. That is, it should be understood that all modifications, embodiments, and other examples based on the technical idea of the present invention are included in the present invention.

Example 1
As a specific example, Ni (CH ₃ COO) ₂ .4H ₂ O was dissolved in a water / methanol mixed solution. Folded Ni mesh (200 mesh) was immersed in this solution, sealed and kept at 60 ° C. for 12 hours to obtain a layered nickel hydroxide acetate salt as a template film. This was further heat-treated at 300 ° C. for 10 minutes to obtain porous nanocrystalline nickel oxide (NiO).
Electrochemical evaluation was performed using the electrode thus prepared as a negative electrode, using metallic Li for the counter and reference electrodes, and an EC / DEC mixed solvent containing 1M LiClO ₄ as the electrolyte. .

FIG. 1a shows an SEM image of a NiO film produced on a nickel mesh. The low magnification image shows that all of the nickel mesh is covered with the NiO film. It can be said that this core-shell Ni / NiO morphology was obtained by the CBD method utilizing heterogeneous nucleation in solution.
In general, a thin-film Li battery electrode material tends to have high characteristics due to its low resistance and short diffusion length. However, considering industrial practical use, the thin film material has a problem that the thin film exists only on one surface of the flat substrate and the amount of the surface active material is small.

In this core shell form, all surfaces such as mesh can be coated, that is, all the three-dimensional high surface area supports can be covered with the active material, so the amount of active material increases and the problem is solved. it can. Furthermore, the gap in the nickel mesh behaves as a diffusion path for the electrolyte and Li ions.
Nickel wire is low resistance and acts as an electronic mission path. It can be seen from the high-magnification SEM image (b in FIG. 1) and TEM image (c in FIG. 1) that the NiO film is composed of nanosheets.

Furthermore, each sheet is composed of about 5 nm nanoparticles and mesopores. The presence of mesopores could also be confirmed by measurement by nitrogen adsorption, the pore size dispersion was 1 to 15 nm, and the peak position was about 3 nm.
These mesopores behave as diffusion paths for electrolytes and Li ions. Furthermore, a NiO film surface having a high surface area (BET surface area of 215 m ² / g) has an effect of reducing the diffusion length and current density in the active material.

Cyclic voltammetry is shown in Figure 2a for the electrochemical properties of the NiO electrode. All curves show a 0.9V cathode peak and a 2.3V anode peak. These are due to the electrochemical redox of NiO and Li. The reaction mechanism can be considered as follows.
NiO + 2Li ⁺ + 2e ^- ⇔ Li ₂ O + Ni
2Li ⇔ 2Li ^{+ +} 2e ^-
NiO + 2Li ⇔ Li ₂ O + Ni
The cathode peak of 0.7V in the first cycle is an irreversible peak due to SEI. In addition, peaks due to surface redox species are also observed in the second to twentieth cycles.

Next, Li storage characteristics will be described. FIG. 2b shows the charge / discharge characteristics of 0.1 A / g, 1 A / g, 5 A / g and 10 A / g. All show the same high capacity of 900, 860, 750, and 695 mAh / g. In particular, even at a high output density of 10 A / g, it shows almost the same characteristics as 714 mAh / g, which is a theoretical capacity of 695 mAh / g.
The reason why the capacity from 0.1 A / g to 5 A / g exceeds the theoretical capacity is thought to be due to the capacity of surface reactive species found in CV.
As described above, when the electrode material of the present invention is used as an electrode of a secondary battery, the charge (or discharge) rate is 10 times (1.0 A / g) from 0.1 A / g, and further 100 times (10 A / g). Even if it raises to g), it has the outstanding effect that the energy density of charge (or discharge) can obtain the secondary battery which keeps 50 to 70% or more at the rate of 0.1 A / g.

2c and 2d show the discharge capacity and the Coulomb efficiency with respect to the number of cycles. As shown in FIGS. 2c and 2d, the cycle characteristics are constant up to a high power density of 10 A / g, indicating high cycle characteristics. The coulomb efficiency is also a high value of 95% or more.
In general, since the volume change is accompanied by the reaction of lithium insertion / desorption, the capacity is reduced by repeating the charge / discharge process. Since this electrode is a nanoparticle and a mesoporous material produced by a self-template method, it can be said that volume change can be mitigated, and as a result, high cycle characteristics were exhibited.

  FIG. 2e shows the relationship between capacitance and current density. It can be seen that up to 20 A / g, a high capacity almost consistent with the capacity of 0.1 A / g is maintained. Even at a very high current density of 50 A / g, 195 mA / g, which is typically used, shows a capacity that is half the theoretical capacity of graphite carbon (372 mA / g) as the negative electrode material. The usefulness of type Ni / NiO electrodes can be seen.

(Example 2)
As a specific example (an example of cobalt oxide), Co (CH ₃ COO) ₂ .4H ₂ O was dissolved in a water / methanol mixed solution. Folded Ni mesh (200 mesh) was immersed in this solution, sealed and kept at 60 ° C. for 12 hours to obtain a layered cobalt hydroxide acetate salt as a template film. This was further heat-treated at 300 ° C. for 10 minutes to obtain porous nanocrystalline cobalt oxide (Co ₃ O ₄ ).
Electrochemical evaluation was performed using the electrode thus prepared as a negative electrode, using metallic Li for the counter and reference electrodes, and an EC / DEC mixed solvent containing 1M LiClO ₄ as the electrolyte. .

FIG. 3 a shows an SEM image of the Co ₃ O ₄ film produced on the nickel mesh. The low magnification image shows that all of the nickel mesh is covered with the Co ₃ O ₄ film. It can be seen from the high-magnification SEM image (b in FIG. 3) that the Co ₃ O ₄ film is composed of nanosheets. The mesoporous nanocrystalline Co ₃ O ₄ coating produced on this nickel mesh can also obtain the same effect as the nickel oxide coating shown in Example 1.
FIG. 4 shows cycle characteristics at a relatively large current density of 1 A / g of a lithium battery using a nickel-mesh core-shell electrode using a Co ₃ O ₄ film. As shown in FIG. 4, it has a high capacity and good cycle characteristics.

(Example 3)
As a specific example (an example of iron oxide), Fe (NO3) 3 · 9H2O was dissolved in a water / methanol mixed solution to which urea was added. Folded Ni mesh (200mesh) was immersed in this solution, sealed and kept at 60 ° C for 24 hours to obtain a template film. This was further heat-treated at 300 ° C. for 10 minutes to obtain porous nanocrystalline iron oxide (Fe ₂ O ₃ ).
Electrochemical evaluation was performed using the electrode thus prepared as a positive electrode, metallic Li as the counter electrode and reference electrode, and a mixed solvent of PC containing 1M LiClO ₄ as the electrolyte.

FIG. 5 a shows an SEM image of the Fe ₂ O ₃ film produced on the nickel mesh. The low magnification image shows that all of the nickel mesh is covered with the Fe ₂ O ₃ film. It can be seen from the high-magnification SEM image (b in FIG. 5) that the Fe ₂ O ₃ film is composed of nanosheets. The mesoporous nanocrystalline Fe ₂ O ₃ coating produced on this nickel mesh can also achieve the same effect as the nickel oxide coating shown in Example 1.
FIG. 6 shows cycle characteristics at a relatively large current density of 1 A / g of a lithium battery using a nickel-mesh core-shell electrode using an Fe ₂ O ₃ film. As shown in FIG. 6, it has high capacity and good cycle characteristics. Fe ₂ O ₃ is a positive electrode material. Therefore, it can be used also as a positive electrode material of a lithium ion secondary battery.

  Transition metal oxide coatings composed of mesoporous nanocrystals formed on a nickel mesh reduce the particle size to reduce the diffusion length of Li in the active material and reduce the current density during rapid charge and discharge processes It has an excellent effect of improving the cycle characteristics in the rapid charge / discharge process and further improving the electron conductivity of the electrode material, and is extremely useful as an electrode material for lithium ion secondary batteries. .

1A and 1B are SEM images of Ni / NiO core-shell electrodes, and c in FIG. 1 is a TEM image of Ni / NiO core-shell electrodes. It is a figure which shows the CV characteristic of the Li ion secondary battery using the electrode of Ni / NiO. It is a figure which shows the charging / discharging characteristic of the Li ion secondary battery using the electrode of Ni / NiO. It is a figure which shows the cycling characteristics of Li ion secondary battery charging / discharging using the electrode of Ni / NiO. It is a figure which shows the Coulomb efficiency characteristic of the Li ion secondary battery using the electrode of Ni / NiO. It is a figure which shows the relationship between the current density of a Li ion secondary battery using the electrode of Ni / NiO, and a capacity | capacitance. a and b are SEM images of Ni / Co ₃ O ₄ core-shell electrodes. It is a diagram showing the cycle characteristics of the lithium secondary battery charging and discharging using an electrode of Ni / Co ₃ O _4. a and b are SEM images of Ni / Fe ₂ O ₃ core-shell electrodes. It is a diagram showing the cycle characteristics of the lithium secondary battery charging and discharging using an electrode of Ni / Fe ₂ O _3.

Claims (13)

On a nickel mesh, nickel oxide, selected from the group consisting of cobalt oxide and iron oxide, a transition metal oxide composed of mesoporous nanocrystalline film is formed, as a diffusion path of the electrolyte solution and ions in the gap nickel mesh An electrode for a lithium ion secondary battery characterized by having a function .   The electrode for a lithium ion secondary battery according to claim 1, wherein the nickel mesh has 50 to 400 mesh. Lithium ion secondary battery electrode according to claim 1 or 2, characterized in that it consists mesopores of nanoparticles and 2 to 50 nm of 2 to 50 nm. The electrode for a lithium ion secondary battery according to any one of claims 1 to 3 , comprising a BET specific surface area of 20 to 300 m ² / g. On the nickel mesh, a transition metal oxide film made of mesoporous nanocrystals selected from the group consisting of nickel oxide, cobalt oxide and iron oxide is formed, and the electrolyte mesh and ion diffusion paths are formed in the gaps of the nickel mesh. A method for producing an electrode for a lithium ion secondary battery characterized by having a function,
50 to 400 mesh directly on a nickel mesh, to precipitate a hydroxide of the transition metal, by this thermal decomposition, and characterized by forming a mesoporous nanocrystalline transition metal oxide coating on the nickel mesh A method for manufacturing an electrode for a lithium ion secondary battery. 6. The mesoporous nanocrystalline nickel oxide film is formed on the nickel mesh by directly depositing nickel hydroxide on a 50-400 mesh nickel mesh and thermally decomposing it. The manufacturing method of the electrode for lithium ion secondary batteries of description . Ni (CH ₃ COO) ₂ · 4H ₂ O is dissolved in a mixed solution of water / methanol, and a nickel mesh is immersed in the nickel mesh to form a layered nickel hydroxide acetate as a template film on the nickel mesh, 6. The method for producing an electrode for a lithium ion secondary battery according to claim 5, wherein a mesoporous nanocrystalline nickel oxide coating is further formed on the nickel mesh by heat-treating the same. The mesoporous nanocrystalline cobalt oxide film is formed on the nickel mesh by directly depositing a cobalt hydroxide on a 50-400 mesh nickel mesh and thermally decomposing it. The manufacturing method of the electrode for lithium ion secondary batteries of description . Co (CH ₃ COO) ₂ · 4H ₂ O is dissolved in a mixed solution of water / methanol, and a nickel mesh is immersed therein to form a layered cobalt hydroxide acetate salt as a template film on the nickel mesh. The method for producing an electrode for a lithium ion secondary battery according to claim 5, wherein a mesoporous nanocrystalline cobalt oxide film is further formed on the nickel mesh by heat treatment. A mesoporous nanocrystalline iron (III) oxide film is formed by directly depositing iron (III) hydroxide or iron (III) oxide hydroxide (FeOOH) on a 50-400 mesh nickel mesh and thermally decomposing it. The method for manufacturing an electrode for a lithium ion secondary battery according to claim 5, wherein the electrode is formed on the nickel mesh. Fe (NO ₃ ) ₃ · 9H ₂ O is dissolved in a water / methanol mixed solution to which urea is added, and a nickel mesh is immersed in this, and the iron (III) hydroxide as a template film is formed on the nickel mesh. Alternatively, iron (III) oxide hydroxide (FeOOH) is formed, and this is further subjected to heat treatment at 300 ° C. for 10 minutes, whereby porous nanocrystalline iron oxide (Fe ₂ O ₃ ) is deposited on the nickel mesh. The method for producing an electrode for a lithium ion secondary battery according to claim 5, wherein the electrode is formed. A secondary battery using the electrode according to any one of claims 1 to 4 , wherein the charge or discharge energy density when the charge or discharge rate is 10 A / g is 0.1 A or less. The secondary battery is characterized by maintaining at least 50% of the amount of / g. The secondary battery using the electrode according to any one of claims 1 to 4 , wherein the charge or discharge energy density when the charge or discharge rate is 10 A / g, the charge or discharge rate is 0.1 A / g. The secondary battery is characterized by maintaining at least 70% of the above.