JP2002334697A - Non-aqueous secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-aqueous secondary battery that has a large energy density and high capacity and is superior in cycle characteristics. SOLUTION: This is a non-aqueous secondary battery that has a negative electrode having a carbon nano tube 1 and an element 3 capable of forming an alloy with lithium is arranged in the hollow section 2 of the carbon nano tube 1. It is desirable that the element 3 capable of forming an alloy with lithium is at least one kind of element selected from a group of zinc, cadmium, aluminum, gallium, indium, thallium, silicon, germanium, tin, lead, antimony, and bismuth. And it is desirable that the ratio of carbon nano tube to the total negative electrode is 5-100%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a non-aqueous secondary battery having a high energy density, a high capacity, and excellent cycle characteristics.

[0002]

2. Description of the Related Art As a negative electrode material of a lithium secondary battery,
If metal lithium, which is an active material, can be used as it is, the potential becomes the lowest, and thus it is most desirable from the viewpoint of energy density. However, during charging, active lithium dendritic crystals or mossy crystals having a large specific surface area precipitate on the surface of the negative electrode, and these crystals react with the solvent in the electrolytic solution and are likely to be inactivated. Therefore, the capacity of the negative electrode rapidly decreases, so that it is necessary to previously fill the negative electrode with a large amount of metallic lithium. In addition, the dendritic crystals (dendrites) that have precipitated may penetrate the separator and cause an internal short circuit, causing a problem in safety and a short cycle life.

[0003] In order to suppress the generation of dendrites during such charging, attempts have been made to use a Li-Al alloy or an alloy of Li and wood metal, which is a fusible alloy, as a negative electrode material. In the case of a metal capable of forming an alloy with lithium and an alloy containing at least one of those metals, a relatively high capacity is electrochemically exhibited at an initial charge / discharge cycle stage.

[0004] However, by repeatedly alloying with lithium and desorbing lithium by charging and discharging, the crystal structure of the original skeletal alloy is maintained, but a phase change occurs or the skeletal alloy of the element is produced. In some cases, the crystal structure changes to a different crystal structure. In such a case, the metal or alloy particles, which are the host material of lithium as the active material, expand and shrink, and the crystal grains of the metal or alloy are cracked by the progress of the charge / discharge cycle, and the miniaturization of the particles proceeds. become. This miniaturization phenomenon increases the electrical resistance between the negative electrode material particles and increases the resistance polarization during charge and discharge, so that a cycle life characteristic that is practically satisfactory cannot be exhibited.

In recent years, a system using a carbon material such as graphite, which can occlude and release lithium ions by charge and discharge, as a host material for a negative electrode material, a lithium-containing transition metal oxide as a positive electrode material, and an organic electrolyte has been proposed. It has already been put to practical use under the name of a so-called lithium ion secondary battery.

As disclosed in JP-A-7-315822 for the purpose of further increasing the capacity of the negative electrode, a composite of a graphitized carbonaceous host material and, for example, silicon incorporated in the host material is disclosed. It has been proposed to use a material for a negative electrode material. According to this proposal, the capacity is increased and the cycle life is improved as compared with a negative electrode using silicon alone as a lithium host material. However, because of the small chemical bonding force between silicon and carbon, volume expansion due to occlusion of lithium in silicon cannot be completely suppressed by carbon around silicon, and satisfactory cycle characteristics have been achieved. Absent.

[0007] As in Japanese Patent Application Laid-Open No. Hei 7-315822, as a negative electrode material for achieving a long cycle life characteristic with high capacity, use of an iron silicide such as Fe ₂ Si ₃ , FeSi, and FeSi _{2 is} disclosed. Japanese Patent Application Laid-Open No. Hei 7-240201 discloses that a silicide of a non-ferrous metal element which is a transition element is used in Japanese Patent Application Laid-Open No. 5-159780. Further, a host material composed of an intermetallic compound containing at least one element of a 4B group element, P and Sb, and having a crystal structure of any of CaF ₂ type, ZnS type and AlLiSi type may be used as the negative electrode material. Proposed.

However, the amount of lithium inserted and extracted between the crystal lattices of these negative electrode materials is limited, and is not a satisfactory level in terms of increasing the capacity.

[0009]

In recent years, carbon nanotubes having a graphite structure have been actively studied, and have been studied as negative electrode materials for lithium batteries. Carbon nanotubes have a structure in which a sheet of carbon formed of a six-membered ring is spirally wound into several to several tens of layers around a hollow part, and both ends are hemispherical including a five-membered ring. The hollow part is closed. By opening both end portions of the carbon nanotube, the hollow portion can be in direct contact with the electrolytic solution.

This carbon nanotube has a small expansion and contraction due to the absorption of lithium and the characteristic structure of the carbon nanotube itself allows lithium to be absorbed not only at the interlayer but also at an internal site. It has been expected as a carbon material having a high capacity exceeding the theoretical capacity of 372 mAh / g.

[0011] However, there is a problem that since the lithium absorbed during charging lacks the desorbing property at the time of discharging, the irreversible capacity becomes large as a result, and practical use is not achieved. It has been pointed out that lithium occluded in the hollow portion of the carbon nanotube exists in a state of lithium cluster and has poor reversibility.

At present, natural graphite, artificial graphite and the like are used as a negative electrode active material of a lithium secondary battery. However, the theoretical capacity is limited to 372 mAh / g, and improvement in volume energy density is required to increase the capacity. is needed.

Therefore, an object of the present invention is to provide a non-aqueous secondary battery having a high energy density, a high capacity, and excellent cycle characteristics in order to solve the above-mentioned conventional problems.

[0014]

In order to achieve the above object, a non-aqueous secondary battery according to the present invention is a non-aqueous secondary battery provided with a negative electrode having carbon nanotubes, wherein a non-aqueous secondary battery is provided in a hollow portion of the carbon nanotube. An element capable of forming an alloy with lithium is provided.

Further, in the non-aqueous secondary battery of the present invention, the elements capable of forming an alloy with lithium include zinc, cadmium, aluminum, gallium, indium, thallium, silicon, germanium, tin, lead, and antimony. , And at least one element selected from the group consisting of bismuth.

Further, in the non-aqueous secondary battery of the present invention, the ratio of the carbon nanotube to the whole negative electrode is 5 to 10%.
It is preferably 0% by mass.

[0017]

FIG. 1 is a schematic view of a carbon nanotube used in the present invention. The non-aqueous secondary battery of the present invention is a non-aqueous secondary battery provided with a negative electrode having a carbon nanotube 1, wherein an element 3 capable of forming an alloy with lithium is formed in a hollow portion 2 of the carbon nanotube 1. It is arranged. For parts other than the negative electrode,
Those used for ordinary non-aqueous secondary batteries can be used. For example, a lithium metal oxide compound can be used as the positive electrode material.

At present, graphite used as a negative electrode material of a lithium ion battery expands by about 10% in the c-axis direction at the time of charging. With deterioration of contact between the electrode active material and the current collector, cycle characteristics and load characteristics have been reduced.

A carbon nanotube is a kind of carbon-based material having a graphite structure, but its crystallinity is located between graphite and a turbostratic structure, and the center of the tube has a hollow structure. The inner diameter of the hollow portion is several nm, and a shape having an inclined structure is obtained by open processing in which both ends of the tube are opened using sulfuric acid and nitric acid. Due to such a structure, expansion and shrinkage due to insertion and desorption of lithium are smaller than that of graphite, and elements capable of forming an alloy with lithium existing in the hollow portion are also fine powders due to alloying with Li. Is suppressed, and can contribute to charge and discharge with good reversibility. In addition, the activity of elements that exist in the form of nanoparticles capable of forming an alloy with lithium is also maintained,
The function as an active material can be maintained in good balance with the carbon nanotube. At the time of charging, lithium is electrochemically alloyed with an element that can be alloyed first, and the alloyed and expanded particles form a hollow inner wall of the carbon nanotube as the inner diameter of the hollow decreases as lithium is inserted between the carbon nanotube graphite layers. Contact is further strengthened, and sufficient conductivity can be maintained. At the time of discharge, the reaction proceeds by the reverse mechanism, and the particles that can be alloyed all the time and the electron conductivity of the carbon nanotube are maintained. Therefore, pulverization accompanying the progress of the alloy cycle can be prevented, and the characteristics associated with the charge / discharge cycle can be improved. In addition, since the reaction between the lithium and the alloy occurs in the hollow portion, there is no risk of lithium being deposited on the surface of the carbon nanotube, so that an internal short circuit due to dendrite can be prevented. The amount of the element alloying with lithium can be selectively determined depending on the atmosphere at the time of carbon nanotube synthesis, or the amount and material of the catalyst.

The method for synthesizing carbon nanotubes is not particularly limited. It is possible to synthesize a desired composition by controlling the composition and atmosphere of a catalyst using a method capable of synthesizing a conventional carbon nanotube, for example, a gas phase synthesis method, a thermal decomposition method, a CVD method, an arc discharge method, or the like. it can.

In particular, in the gas phase synthesis method, since carbon nanotubes grow with metal particles serving as a catalyst as nuclei, the concentration of metal particles contained in the hollow part can be controlled by the concentration of the target element used as a catalyst. Is a more desirable synthesis method.

The element capable of forming an alloy with lithium is at least one selected from the group consisting of zinc, cadmium, aluminum, gallium, indium, thallium, silicon, germanium, tin, lead, antimony, and bismuth. One type of element can be used. Among them, silicon has a larger lithium storage capacity,
It is particularly preferable in that high capacity can be achieved.

The ratio of the carbon nanotubes to the whole negative electrode is 5 to 100% by mass. This ratio is 10
The case of 0% by mass is a case where only the carbon nanotube is used alone as the active material of the negative electrode. In other cases, the carbon nanotube is mixed with one or more other active materials, binders, and the like. If the ratio of the carbon nanotubes to the whole negative electrode is within this range, the effect can be sufficiently exhibited, but a more preferable range is 50 to 100% by mass.

[0024]

EXAMPLES Hereinafter, the present invention will be described specifically with reference to Examples, but the present invention is not limited thereto.

Example 1 In order to use Si as an element capable of forming an alloy with lithium, SiO ₂ having a particle size of 1 to 15 μm was prepared as a Si supply material. Also,
Particle size 1 as a catalyst for the synthesis of carbon nanotubes
215 μm Fe ₂ O ₃ was prepared. This SiO ₂ and F
After mixing e ₂ O ₃ powder at a ratio of 1: 1 mol, the mixture was sintered in an atmospheric furnace at a temperature of 1400 ° C. for 10 hours to prepare a composite oxide. This composite oxide was subjected to partial reduction of Fe in an atmosphere of H ₂ gas, and then carbon nanotubes were synthesized at 600 ° C. in an atmosphere of C ₂ H ₄ gas and H ₂ gas. The synthesized carbon nanotubes are transferred to a transmission electron microscope (TE
M) and X-ray fluorescence analysis were performed to quantify the composition. As a result, the content of Si contained in the hollow portion of the carbon nanotube was 2% by mass.

The obtained carbon nanotubes are used as a negative electrode active material, polyvinylidene fluoride (PVDF) is used as a binder, and 45 g of this negative electrode active material and 5 g of PVDF are mixed to form a paste. After coating, drying and compression molding using a calender roll, an electrode was prepared. This electrode is used as a working electrode, metal Li is used for a counter electrode and a reference electrode, and a mixed solution of ethylene carbonate (EC) and methyl ethyl carbonate (MEC) in a volume ratio of 1: 2 is 1.2 mol / dm. of LiPF ₆ ³ was prepared by dissolving. Using these, a model cell was assembled and charge / discharge characteristics were examined.

(Example 2) The same SiO ₂ and Fe ₂ O ₃ powders as those prepared in Example 1 were mixed at a ratio of 5: 1 mol, and the mixture was heated in an air furnace at a temperature of 1400 ° C. for 10 hours. The composite oxide was prepared by sintering. This composite oxide was subjected to partial reduction of Fe in an atmosphere of H ₂ gas, and then carbon nanotubes were synthesized at 600 ° C. in an atmosphere of C ₂ H ₄ gas and H ₂ gas. Synthesized carbon nanotubes in TEM
And X-ray fluorescence analysis were performed to quantify the composition. As a result, the content of Si contained in the hollow portion of the carbon nanotube was 10% by mass.

The obtained carbon nanotube was used as a negative electrode active material.
The anode active material 4 is made of PVDF as a binder.
5g and 5g of PVDF are mixed to form a paste,
Apply the paste to a 10μm thick copper foil, dry
An electrode was prepared by compression molding using a Darroll. this
Using the electrode as a working electrode, metal Li as a counter electrode and a reference electrode,
EC and MEC mixed solution of 1: 2 by volume in electrolyte
1.2 mol / dm ^ThreeLiPF₆For use
Was. Assemble the model cell using these and charge and discharge
The sex was examined.

Example 3 The same powders of SiO ₂ and Fe ₂ O ₃ as those prepared in Example 1 were mixed at a ratio of 5: 1 mol, and then, in an air furnace at a temperature of 1400 ° C. for 10 hours. The composite oxide was prepared by sintering. This composite oxide was subjected to partial reduction of Fe in an atmosphere of H ₂ gas, and then carbon nanotubes were synthesized at 600 ° C. in an atmosphere of C ₂ H ₄ gas and H ₂ gas. Synthesized carbon nanotubes in TEM
And X-ray fluorescence analysis were performed to quantify the composition. As a result, the content of Si contained in the hollow portion of the carbon nanotube was 10% by mass.

The obtained carbon nanotubes and other mesocarbon microbeads were mixed at a mass ratio of 3: 1, and this mixture was used as a negative electrode active material. Using PVDF as a binder, 45 g of this negative electrode active material and 5 g of PVDF were mixed. The paste was applied to a copper foil having a thickness of 10 μm, dried, and compression-molded using a calender roll to form an electrode. This electrode is used as a working electrode, metallic Li is used for a counter electrode and a reference electrode, and 1.2 mol / dm ³ LiP is added to a mixed solution of EC and MEC in a volume ratio of 1: 2 as an electrolytic solution.
It was prepared by dissolving the F _6. Using these, a model cell was assembled and charge / discharge characteristics were examined.

Comparative Example 1 The same powder of Fe ₂ O ₃ as that prepared in Example 1 was heated in an atmospheric furnace at a temperature of 1400 ° C. for 10 minutes.
After sintering for a period of time, this was subjected to partial reduction of Fe in an atmosphere of H ₂ gas, and subsequently carbon nanotubes were synthesized at 600 ° C. in an atmosphere of C ₂ H ₄ gas and H ₂ gas.

The obtained carbon nanotube was used as a negative electrode active material.
The anode active material 4 is made of PVDF as a binder.
5g and 5g of PVDF are mixed to form a paste,
Apply the paste to a 10μm thick copper foil, dry
An electrode was prepared by compression molding using a Darroll. this
Using the electrode as a working electrode, metal Li as a counter electrode and a reference electrode,
EC and MEC mixed solution of 1: 2 by volume in electrolyte
1.2 mol / dm ^ThreeLiPF₆For use
Was. Assemble the model cell using these and charge and discharge
The sex was examined.

Table 1 shows the results of the charge / discharge characteristics of Examples 1 to 3 and Comparative Example 1.

[0034]

[Table 1]

As is clear from Table 1, in Examples 1 to 3 in which Si capable of forming an alloy with lithium was disposed in the hollow portion of the carbon nanotube, the charge capacity and the discharge capacity in the first cycle were compared with those in Comparative Example 1. It turns out that it is larger than. In particular, the discharge capacity of Example 2 in which the mixing ratio of SiO ₂ was large and carbon nanotubes were used as the negative electrode active material except for the binder was the most excellent. Also, 50
The discharge capacity at the cycle also shows the same tendency as described above.

From the above, it can be seen that the non-aqueous secondary battery of the present invention has a higher energy density, a higher capacity, and superior cycle characteristics as compared with conventional batteries.

Further, since the non-aqueous secondary battery of the present invention uses a negative electrode active material mainly composed of carbon nanotubes, the generation of lithium dendrites is suppressed, the safety of the battery is improved, and The discharge characteristics are also improved.

Further, since the expansion and contraction of the carbon nanotube due to charge and discharge is smaller than that of graphite, the load characteristics of the nonaqueous secondary battery of the present invention are also improved.

[0039]

As described above, according to the non-aqueous secondary battery of the present invention, the energy density is high, the capacity is high, the cycle characteristics are excellent, the reliability is high in terms of safety, the self-discharge is small, A non-aqueous secondary battery having good load characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic view of a carbon nanotube used in the present invention. DESCRIPTION OF SYMBOLS 1 Carbon nanotube 2 Hollow part 3 Element which can form an alloy with lithium

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5H029 AJ02 AJ03 AJ05 AK03 AK11 AL06 AM03 AM05 AM07 HJ01 5H050 AA02 AA07 AA08 BA15 BA16 BA17 CA07 CA17 CB07 FA07 HA01

Claims (3)

[Claims] 1. A non-aqueous secondary battery provided with a negative electrode having carbon nanotubes, wherein an element capable of forming an alloy with lithium is arranged in a hollow portion of the carbon nanotube. Rechargeable battery. 2. The element capable of forming an alloy with lithium includes zinc, cadmium, aluminum, gallium, indium, thallium, silicon, germanium,
The non-aqueous secondary battery according to claim 1, wherein the non-aqueous secondary battery is at least one element selected from the group consisting of tin, lead, antimony, and bismuth. 3. The method according to claim 1, wherein the ratio of the carbon nanotubes to the whole negative electrode is 5 to 100% by mass.
The non-aqueous secondary battery according to 1.