JP3237456B2 - Electrode material for lithium secondary battery and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode material used in a lithium secondary battery, and more particularly to a negative electrode material.

[0002]

2. Description of the Related Art In recent years, development of secondary batteries of small size, light weight and high energy density has become active.

[0003] As a negative electrode active material for the secondary battery, a material using an alkali metal, especially lithium, has attracted particular attention.

In this case, it is already known that it is useful to use a carbon material as an electrode, and some electrodes have actually been proposed as a negative electrode.

[0005] When lithium is used as the negative electrode active material, internal short-circuit and remarkable reduction in charge / discharge efficiency due to the precipitation of lithium in the form of dendrite during charging may occur.

However, in this case, it is possible to avoid such a phenomenon by using a carbon material having a function of electrochemically intercalating and deintercalating lithium ions.

[0007] On the other hand, there are some problems that must be solved by using a carbon material, such as a large charge / discharge capacity, a loss of charge / discharge capacity, and a decrease in battery characteristics due to deterioration of an electrolytic solution. Is mentioned.

Among them, the cause of the deterioration of the electrolyte is as follows.
It is considered that a gas is generated due to the decomposition of the solvent and the electrolyte, and an insulating film is formed on the electrode.
The charge / discharge repetition characteristics of the battery characteristics are deteriorated.

As for the solvent of the electrolytic solution, the cyclic ester structure is broken to generate carbon dioxide gas and hydrogen,
Alternatively, the electrolyte itself decomposes and reacts with lithium ions of the electrolyte to form an insulating film.

[0010] It is also known that the degree of such a phenomenon varies depending on the type of carbon material.

Specifically, differences in particle size and surface condition have been pointed out, but in general, graphitization has progressed.
It can be said that the solvent tends to be easily decomposed.

It is also known that an amorphous carbon material has a high initial charge but a large discharge loss.

For this reason, there have been proposed carbon materials, electrode materials, electrolytes, electrolytes, electrode binder resins and the like for the purpose of solving the respective problems.

More specifically, as a carbon material, there are differences in structural aspects such as starting materials, firing conditions, crystallinity, and crystallinity. Carbon materials having a specified crystallite size have been proposed.

[0015] Proposals have also been made for optimum ranges of the size, distribution, density, etc. of the carbon particle size.

Regarding the particle diameter, for example, when the particle diameter is smaller than 5 μm or less, the generation of gas is large, and the repetition characteristics of charge / discharge become poor. , The atomic ratio of C to H, 0
It is also known that battery characteristics vary greatly depending on the presence of atoms and differences in the chemical structure of the surface.

Among the components of the electrolytic solution, the electrolyte may be LiPF ₆ , LiBF ₄ , LiClO ₄ , LiSO ₃ C
F ₃ and the like are often used.

As the solvent for the electrolytic solution, cyclic carbonates such as propylene carbonate and ethylene carbonate are often used due to their characteristics such as stability, dielectric constant and viscosity.

Of these single solvents, those which completely satisfy the required specifications as a solvent cannot be obtained in many cases, and mixed solvent systems have been proposed. For example, dimethoxyethane, tetrahydrofuran, or A mixed solvent with a solvent having a similar structure is often used.

Further, although there has been reported an example in which a crown ether compound is used as an additive in an electrolytic solution to improve charge / discharge efficiency and charge / discharge repetition stability, further improvement in performance has been reported. Improvement is required.

[0021]

SUMMARY OF THE INVENTION The present invention relates to an electrode material used in the above-mentioned lithium secondary battery, particularly to a carbon material used as a negative electrode, the amount and efficiency of charge / discharge caused by an electrolyte, and the repetition characteristics of charge / discharge. It is another object of the present invention to provide an electrode material for a lithium secondary battery capable of realizing a high performance lithium secondary battery by solving problems such as deterioration of battery performance such as a standing life.

[0022]

As described above, the carbon materials used for the negative electrode greatly differ in battery characteristics depending on the structure, starting materials and manufacturing methods. , Mainly degradation of electrolyte components,
This is due to the interaction between lithium ions and the carbon material.

The degree of deterioration caused by these factors differs depending on structural differences of carbon materials, manufacturing conditions, and the like.

The present invention focuses on the structure or surface state of the powdery or film-like carbon material itself, not on the electrolyte component side, and controls the carbon material itself or the surface state with polypyrrole to improve battery performance. Specifically, the amount of lithium ions charged to the carbon compound is increased, the discharge efficiency is improved, the charging can be performed efficiently, and the charging speed is not improved.

According to the study of the present inventor, as a cause of lowering the charge / discharge efficiency and the lowering of the repetition stability, a chemically or electrochemically active structure on the surface of the carbon material is caused by lithium ions. It is conceivable that trapping is performed so that doping including intercalation and undoping including deintercalation hardly occur.

Therefore, if such a structure is destroyed or removed in advance, the battery characteristics can be expected to be improved.

Therefore, the main constitution of the present invention is that the heat-treated carbon material and polypyrrole are formed by electrolytic polymerization or chemical polymerization.
It is an electrode material for a lithium secondary battery formed by compounding in some cases.

In this case, it is possible to change not only the surface condition of the carbon material but also the structure of the carbon material itself.

First, it is observed that the surface of the carbon material after the heat treatment has a structure having a certain functional group which cannot be completely the same as the bulk structure, and the surface state is changed.

When a high-temperature heat treatment is performed after the composite of the carbon material and polypyrrole, the region where the polypyrrole exists is hard to form a graphite structure, so that a region where the progress of graphitization is different is also generated. is there.

Here, specifically, this is an electrode material for a secondary battery in which a carbon material and polypyrrole are carbonized or graphitized and compounded.

However, when polypyrrole is compounded after carbonization or graphitization, the surface state is mainly changed. In particular, when polypyrrole is compounded after graphitization, the carbon material is reduced. It is thought that changing the structure has little contribution and substantially changes only the surface state.

In any case, in the above case, the internal structure or surface state of the carbonized material or the graphitized material is adjusted by doping and / or undoping of lithium ions by compounding polypyrrole. Become.

Next, as a method for producing such an electrode material for a lithium secondary battery, a step of preparing a carbon material, a carbonized material or a graphitized material, and forming a polypyrrole layer on the surface of each of the above materials are described. And a method for producing an electrode material for a lithium secondary battery in which a heat treatment is further performed in a state where a polypyrrole layer is formed on the surface of each of the above materials.

Here, the heat treatment to be further performed is a heat treatment in a temperature range in which carbonization or graphitization is performed as a whole in the case of a carbon material, and in the case of a carbonized material, polypyrrole is also carbonized or graphite is entirely formed. This is a heat treatment in a temperature range in which the material is a graphitized material, or a graphite material in the case of polypyrrole.

The step of preparing the carbonized material includes a heat treatment step of heat-treating the carbon material at a temperature of 500 ° C. or more and 1200 ° C. or less, and the step of preparing the graphitized material includes a heat treatment of 1200 ° C. to 3000 ° C. It is preferable to include a heat treatment step of performing heat treatment in the range.

Similarly, the step of carbonizing the polypyrrole includes a heat treatment step of heat-treating the polypyrrole at a temperature of 500 ° C. to 1200 ° C., and the step of graphitizing the polypyrrole includes a step of 1200 ° C. to 3000 ° C. It is also preferable to include a heat treatment step of performing heat treatment in the range.

Here, the term “carbonization” or “graphitization” means that the starting material is a carbon material that has been further crystallized from the starting carbon material. Graphitization is more crystalline than carbonization. It means that progress has been made.

If the material can be carbonized or graphitized, carbonization is performed by heat treatment in the range of 500 ° C. or more and 1200 ° C., and graphitization is performed between 1200 ° C. and 300 ° C.
Generally, the heat treatment is performed at a temperature of 0 ° C. or lower.

The step of forming the polypyrrole layer may include a step of preparing a pyrrole monomer and a step of electrolytically polymerizing the pyrrole monomer, or a step of preparing the pyrrole monomer. And a step of chemically polymerizing the pyrrole monomer.

In the step of forming the polypyrrole layer, it is preferable to use an electrolyte containing an aromatic compound or an oxidizing agent containing an aromatic compound.

That is, the oxidizing agent or the anion is contained in the polypyrrole layer regardless of the electrolytic polymerization method or the chemical polymerization method. The agent or anion must have an aromatic ring.

Otherwise, the polypyrrole layer disappears due to the heat treatment. By the way, when used as an electrode of a battery, after heat treatment, it is subjected to a pulverizing treatment to obtain a desired powder size, but by optimizing the pulverizing treatment, it is not possible to further improve the battery characteristics. Of course.

Therefore, as the powder, not only the size but also the influence of the shape are great. The shape can be from amorphous to spherical or fibrous.

As a raw material for producing a carbonized material or a graphitized material, a petroleum pitch-based material, polyimide, phenol resin or the like can be suitably used.

[0046]

According to the above construction, the trapping of lithium ions, which is the active material of the battery, is reduced, the amount of doping including intercalation is increased, and deintercalation and dedoping are efficient. Well done.

Therefore, the charging characteristics and discharging characteristics of lithium ions to the carbon material electrode are improved, so that the battery characteristics such as the charge / discharge amount, charging / discharging efficiency, charging speed, and repetition stability of the battery characteristics can be improved. Can be

[0048]

EXAMPLES Examples of the present invention will be described in detail below in comparison with comparative examples.

Embodiment 1 Hereinafter, a first embodiment of the present invention will be described.

In this embodiment, the cathode material is L
The firing was performed in air at 900 ° C. using iCoO ₃ .

Then, polyvinylidene fluoride was used as a binder, and a slurry was formed using an organic solvent.

Thereafter, the composition was applied on an aluminum foil, dried, and then compression-molded to form a positive electrode.

On the other hand, a polyimide powder was used as a starting material of the carbon material for the negative electrode material, and calcined at a temperature of 800 ° C. to complete the carbonization.

Next, a polypyrrole film having a thickness of about 1 μm was formed on the surface of the carbonized material thus carbonized by electrolytic polymerization using a polyanion composed of a phenoxy resin as an electrolyte.

Thereafter, a pulverizing treatment was carried out to make the particle size uniform by ball milling. Then, polyvinylidene fluoride was used as a binder, and a slurry was formed using an organic solvent.

Thereafter, the composition was applied on an aluminum foil, dried, and then compression-molded to form a negative electrode.

The plane spacing d002 of the (002) plane after the heat treatment at 800 ° C. was 3.75 °, and the crystal size Lc in the c-axis direction was 20 °.

As a basic composition, the solvent is
LiPF for anchor carbonate and electrolyte ₆Of the combination
Using the solution, a cylindrical lithium secondary battery was produced.

Using the battery thus manufactured, the loss of charge / discharge amount and the repetition stability were evaluated.

The results are summarized in Sample 1 in (Table 1).

[0061]

[Table 1]

Example 2 A lithium secondary battery was produced in the same manner as in Example 1 except that the heat treatment temperature after forming the polypyrrole film was 2700 ° C., and the evaluation results shown in Sample 2 in Table 1 were obtained. I got

Example 3 The procedure of Example 2 was repeated except that the starting material to be carbonized was petroleum pitch and a polypyrrole film was obtained by a chemical polymerization method using benzenesulfonic acid as an oxidizing agent instead of the electrolytic polymerization method. Similarly, a lithium secondary battery is manufactured,
The evaluation result shown in Sample 3 of (Table 1) was obtained.

Incidentally, d0 of the carbonized material after the heat treatment at 800 ° C.
02 was 3.47 ° and the crystal size Lc was 40 °.

(Example 4) The petroleum pitch of the starting material was 27
A lithium secondary battery was fabricated in the same manner as in Example 3, except that the graphite was formed by heat treatment at 00 ° C., and the evaluation result shown in Sample 4 of (Table 1) was obtained.

The d002 of the graphite after the heat treatment at 2700 ° C. was 3.37 ° and Lc was 380 °.

Example 5 In this example, a polyimide film was used as a starting material, and a polypyrrole film was produced in the same manner as in Example 3 without performing heat treatment on the polyimide film. Was performed to produce a graphitized film.

Then, this film was pulverized in the same manner as in the above-mentioned example to produce a lithium secondary battery.

The results are shown in Sample 5 of (Table 1). (Comparative Example 1) Comparative Example 1 was performed in the same manner as in Example 1 except that a carbon material was used, which was fired at a temperature of 800 ° C to complete carbonization and did not form a polypyrrole film.
In the same manner as in the above, a lithium secondary battery was produced.

The results are shown in Comparative Example 1 in (Table 2).

[0071]

[Table 2]

Comparative Example 2 As Comparative Example 2, 2700 ° C.
A lithium secondary battery was produced in the same manner as in Example 1, except that a carbon material which was only fired at the temperature described above to complete the graphitization and did not form a polypyrrole film was used.

The results are shown in Comparative Example 2 in (Table 2).
Referring to the evaluation results of each of the above Examples and Comparative Examples, according to the results of repeating charging and discharging up to 40 cycles, a remarkable difference in repetition stability of charging and discharging characteristics was observed,
It was confirmed that the effect obtained by forming a polypyrrole film on the carbon material and heat-treating the composite material was exhibited.

Although other examples using a phenol resin or the like are omitted, even if a polypyrrole film is formed directly on a carbon material as a starting material, the polypyrrole film may be formed after carbonization or graphitization. Even if it is formed, similar effects have been confirmed as long as heat treatment is performed using a starting material that can be carbonized or graphitized.

[0075]

As described above, according to the present invention, a carbon material capable of producing a carbonized material or a graphitized material suitably used as a negative electrode material and polypyrrol are compounded to obtain a charge / discharge amount, It is possible to provide a lithium secondary battery having excellent efficiency and repetition stability.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-46756 (JP, A) JP-A-61-214371 (JP, A) JP-A-4-220948 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) H01M 4/02-4/04 H01M 4/58 H01M 10/40

Claims (17)

(57) [Claims] 1. An electrode material for a lithium secondary battery formed by complexing a heat-treated carbon material and polypyrrole by electrolytic polymerization or chemical polymerization . 2. The electrode material for a lithium secondary battery according to claim 1, wherein the carbon material and polypyrrole are carbonized or graphitized to form a composite. (3) Polypyrrole is compounded on the surface of the carbon material.
The electrode material for a lithium secondary battery according to claim 1, wherein 4. A step of preparing a carbon material, a step of forming a polypyrrole layer on the surface of the carbon material, and a step of carbonizing or graphitizing the polypyrrole layer with the polypyrrole layer formed on the surface of the carbon material. A method for producing an electrode material for a lithium secondary battery having the same. 5. A step of preparing a carbonized material, a step of forming a polypyrrole layer on the surface of the carbonized material, and a step of also carbonizing the polypyrrole layer formed on the surface of the carbonized material. A method for producing an electrode material for a lithium secondary battery. 6. A step of preparing a carbonized material, a step of forming a polypyrrole layer on the surface of the carbonized material, and a step of graphitizing the polypyrrole layer on the surface of the carbonized material. A method for producing an electrode material for a lithium secondary battery having the same. 7. A step of providing a graphitized material;
A method for producing an electrode material for a lithium secondary battery, comprising: a step of forming a polypyrrole layer on the surface of the graphitized material; and a step of also graphitizing the polypyrrole layer formed on the surface of the graphitized material. 8. The method for producing an electrode material for a lithium secondary battery according to claim 5, wherein the step of preparing the carbonized material includes a heat treatment step of heat-treating the carbon material at a temperature of 500 ° C. to 1200 ° C. . 9. The step of providing a graphitized material,
The method for producing an electrode material for a lithium secondary battery according to claim 7, further comprising a heat treatment step of performing a heat treatment at a temperature of 1200 ° C or more and 3000 ° C or less. 10. The method for producing an electrode material for a lithium secondary battery according to claim 4, wherein the step of carbonizing the polypyrrole includes a heat treatment step of heat-treating the polypyrrole at a temperature of 500 ° C. or more and 1200 ° C. or less. 11. The method for producing an electrode material for a lithium secondary battery according to claim 4, wherein the step of graphitizing the polypyrrole includes a heat treatment step of performing a heat treatment at a temperature of 1200 ° C. or more and 3000 ° C. or less. 12. The lithium secondary battery according to claim 4, wherein the step of forming a polypyrrole layer includes a step of preparing a pyrrole monomer and a step of electrolytically polymerizing the pyrrole monomer. Method of manufacturing electrode material for use. 13. The lithium secondary battery according to claim 4, wherein the step of forming a polypyrrole layer includes a step of preparing a pyrrole monomer and a step of chemically polymerizing the pyrrole monomer. Method of manufacturing electrode material for use. 14. The method according to claim 1, wherein a compound containing an aromatic compound is used as an electrolyte in the step of forming the polypyrrole layer.
3. The method for producing an electrode material for a lithium secondary battery according to item 2. 15. The method according to claim 1, wherein in the step of forming the polypyrrole layer, a compound containing an aromatic compound is used as the oxidizing agent.
4. The method for producing an electrode material for a lithium secondary battery according to item 3. 16. The lithium secondary battery according to claim 4, further comprising a treatment step of treating a carbon material having a carbonized or graphitized polypyrrole layer into an amorphous, spherical, or fibrous form. Manufacturing method of electrode material. 17. The lithium 2 according to claim 16, wherein the carbon material is petroleum pitch, polyimide or phenol resin.
Method for producing electrode material for secondary battery.