JP5536364B2 - Negative electrode material for lithium secondary battery and method for producing the same - Google Patents

Description

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

  Most of the conventional lithium secondary batteries use a lithium-containing transition metal oxide such as lithium cobaltate, lithium manganate, or nickel acid for the positive electrode, and a carbon-based material such as graphite for the negative electrode. However, the capacity density and energy density of lithium secondary batteries put into practical use with a combination of these materials are almost reaching their limits. On the other hand, high capacity and high energy density are desired for batteries as the performance of equipment using lithium secondary batteries increases.

  In order to achieve higher capacity and higher energy density of batteries, it is necessary to change the active material to a material that can achieve higher capacity.

  Among them, when metallic lithium is used as the negative electrode active material, the energy density per weight and volume can be extremely higher than that of the graphite-based carbon material. And has the potential problem of causing an internal short circuit.

  In addition, a lithium secondary battery using silicon (Si), tin, or the like as a negative electrode active material, which can be alloyed with lithium during charging, has been reported as a material for realizing high capacity.

  Si is industrially produced using silicon dioxide as a raw material, but the silicon dioxide reserves are abundant in the world. Moreover, since the theoretical capacity when Si is used as a negative electrode active material is large, lithium secondary batteries using Si as a negative electrode have been proposed (see Patent Documents 1 to 4).

  However, when Si is used as a negative electrode for a lithium secondary battery, when alloying with lithium, a large volume change is caused, resulting in a decrease in current collecting performance. In particular, when charging / discharging is repeated, the characteristic deterioration becomes remarkable.

JP 2001-216916 A JP 2006-196338 A JP 2006-260784 A JP 2007-157709 A

  Generally, a carbonate-based polar non-aqueous organic solvent is used as an electrolyte for a lithium secondary battery. In a lithium secondary battery using Si as the negative electrode active material, when this electrolytic solution is used, Si of the negative electrode active material reacts with the electrolytic solution during the first charge, and an excessive amount of charge is used. By such an irreversible reaction, a passivation layer such as a solid electrolyte interface (SEI) film is formed on the Si surface. Such an SEI film makes it possible to maintain stable charge and discharge without further decomposition of the electrolytic solution. However, the SEI film gradually cracks due to the expansion and contraction of the active material that occurs as the battery is repeatedly charged and discharged, and is detached from the electrode surface. Therefore, the electrolytic solution is in direct contact with the active material, and the decomposition reaction of the electrolyte is continuously generated. Once the crack is generated, the crack continues to develop due to charging / discharging of the battery, and the active material itself is cracked, and the deterioration proceeds. In particular, when Si is an active material, the volume change due to charging / discharging is large, and the deterioration of the active material as described above is conspicuous. In fact, when Si is used for the negative electrode, it is known that the maximum volume fluctuation is about four times.

  An object of the present invention is to solve the conventional problems, and to provide a negative electrode material for a lithium secondary battery in which deterioration of an active material of an electrode for a lithium secondary battery is suppressed and cycle characteristics are improved, and a method for producing the same. And

  The negative electrode material for a lithium secondary battery according to the present invention is characterized by using Si particles having an average particle diameter of 20 μm or less coated with a mixture containing an organosilicon compound and carbon.

  In order to suppress the cracking of the Si particles during expansion and contraction during charge and discharge, it is effective to make the particles themselves fine. As a result of the experiment, it was found that it can be produced by a simple production method at 20 μm or less to several μm, and the cycle characteristics can be sufficiently improved.

In the negative electrode material for a lithium secondary battery according to the present invention, the organosilicon compound is a compound represented by the following general formula (1).
R ¹ _a Si (OR ² ) _b (OH) _{4- (a + b)} (1)
(R ¹ represents a monovalent organic group, R ² represents a lower alkyl group having 1 to 8 carbon atoms, a represents an integer of 0 to 2, and b represents an integer of 0 to 4. However, 0 ≦ a + b ≦ 4)), which is a product formed by polycondensation of one or more organosilicon compound precursors.

  The organosilicon compound has flexibility because it contains an organic chain. The organic chain contributes to the flexibility of the structure, but becomes a steric hindrance during hydrolysis and polycondensation reactions, and suppresses the progress of the reaction. In order to allow the reaction necessary for forming a skeleton with flexibility, it is preferable that 5 to 90% of all Si atoms in the organosilicon compound are bonded to a monovalent organic group. .

  According to this configuration, it is possible to produce a negative electrode having a structure in which the organosilicon compound has flexibility and a sufficient skeleton is formed and is resistant to expansion and contraction.

  Further, coating carbon on Si itself is very effective in improving cycle characteristics. The coating amount of carbon is preferably 10 wt% to 50 wt% with respect to the Si particles by weight.

  In addition to the Si alkoxide represented by the formula (1), a metal alkoxide of Al, Ti, or Zr can be used as a precursor of the organosilicon compound. By using a metal alkoxide of Al, Ti, or Zr as a precursor, the formation reaction of the organic silicon compound is promoted, and the film-forming property of the coating is improved.

  Further, it is preferable to use ball milling as a method for producing a film on Si fine particles from the viewpoint of a method for producing a film having good adhesion and industrial simplicity.

  The negative electrode material for a lithium secondary battery of the present invention is a negative electrode material that is resistant to expansion and contraction by using Si particles having an average particle size of 20 μm or less coated with a mixture containing an organosilicon compound and carbon, and a negative electrode with good cycle characteristics Become a material.

The figure which shows the charging / discharging test result by the lithium secondary battery of this invention Example and a comparative example

Hereinafter, embodiments of the present invention will be described in detail.
<Negative electrode>
In the present invention, as a material constituting the negative electrode, Si particles are used, and the surface is coated with an organic silicon compound and carbon using ball milling.

  In order to suppress the cracking of the Si particles during expansion and contraction during charge and discharge, it is effective to make the particles themselves fine. This is because if the particles are small to a certain extent, the strain applied to the particles when expanded during charging can be suppressed, so that the development of cracks can be prevented. A smaller particle size is preferable from the viewpoint that the strain can be suppressed, but the smaller the particle size, the more difficult it becomes industrially and the higher the production cost. As a result of the experiment, it was found that it can be produced by a simple production method by ball milling at 20 μm or less to several μm, and the cycle characteristics can be sufficiently improved.

In the negative electrode material for a lithium secondary battery according to the present invention, the organosilicon compound is a compound represented by the following general formula (1).
R ¹ _a Si (OR ² ) _b (OH) _{4- (a + b)} (1)
(R ¹ represents a monovalent organic group, R ² represents a lower alkyl group having 1 to 8 carbon atoms, a represents an integer of 0 to 2, and b represents an integer of 0 to 4. However, 0 ≦ a + b ≦ 4)), which is a product formed by polycondensation of one or more organosilicon compound precursors. The organosilicon compound precursor represented by (1) forms an organosilicon compound through a reaction process generally known as a sol-gel method, that is, hydrolysis and polycondensation.

Further, the organic group represented by R ¹ may be any organic group as long as it does not easily become a steric hindrance during polycondensation and can sufficiently develop a skeleton necessary for film formation. . For example, examples of the organic group represented by R ¹ include an unsubstituted monovalent hydrocarbon group such as an alkyl group having 1 to 10 carbon atoms, a cycloalkyl group, an alkenyl group, an aryl group, and an aralkyl group, and hydrogen of these groups. Some or all of the atoms may be halogen atoms (chlorine, fluorine, bromine atoms, etc.), cyanoalkylene, oxyalkylene groups such as oxyethylene groups, polyoxyalkylene groups such as polyoxyethylene groups, (meth) acryl groups, (meth ) In substituted monovalent hydrocarbon groups substituted with functional groups such as acryloxy group, acryloyl group, methacryloyl group, mercapto group, amino group, amide group, ureido group, and epoxy group, these unsubstituted or substituted monovalent hydrocarbon groups, -O -, - NH -, - N (CH 3) -, - N (C 6 H 5) -, cited _{C 6 H 5 NH-, H 2} NCH 2 CH 2 NH- or the like is interposed group It is possible.

More specific examples of the organic group represented by R ¹ include alkyl groups such as CH ₃ —, CH ₃ CH ₂ —, CH ₃ CH ₂ CH ₂ —, CH ₂ ═CH—, CH ₂ ═CHCH ₂ —. , An alkenyl group such as CH ₂ ═C (CH ₃ ) —, an aryl group such as C ₆ H ₅ —, ClCH ₂ —, ClCH ₂ CH ₂ CH ₂ —, CF ₃ CH ₂ CH ₂ —, CNCH ₂ CH ₂ — CH ₃ — (CH ₂ CH ₂ O) m—CH ₂ CH ₂ CH ₂ — (where m is an integer of 1 or more), CH ₂ (O) CHCH ₂ OCH ₂ CH ₂ CH ₂ — (provided that CH ₂ (O) CHCH ₂ represents a glycidyl group), CH ₂ = CHCOOCH _2- , CH ₂ = CHCOOCH ₂ CH ₂ CH _2- , CH ₂ = CCH ₃ COOCH ₂ CH ₂ CH _2- , HSCH ₂ CH ₂ CH ₂ —, NH ₂ CH ₂ CH ₂ CH ₂ —, NH ₂ CH ₂ CH _{2 NHCH 2 CH 2 CH 2 -,} NH 2 CONHCH 2 CH 2 CH 2 - and the like.

Among the organic groups described above, in particular, alkyl groups and alkenyl groups having 1 to 5 carbon atoms are less likely to become a steric hindrance in the polycondensation reaction compared to other organic groups, and can form a film on the surface of Si particles. This is preferable because the necessary skeleton can be sufficiently developed. In the general formula (1), when the integer a is 2, the two organic groups represented by R ¹ may be the same organic group or different organic groups.

In the general formula (1), the lower alkyl group having 1 to 8 carbon atoms represented by R ² may be linear or branched, and specific examples include: , —CH ₃ , —CH ₂ CH ₃ , —CH (CH ₃ ), —CH ₂ CH ₂ CH _{3 and the} like. In the Si alkoxide represented by the general formula (1), when the integer b is 2 or more, all the b R ^{2 s} may be the same group or different groups.

  Moreover, it is preferable that 5 to 90% of Si atoms among Si atoms contained in the organosilicon compound are bonded to a monovalent organic group. Such an organic group bonded to the Si atom imparts flexibility to the organosilicon compound covering the surface of the Si particle. The monovalent organic group may be any one as long as it can improve the film flexibility, and is not particularly limited. 5 alkyl or alkenyl groups are preferred.

  Further, since Si itself is inferior in conductivity, contact with a current collector and contact with a conductive material are very important. In order to increase the contact area with the conductive material, it is effective to reduce the particle size as described above. It is also very effective to coat Si itself with carbon. As for the amount of carbon coated on the Si particles, a larger amount is preferable in terms of enhancing the conductivity, but if it is too much, the energy density of the entire negative electrode is reduced. From the above viewpoint, the coating amount of carbon is preferably 10 wt% to 50 wt% with respect to the Si particles by weight. According to this configuration, a negative electrode having high current collecting performance can be produced without excessively reducing the energy density of the Si particles.

  In addition to the Si alkoxide represented by the formula (1), a metal alkoxide of Al, Ti, or Zr can be used as a precursor of the organosilicon compound. By using a metal alkoxide of Al, Ti, or Zr as a precursor, the formation reaction of the organic silicon compound is promoted, and the film-forming property of the coating is improved. This results in an improvement in cycle characteristics.

  Further, it is preferable to use ball milling as a method for producing a film on Si fine particles from the viewpoint of a method for producing a film having good adhesion and industrial simplicity.

<Non-aqueous electrolyte>
In the present invention, the non-aqueous electrolyte is not particularly limited, but as a solvent, ethylene carbonate (hereinafter abbreviated as EC), propylene carbonate, butylene carbonate, diethyl carbonate (hereinafter abbreviated as DEC), dimethyl carbonate, methyl ethyl carbonate, Esters such as γ-butyrolactone, ethers such as tetrahydrofuran, 2-methyltetrahydrofuran, dioxane, dioxolane, diethyl ether, dimethoxyethane, diethoxyethane, methoxyethoxyethane, dimethyl sulfoxide, sulfolane, methylsulfolane, acetonitrile, methyl formate A polar solvent such as methyl acetate can be used. These solvents may be used alone, or two or more kinds may be mixed and used as a mixed solvent.

The non-aqueous electrolyte may contain an electrolyte salt. Examples of the electrolyte salt include LiClO ₄ , LiBF ₄ (lithium borofluoride), LiPF ₆ (lithium hexafluorophosphate), LiCF ₃ SO ₃ (lithium trifluoromethanesulfonate), LiF (lithium fluoride), LiCl (lithium chloride). ), LiBr (lithium bromide), LiI (lithium iodide),
Examples thereof include lithium salts such as AlCl ₄ Li (lithium tetrachloride aluminate). These may be used alone or in combination of two or more.

  The concentration of the electrolyte salt is not particularly limited, but is preferably 0.5 mol / L to 2.5 mol / L, more preferably 1.0 mol / L to 2.2 mol / L. When the concentration of the electrolyte salt is less than 0.5 mol / L, the carrier concentration for carrying charges in the non-aqueous electrolyte is lowered, and the resistance of the non-aqueous electrolyte may be increased. Moreover, when the concentration of the electrolyte salt is higher than 2.5 mol / L, the degree of dissociation of the salt itself is lowered, and the carrier concentration in the nonaqueous electrolytic solution may not be increased.

<Application>
The use of the lithium secondary battery of the present invention is not particularly limited. For example, when mounted on an electronic device, a notebook computer, a sub-notebook computer, a pen input computer, a pocket (palmtop) computer, an electronic book player, a mobile phone, Cordless phone, handy terminal, portable fax, portable copy, portable printer, headphone stereo, video movie, LCD TV, handy cleaner, portable music player, electric shaver, electronic translator, transceiver, power tool, electronic notebook, calculator, IC recorders, radios, backup power supplies, etc. Others for consumer use include automobiles, electric vehicles, motors, lighting equipment, toys, game equipment, road conditioners, irons, watches, strobes, cameras, medical equipment (pacemakers, hearing aids, shoulder massagers, etc.). Furthermore, it can be used for various military use and space use. Moreover, it can also combine with another secondary battery and a solar cell.

  Hereinafter, the present invention will be specifically described with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[Example 1]
A lithium secondary battery according to Example 1 of the present invention was produced according to the following procedure.
<Preparation of Si fine particles>
10 g of Si particles (purity 99.999%, particle size 75 μm) are put in a 250 ml silicon nitride container, and 15 silicon nitride balls (φ20 mm) are used for 30 minutes in a planetary ball mill. Then, ball milling was performed in an air atmosphere. When the average particle diameter of the particles was measured, the average particle diameter was 18 μm.

<Preparation of organosilicon compound>
Mechiruto Rie Tokishishiran, tetraethoxysilane, 0.01 N hydrochloric acid, ethanol, at a molar ratio of 1: 1: 7: 9 were weighed so as to satisfy the 30g in total volume, stirred with sample tubes 3 hours, 30 at 50 ° C. By drying for a minute, a gel-like organosilicon compound was obtained.

<Coating of organosilicon compound and carbon on Si fine particles>
First, 5 g of the Si fine particles, 5 g of the organosilicon compound, and 2 g of acetylene black were put into a 250 ml silicon nitride container, and further ball milled for 1 hour to obtain Si fine particles whose surface was coated with the organosilicon compound and carbon. Obtained. 2 g of the surface-coated Si fine particles are used as a negative electrode active material, 2 g of graphite particles (SFG-6, Timcal) having an average particle diameter of 6 μm as a conductive auxiliary, and polyvinylidene fluoride (hereinafter abbreviated as PVDF) as a binder ( 1 g of KF1000 (manufactured by Kureha Co., Ltd.) was mixed with N-methyl-2-pyrrolidone (hereinafter abbreviated as NMP) as a solvent, and then kneaded and dispersed in an agate mortar to prepare a negative electrode paste. The prepared paste was applied to one side of a 10 μm thick copper foil as a negative electrode current collector using a bar coater. The coated electrode was air dried at 50 ° C. for 20 minutes, then pressed using a hydraulic press, the coated part was cut into a size of 20 mm × 20 mm, and further vacuum dried at 150 ° C. for 8 hours to form a negative electrode plate Formed.

<Assembly of single cell>
A tripolar cell was prepared using metallic lithium as the counter electrode and reference electrode of this negative electrode plate, and filled with 1 mol / L of LiPF ₆ / EC + DEC (volume ratio 1: 2) using a 50 ml sample bottle. A single cell was created.

<Cycle characteristic test>
The single cell was repeatedly charged and discharged at a current of 100 mA / g in the range of 0.001 V to 1.5 V, and the obtained charge and discharge results are shown in FIG. As shown in the results of FIG. 1, the capacity retention rate was as high as 66.7% at 10 cycles.

[Example 2]
As a manufacturing method of an organic silicon compound, Mechiruto Rie Tokishishiran, titanium (IV) n-butoxide, 0.01 N hydrochloric acid, ethanol, in a molar ratio of 4: 1: 16: 19 was weighed so as to provide 30g in total amount, The mixture was stirred for 3 hours in the sample tube and dried at 50 ° C. for 30 minutes to obtain a gel-like organosilicon compound. Other manufacturing procedures were the same as in Example 1. As shown in the results of FIG. 1, the capacity retention rate was as high as 66.2% at 10 cycles.

[Example 3]
As a manufacturing method of an organic silicon compound, Mechiruto Rie Tokishishiran, aluminum sec- butoxide, 0.01 N hydrochloric acid, ethanol, in a molar ratio of 4: 1: 15: 20 and weighed so as to provide 30g in total volume, the sample tube The mixture was stirred for 3 hours and dried at 50 ° C. for 30 minutes to obtain a gel-like organosilicon compound. Other manufacturing procedures were the same as in Example 1. As shown in the results of FIG. 1, the capacity retention rate was as high as 68.5% at 10 cycles.

[Example 4]
As a manufacturing method of an organic silicon compound, Mechiruto Rie Tokishishiran, tetra-n-propoxy zirconium, 0.01 N hydrochloric acid, ethanol, in a molar ratio of 4: 1: 16: 19 was weighed so as to provide 30g in total volume, the sample tube The mixture was stirred for 3 hours and dried at 50 ° C. for 30 minutes to obtain a gel-like organosilicon compound. Other manufacturing procedures were the same as in Example 1. As shown in the results of FIG. 1, the capacity retention rate was as high as 69.4% at 10 cycles.

[Comparative Example 1]
Using untreated Si particles (manufactured by High-Purity Chemical Laboratory, purity 99.999%, particle size 75 μm) as the cathode active material, a single cell was assembled and a cycle characteristic test was conducted in the same manner as in Example 1. . As shown in the results of FIG. 1, the capacity retention rate was 4.9% at 10 cycles.

[Comparative Example 2]
Place 5g of Si particles (purity 99.999%, particle size 75μm) into a 250ml silicon nitride container, and use 15 pieces of silicon nitride balls (φ20mm) for 10 minutes planetary ball mill Then, ball milling was performed in an air atmosphere. When the average particle diameter of the particles was measured, the average particle diameter was 30 μm.

  Hereinafter, using the same procedure as in Example 1, organosilicon compound, carbon coating, single cell assembly, and cycle test were performed on Si particles. As shown in the results of FIG. 1, the capacity retention rate was 21.4% at 10 cycles.

  From the above, it is suggested that the lithium secondary battery using the negative electrode material of the present invention is suppressed from deterioration against expansion and contraction during charge and discharge, and as a result, the cycle characteristics are improved.

Claims (5)

As a negative electrode material for a lithium secondary battery, Si particles having an average particle diameter of 20 μm or less coated with a mixture of a single layer containing an organosilicon compound and carbon are used .
The organosilicon compound is represented by the following general formula (1)
R ¹ _a Si (OR ² ) _b (OH) _{4- (a + b)} (1)
(R ¹ represents a monovalent organic group, R ² represents a lower alkyl group having 1 to 8 carbon atoms, a represents an integer of 0 to 2, and b represents an integer of 0 to 4. However, 0 <= A + b <= 4.) The negative electrode material for lithium secondary batteries characterized by being a product produced | generated by the polycondensation of the 1 or more types of organosilicon compound precursor shown . 2. The negative electrode material for a lithium secondary battery according to claim 1, wherein 5 to 90% of all Si atoms in the organosilicon compound are bonded to a monovalent organic group. The weight ratio of carbon is coated on the Si particles, the negative electrode material for lithium secondary battery according to claim 1 or 2, characterized in that a 10 wt% 50 wt% relative to the Si particles. Organosilicon compounds, Al, Ti, negative electrode material for lithium secondary battery according to any one of claims 1 to 3, characterized in that it contains at least one metal selected from the group consisting of Zr. A method for producing a negative electrode material for a lithium secondary battery according to any one of claims 1 to 4 , wherein ball milling is used as a method for coating a single layer mixture of an organosilicon compound and carbon on Si particles. A method for producing a negative electrode material for a lithium secondary battery.

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