KR101577889B1 - Anode active material for lithium secondary battery and anode comprising the same - Google Patents

Abstract

The present invention relates to a negative electrode active material for a lithium secondary battery and a negative electrode active material slurry containing the same, and more particularly, to a slurry containing artificial graphite having a specific surface area of 4.0 m 2 / g or less and a carbonaceous material having a specific surface area of 10 m 2 / A negative electrode active material having improved dispersibility, a charge produced using the negative active material, a negative electrode having improved discharge rate, and a lithium secondary battery comprising the same.

Description

[0001] The present invention relates to an anode active material for a lithium secondary battery, and an anode comprising the anode active material.

The present invention relates to a negative electrode active material for a lithium secondary battery and a negative electrode comprising the same, and more particularly, to a negative electrode active material including artificial graphite having a small specific surface area and a carbonaceous material having a large specific surface area, and a negative electrode comprising the same.

Recently, miniaturization and weight reduction of electronic equipment and general use of portable electronic devices have led to active research on lightweight lithium secondary batteries having high energy density as their power source.

The lithium secondary battery generates electrical energy by oxidation and reduction reactions when lithium ions are inserted and removed from the negative electrode and the positive electrode. Therefore, the lithium secondary battery is manufactured by using a material capable of inserting and removing lithium ions as a cathode and an anode, and filling an organic electrolytic solution or a polymer electrolyte between the cathode and the anode. The negative electrode and the positive electrode are prepared by mixing an optional additive and each electrode active material to prepare an electrode slurry, applying the slurry on an electrode current collector, and then drying.

Since the lithium secondary battery uses an organic electrolyte solution, the lithium secondary battery exhibits a high energy density that exhibits a discharge voltage two times higher than that of a conventional battery using an aqueous alkaline solution.

On the other hand, a chalcogenide compound is used as a positive electrode active material of a lithium secondary battery, and examples thereof include LiCoO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _{1 - x} Co _x O ₂ (0 < x < 1), and the like.

Until recently, lithium metal was used as an anode active material of a lithium secondary battery. However, due to the short circuit of the battery due to the formation of dendrite, it is known that there is a risk of explosion, and it is replaced with a carbonaceous active material instead of lithium metal.

Examples of the carbonaceous active material include amorphous carbon such as soft carbon and hard carbon, and crystalline carbon such as natural graphite or artificial graphite having a theoretical limit capacity of 372 mAh / g.

The amorphous carbon has a large capacity, but has a disadvantage of large irreversibility in charge and discharge processes, and the crystalline carbon, such as natural graphite, has a disadvantage that life deterioration is severe.

There has been proposed a method of physically mixing hard carbon, which is an amorphous carbon, and natural graphite, which is a crystalline carbon, to produce an anode active material having improved lifetime deterioration. However, in the case of the above method, when the content of the crystalline carbon exceeds 40% by weight based on the total weight of the mixture, the lifetime of the lithium secondary battery still decreases.

Recently, attempts have been made to use hydrophobic artificial graphite having a large specific surface area, which is improved natural graphite.

However, hydrophobic artificial graphite has a very low dispersibility in a slurry of a water-based active material other than an organic solvent such as N-methyl-pyrrolidone, which makes it difficult to obtain a desired level of physical property of a battery.

Therefore, it is urgent to develop a negative electrode active material having a novel constitution for improving the dispersibility of the negative electrode active material.

Disclosed is a negative active material having improved dispersibility and a slurry for a water-based negative active material containing the same.

Also, the present invention provides an electrode having improved lifetime characteristics by using the negative electrode active material slurry and a lithium secondary battery having the electrode.

In order to solve the above problems, according to one embodiment of the present invention,

And a carbonaceous material having a specific surface area of 10 m &lt; 2 &gt; / g or more.

Also provided is a negative electrode active material slurry comprising (a) the negative electrode active material, (b) a binder resin, and (c) a polar solvent.

In addition, the present invention provides a negative electrode comprising the negative electrode active material slurry.

Further, the present invention provides a lithium secondary battery comprising the negative electrode, the positive electrode, the separator, and the electrolyte in which the lithium salt is dissolved.

In the present invention, a negative electrode active material slurry having a high dispersibility in an aqueous system can be prepared by using a negative electrode active material mixed with hydrophobic artificial graphite having a small specific surface area and a carbonaceous material having a large specific surface area. Also, a negative electrode having improved charging and discharging characteristics and a lithium secondary battery including the same can be manufactured using the same.

FIG. 1 is a graph showing the results of measurement of dispersibility (TG) of various negative electrode active materials according to mixing ratio of artificial graphite and carbonaceous material according to Experimental Example 1 of the present invention.
FIG. 2 is a graph showing the results of measurement of dispersibility (TG) of various negative electrode active materials according to the specific surface area of the carbonaceous material according to Experimental Example 2 of the present invention.
3 is a graph showing charge and discharge rate characteristics of a secondary battery according to Experimental Example 3 of the present invention.

Hereinafter, the present invention will be described in detail.

The present invention provides a negative electrode active material (a) comprising artificial graphite having a specific surface area of 4.0 m 2 / g or less and a carbonaceous material having a specific surface area of 10 m 2 / g or more.

Specifically, the artificial graphite may use hydrophobic artificial graphite having a specific surface area of 1.0 m 2 / g to 4.0 m 2 / g.

The hydrophobic artificial graphite has very low dispersibility in the aqueous negative electrode active material slurry, and it may be difficult to obtain the desired physical properties of the secondary battery. However, when artificial graphite having a specific surface area in the above range is used together with a carbonaceous material having a specific specific surface area, the dispersibility of artificial graphite in an aqueous negative electrode active material slurry can be increased. As a result, a uniform slurry of the negative electrode active material can be produced through high viscosity mixing. Therefore, the charge, discharge rate (C-rate) and lifetime characteristics can be improved when the secondary battery is applied.

The carbon-based material may have a specific surface area of 10 m 2 / g or more, preferably 10 m 2 / g to 100 m 2 / g, and more preferably 10 m 2 / g to 20 m 2 / g. If the specific surface area of the carbonaceous material is less than the above range, it is difficult to improve the dispersibility of artificial graphite in the aqueous system. If the specific surface area of the carbonaceous material exceeds the above range, Initial efficiency reduction and storage and lifetime performance may be degraded. Specifically, when the specific surface area of the carbon-based material becomes larger than necessary, the solvent (water) may be used in excess and the solids content in the negative electrode active material slurry may be lowered.

This is because when the electrode is evaporated, the remaining amount of the water evaporates and the uniformity of the electrode is lowered, and a large amount of binder may be required to bond the carbon-based material having a large specific surface area. However, if the binder content is insufficient, the adhesive strength of the electrode may be lowered. In this case, the formation of the SEI layer may be insufficient at the time of the first charging of the secondary battery, which may lead to deterioration of the efficiency, discharge capacity and lifetime characteristics of the secondary battery.

According to an embodiment of the present invention, the specific surface area of the artificial graphite and the carbon-based material may be measured by a BET (Brunauer-Emmett-Teller) method. For example, it can be measured by a BET 6-point method by a nitrogen gas adsorption / distribution method using a porosimetry analyzer (Bell Japan Inc, Belsorp-II mini).

The carbon-based material may include any one selected from the group consisting of carbon nanotubes, scaly natural graphite, and scaly natural graphite whose surface is coated with amorphous carbon, or a mixture of two or more thereof.

In the negative active material of the present invention, the artificial graphite and the carbonaceous material may be mixed in a weight ratio of 7: 3 to 5: 5. When the content of the carbonaceous material is less than 3, that is, less than 30% by weight based on the total weight of the negative electrode active material slurry, the effect of improving the dispersibility is insufficient and the improvement of the charge and discharge rate of the battery is not expected. Is more than 5, that is, exceeds 50 wt% based on the total weight of the negative electrode active material slurry, the initial efficiency and capacity of the battery may be reduced.

Also, according to an embodiment of the present invention, there is provided a negative electrode active material slurry comprising (a) the negative electrode active material, (b) a binder resin, and (c) a polar solvent.

At this time, as the polar solvent, a sufficient amount of water can be used.

The negative electrode active material may be included in an amount of about 40 to 50 wt% based on the total weight of the negative electrode active material slurry. If the content of the negative electrode active material is less than 40% by weight, the content of water in the entire slurry is large, so that not only the formation of the electrode is not smooth but also the trace of vaporization of the bubbles on the electrode surface may occur while the water evaporates. If the content of the negative electrode active material is more than 50% by weight, the binder may not be easily dispersed due to high viscosity, and the die filter of the die coater may be clogged.

The binder resin is not limited as long as it is an aqueous binder that can be used in the production of a negative electrode. Examples of the binder resin include, but not limited to, a rubber binder such as acrylonitrile-butadiene rubber, styrene-butadiene rubber (SBR) and acrylic rubber, And carboxymethylcellulose, and carboxymethylcellulose or styrene-butadiene rubber can be used in particular.

The binder resin may be contained in an amount of about 30 wt% or less, specifically 10-30 wt% based on the total weight of the negative electrode active material slurry. If the content of the binder is less than 10% by weight, the adhesive force of the electrode decreases, and swelling increases when the electrode is repeatedly charged and discharged. On the other hand, when the binder content exceeds 30% by weight, the resistance by the binder increases, the electric conductivity of the electrode decreases, and the initial efficiency reduction and output characteristics of the battery may be reduced.

The negative electrode active material slurry of the present invention may further contain oxalic acid as an additive.

The present invention also provides a negative electrode prepared by coating the negative electrode active material slurry on a negative electrode current collector having an appropriate thickness and length, or by forming the active material slurry into a film shape.

The present invention also provides a lithium secondary battery comprising the negative electrode, the positive electrode, the separator, and the electrolyte in which the lithium salt is dissolved.

The positive electrode is prepared by applying a positive electrode active material onto a positive electrode collector and then drying the positive electrode active material. In this case, the cathode active material may be a conventional cathode active material that can be used for a cathode of a general secondary battery. Examples of the cathode active material include LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li (Ni _a Co _b Mn _c O ₂ (0 <a <1, 0 <b <1, 0 <c <1, a + b + c = 1), LiNi _{1 -Y} Co _Y O ₂ , LiCo _{1 - Y} Mn _Y O ₂ , LiNi _{1 - Y} Mn _Y O ₂ (Here, 0≤Y <1), Li ( Ni a Co b Mn c) O 4 (0 <a <2, 0 <b <2, 0 <c <2, a + b + c = 2), LiMn _{2 - z} Ni _z O ₄ , LiMn _{2 - z} Co _z O ₄ (Where 0 <Z <2), LiCoPO ₄ , LiFePO ₄ Or a mixture thereof. Further, lithium manganese-based oxides such as Li _{1 + a} Mn _{2 - a} O ₄ (Where 0? A? 0.33), LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ , LiMn _{2 - a} M _a O ₂ (Where M = Co, Ni, Fe, Cr, Zn or Ta and 0.01? A? 0.1) or Li ₂ Mn ₃ MO ₈ (Where M = Fe, Co, Ni, Cu, or Zn), and the like.

The separator may be a polyethylene separator, a polypropylene separator, a polyethylene / polypropylene two-layer separator, a polyethylene / polypropylene / polyethylene three-layer separator, or a polypropylene / polyethylene / polypropylene three-layer separator.

Examples of the above lithium salt is not limited to _{LiPF 6, LiAsF 6, LiBF 4} , LiSbF 6, LiAlCl 4, LiClO 4, CF 3 SO 3L i, C 4 F 9 SO 3 Li, CF 3 COOLi, (CF 3 CO) ₂ NLi, (CF ₃ SO ₂ ) ₂ NLi or (C ₂ F ₅ SO ₂ ) NLi.

Examples of the electrolyte include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel polymer electrolyte, a solid inorganic electrolyte, and a molten inorganic electrolyte, which are usable in the production of a secondary battery, but are not limited thereto.

The lithium ion secondary battery of the present invention can be used as a power source for various electronic products. For example, a portable telephone, a mobile phone, a game machine, a portable television, a notebook computer, a calculator, and the like, but is not limited thereto.

Hereinafter, the present invention will be described in detail with reference to examples and comparative examples. However, the embodiments according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. Embodiments of the present invention are provided to more fully describe the present invention to those skilled in the art.

Example

(Example 1)

Artificial graphite having a specific surface area of 2 m 2 / g and scaly natural graphite having a specific surface area of 10 m 2 / g were mixed at a relative weight ratio of 7: 3 to prepare an anode active material.

Subsequently, the negative active material (46 g), SBR binder (20 g) and distilled water were mixed to prepare a negative electrode active material slurry having a solid content of 46%.

Next, the negative electrode active material slurry was applied to a copper foil and dried at 130 DEG C for 10 minutes to prepare a negative electrode.

Next, a separator (E16MMS manufactured by Tonen Co., Ltd.), ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a ratio of 1: 1 between the cathode and the anode, and then coated with lithium cobalt oxide (LiCoO ₂ ) injecting an electrolyte 1M LiPF ₆ is dissolved in a solvent mixture of 1 volume ratio to prepare a half cell of a coin.

(Example 2)

A negative electrode active material, a negative electrode and a coin type half cell comprising the same were prepared in the same manner as in Example 1, except that the mixing ratio of the artificial graphite and the carbonaceous material was changed to a relative weight ratio of 6: 4.

(Example 3)

A negative electrode active material, a negative electrode and a coin type half cell comprising the same were prepared in the same manner as in Example 1, except that the mixing ratio of the artificial graphite and the carbon-based material was set at a relative weight ratio of 5: 5.

(Example 4)

Except that artificial graphite having a specific surface area of 2 m 2 / g and graphite natural graphite having a specific surface area of 20 m 2 / g were mixed at a relative weight ratio of 5: 5 to prepare a negative electrode active material. To prepare a negative electrode active material, a negative electrode and a coin type half cell comprising the same.

(Comparative Example 1)

A negative electrode including a negative electrode active material and a coin type half cell were produced in the same manner as in Example 1 except that the negative electrode active material consisting only of artificial graphite having a specific surface area of 0.6 m 2 / g was used.

(Comparative Example 2)

A negative electrode active material, a negative electrode and a coin type half cell comprising the same were prepared in the same manner as in Example 1, except that the mixing ratio of the artificial graphite and the carbon-based material was set at a relative weight ratio of 8: 2.

(Comparative Example 3)

A negative electrode active material, a negative electrode and a coin type half cell comprising the same were prepared in the same manner as in Example 1, except that the mixing ratio of the artificial graphite and the carbon-based material was changed to a relative weight ratio of 4: 6.

(Comparative Example 4)

Except that artificial graphite having a specific surface area of 2 m 2 / g and graphite natural graphite having a specific surface area of 3 m 2 / g were mixed at a relative weight ratio of 5: 5 to prepare a negative electrode active material. To prepare a negative electrode active material, a negative electrode and a coin type half cell comprising the same.

(Comparative Example 5)

Except that artificial graphite having a specific surface area of 2 m 2 / g and graphite natural graphite having a specific surface area of 5 m 2 / g were mixed at a relative weight ratio of 5: 5 to prepare a negative electrode active material. To prepare a negative electrode active material, a negative electrode and a coin type half cell comprising the same.

(Experimental Example 1: Measurement of dispersibility according to carbon content)

Each of the anode active material slurries prepared in Examples 1 to 3 and Comparative Examples 1 to 3 was prepared and stored in a 20-mL-sized glass shell for one week at room temperature. Three mL of each supernatant was extracted, and the solvent was evaporated. And the weight of the solid was measured. The measurement was carried out three times, and the results are shown in Fig.

 As can be seen from FIG. 1, when the anode active material mixed with hydrophobic artificial graphite by adding 30% to 50% of carbonaceous material was used as in Examples 1 to 3, the weight of solids remaining after evaporation of solvent The solid weight was about 0.8 g when 100% artificial graphite was used, and 20% and 60%, respectively, as in Comparative Examples 2 and 3. The solid weight From about 0.8 g to 0.9 g, it can be confirmed that the solid content weight is remarkably low to about 20% to 50% as compared with Examples 1 to 3 of the present invention.

It was found that the hydrophobic artificial graphite and the carbonic material having a specific surface area of 10 m &lt; 2 &gt; / g or more were applied at a specific mixing ratio as in Examples 1 to 3 of the present application to improve the dispersibility of the hydrophobic artificial graphite, .

(Experimental Example 2: Measurement of dispersibility according to specific surface area (BET) of carbonaceous material)

Each of the negative electrode active material slurries prepared in Examples 3 and 4 and Comparative Examples 4 and 5 was prepared and stored in a 20-mL-sized glass beaker at room temperature for one week. Subsequently, 3 mL of the supernatant was extracted and the solvent was evaporated. , The degree of improvement in the dispersibility of artificial graphite in an aqueous system was determined. The measurement was carried out three times, and the results are shown in Fig.

2, when a carbon-based material having a specific surface area (BET) of 3 m 2 / g and 5 m 2 / g is used as in Comparative Examples 4 and 5, the dispersibility of artificial graphite in the aqueous system is sufficiently It can be understood that the solid content remaining after the evaporation of the solvent is relatively decreased as compared with Examples 1 and 2 of the present invention.

2, carbon-based materials having a specific surface area of 10 m 2 / g and 20 m 2 / g were added to hydrophobic artificial graphite at a ratio of 50:50 as in Examples 3 and 4 of the present application In the case of using the negative electrode active material, the weight of the solid remaining after evaporation of the solvent is about 1.25 g to 1.35 g, whereas when the specific surface area of the carbonaceous material is too small as 3 m 2 / g as in Comparative Example 4, It was only about 0.6 g. In the case of Comparative Example 5 having a specific surface area of 5 m 2 / g of the carbon-based material, the solid content weight was increased as compared with Comparative Example 4, but 1.0 g was still significantly lower than that of the Example.

(Experimental Example 3: Measurement of charge and discharge rate)

Each of the coin-type half-cells manufactured in Examples 1 to 3 and Comparative Example 1 prepared above was charged and discharged at a room temperature in a voltage range of 0.005 V to 1.5 V, and characteristics were compared. The charge was rated at 0.1 C-rate and the discharge was rated at 0.1 C to 10.0 C with increasing current. Charging was CCCV mode, CV charging was done at 0.005V until 0.005C, and discharge was discharged to 0.5V in CC mode.

As shown in FIG. 3, at the 10 C-rate, the batteries of Examples 1 to 3 were about 95% or more, while those of Comparative Example 1 were as low as 94% or less.

From this, it is confirmed that the charge and discharge characteristics of the lithium secondary battery including the negative electrode active material according to the present invention are remarkably excellent.

According to an embodiment of the present invention, by mixing the hydrophobic artificial graphite having a small specific surface area with the carbon-based material having a large specific surface area, it is possible to induce high dispersibility of artificial graphite in the slurry of the water based negative electrode active material, It can be predicted that this is caused by the uniform preparation of the negative electrode active material slurry.

Claims (13)

Artificial graphite having a specific surface area of 4.0 m &lt; 2 &gt; / g or less, and
Based material having a specific surface area of 10 m &lt; 2 &gt; / g to 20 m &lt; 2 &gt; / g,
The specific surface area of the carbon-based material is 5 to 10 times the specific surface area of artificial graphite,
The mixing ratio (weight ratio) of the artificial graphite and the carbonaceous material is 7: 3 to 5: 5,
Wherein the carbon-based material includes carbon nanotubes, natural graphite, or a mixture thereof.
The method according to claim 1,
Wherein the specific surface area of the artificial graphite is 1.0 m &lt; 2 &gt; / g to 4.0 m &lt; 2 &gt; / g.
delete delete delete delete A negative electrode active material slurry comprising (a) the negative electrode active material according to claim 1, (b) a binder resin, and (c) a polar solvent.
The method of claim 7,
Wherein the negative electrode active material is contained in an amount of 40 to 50 wt% based on the total weight of the negative electrode active material slurry.
The method of claim 7,
Wherein the binder resin is any one selected from the group consisting of acrylonitrile-butadiene rubber, styrene-butadiene rubber, acrylic rubber, hydroxyethyl cellulose and carboxymethyl cellulose, or a mixture of two or more thereof. .
The method of claim 7,
Wherein the binder resin is contained in an amount of 10 to 30% by weight based on the total weight of the negative electrode active material slurry.
The method of claim 7,
Wherein the polar solvent is water.
A negative electrode comprising the negative electrode active material slurry according to claim 7.
A cathode, an anode, a separator, and an electrolyte in which a lithium salt is dissolved,
Wherein the negative electrode is the negative electrode according to claim 12.

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