KR20170127240A - Electrode, method for fabricating the same, and lithium secondary battery comprising the same - Google Patents

Abstract

The present invention relates to: an electrode which includes an active material, a conductive material including a linear conductive material and a planar conductive material at a weight ratio of 3:7 or 7:3, and a binder; a manufacturing method thereof; and a lithium secondary battery including the same. One objective of the present invention is to provide the electrode capable of reducing production costs, improving productivity, and reducing damage to the electrode during a rolling process. Another objective of the present invention is to provide the lithium secondary battery capable of having an excellent capacity retention rate and excellent resistance characteristics even if charging and discharging are repeated at a high speed. The other objective of the present invention is to provide the lithium secondary battery capable of reducing a swelling phenomenon of a battery caused by side reactions of the conductive material and an electrolyte.

Description

TECHNICAL FIELD The present invention relates to an electrode, a method of manufacturing the same, and a lithium secondary battery including the same. BACKGROUND OF THE INVENTION 1. Field of the Invention [0001]

The present invention relates to an electrode, a method for producing the same, and a lithium secondary battery including the same.

As technology development and demand for mobile devices increase, the demand for secondary batteries as energy sources is rapidly increasing. Of these secondary batteries, lithium secondary batteries having high energy density and operating potential, long cycle life, and low self-discharge rate are commercially available and widely used.

In recent years, as interest in environmental problems has increased, there has been a growing interest in environmental pollutants, such as gasoline vehicles (EVs), hybrid electric vehicles (HEVs), and the like, which can replace fossil- And the like. As a power source for such electric vehicles (EV) and hybrid vehicles (HEV), a lithium ion battery having high output, high capacity retention rate, excellent charge / discharge characteristics and high temperature stability .

KR 2014-0002646 A

An object of the present invention is to provide an electrode which can reduce the production cost and can improve the productivity while reducing the damage of the electrode during the rolling process.

Another object of the present invention is to provide a lithium rechargeable battery excellent in capacity retention rate and resistance characteristic even when charging / discharging is repeated at a high temperature.

Another object of the present invention is to provide a lithium secondary battery capable of reducing a swelling phenomenon of a battery due to a side reaction between a conductive material and an electrolyte.

In order to solve the above-described problems, A conductive material comprising a linear conductive material and a planar conductive material in a weight ratio of 3: 7 to 7: 3; And a binder.

The present invention also provides a method for forming an electrode comprising a conductive material, a binder and a solvent, which comprises an active material, a linear conductive material and a planar conductive material in a weight ratio of 3: 7 to 7: 3, ; And a second step of rolling the composition for electrode formation.

Further, the present invention provides a lithium secondary battery including the electrode.

The electrode according to the present invention can use a smaller amount of solvent than in the conventional process. As a result, the production cost of the electrode is reduced, the manufacturing time is shortened, and the productivity is improved.

In the electrode according to the present invention, damage to the active material contained in the electrode is reduced during the rolling process, and uniform rolling density can be achieved over the entire area of the electrode.

The lithium secondary battery according to the present invention is excellent in capacity retention rate and resistance characteristic even when charging / discharging is repeated at a high temperature.

The lithium secondary battery according to the present invention can reduce the swelling phenomenon of the battery due to the side reaction between the conductive material and the electrolyte.

FIG. 1 is a SEM photograph of a carbon nanotube used as a linear conductive material in Examples 1 to 3. FIG.
Fig. 2 is an SEM photograph of graphene used as a planar conductive material in Examples 1 to 3; Fig.
3 is a SEM photograph of the anode of Example 1. Fig.
4 is an SEM photograph enlarged in Fig.
5 is an SEM photograph of the anode of Example 2;
6 is an SEM photograph enlarged in Fig.
7 shows measured values obtained by repeating charging the lithium secondary battery of Example 3 and the lithium secondary battery of Comparative Example 3 at 45 ° C from SOC100 to SOC0 at 10 C and repeating the cycle up to 900 cycles.
8 shows the measured values of the capacity retention rate and the resistance retention ratio of the lithium secondary battery of Example 3 and the lithium secondary battery of Comparative Example 3 that were changed over time after charging the secondary battery at 60 DEG C with SOC100.
9 is a graph showing the measured values of the lithium secondary battery of Example 4 and the lithium secondary battery of Comparative Example 3 repeatedly charged up to 900 C from SOC 80 to SOC 20 at 45 ° C and discharging at 10 ° C.
10 shows the measured values of the capacity retention ratio and the resistance retention ratio at 60 캜 after charging the lithium secondary battery of Example 4 and the lithium secondary battery of Comparative Example 3 with SOC100 at room temperature.

Hereinafter, the present invention will be described in detail in order to facilitate understanding of the present invention.

The terms and words used in the present specification and claims should not be construed in an ordinary or dictionary sense and the inventor can properly define the concept of the term to describe its invention in the best possible way It should be construed as meaning and concept consistent with the technical idea of the present invention.

An electrode according to an embodiment of the present invention includes an active material; A conductive material comprising a linear conductive material and a planar conductive material in a weight ratio of 3: 7 to 7: 3; And a binder.

The active material contained in the electrode according to an embodiment of the present invention stores the energy of the lithium secondary battery, and the active material may be a cathode active material or an anode active material. However, in the electrode according to an embodiment of the present invention, have.

The cathode active material is a compound capable of reversibly intercalating and deintercalating lithium, and examples thereof include spheres, polyhedrons, fibers, platelets, and scales.

Specific examples of the positive electrode active material include lithium cobalt oxide such as Li _x CoO ₂ (0.5 <x <1.3); Lithium nickel oxides such as Li _x NiO ₂ (0.5 < x &lt;1.3); Lithium manganese oxides such as Li _{1 + x} Mn _{2 - x} O ₄ (0 _{? X?} 0.33), LiMnO ₃ , LiMn ₂ O ₃ , or Li _x MnO ₂ (0.5 <x <1.3); Lithium copper oxide such as Li ₂ CuO ₂ ; Lithium iron oxides such as LiFe ₃ O ₄ ; Lithium nickel cobalt manganese oxide such as Li [Ni _x Co _y Mn _z ] O ₂ (x + y + z = 1, 0 <x <1, 0 <y <1, 0 <z <1); Lithium nickel cobalt aluminum oxide such as Li [Ni _x Co _y Al _z ] O ₂ (x + y + z = 1, 0 <x <1, 0 <y <1, 0 <z <1); Lithium vanadium compounds such as LiV ₃ O ₈ ; Nickel-type lithium nickel oxides such as LiNi _{1 - x} M _x O ₂ (M = Co, Mn, Al, Cu, Fe, Mg, B or Ga and 0.01? _X? 0.3); _{LiMn 2 - x M x O 2} (M = Co, Ni, Fe, Cr, Zn or Ta, 0.01≤x≤0.1), or _{Li 2 Mn 3 MO 8 (M} = Fe, Co, Ni, Cu or Zn), etc. Lithium manganese composite oxide; LiMn ₂ O _{4 in} which a part of lithium is substituted with an alkaline earth metal ion; Disulfide compounds; Vanadium oxides such as V ₂ O ₅ or Cu ₂ V ₂ O ₇ ; Or Fe ₂ (MoO _{4) 3} and the like, more specifically, _{Li [Ni x Co y Mn z} ] O 2 (x + y + z = 1, 0.3≤x≤0.8, 0.1≤y≤0.5, 0.1≤z≤0.5) such as a lithium nickel cobalt manganese oxide or _{Li [Ni x Co y Al z} ] O 2 (x + y + z = 1, 0.75≤x≤0.85, 0.1≤y≤0.17, 0 <z≤ of 0.08), or the like. These may be contained in the positive electrode active material in one or two or more kinds.

Specific examples of the negative electrode active material include low crystalline carbon such as hard carbon or soft carbon; Natural graphite, kish graphite, pyrolytic carbon, mesophase pitches, mesophase pitch based carbon fiber, meso-carbon microbeads, petroleum-derived Highly crystalline carbons such as petroleum derived cokes or coal tar pitch derived cokes; Li _x Fe ₂ O ₃ (0 _{≤ x ≤} 1), Li _x WO ₂ (0 _{≤ x ≤} 1), Sn _x Me _{1 - x} Me _y O _z (Me = Mn, Fe, Pb or Ge; Me ' = Al, B, P, Si, Group 1, Group 2, Group 3 elements or halogens of the periodic table, and 0 <x <1> 1 <y>3; Lithium metal; Lithium alloy; Silicon alloy; Silicon oxides; Tin alloy; Metal oxides such as SnO, SnO ₂ , PbO, PbO ₂ , Pb ₂ O ₃ , Pb ₃ O ₄ , GeO, GeO ₂ , Bi ₂ O ₃ , or Bi ₂ O ₅ ; Conductive polymers such as polyacetylene; Li-Co-Ni-based materials; Titanium oxide; Or lithium titanium oxide, and one or more of these compounds may be included.

The active material may be contained in an amount of 95.0% by weight to 98.0% by weight based on the total weight of the electrode. If the above range is satisfied, the amount of the conductive material to be used is reduced and the active material content is increased, the anode loading amount (coating amount including the active material / conductive material / binder) decreases at the same mAh / There is an effect that the thickness is reduced. As a result, it is possible to provide a lithium secondary battery having high output characteristics and improved energy density (Wh / L).

The conductive material included in the electrode according to one embodiment of the present invention is included to improve the electrical conductivity of the active material and includes a linear conductive material and a planar conductive material in a weight ratio of 3: 7 to 7: 3.

When the linear conductive material and the planar conductive material are contained while satisfying the weight ratio described above, the viscosity of the electrode composition can be maintained to a degree that enables coating on the electrode current collector even when a smaller amount of solvent is used in forming the electrode composition . Accordingly, the amount of the solvent used is reduced, so that the production cost of the electrode is reduced, the drying time is shortened, and the productivity can be improved.

In addition, since a smaller amount of solvent is used in forming the electrode composition, the thickness of the electrode composition formed on the electrode current collector becomes thinner than the conventional electrode composition even when an electrode composition including a positive active material, a conductive material and a binder is used . Accordingly, since the rolling can be performed at a relatively low intensity in the rolling process for rolling to the target electrode thickness, the damage to the active material is small and the rolling density of the whole area of the electrode can be equalized.

Further, when a side reaction occurs between the conductive material and the electrolyte unexpectedly at a high temperature, side reactions may occur at the surface of the conductive material exhibiting conductivity. If the linear conductive material having a large specific surface area and the planar conductive material having a small specific surface area are properly mixed, the specific surface area is reduced compared to the amount of the conductive material, thereby reducing the space in which side reactions can occur. As a result, the hot side reaction is lower than when the linear conductive material is included alone, and the swelling phenomenon of the battery can be reduced.

On the other hand, the linear conductive material may be present in the electrode in a twisted shape rather than a straight line in the electrode, more specifically, a shape in which a plurality of carbon nanotubes are not limited to a specific orientation but are entangled with each other without a certain shape. Due to this form, it is difficult to realize an electrical network in the electrode as long as it has inherent. However, in the case of the surface-type conductive material, since it exists in the electrode as it is in its original size, it is possible to realize an electrical network in the electrode as intended.

When the linear conductive material is contained in an amount greater than the above-mentioned range, the linear conductive material is excessively applied to the surface of the active material. Therefore, the linear conductive material is excessively coated on the surface of the active material, . However, if the planar conductive material is included in the above-described ratio, not only this phenomenon can be prevented but also the movement of lithium ions around the active material can be smoothly performed.

Since the linear conductive material is distributed on the active material and the planar conductive material is distributed between the active materials, it is possible to realize an even electrical network by increasing the electrical contact between the active materials. Accordingly, even when the active material repeatedly shrinks and expands at the time of charging / discharging lithium secondary battery at a high temperature, shorting between the active material and the conductive material is suppressed, thereby reducing the rate of increase in resistance and improving the capacity retention rate.

The ratio of the average diameter to the length of the preceding conductive material may be from 1: 100 to 1: 300.

The average diameter of the linear conductive material may be 8 nm to 12 nm while the ratio of the average diameter to the length is satisfied, and the length of the linear conductive material may be 0.8 to 3.6 탆. If the linear conductive material satisfies the above average diameter and length, the specific surface area of the preceding conductive material may be 200 m2 / g to 330 m2 / g, and more specifically 250 m2 / g to 260 m2 / g. If the above range is satisfied, the number of absolute strands per volume is appropriately included, thereby making it possible to manufacture an electrode with a high energy density and easy dispersion of the linear conductive material in the electrode. In addition, the conductivity and strength of the electrode and the service life of the electrode at room temperature and high temperature can be improved. In addition, coating stability is improved when an electrode slurry is formed, and excellent storage performance can be exhibited at a high temperature.

Here, the specific surface area of the linear conductive material means a specific surface area per g including a plurality of linear conductive materials.

Here, the average diameter and the length can be measured using a field emission scanning electron microscope. The specific surface area can be measured by a BET (Brunauer-Emmett-TELLER: BET) method. Specifically, the specific surface area can be calculated as the amount of nitrogen gas adsorbed at a liquid nitrogen temperature (77 K) using BEL Japan's BELSORP-mino II .

Specific examples of the linear conductive material include carbon nanotubes, carbon nanorods, and carbon nanofibers.

Since the carbon nanotubes have a very high strength and a high resistance to fracture, it is possible to prevent repetition of charge and discharge and deformation of the electrode due to external force. In addition, oxidation of the electrode surface in a non-ideal battery environment such as high temperature and overcharge can be prevented, so that the safety of the battery can be greatly improved. The carbon nanotubes are classified into single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs) and multi-walled carbon nanotubes (DWCNTs) according to the number of carbon atoms MWCNT, and multi-walled carbon nanotubes. The single-walled carbon nanotube is excellent in electrical conductivity and thermal conductivity. The double walled carbon nanotubes are excellent in electrical conductivity and mechanical properties. The multi-walled carbon nanotubes are excellent in mechanical properties and can be mass-synthesized, so that they are easy to manufacture and have a wide application range.

The carbon nanotubes may include a main catalyst or a cocatalyst-derived metal element such as Fe, Ni, Co, Mo, V, or Cr used in the manufacturing process as an impurity. Of the impurities, one or a combination of Fe, Ni, and Mo may be included in an amount of 3 mg or less, specifically, 2 mg or less, based on 1 kg of the carbon nanotubes. If the carbon nanotubes do not contain Fe as an impurity, excellent conductivity can be exhibited without concern for side reaction in the electrode.

Meanwhile, the content of the impurities in the carbon nanotubes can be analyzed using inductively coupled plasma (ICP).

The carbon nanotubes may be commercially available or may be used directly. More specifically, it can be manufactured by a conventional method such as an arc discharge method, a laser evaporation method, or a chemical vapor deposition method. In the course of manufacturing, by controlling the kind of the catalyst, the heat treatment temperature, Physical properties can be realized.

Unlike the linear conductive material, the planar conductive material may be distributed among the active materials and may exist in a laminated form of 10 to 20 sheets.

The aspect ratio of the planar conductive material, that is, the aspect ratio may be 1: 1 to 1:10.

If the aspect ratio described above is satisfied, the width of the planar conductive material may be 1 탆 to 10 탆, the length may be 1 탆 to 100 탆, and the thickness may be 5 nm to 30 nm. The specific surface area of the planar conductive material may be from 40 m2 / g to 60 m2 / g, provided that the planar conductive material satisfies the abovementioned width, length and thickness. When the above-described range is satisfied, a longer electrical network can be formed when only the linear conductive material is contained in the electrode. Therefore, even if the active material repeatedly shrinks and expands due to repetitive charging and discharging, increase in resistance due to contact instability between the conductive material and the active material can be suppressed. In addition, since the surface area of the planar conductive material is small and the planar conductive material replaces the linear conductive material, even if a side reaction with an unexpected electrolyte occurs at a high temperature, the scale of the side reaction is not large and the swelling phenomenon Swelling phenomenon due to gas generation) is reduced.

On the other hand, in the case of a high-output lithium secondary battery, an electrode having high porosity can be used to facilitate the movement of lithium ions. Due to the high porosity, a large amount of conductive material may be required. The advantage of the planar conductive material is that it is more advantageous than the linear conductive material to fill wide and long pores.

Here, the average diameter and the length can be measured using a field emission scanning electron microscope. The specific surface area can be measured by a BET (Brunauer-Emmett-TELLER: BET) method. Specifically, the specific surface area can be calculated as the amount of nitrogen gas adsorbed at a liquid nitrogen temperature (77 K) using BEL Japan's BELSORP-mino II .

Specific examples of the planar conductive material include graphene and graphite.

The graphene may be contained in an electrode according to an embodiment of the present invention in a laminated form of 10 to 20 sheets. The graphene refers to a thin film made of carbon atoms and having a thickness of one atom. The graphene is excellent in physical strength, thermal conductivity and flexibility, and has low resistance.

The graphene may be commercially available and used, or it may be used by direct manufacture. More specifically, it can be produced by a conventional method such as a physical stripping method, a chemical vapor deposition method, a chemical stripping method, or an epitaxial synthesis method. In the course of the production, by controlling the kind of the catalyst and the heat treatment temperature, Physical properties can be realized.

The graphite can be used as a planar conductive material by delamination through a wet milling process. The graphite can be used both natural and artificial, and can be obtained from nature or by heat treating the carbon source at high temperature.

The conductive material may be included in an amount of 1.0 wt% to 2.5 wt% with respect to the total weight of the electrode. When the above-described range is satisfied, the low electrical conductivity of the active material can be compensated.

The binder included in one embodiment of the present invention is included to provide an adhesive force between the electrode slurry and the electrode current collector and to provide a bonding force between the active material and the conductive material in the electrode slurry.

The weight average molecular weight of the binder may be more than 750,000 g / mol and 900,000 g / mol. When the weight average molecular weight is satisfied, both the bonding force between the linear conductive material and the active material and the bonding force between the planar conductive material and the active material can be improved.

Specific examples of the binder include polyvinylidene fluoride, polyhexafluoropropylene, polytetrafluoroethylene, and copolymers of polyhexafluoropropylene-polyvinylidene fluoride. These may be used alone or in combination of two or more.

The binder may be contained in an amount of 1.0 wt% to 2.5 wt% based on the total weight of the electrode. When the above-mentioned range is satisfied, the adhesive force between the electrode slurry and the electrode current collector and the bonding force between the active material and the conductive material in the electrode slurry are excellent.

The electrode according to an embodiment of the present invention may further include a dispersing agent to improve dispersibility of the conductive material.

The dispersant may be a nitrile rubber, more specifically, a partially or totally hydrogenated nitrile butadiene rubber. The hydrogenated nitrile-butadiene-based rubber may include a conjugated diene-derived structural unit, a hydrogenated conjugated diene-derived structural unit, and an?,? - unsaturated nitrile derived structural unit.

The hydrogenated acrylonitrile-butadiene rubber (H-NBR) may have a weight average molecular weight of 10,000 g / mol to 700,000 g / mol, more specifically 10,000 g / mol to 300,000 g / mol.

The partially hydrogenated acrylonitrile-butadiene rubber (H-NBR) has a polydispersity index PDI (Mw / Mn ratio, Mw is weight average molecular weight and Mn is number average Molecular weight). When the partially hydrogenated acrylonitrile-butadiene rubber satisfies the weight average molecular weight and the polydispersity index within the above-mentioned range, the carbon nanotubes can be uniformly dispersed in the solvent. The weight average molecular weight and the number average molecular weight are molecular weight in terms of polystyrene analyzed by gel permeation chromatography (GPC).

Hereinafter, a method of manufacturing an electrode according to an embodiment of the present invention will be described.

A method of manufacturing an electrode according to an embodiment of the present invention is a method of manufacturing an electrode including an active material, a conductive material including a linear conductive material and a planar conductive material in a weight ratio of 3: 7 to 7: 3, To form a composition for forming a first layer.

The first step is a step of forming the electrode forming composition on the current collector and drying and forming the electrode forming composition on a separate support and then peeling the support from the support to form a film on the current collector And then laminating them.

The collector may be a cathode current collector or a cathode current collector, and may have a thickness of 3 to 500 탆. The current collector has high conductivity and can easily adhere to the composition for electrode formation and is not reactive in the voltage range of the battery. Specific examples of the cathode current collector include aluminum, nickel, and alloys thereof. Specific examples of the negative electrode current collector include copper, gold, nickel, and alloys thereof.

Wherein the composition for electrode formation is produced by dispersing a conductive material containing the linear conductive material and the planar conductive material in a weight ratio of 3: 7 to 7: 3 in a solvent to prepare a conductive material dispersion, , And if necessary, further adding a solvent. The conductive material dispersion may further include a dispersant.

The conductive material and the solvent may be mixed using a mixing device such as a homogenizer, a bead mill, a ball mill, a basket mill, an impact mill, a universal stirrer, a clear mixer or a TK mixer. The mixing of the conductive material and the solvent may be performed by a cavitation dispersion treatment in order to improve the mixing property and dispersibility. Here, the cavitation dispersion treatment is a dispersion treatment method using a shock wave generated by rupture of vacuum bubbles formed in water when high energy is applied to a liquid. By this method, it is possible to disperse have. The cavitation dispersion treatment may be performed by ultrasonic wave, jet mill, or shear dispersion treatment.

The viscosity of the electrode forming composition may be 3,000 cps to 15,000 cps at room temperature. When the viscosity range described above is satisfied, the process is easy.

In the first step, the electrode forming composition is formed on the current collector by forming a doctor blade, a comma coating, or the like on the current collector. , Screen printing or gravure coating, or the like.

The electrode forming composition may be formed on the current collector in a loading amount of 55 mg / cm 2 to 30 mg / cm 2, and specifically, a loading amount of 6 mg / cm 2 to 10 mg / cm 2. When formed in the above-described amounts, the charging / discharging rate characteristics and the charge / discharge life characteristics of the lithium secondary battery as a final product can be improved.

The electrode forming composition formed on the current collector may be dried at a temperature of 100 ° C to 150 ° C. When the drying process is performed, the solvent included in the electrode forming composition may be removed.

The conductive material, the binder and the dispersing agent containing the active material, the linear conductive material and the planar conductive material in the weight ratio of 3: 7 to 7: 3, which are contained in the composition for electrode formation, have been described above and thus will be omitted.

The solvent included in the electrode forming composition may be an amide polar organic solvent such as dimethylformamide (DMF), diethylformamide (DEF), dimethylacetamide (DMAc) or N-methylpyrrolidone (NMP); Propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl -2-propanol (tert-butanol), pentanol, hexanol, heptanol or octanol; Glycols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, or hexylene glycol; Polyhydric alcohols such as glycerin, trimethylol propane, pentaerythritol, or sorbitol; Ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, tetraethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monoethyl ether, triethylene glycol monoethyl ether, tetraethylene Glycol ethers such as glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether or tetraethylene glycol monobutyl ether; Ketones such as acetone, methyl ethyl ketone, methyl propyl ketone, or cyclopentanone; And esters such as ethyl acetate,? -Butyllactone and? -Propiolactone, and the like, and mixtures of two or more of them may be used. Considering the effect of improving the dispersibility of the linear conductive material and the planar conductive material, the solvent may be an amide-based polar solvent.

A method of manufacturing an electrode according to an embodiment of the present invention includes a second step of rolling the electrode forming composition.

Since the composition for electrode formation of the present invention uses about 30% less solvent than the conventional electrode composition, even when an electrode composition including a positive active material, a conductive material, and a binder is used, the electrode composition Is thinner than the conventional one. As a result, since the rolling can be performed at a weaker strength than that of the conventional method, the damage to the active material can be reduced and the rolling density of the entire area of the electrode can be equalized.

The method for manufacturing an electrode according to an embodiment of the present invention may further include drying the rolled electrode forming composition. The drying step may be a step of removing moisture in the electrode forming composition. The drying may be performed at a temperature of 100 ° C to 200 ° C for 8 hours to 12 hours. The drying step may be performed in an oven in a vacuum state without air circulation.

Hereinafter, a lithium secondary battery according to another embodiment of the present invention will be described.

A lithium secondary battery according to another embodiment of the present invention includes an anode assembly including an anode, a cathode, and a separator disposed between the anode and the cathode, and an electrolyte. The anode or the cathode may be an electrode according to an embodiment of the present invention, and more specifically, the anode may be an electrode according to an embodiment of the present invention.

The separation membrane prevents a short circuit between the positive electrode and the negative electrode, and provides a movement path of lithium ions. The separator may be an insulating thin film having high ion permeability and mechanical strength. Specific examples of the separation membrane include polyolefin-based polymer membranes such as polypropylene and polyethylene, or their multiple membranes, microporous films, woven fabrics, and nonwoven fabrics. When a solid electrolyte such as a polymer is used as an electrolyte to be described later, the solid electrolyte can also serve as a separation membrane.

The electrolyte may be an electrolyte containing a lithium salt. Examples of the lithium salt of the anion is ^{F -, Cl -, Br -} , I -, NO 3 -, N (CN) 2 -, BF 4 -, ClO 4 -, PF 6 -, (CF 3) 2 PF 4 _{^{-, (CF 3) 3 PF}} 3 -, (CF 3) 4 PF 2 -, (CF 3) 5 PF -, (CF 3) 6 P -, CF 3 SO 3 -, CF 3 CF 2 SO 3 -, _{(CF 3 SO 2) 2 N} -, (FSO 2) 2 N -, CF 3 CF 2 (CF 3) 2 CO -, (CF 3 SO 2) 2 CH -, (SF 5) 3 C -, (CF ₃ SO ₂ ) ₃ C ^- , CF ₃ (CF ₂ ) ₇ SO ₃ ^- , CF ₃ CO ₂ ^- , SCN ^- or (CF ₃ CF ₂ SO ₂ ) ₂ N ^- . These may be contained in the electrolyte either singly or in combination.

The outer shape of the lithium secondary battery according to another embodiment of the present invention is not particularly limited, but specific examples thereof include a cylindrical shape, a square shape, a pouch shape, or a coin shape using a can.

The lithium secondary battery according to another embodiment of the present invention can be used not only in a battery cell used as a power source for a small device but also as a unit cell in a middle or large battery module including a plurality of battery cells. Specific examples of the medium and large-sized devices include, but are not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, or electric power storage systems.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the present invention as defined by the appended claims. Such variations and modifications are intended to be within the scope of the appended claims.

Example  1 and Example  2: Preparation of anode

Carbon nanotubes and N-methyl pyrrolidone (NMP) were mixed as the linear conductive material described in Table 1 below and a linear conductive material dispersion was formed using beads mill. Grapine and N-methylpyrrolidone (NMP) were mixed as the planar conductive material described in Table 1 below, and a planar conductive material dispersion was formed by using a bead mill. The linear conductive material dispersion and the planar conductive material were mixed so that the linear conductive material and the planar conductive material were mixed at the weight ratios shown in Table 1, and then Li [Ni _0.3 Co _0.4 Al _0.3 ] O ₂ and the binder 0.0 &gt; PVdF &lt; / RTI &gt; Then, the slurry for electrode formation was completed by mixing for 1 hour using a TK mixer and a homogenizer. On the other hand, the weight ratios of the sum of the positive electrode active material, the conductive material and the binder and the N-methyl pyrrolidone (NMP) shown in Table 1 are shown in Table 2.

Thereafter, a composition for forming an electrode was formed on an aluminum (Al) thin film having a thickness of 20 μm by a screen printing method, and then primary drying was performed at a temperature of the circulating air of 140 ° C. At this time, the thickness of the primary dried anode was 64 占 퐉. Then, after roll pressing, the cathode was dried at 100 ° C for 24 hours to prepare a positive electrode having a thickness of 44 μm.

Meanwhile, the properties of carbon nanotubes and graphenes used as a conductive material are shown in Table 3 below. FIG. 1 shows SEM pictures of carbon nanotubes and FIG. 2 shows SEM pictures of graphenes.

3 to 6 are SEM photographs of the anode. Referring to FIGS. 3 to 6, carbon nanotubes, which are linear conductive materials, are mainly distributed on the cathode active material, and graphene, which is a planar conductive material, is distributed among the cathode active materials.

division Cathode active material Conductive material bookbinder ingredient
(Weight ratio) content
(weight%) ingredient
(Weight ratio) content
(weight%) ingredient content
(weight%) Example 1 Li [Ni _0.3 Co _0.4 Al _0.3 ] O ₂ 97.4 Carbon nanotubes:
Graffin = 7: 3 1.2 PVdF 1.4 Example 2 Carbon nanotubes:
Graffin = 6: 4

division Sum of cathode active material, conductive material and binder menstruum Viscosity
(cps) Weight ratio Example 1 55 45 7,500 Example 2 56 44 7,800

size Secondary structural feature Specific surface area Carbon nanotube Average diameter: 10 nm
Average length: 1 탆 A plurality of carbon nanotubes are not limited to specific orientations but are entangled with each other without a certain shape 255㎡ / g Grapina Average width: 5㎛
Average length: 5㎛
Average thickness: 5 nm 10 to 20 or more graphene laminated shapes 55 m2 / g

Comparative Example  1 and Comparative Example  2: Preparation of anode

Carbon nanotubes or graphene were mixed with N-methylpyrrolidone (NMP) as a conductive material and a conductive material dispersion was formed using a beads mill. Thereby forming a conductive material dispersion. Li [Ni _0.3 Co _0.4 Al _0.3 ] O ₂ as a cathode active material and PVdF as a binder were added to the conductive material dispersion and mixed with a TK mixer and a homogenizer for 1 hour to complete a slurry for electrode formation. On the other hand, the weight ratios of the sum of the cathode active material, the conductive material and the binder and the N-methyl pyrrolidone (NMP) shown in Table 4 are shown in Table 5.

Thereafter, a composition for forming an electrode was formed on an aluminum (Al) thin film having a thickness of 20 μm by a screen printing method, and then the temperature of circulated air was first dried at 140 ° C. At this time, the thickness of the primary dried anode was 64 占 퐉. Subsequently, after roll pressing, the anode was dried at 130 DEG C for 12 hours to prepare a cathode having a thickness of 44 mu m.

On the other hand, the characteristics of the carbon nanotubes and graphenes used as the conductive material in the preparation of the positive electrode of Comparative Example 1 and Comparative Example 2 are as shown in Table 4 above.

division Cathode active material Conductive material bookbinder ingredient
(Weight ratio) content
(weight%) ingredient
(Weight ratio) content
(weight%) ingredient content
(weight%) Comparative Example 1 Li [Ni _0.3 Co _0.4 Al _0.3 ] O ₂ 97.4 Carbon nanotube 1.2 PVdF 1.4 Comparative Example 2 Li [Ni _0.3 Co _0.4 Al _0.3 ] O ₂ 97.4 Grapina 1.2 PVdF 1.4

division Sum of cathode active material, conductive material and binder menstruum Viscosity
(cps) Weight ratio Comparative Example 1 49 51 7,500 Comparative Example 2 65 35 7,300

Example  3, Example  4, Comparative Example  3 and Comparative Example  4: The lithium secondary battery  Produce

&Lt; Preparation of negative electrode &

A mixture containing 96.3% by weight of carbon powder, 1.0% by weight of super-p as a conductive material, 1.5% by weight of styrene-butadiene rubber (SBR) as a binder and 1.2% by weight of carboxymethylcellulose (CMC) as a thickener was prepared as an anode active material. 200 g of the mixture and 220 ml of secondary distilled water were mixed to prepare a negative electrode active material slurry. The prepared negative electrode active material slurry was applied to a copper (Cu) thin film as an anode current collector having a thickness of 10 탆, dried, and rolled to prepare a negative electrode.

&Lt; Preparation of electrolyte &

LiPF ₆ was added to a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3: 7 to prepare a 1M LiPF ₆ electrolyte.

&Lt; Production of lithium secondary battery >

After a mixed separator of polyethylene and polypropylene was interposed between the positive electrode and the negative electrode described in Table 6 below, a pouch type polymer battery was prepared by a conventional method, and the electrolyte was injected to complete a lithium secondary battery.

division anode Example 3 Example 1 Example 4 Example 2 Comparative Example 3 Comparative Example 1 Comparative Example 4 Comparative Example 2

Test Example  1: Initial output evaluation

The initial outputs of the lithium rechargeable batteries of Examples 3, 4, 3 and 4 were measured at SOC50, and the results are shown in Table 7.

division Initial Output (DC-IR) (Ω) Example 3 0.7 Example 4 0.7 Comparative Example 3 0.7 Comparative Example 4 1.8

Referring to Table 7, the initial output of the lithium secondary batteries of Example 3, Example 4, and Comparative Example 3 was 0.7 OMEGA, so that it was able to function as a battery. However, in the case of the lithium secondary battery of Comparative Example 4, since the conductivity in the electrode was not smoothly provided, the initial output was 1.8 Ω. As a result, it was found that the lithium secondary battery of Comparative Example 4 was not able to function as a battery due to a high initial output.

Test Example 2: at 45 [deg.] C  Evaluation of Capacitance and Resistance Characteristics 1

The lithium secondary battery of Example 3 and the lithium secondary battery of Comparative Example 3 were charged at 10 ° C from 45 ° C to 4.2 V and then charged and discharged at 10 ° C from SOC 100 to SOC 0 at 45 ° C and repeatedly carried out up to 900 cycles. The capacity retention rate (%) and resistance increase rate (%) measured at cycle, 600 cycle and 900 cycle are shown in Table 8 and FIG.

Capacity retention rate (%) Rate of resistance increase (%) 300 cycles 600 cycles 900 cycles 300 cycles 600 cycles 900 cycles Example 3 93.1 91.3 90.0 6.0 9.4 7.9 Comparative Example 3 93.3 91.6 90.4 5.7 12.6 15.5

Referring to Table 8 and FIG. 7, the capacity retention ratios of the lithium secondary battery of Example 4 and the lithium secondary battery of Comparative Example 3 were the same, but the rate of resistance increase was similar to that of the lithium secondary battery of Example 4 according to one embodiment of the present invention. Is lower than that of the lithium secondary battery of Comparative Example 1. Since the lithium secondary battery according to an embodiment of the present invention includes the linear conductive material and the planar conductive material in a weight ratio of 7: 3, even if the cathode active material repeatedly shrinks and expands at a high temperature, the electric short- Low.

Test Example 3: at 60 캜  Evaluation of Capacitance and Resistance Characteristics 1

The lithium secondary battery of Example 3 and the lithium secondary battery of Comparative Example 3 were charged at 4.2 V of SOC100 at 60 占 폚. Capacity retention and resistance retention were evaluated after 1 week, 2 weeks, 3 weeks, and 4 weeks, and these are shown in Table 9 and FIG.

Capacity retention rate (%) Rate of resistance increase (%) 1 week 2 weeks 4 weeks 1 week 2 weeks 4 weeks Example 3 88.9% 87.2% 84.5% 20.4% 26.1% 37.5% Comparative Example 3 90.8% 86.3% 83.8% 20.6% 35.7% 46.3%

Referring to Table 9 and FIG. 8, the capacity retention ratio of the lithium secondary battery of Example 3 and the capacity retention ratio of the lithium secondary battery of Comparative Example 3 were equal to each other even after a lapse of time. However, the resistance increase rate of the lithium secondary battery of Example 3 was lower than that of the lithium secondary battery of Comparative Example 3 as time elapsed. That is, since the lithium secondary battery according to an embodiment of the present invention includes the linear conductive material and the planar conductive material at a weight ratio of 7: 3, even if the cathode active material repeatedly shrinks and expands at a high temperature, the electric short- The growth rate is low.

Test Example 4: at 45 ° C  Capacity and resistance characteristics evaluation 2

The lithium secondary battery of Example 4 and the lithium secondary battery of Comparative Example 3 were repeatedly charged up to 900 cycles at SOC 80 to SOC 20 at 45 ° C and discharged at a rate of 10 C, and the capacity retention ratios measured at 300 cycles, 600 cycles, and 900 cycles %) And resistance increase rate (%) are shown in Table 10 and FIG.

Capacity retention rate (%) 300 cycles 600 cycles 900 cycles Example 4 96.5% 95.1% 93.9% Comparative Example 3 96.7% 95.1% 93.5%

Referring to Table 10 and FIG. 9, the capacity retention ratios of the lithium secondary battery of Example 4 and the lithium secondary battery of Comparative Example 3 were the same.

Test Example 5: at 60 DEG C  Capacity and resistance characteristics evaluation 2

The capacity retention ratios and resistance retention ratios of the lithium secondary batteries of Example 4 and Comparative Example 3 varied with time at 60 DEG C after charging the secondary battery at 4.2 V of SOC100 at room temperature, 10.

Capacity retention rate (%) Rate of resistance increase (%) 3 weeks 5 weeks 3 weeks 4 weeks Example 4 79.5% 80.0% 33.3% 42.0% Comparative Example 3 70.5% 72.6% 50.4% 55.2%

Referring to Table 11 and FIG. 10, as the time passed, the capacity retention rate of the lithium secondary battery of Example 4 was superior to that of the lithium secondary battery of Comparative Example 3. The resistance increase rate of the lithium secondary battery of Example 4 was lower than that of the lithium secondary battery of Comparative Example 3. That is, since the lithium secondary battery according to an embodiment of the present invention includes the linear conductive material and the planar conductive material in a weight ratio of 6: 4, even if the cathode active material repeatedly shrinks and expands at a high temperature, The capacity retention rate is relatively slowly lowered, and the electrical short circuit phenomenon is suppressed and the resistance increase rate is low.

Claims (19)

Active material;
A conductive material comprising a linear conductive material and a planar conductive material in a weight ratio of 3: 7 to 7: 3; And
And a binder.
The method according to claim 1,
Wherein the ratio of the average diameter to the length of the linear conductive material is from 1: 100 to 1: 300.
The method of claim 2,
Wherein the linear conductive material has an average diameter of 8 nm to 12 nm.
The method of claim 2,
Wherein the linear conductive material has a length of 0.8 mu m to 3.6 mu m.
The method of claim 2,
Wherein the linear conductive material has a specific surface area of 200 m &lt; 2 &gt; / g to 330 m &lt; 2 &gt; / g.
The method according to claim 1,
Wherein the linear conductive material is at least one selected from the group consisting of carbon nanotubes, carbon nanorods, and carbon nanofibers.
The method according to claim 1,
Wherein the planar conductive material has an aspect ratio of 1: 1 to 1:10.
The method of claim 7,
And the width of the planar conductive material is 1 占 퐉 to 10 占 퐉.
The method of claim 7,
And the length of the planar conductive material is 1 占 퐉 to 100 占 퐉.
The method of claim 7,
Wherein the planar conductive material has a thickness of 5 to 30 nm.
The method of claim 7,
And the specific surface area of the planar conductive material is 40 m &lt; 2 &gt; / g to 60 m &lt; 2 &gt; / g.
The method according to claim 1,
Wherein the planar conductive material is graphene or graphite.
The method according to claim 1,
Wherein the binder has a weight average molecular weight of 750,000 g / mol to 900,000 g / mol or less.
14. The method of claim 13,
The binder is characterized in that the binder is at least one selected from the group consisting of polyvinylidene fluoride, polyhexafluoropropylene, polytetrafluoroethylene and copolymers of polyhexafluoropropylene-polyvinylidene fluoride Electrode.
The method according to claim 1,
With respect to the total weight of the electrode,
95.0 wt% to 98.0 wt% of the active material;
1.0 wt% to 2.5 wt% of the conductive material; And
And 1.0 wt% to 2.5 wt% of the binder.
The method according to claim 1,
Wherein the electrode further comprises a dispersing agent.
A first step of forming an electrode forming composition comprising a conductive material, a binder and a solvent, which comprises an active material, a linear conductive material and a planar conductive material in a weight ratio of 3: 7 to 7: 3 on a current collector; And
And a second step of rolling the composition for electrode formation.
A lithium secondary battery comprising the electrode according to any one of claims 1 to 16.
19. The method of claim 18,
And the electrode is a positive electrode.

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