KR101745416B1 - Cathode active material for lithium secondary battery and method of preparing the same - Google Patents

Abstract

One embodiment of the present invention provides a cathode active material for a lithium secondary battery comprising a lithium-containing transition metal oxide and a graphene nanoribbons coated on the surface of the lithium-containing transition metal oxide, and a method of manufacturing the same.

Description

[0001] The present invention relates to a cathode active material for a lithium secondary battery, and a cathode active material for a lithium secondary battery,

More particularly, the present invention relates to a cathode active material for a lithium secondary battery comprising a lithium-containing transition metal oxide surface-modified with graphene nanoribbons and a method for producing the same.

As the application fields of lithium secondary batteries such as portable electronic devices and electric vehicles are expanded, demand for high power, large capacity, and long life lithium secondary batteries is increasing, and research and development of lithium secondary battery materials satisfying high efficiency, low cost, .

However, lithium secondary battery has a problem that life span rapidly drops as charging and discharging are repeated. This degradation in lifetime is caused by a side reaction between the anode and the electrolyte, and can become more severe under high voltage and high temperature conditions.

In order to develop a high-voltage lithium secondary battery, it is very important to control a side reaction between a cathode active material and an electrolyte or an electrode interface reaction.

In this regard, techniques for coating a metal oxide including Mg, Al, Co, K, Na, Zr, or Ca on the surface thereof have been proposed in order to stabilize the structure of the cathode active material.

However, since such oxides are dispersed in the form of nanoparticles on a part of the surface of the cathode active material, there is a limit in that the front surface of the cathode active material can not be protected. That is, the effect of surface modification of the cathode active material by the oxide coating layer is inevitably limited, and furthermore, the oxide coating layer acts as an ion-insulating layer, which hinders the movement of lithium ions.

Disclosure of Invention Technical Problem [8] Accordingly, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a positive electrode active material for a lithium secondary battery which can improve structural stability and improve electrochemical characteristics and life characteristics of a battery, will be.

In order to achieve the above object, one aspect of the present invention provides a cathode active material for a lithium secondary battery comprising a lithium-containing transition metal oxide and a graphene nanoribbons coated on the surface of the lithium-containing transition metal oxide.

In one embodiment, the lithium-containing transition metal oxide may be represented by the following formula (1).

[Chemical Formula 1]

LiM _a X _b

Wherein M is at least one member selected from the group consisting of Co, Ni and Mn, X is at least one member selected from the group consisting of oxygen atom, phosphoric acid group, carbonate group and nitric acid group, And the number of moles of X, which is a real number satisfying the above formula (1).

In one embodiment, the content of the graphene nanoribbons may be 10 wt% or less based on the total weight of the cathode active material.

According to another aspect of the present invention, there is provided a positive electrode for a lithium secondary battery in which a slurry containing a positive electrode active material for a lithium secondary battery, a binder, and a conductive material is applied to a current collector.

In one embodiment, the binder is selected from the group consisting of polyvinylidene fluoride, polyvinyl fluoride, polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, , Ethylene-propylene (EPDM) rubber, styrene-butylene rubber, fluorine rubber, and mixtures of two or more thereof.

In one embodiment, the content of the cathode active material for the lithium secondary battery may be 70 wt% to 95 wt% based on the total weight of the slurry.

According to another aspect of the present invention, there is provided a method of manufacturing a carbon nanotube, including: (a) preparing a graphene nanoribbons by cutting one surface of a carbon nanotube in a direction of a central axis; (b) dispersing the graphene nanoribbons in a suspension containing a lithium-containing transition metal oxide to prepare a dispersion; And (c) heat treating the dispersion to coat the surface of the lithium-containing transition metal oxide with the graphene nanoribbons.

In one embodiment, the carbon nanotubes may be selected from the group consisting of single-wall carbon nanotubes, multi-wall carbon nanotubes, multiple-wall carbon nanotubes, and mixtures of two or more thereof.

In one embodiment, the carbon nanotubes may be sequentially subjected to acid and base treatment in the step (a).

In one embodiment, the acid may be one selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, hydrogen peroxide, and mixtures of two or more thereof.

In one embodiment, the base may be hydrazine hydrate.

In one embodiment, the lithium-containing transition metal oxide may be represented by the following formula (1).

[Chemical Formula 1]

LiM _a X _b

Wherein M is at least one member selected from the group consisting of Co, Ni and Mn, X is at least one member selected from the group consisting of oxygen atom, phosphoric acid group, carbonate group and nitric acid group, And the number of moles of X, which is a real number satisfying the above formula (1).

According to one aspect of the present invention, the surface of lithium-containing transition metal oxide as a cathode active material is coated with a graphene nanoribbon to improve the structural stability of the cathode active material, and the electrochemical characteristics and lifetime The characteristics can be greatly improved.

It should be understood that the effects of the present invention are not limited to the above effects and include all effects that can be deduced from the detailed description of the present invention or the configuration of the invention described in the claims.

1 is a schematic view illustrating a structure of a cathode active material for a lithium secondary battery according to an embodiment of the present invention.
2 shows an XRD pattern of a cathode active material for a lithium secondary battery according to an embodiment of the present invention.
3 shows HR-Raman spectrum of a cathode active material for a lithium secondary battery according to an embodiment of the present invention.
FIG. 4 shows a thermogravimetric analysis (TGA) result of a cathode active material for a lithium secondary battery according to an embodiment of the present invention.
5 is an FE-SEM image of a cathode active material for a lithium secondary battery according to an embodiment of the present invention.
6 is an FE-TEM image of a cathode active material for a lithium secondary battery according to an embodiment of the present invention.
7 shows an electrochemical impedance analysis (EIS) result of a lithium secondary battery according to an embodiment of the present invention.
8 is a current-voltage curve of a lithium secondary battery according to an embodiment of the present invention.
FIG. 9 shows the initial charge / discharge change of the lithium secondary battery according to an embodiment of the present invention.
10 shows the cycling performance of a lithium secondary battery according to an embodiment of the present invention.
11 illustrates a rate capability of a lithium secondary battery according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described with reference to the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

Throughout the specification, when a part is referred to as being "connected" to another part, it includes not only "directly connected" but also "indirectly connected" . Also, when an element is referred to as "comprising ", it means that it can include other elements, not excluding other elements unless specifically stated otherwise.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

For lithium secondary battery Cathode active material

1 is a schematic view illustrating a structure of a cathode active material for a lithium secondary battery according to an embodiment of the present invention. Referring to FIG. 1, a cathode active material for a lithium secondary battery according to an aspect of the present invention may include a lithium-containing transition metal oxide and a graphene nanoribbon coated on the surface of the lithium-containing transition metal oxide.

The lithium-containing transition metal oxide may be represented by the following formula (1).

[Chemical Formula 1]

LiM _a X _b

Wherein M is at least one member selected from the group consisting of Co, Ni and Mn, X is at least one member selected from the group consisting of oxygen atom, phosphoric acid group, carbonate group and nitric acid group, And the number of moles of X, which is a real number satisfying the above formula (1).

For example, when the lithium containing transition metal compound is LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li (Ni _a Co _b Mn _c ) O ₂ (0 <a <1, 0 <b <<c<1, a + b + c = 1), LiNi 1 - y Co y O 2 (0≤y <1), LiCo 1-y Mn y O 2 (0≤y <1), LiNi 1 - y _{Mn y O 2 (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 (0 <z <2), LiMn _{2 - z} Co _z O ₄ (0 <z <2), LiCoPO ₄ , LiFePO ₄ and mixtures of two or more thereof And preferably LiMnO ₂ or LiMn ₂ O ₄ , but is not limited thereto.

The graphene nanoribbons refer to graphene in the form of a ribbon having a width corresponding to the diameter of the carbon nanotubes produced by cutting one surface of the carbon nanotube in a central axis direction by a chemical method.

Conventionally, a cathode active material coated with carbon nanotubes has been proposed. However, since the surface of the cathode active material can not be uniformly coated on the entire surface, there is a limitation in preventing direct contact between the electrode and the electrolyte. On the other hand, It is possible to overcome the limit of the tube and greatly improve the electrochemical characteristics and lifetime characteristics of the battery.

That is, since the graphene nanoribbons coated on the surface of the lithium-containing transition metal oxide form a uniform and strong bond over the entire surface, the structure of the cathode active material can be stabilized, and Li ⁺ ions And Mn &lt; ^{2 +} &gt; can be suppressed and the electrochemical characteristics of the lithium secondary battery can be improved.

Generally, an electrode of a lithium secondary battery reacts with an electrolyte to form a solid electrolyte interface (SEI) layer. The SEI layer generates interfacial resistance between electrochemical reactions to lower the Li ⁺ ion mobility.

The graphene nanoribbons can reduce the interfacial resistance and improve the electrical conductivity of the lithium-containing transition metal oxide particles. In particular, the edge effect of the graphene nanoribbons can increase the Li capacity, and the graphene nanoribbons can form a relatively strong bond with lithium compared to carbon-based materials such as conventional graphene and carbon nanotubes can do.

In addition, the structure of the cathode active material may affect the charging / discharging characteristics of the lithium secondary battery, that is, the lifetime characteristics. When the lithium-containing transition metal oxide is LiMn ₂ O ₄ , Mn ^{2 +} ions are eluted by LiPF ₆ present in the electrolyte between charge and discharge, and the electrode and the electrolyte can be in direct contact with each other. It may cause a decrease in capacity. On the other hand, graphene nanoribbons reduce the local concentration of HF, thereby suppressing side reactions of Li ⁺ ions between charging and discharging to improve mobility and further improve the reversibility capacity and rate capability of lithium secondary batteries .

However, if the content of the graphene nanoribbons is excessive, the mobility of Li ⁺ ions between the cell drives may be lowered. Therefore, it may be adjusted to 10 wt% or less based on the total weight of the cathode active material, % To 10% by weight.

For lithium secondary battery  anode

Another aspect of the present invention provides a positive electrode for a lithium secondary battery in which a slurry containing the positive electrode active material for a lithium secondary battery, a binder, and a conductive material is applied to a current collector.

Wherein the binder is selected from the group consisting of polyvinylidene fluoride, polyvinyl fluoride, polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ) Rubber, styrene-butylene rubber, fluorine rubber, and a mixture of two or more thereof, preferably polyvinylidene fluoride, but is not limited thereto. The content of the binder may be 1 wt% to 15 wt%, preferably 5 wt% to 13 wt% based on the total weight of the slurry.

The conductive material is not particularly limited as long as it has conductivity without causing chemical changes in the battery. For example, the conductive material may be graphite such as natural graphite or artificial graphite; Carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Polyphenylene derivatives, and the like. The content of the conductive material may be 1 wt% to 30 wt%, preferably 5 wt% to 20 wt%, based on the total weight of the slurry.

The content of the cathode active material for the lithium secondary battery may be 70 wt% to 95 wt% based on the total weight of the slurry. If the content of the cathode active material for the lithium secondary battery is less than 70% by weight, the life characteristics and conductivity of the battery may be deteriorated. If the content is more than 95% by weight, the content of the binder and the conductive material may be decreased.

The current collector is not particularly limited as long as it has conductivity without causing chemical changes in the battery. For example, the current collector may be a surface treated with carbon, nickel, titanium, silver or the like on the surface of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel, aluminum-cadmium alloy, . Further, fine unevenness can be formed on the surface to enhance the bonding force of the cathode active material and can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric. The current collector may have a thickness of 3 탆 to 500 탆.

The lithium secondary battery may have a structure in which a lithium salt-containing electrolyte is impregnated in an electrode assembly having a separator interposed between an anode and a cathode.

The separation membrane is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength can be used. The pore diameter of the separator is generally 0.01 탆 to 10 탆, and the thickness may be 5 탆 to 300 탆. For example, the separator may be an olefinic polymer such as a chemical resistant and hydrophobic polypropylene; A sheet or a nonwoven fabric made of glass fiber, polyethylene, or the like. In addition, when a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may serve as a separator.

The lithium salt-containing electrolyte may be an electrolyte and a lithium salt, and the electrolyte may be water, a non-aqueous organic solvent, an organic solid electrolyte, an inorganic solid electrolyte, or the like, but is not limited thereto.

Wherein the non-aqueous organic solvent is at least one selected from the group consisting of N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma -butylolactone, 1,2-dimethoxyethane, The organic solvent is selected from the group consisting of franc, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, , Trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl pyrophosphate, ethyl propionate But may not be limited to, non-magnetic organic solvents.

Wherein the organic solid electrolyte is selected from the group consisting of a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene, But it is not limited thereto.

Wherein the inorganic solid electrolyte is selected from the group consisting of Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Li ₄ SiO ₄ -LiI -LiOH, Li ₃ PO ₄ -Li ₂ S-SiS ₂ and the like, but the present invention is not limited thereto.

The lithium salt is a substance which can be dissolved in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , _{LiAsF 6, LiSbF 6, LiAlCl 4} , CH 3 SO 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate, but already the like de, but are not limited to .

In order to improve charge / discharge characteristics, flame retardancy and the like, the electrolyte is required to contain at least one selected from the group consisting of pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, hexamphosphoric triamide, Dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride and the like may be added. Further, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride can be further added to impart nonflammability to the electrolyte, and carbon dioxide gas, FEC (Fluoro-Ethylene Carbonate), PRS (Propene sultone ) May be further added.

For lithium secondary battery Cathode active material  Manufacturing method

According to another aspect of the present invention, there is provided a method of manufacturing a carbon nanotube, comprising: (a) preparing a graphene nanoribbons by cutting one surface of a carbon nanotube in a central axis direction; (b) dispersing the graphene nanoribbons in a suspension containing a lithium-containing transition metal oxide to prepare a dispersion; And (c) heat treating the dispersion to coat the surface of the lithium-containing transition metal oxide with the graphene nanoribbons.

In the step (a), one surface of the carbon nanotube may be cut in the direction of the central axis by sequentially treating the carbon nanotubes with an acid and a base.

Firstly, a certain amount of carbon nanotubes are added to an acid solution, dispersed and stirred to prepare a suspension, and then a metal oxide such as KMnO ₄ is further added to the suspension and heated under a temperature condition of 100 ° C or lower to prepare a carbon nanotube That is, the side surface, along the central axis direction.

The carbon nanotubes may be selected from the group consisting of single-walled carbon nanotubes, multi-walled carbon nanotubes, multiple-walled carbon nanotubes, and a mixture of two or more thereof. Preferably, And may be a multi-walled carbon nanotube, but the present invention is not limited thereto. The acid may be one selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, hydrogen peroxide, and a mixture of two or more thereof, preferably hydrochloric acid or sulfuric acid, but is not limited thereto.

The term "dispersion" or "suspension " as used herein means a material in which solid particles are suspended and suspended in a liquid.

When one side of the carbon nanotube is cut, a sheet-like carbon-based material, that is, graphene may be produced. The resulting graphene may be a ribbon having a width corresponding to the diameter of the carbon nanotube, , and graphene nano-ribbon.

Since a large amount of oxygen molecules may remain on the surface of the graphene nanoribbons produced through the acid treatment, the suspension may be treated with a basic substance such as hydrazine hydrate to reduce or remove residual oxygen molecules.

In the step (b), the graphene nanoribbons may be dispersed in a suspension containing a lithium-containing transition metal oxide to prepare a dispersion. The suspension is different from the suspension prepared in the step (a), which means that the lithium-containing transition metal oxide is dispersed in an organic solvent.

The lithium-containing transition metal oxide may be represented by the following formula (1).

[Chemical Formula 1]

LiM _a X _b

Wherein M is at least one member selected from the group consisting of Co, Ni and Mn, X is at least one member selected from the group consisting of oxygen atom, phosphoric acid group, carbonate group and nitric acid group, And the number of moles of X, which is a real number satisfying the above formula (1). The types of the lithium-containing transition metal oxides that can be used are as described above.

In the step (c), the surface of the lithium-containing transition metal oxide may be coated with the graphene nanoribbons by heat-treating the dispersion. That is, the dispersion may be dried and heated to bond the graphene nanoribbons to the surface of the lithium-containing transition metal oxide.

Specifically, the product of step (b) may be dried to obtain a particulate phase, and the particulate phase may be heat-treated to densify or densify the graphene nanoribbons completely capturing the lithium-containing transition metal oxide, The entire surface of the lithium-containing transition metal oxide can be coated by the fin nanoribbons.

 The heat treatment may be performed in a known heating apparatus such as an oven, an electric furnace, a vacuum furnace, etc., and the heating temperature may be 200 ° C to 500 ° C, preferably 300 ° C to 400 ° C.

Hereinafter, embodiments of the present invention will be described in detail.

Example

100 mg of multi-walled carbon nanotubes (hereinafter referred to as "MWCNT") was added to 50 ml concentrated H- ₂ SO ₄ and ultrasonically dispersed for 1 hour and then stirred for 15 hours to prepare a suspension. 500 mg of KMnO ₄ was added to the suspension, and the mixture was heated at 80 ° C to 90 ° C for 3 hours. Then, a certain amount of hydrogen peroxide (H ₂ O ₂ ) was administered. The suspension was diluted with DIW until the pH of the suspension became neutral, And washed. Further, the suspension was treated with hydrazine hydrate at 80 ° C to 100 ° C for 12 hours to reduce oxygen molecules remaining on the surface of graphene nanoribbon (hereinafter referred to as "GNR"), and DIW and ethanol The suspension was washed several times with.

On the other hand, LiMn ₂ O ₄ particles (hereinafter referred to as "LMO") were dispersed in ethanol by ultrasonication for 2 hours, followed by 5.0 wt% of GNR and ultrasonic dispersion for 1 hour to prepare a dispersion. The dispersion was dried to evaporate ethanol, and then heated at 300 캜 for 3 hours under a nitrogen atmosphere to obtain GNR-coated LMO particles (hereinafter referred to as "GNR-LMO").

Comparative Example  One

LMO particles coated with MWCNT (hereinafter referred to as "CNT-LMO") were prepared in the same manner as in Example 1, except that MWCNT was used instead of GNR.

Comparative Example  2

LNO particles treated with GNR or MWCNT and not coated were prepared.

Experimental Example  One : Cathode active material  Structural characteristics

XRD, HR-Raman spectrum and TGA (heating rate: 5 ° C / min) analysis were performed to identify the structural characteristics of each of GNR-LMO, CNT-LMO and LMO of the above Examples and Comparative Examples 1 and 2, -SEM, and FE-TEM images.

Curves a, b and c in Fig. 2 represent XRD patterns of LMO, CNT-LMO and GNR-LMO, respectively, and are 18.8 °, 36.3 °, 38.0 °, 44.1 °, 48.3 °, 58.3 °, 64.0 °, Peaks are observed at 67.3 DEG, 75.8 DEG and 76.8 DEG, which suggests that the LMO has a cubic spinel structure. On the other hand, the curves d and e show the XRD patterns of MWCNT and GNR, respectively. Comparing these curves to curves b and c, peaks of carbon-based materials are not observed in CNT-LMO and GNR-LMO, But did not affect the spinel structure of LMO.

3, the HR-Raman spectra of LMO and GNR-LMO are common in that they represent an active mode at about 500 c / m to 700 c / m. However, in the case of GNR-LMO, 1340 c / m and 1596 c / m, additional D and G bands are observed, which are each analyzed to be due to defects by sp ² -bonded carbons and the presence of sp ² hybrid carbon atoms containing graphene sheets.

Also, referring to FIG. 4, GNR-LMO started to lose weight at about 200 ° C., and graphene oxidized or decomposed at about 450 ° C., resulting in abrupt weight loss. At about 700 ° C., The degradation resulted in no further weight loss. As a result of TGA analysis, it was confirmed that the calculated content of GNR was about 5.0 wt%, which is the same as that of GNR in the production of the cathode active material.

5 and 6 are FE-SEM and FE-TEM images of a cathode active material for a lithium secondary battery according to an embodiment of the present invention, respectively. First, comparing FIGS. 5 (b) and 5 (c), it was confirmed that the surface of the LMO was densely coated as compared to CNT-LMO in the case of GNR-LMO. 6 (c) and 6 (d), CNT-LMO is weak and irregular in contact with MWCNT and LMO, whereas in GNR-LMO, the interface between LMO and GNR is smooth, As shown in Fig.

Manufacturing example

80% by weight of GNR-LMO, 10% by weight of carbon black and 10% by weight of polyvinylidene fluoride in the above examples were kneaded in a N-methyl-2-pyrrolidone solvent to prepare a slurry. The slurry was applied onto Al foil and dried under vacuum condition at 120 DEG C for 12 hours to prepare coin-shaped cells. The coin-type cell was applied as a positive electrode of a lithium secondary battery, and the following materials or materials were used as a negative electrode, an electrolyte, and a separator, respectively, to constitute a lithium secondary battery.

- Cathode: Lithium metal

Electrolyte: 1M LiPF ₆ in ethylene carbonate (EC) / diethyl carbonate (DEC) = 50/50 (v / v)

- Membrane: microporous polypropylene

Comparative Manufacturing Example  One

Lithium produced a secondary battery in the same manner as in Preparation Example 1, except that 80 wt% of CNT-LMO of Comparative Example 1 was kneaded instead of GNR-LMO.

Comparative Manufacturing Example  2

Lithium produced a secondary battery in the same manner as in Preparation Example 1, except that 80% by weight of LMO of Comparative Example 2 was kneaded instead of GNR-LMO.

Experimental Example  2 : The lithium secondary battery  Electrochemical properties

In order to evaluate the electrochemical characteristics of the lithium secondary batteries produced in the above Production Examples and Comparative Production Examples 1 and 2, EIS, current-voltage, initial charge / discharge change, lifetime characteristics and rate characteristics were measured. The measurement conditions for each characteristic are shown in Table 1 below.

Electrochemical properties Measuring conditions EIS AC, 0.2mV, 10 ^-2 to 10 ^-6 Hz Current-voltage curve 3.0 to 4.5 V, 0.2 mV / s, initial three cycles Initial charge / discharge change 3.0 to 4.5V, 0.2C rate Life characteristics 3.0 to 4.5 V, 1.0 C rate, 50 cycles Rate characteristic 3.0 to 4.5 V, 0.2 C, 0.5 C, 1.0 C, 2.0 C, 5.0 C

First, referring to FIG. 7, each curve shows a semicircular shape in the high frequency region and a straight line shape in the low frequency region at a constant slope. In general, the diameter of the semicircle is related to the charge transfer resistance at the electrode / electrolyte interface, and the straight line corresponds to Warburg impedance causing diffusion of Li ⁺ ions into the electrode. That is, the impedance values of LMO, CNT-LMO, and GNR-LMO were about 165, 147, and 95Ω, respectively, indicating that GNR-LMO is a conductive structure having a significantly reduced interface resistance.

Referring to FIG. 8, two pairs of peaks are observed at LMO, CNT-LMO and GNR-LMO at 3.9 V / 4.1 V and 4.05 V / 4.2 V, The typical redox behaviors were shown. However, in the case of CNT-LMO and GNR-LMO, the redox peaks were relatively clear compared with LMO, which is attributed to the fact that the coating with carbon-based material effectively suppressed the direct contact between the electrode and the electrolyte. Particularly, in CNT-LMO and GNR-LMO, peak shift did not occur, resulting in less Li ⁺ ion loss and excellent reversibility. Further, the potential difference? E can be calculated from the two pairs of redox peaks in the third circulation. The calculated ΔE values for LMO, CNT-LMO and GNR-LMO are about 310mV / 270mV, 210mV / 170mV and 200mV / 150mV, respectively, and the polarization at the GNR-LMO electrode with the lowest ΔE value Respectively.

9, the charge-discharge potential difference in the order of LMO, CNT-LMO, and GNR-LMO decreased in the order of the charge and discharge characteristics of the lithium secondary battery. These results also show that the polarization in the GNR- Backed.

10, the discharge capacities of the CNT-LMO and GNR-LMO electrodes were measured to be about 105.2 mAh / g and 110.7 mAh / g, respectively, which was improved compared to 100.4 mAh / g of LMO, capacity retention), the GNR-LMO electrode is measured to be the highest at about 90%, and the cyclic behavior of the GNR-LMO electrode is relatively stable. These results can also be confirmed from FIG. 11 and Table 2 below. The discharge capacity of LMO, CNT-LMO and GNR-LMO electrodes decreased as the current ratio increased, but the discharge capacity of the GNR-LMO electrode was the highest at each current rate. This tendency was more pronounced as the current ratio increased.

Current ratio (C) Discharge capacity (mAh / g) LMO CNT-LMO GNR-LMO 0.2 114.8 117.6 120.9 0.5 105.7 109.3 114.7 1.0 99.9 104.1 110.7 2.0 88.3 92.4 97.8 5.0 70.1 78.7 86.6

It will be understood by those skilled in the art that the foregoing description of the present invention is for illustrative purposes only and that those of ordinary skill in the art can readily understand that various changes and modifications may be made without departing from the spirit or essential characteristics of the present invention. will be. It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive. For example, each component described as a single entity may be distributed and implemented, and components described as being distributed may also be implemented in a combined form.

The scope of the present invention is defined by the appended claims, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be construed as being included within the scope of the present invention.

Claims (12)

A lithium-containing transition metal oxide having a spinel structure, and
And a graphene nanoribbons coated on the entire surface of the lithium-containing transition metal oxide. The method according to claim 1,
Wherein the lithium-containing transition metal oxide is represented by the following formula (1): &lt; EMI ID =
[Chemical Formula 1]
LiM _a X _b
In this formula,
M is one or more selected from the group consisting of Co, Ni, and Mn,
X is at least one selected from the group consisting of an oxygen atom, a phosphoric acid group, a carbonic acid group, and a nitric group,
a and b each represent the number of moles of M and X, and is a real number satisfying the above formula (1). The method according to claim 1,
Wherein the content of the graphene nanoribbons is 10 wt% or less based on the total weight of the cathode active material. A positive electrode for a lithium secondary battery according to any one of claims 1 to 3, wherein a slurry containing a positive electrode active material for a lithium secondary battery, a binder, and a conductive material is applied to the current collector. 5. The method of claim 4,
Wherein the binder is selected from the group consisting of polyvinylidene fluoride, polyvinyl fluoride, polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ) Rubber, styrene-butylene rubber, fluorine rubber, and a mixture of two or more thereof. 5. The method of claim 4,
Wherein the content of the cathode active material for the lithium secondary battery is 70 wt% to 95 wt% based on the total weight of the slurry. (a) preparing a graphene nanoribbons by cutting one surface of a carbon nanotube in a direction of a central axis;
(b) dispersing the graphene nanoribbons in a suspension containing a lithium-containing transition metal oxide having a spinel structure to prepare a dispersion; And
(c) heat treating the dispersion to coat the graphene nanoribbons on the entire surface of the lithium-containing transition metal oxide. 8. The method of claim 7,
Wherein the carbon nanotube is one selected from the group consisting of a single-walled carbon nanotube, a multi-walled carbon nanotube, a multiple-walled carbon nanotube, and a mixture of two or more thereof. 8. The method of claim 7,
Wherein the carbon nanotubes are sequentially treated with an acid and a base in the step (a). 10. The method of claim 9,
Wherein the acid is one selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, hydrogen peroxide, and a mixture of two or more thereof. 10. The method of claim 9,
Wherein the base is hydrazine hydrate. &Lt; RTI ID = 0.0 &gt; 11. &lt; / RTI &gt; 8. The method of claim 7,
Wherein the lithium-containing transition metal oxide is represented by the following formula (1): &lt; EMI ID =
[Chemical Formula 1]
LiM _a X _b
In this formula,
M is one or more selected from the group consisting of Co, Ni, and Mn,
X is at least one selected from the group consisting of an oxygen atom, a phosphoric acid group, a carbonic acid group, and a nitric group,
a and b each represent the number of moles of M and X, and is a real number satisfying the above formula (1).

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