KR20150047052A - Cathode active material, preparation method thereof, and lithium secondary battery comprising the same - Google Patents

Abstract

Provided in the present invention is a method for manufacturing a positive electrode active material, which comprises the steps of mixing a positive electrode active material including lithium impurities to a first organic solvent in which an organic acid is dissolved, stirring, dissolving the lithium impurities, and removing the lithium impurities. The method for manufacturing a positive electrode active material according to an embodiment of the present invention selectively dissolves only lithium impurities existing on the surface without damage of the positive electrode active material to reduce the amount of the lithium impurities included in the positive electrode active material. Also, the positive electrode active material having the amount of the lithium impurities reduced by the method makes the surface of the positive electrode active material more activated when applied to a secondary battery, thereby improving charge and discharge efficiency to increase discharge capacity and durability.

Description

TECHNICAL FIELD The present invention relates to a cathode active material, a method for producing the cathode active material, and a lithium secondary battery including the cathode active material,

More particularly, the present invention relates to a cathode active material, a method for producing the same, and a lithium secondary battery including the same. More specifically, the cathode active material is mixed with a first organic solvent dissociated from organic acid and stirred to dissolve lithium impurities And removing the lithium impurities, a cathode active material produced thereby, and a lithium secondary battery comprising the same.

Lithium rechargeable batteries have been widely used as power sources for portable devices since they appeared in 1991 as small-sized, lightweight, and large-capacity batteries. In recent years, with the rapid development of the electronics, communication, and computer industries, camcorders, mobile phones, and notebook PCs have been remarkably developed and demand for lithium secondary batteries as a power source for driving these portable electronic information communication devices is increasing day by day .

The lithium secondary battery has a problem in that its service life is rapidly deteriorated by repeated charging and discharging. This problem is particularly serious at high temperatures. This is due to the phenomenon that electrolytes are decomposed or deteriorated due to moisture and other influences inside the battery, and the internal resistance of the battery is increased.

To solve these problems, a technique has been developed in which a metal oxide such as Mg, Al, Co, K, Na, or Ca is coated on the surface of a cathode active material by heat treatment. In addition, TiO ₂ was added to LiCoO ₂ active material to improve energy density and high-rate characteristics.

However, the problem of deterioration of lifetime and the problem of generation of gas due to decomposition of electrolyte during charging and discharging are not completely solved yet.

In addition, when impurities are present on the surface of the cathode active material during the electrode manufacturing process of the lithium secondary battery, it is possible not only to affect the change over time in the production step of the electrode slurry during the electrode manufacturing process of the lithium secondary battery, Thereby causing a swelling phenomenon in the lithium secondary battery.

Therefore, various studies have been developed to minimize lithium impurities.

For example, a method of reducing the amount of lithium impurities by converting lithium impurities into LiF by reacting with lithium impurities by heat treatment using a polyvinylidene fluoride (PVdF) type polymer has been developed.

However, according to the above method, an insulator material called LiF can be produced, and the surface of the cathode active material is etched due to the HF gas generated during the heat treatment, thereby deteriorating the high temperature characteristics of the secondary battery.

Also, a method of reducing the amount of lithium impurities by washing the positive electrode active material has been developed. However, in this case, the lithium ion can also escape from the water during washing, thereby deteriorating the life characteristics.

Therefore, there is an urgent need to develop a cathode active material that can minimize the amount of lithium impurities present in the cathode active material for a lithium secondary battery and is structurally stable.

A first object of the present invention is to selectively dissolve lithium impurities present in a cathode active material without damaging the cathode active material, thereby minimizing the swelling phenomenon that may occur due to lithium impurities And a method for producing the positive electrode active material.

A second technical problem to be solved by the present invention is to provide a cathode active material in which the amount of lithium impurities is reduced.

A third object of the present invention is to provide a positive electrode and a lithium secondary battery including the positive electrode active material.

According to an aspect of the present invention, there is provided a lithium secondary battery comprising a positive electrode active material containing a lithium impurity, a negative electrode active material containing lithium impurities, A method for producing an active material is provided.

The present invention also provides a cathode active material comprising lithium impurities in an amount of less than 0.3% by weight based on the total weight of the cathode active material.

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

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

The method of producing a cathode active material according to an embodiment of the present invention can selectively dissolve only lithium impurities existing in a cathode active material without damaging the cathode active material, thereby reducing the amount of lithium impurities contained in the cathode active material .

In addition, the cathode active material in which the amount of lithium impurities is reduced by the above-described method can further increase the charge / discharge efficiency by increasing the surface of the cathode active material when the secondary battery is applied, thereby improving the discharge capacity and the life.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments of the invention and, together with the description of the invention, It should not be construed as limited.
1 (a) and 1 (b) are photographs at 40,000 times and 2,000 times of a scanning electron microscope (SEM) photograph of the surface of the cathode active material prepared in Example 1 of the present invention, respectively.
FIG. 2 is a graph showing pH titration results for examining the amount of lithium impurities in the cathode active material prepared in Example 1 of the present invention. FIG.
FIGS. 3 and 4 are graphs showing the discharge capacities of the lithium secondary batteries manufactured in Example 2 and Comparative Example 2 according to the voltage.
5 is a graph showing life characteristics of lithium secondary batteries manufactured in Example 2 and Comparative Example 2 of the present invention.

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 as limited to ordinary or dictionary terms and the inventor may appropriately define the concept of the term in order to best describe its invention It should be construed as meaning and concept consistent with the technical idea of the present invention.

According to an embodiment of the present invention, there is provided a method for producing a cathode active material, comprising: mixing a cathode active material containing a lithium impurity into a first organic solvent dissociated from organic acid and stirring the lithium organic compound to dissolve lithium impurities to remove lithium impurities; Of the present invention.

Generally, when lithium impurities are present in excess on the surface of the positive electrode active material, the lithium impurities may swell in the lithium secondary battery due to the reaction with the electrolyte injected into the lithium secondary battery, that is, cause swelling .

In the method of manufacturing a cathode active material according to an embodiment of the present invention, the cathode active material is washed using a solution in which the organic acid is dissociated in the first organic solvent to prevent the lithium ion desorption phenomenon by moisture, thereby damaging the cathode active material. The amount of lithium impurities contained in the cathode active material can be reduced by selectively dissolving only the lithium impurities existing in the cathode active material.

Specifically, a method of manufacturing a cathode active material according to an embodiment of the present invention includes mixing a cathode active material containing lithium impurities into a first organic solvent dissociated from organic acid, and stirring the lithium salt to dissolve lithium impurities to remove lithium impurities . ≪ / RTI >

The lithium impurity may include at least one of LiOH and Li ₂ CO ₃ . Specifically, when the cathode active material is, for example, a lithium transition metal oxide, the lithium impurity may be included in the lithium transition metal oxide, which is a positive electrode active material,

(2)

(1-st) [Li (Li _a Mn _(1-axz) Ni _x CO _z ) O ₂ ] s [Li ₂ CO ₃ ]

0 <x <0.9, 0 <z <0.6, 0 <s <0.05, and 0 <t <0.05.

Generally, the amount of lithium impurities contained in the cathode active material may vary depending on the type and amount of the cathode active material. For example, in an amount of 0.3 wt% or more based on the total weight of the cathode active material.

According to an embodiment of the present invention, the usable organic acid may be dissociated into the first organic solvent and a weak acid capable of minimizing the damage of the cathode active material is preferable. The organic acid may preferably be any one selected from the group consisting of citric acid, tartaric acid and malic acid, or a mixture of two or more thereof, most preferably citric acid.

Since the organic acid has a lower acidity than other acids, only the lithium impurity existing in the cathode active material can be selectively dissolved without damaging the cathode active material. In particular, citric acid effectively dissolves Li ₂ CO ₃ among the lithium impurities .

The organic acid may be dissociated into a first organic solvent and then mixed with a cathode active material and stirred.

The first organic solvent which can be used according to an embodiment of the present invention is not particularly limited as long as it is a solvent capable of dissociating the organic acid. Examples of the first organic solvent include methanol, ethanol, propanol, butanol, isopropyl alcohol and acetone , Or a mixture of two or more thereof.

According to an embodiment of the present invention, the organic acid may be used in an amount of 5 wt% to 30 wt% based on the total weight of the cathode active material containing the lithium impurity.

When the organic acid is less than 5% by weight based on the total weight of the cathode active material containing the lithium impurity, the lithium impurity, which is a desired effect of the present invention, can not be effectively removed. When the amount exceeds 30% by weight, It may not dissociate completely to the first organic solvent.

The weight ratio of the first organic solvent to the organic acid may be 100: 2 to 20.

In the method for producing a cathode active material according to an embodiment of the present invention, the stirring temperature and stirring time are not particularly limited, but it is preferable that the mixing is performed at room temperature for 1 minute to 1 hour, for example.

According to an embodiment of the present invention, after the lithium impurity is dissolved, the cathode active material may be washed with a second organic solvent.

The second organic solvent may remove lithium impurities remaining in the organic acid as well as the organic acid by washing away the organic acid remaining in the cathode active material.

The second organic solvent may be any one selected from the group consisting of methanol, ethanol, propanol, butanol, isopropyl alcohol and acetone, or a mixture of two or more thereof.

According to an embodiment of the present invention, by further adding water to the second organic solvent, lithium impurities remaining on the surface of the cathode active material can be removed more effectively.

The water may be effective for removing LiOH and Li ₂ CO ₃ , which are lithium impurities remaining on the surface of the cathode active material, without being removed by the first organic solvent and the second organic solvent dissociated from the organic acid. However, Depending on the type of active material, the cathode active material may be damaged. Therefore, it is necessary to appropriately control the amount of the second organic solvent and water to be used depending on the kind of the cathode active material.

The method of manufacturing a cathode active material according to an embodiment of the present invention may further include removing the lithium impurity and then filtering and drying the cathode active material from which lithium impurities have been removed.

The filtration and drying may be performed by filtration and drying methods commonly used in the art. For example, the cathode active material may be filtered and dried in an oven at about 80 ° C to 120 ° C for about 5 to 15 hours .

Further, the present invention can provide the cathode active material produced by the above method.

The cathode active material is not particularly limited, and examples thereof include lithium-cobalt oxide, lithium-manganese oxide, lithium-nickel-manganese oxide, lithium-manganese-cobalt oxide and lithium- And LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li (Ni _a Co _b Mn _c ) O ₂ (here, in, 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 - _{(here, 0≤Y <1) Y Mn Y} O 2, Li (Ni a Co b Mn c) O 4 (0 <a <2, 0 <b <2, 0 <c <2, a + b + c = 2) and LiMn _{2 - z} Ni _z O ₄ , LiMn _{2 - z} Co _z O ₄ (where 0 <Z <2), or a mixture of two or more thereof.

The amount of the lithium impurity present in the cathode active material according to the present invention is preferably less than 0.3 wt%, preferably less than 0.2 wt% with respect to the total weight of the cathode active material. Lithium impurities such as LiOH or Li ₂ CO ₃ have a high reactivity with respect to an electrolytic solution. Therefore, when the amount of the lithium impurity existing in the cathode active material is 0.3 wt% or more, excessive swelling phenomenon occurs.

That is, the amount of the lithium impurity may be reduced by about 30% or more compared to that before the removal of the lithium impurity by the cleaning, by the manufacturing method of the cathode active material according to one embodiment of the present invention. Therefore, when the positive electrode active material is applied to a lithium secondary battery, the swelling phenomenon due to side reactions with the electrolyte can be minimized, thereby increasing the charging and reversing efficiency and increasing the discharge capacity. At the same time, Can also be improved.

The present invention also provides a positive electrode comprising the positive electrode active material.

The anode may be prepared by a conventional method known in the art. For example, a slurry may be prepared by mixing and stirring a solvent, a binder, a conductive agent, and a dispersant, if necessary, in a cathode active material, coating the cathode active material with a collector of a metal material, have.

The current collector of the metal material is a metal having high conductivity and can be easily adhered to the slurry of the cathode active material, and any material can be used as long as it is not reactive in the voltage range of the battery. Non-limiting examples of the positive electrode current collector include foil produced by aluminum, nickel, or a combination thereof.

Examples of the solvent for forming the positive electrode include organic solvents such as NMP (N-methylpyrrolidone), DMF (dimethylformamide), acetone, and dimethylacetamide, and water. These solvents may be used alone or in combination of two or more Can be mixed and used. The amount of the solvent used is sufficient to dissolve and disperse the cathode active material, the binder and the conductive agent in consideration of the coating thickness of the slurry and the production yield.

Examples of the binder include polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, Polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM) Sulfonated EPDM, styrene butadiene rubber (SBR), fluorine rubber, poly acrylic acid, and polymers in which hydrogen is substituted with Li, Na, or Ca, or Various kinds of binder polymers such as various copolymers can be used.

The conductive agent is not particularly limited as long as it has electrical conductivity without causing any chemical change in the battery, and examples thereof include graphite such as natural graphite and artificial graphite; Carbon black such as carbon black, acetylene black, Ketjen black, channel black, panes black, lamp black, and thermal black; Conductive fibers such as carbon fiber and metal fiber; Conductive tubes such as carbon nanotubes; Metal powders such as fluorocarbon, aluminum and nickel powder; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The present invention also provides a lithium secondary battery including the positive electrode, the negative electrode, and the separator interposed between the positive electrode and the negative electrode.

As the negative electrode active material used for the negative electrode according to an embodiment of the present invention, a carbon material, lithium metal, silicon, or tin which lithium ions can be occluded and released can be used. Preferably, carbon materials can be used, and carbon materials such as low-crystalline carbon and highly-crystalline carbon can be used. Examples of the low crystalline carbon include soft carbon and hard carbon. Examples of highly crystalline carbon include natural graphite, Kish graphite, pyrolytic carbon, liquid crystal pitch carbon fiber high temperature sintered carbon such as mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches and petroleum or coal tar pitch derived cokes.

The negative electrode collector is generally made to have a thickness of 3 to 500 mu m. Such an anode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and may be formed of a material such as copper, stainless steel, aluminum, nickel, titanium, fired carbon, surface of copper or stainless steel A surface treated with carbon, nickel, titanium, silver or the like, an aluminum-cadmium alloy, or the like can be used. In addition, like the positive electrode collector, fine unevenness can be formed on the surface to enhance the bonding force of the negative electrode active material, and it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams and nonwoven fabrics.

As the binder and the conductive agent used for the cathode, those which can be commonly used in the art can be used as the anode. The negative electrode may be prepared by preparing a negative electrode active material composition by mixing and stirring the negative electrode active material and the additives, applying the negative electrode active material composition to the current collector, and compressing the negative electrode active material composition to produce a negative electrode.

As the separator, a conventional porous polymer film conventionally used as a separator, for example, a polyolefin such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer and an ethylene / methacrylate copolymer A porous polymer film made of a high molecular weight polymer may be used alone or in a laminated manner, or a nonwoven fabric made of a conventional porous nonwoven fabric such as a glass fiber having a high melting point, a polyethylene terephthalate fiber or the like may be used. It is not.

The external shape of the lithium secondary battery of the present invention is not particularly limited, but may be a cylindrical shape, a square shape, a pouch shape, a coin shape, or the like using a can.

The lithium secondary battery according to the present invention can be used not only in a battery cell used as a power source of a small device but also as a unit cell in a middle- or large-sized battery module including a plurality of battery cells.

Preferable examples of the above medium and large-sized devices include, but are not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and electric power storage systems.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to 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. The embodiments of the present invention are provided to enable those skilled in the art to more fully understand the present invention.

Example

EXAMPLES The present invention will be further illustrated by the following examples and experimental examples, but the present invention is not limited by these examples and experimental examples.

&Lt; Preparation of positive electrode active material &

Example  One

As a mixed transition metal precursor was used _{MOOH (M = Ni 0 .6 Mn} 0 .2 Co 0 .2), the mixed transition metal precursor and a stoichiometric ratio of _{Li 2 CO 3 (Li: M} = 1.02: 1) mixed, and the mixture was baked for 10 hours at about 900 ℃ to 920 ℃ in the air to prepare a _{LiNi 0 .6 Mn 0 .2 Co 0} .2 O 2 in.

After dissolving 5 wt% of citric acid (relative to the total weight of LiNi _0.6 Mn _0.2 Co _0.2 O ₂ ) as a first organic acid in 200 ml of ethanol as the first organic solvent, the LiNi _{0 .6} Mn _{0 .2} Co _{0 .2} O ₂ And the mixture was stirred for 5 minutes, and then washed with ethanol as a second organic solvent to remove lithium impurities.

After filtering to remove the impurities that the lithium _{LiNi 0 .6 Mn 0 .2 Co 0} .2 O 2, was about 5 hours drying in an oven at about 100 to 120 ℃ was prepared the positive electrode active material.

Comparative Example  One

As a mixed transition metal precursor was used _{MOOH (M = Ni 0 .6 Mn} 0 .2 Co 0 .2), the mixed transition metal precursor and a stoichiometric ratio of _{Li 2 CO 3 (Li: M} = 1.02: 1) mixed, and the mixture was baked for 10 hours at about 900 ℃ to 920 ℃ in the air to prepare a _{LiNi 0 .6 Mn 0 .2 Co 0} .2 O 2 in.

&Lt; Production of lithium secondary battery >

Example  2

Anode manufacturing

94% by weight of the cathode active material prepared in Example 1, 3% by weight of carbon black as a conductive agent and 3% by weight of polyvinylidene fluoride (PVdF) as a binder were dissolved in N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode mixture slurry. The positive electrode mixture slurry was applied to an aluminum (Al) thin film having a thickness of about 20 탆 and dried to produce a positive electrode, followed by a roll press to prepare a positive electrode.

Cathode manufacture

A negative electrode active material was prepared by mixing 96.3 wt% of carbon powder, 1.0 wt% of super-p as a conductive material, 1.5 wt% and 1.2 wt% of styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) To prepare a negative electrode active material slurry. The negative electrode active material slurry was applied to a copper (Cu) thin film as an anode current collector having a thickness of 10 mu m and dried to produce a negative electrode, followed by roll pressing to produce a negative electrode.

Non-aqueous  Electrolytic solution manufacturing

Meanwhile, LiPF ₆ was added to a nonaqueous electrolyte solvent prepared by mixing ethylene carbonate (EC), dimethyl carbonate (DMC), and dimethyl ether (DME) as electrolytes in a volume ratio of 1: 2: 1 to prepare a 1M LiPF ₆ nonaqueous electrolyte .

Lithium secondary battery manufacturing

The prepared positive electrode and negative electrode were sandwiched between a polyethylene separator and a polypropylene mixed separator, followed by preparing a polymer type battery by a conventional method, and then injecting the prepared non-aqueous electrolyte solution to complete the production of a lithium secondary battery.

Comparative Example  2

A lithium secondary battery was prepared in the same manner as in Example 2 except that the cathode active material prepared in Comparative Example 1 was used.

Experimental Example  1: Scanning electron microscope ( SEM ) Photo analysis

Electron microscopic photographs of the cathode active material prepared in Example 1 were performed, and the results are shown in FIG.

1 (a) is a case of 5.0 Kv 7.6 mm X 40,000, and (b) is a case of 5.0 Kv 7.6 mm X 2,000.

As shown in FIG. 1, it was confirmed that no damage was caused to the surface of the cathode active material by stirring with the cathode active material using citric acid dissociated into ethanol according to the method of Example 1 and then washing with ethanol .

It can be confirmed that even if the first organic solvent and citric acid are used, only the lithium impurity existing in the cathode active material can be selectively dissolved and removed without damaging the cathode active material.

Experimental Example  2: Measurement of the amount of lithium impurity

To determine the amount of lithium impurities in the cathode active material prepared in Example 1 and Comparative Example 1, the pH was measured by a titration method. 5 g of each of the cathode active materials prepared in Example 1 and Comparative Example 1 was dispersed in 100 ml of distilled water and mixed for 5 minutes. The water obtained by filtering was adjusted to pH by titrating the amount of lithium impurity Lt; / RTI &gt; The results are shown in Table 1 below.

Example 1 Comparative Example 1 LiOH 0.10 wt% 0.18 wt% Li ₂ CO ₃ 0.11 wt% 0.16 wt% Total lithium impurity 0.21 wt% 0.34 wt%

As shown in the following Table 1, the amounts of LiOH contained in the cathode active materials of Example 1 and Comparative Example 1 were 0.10 wt% and 0.18 wt%, respectively, and Li ₂ CO ₃ were 0.11 wt% and 0.16 wt%, respectively. That is, in the case of LiOH, Example 1 is reduced by about 45% compared to Comparative Example 1, and Li ₂ CO ₃ is reduced by about 32% compared to Comparative Example 1.

Therefore, it can be seen that the amount of total lithium impurity in Example 1 is reduced by about 38% compared with Comparative Example 1. [

In addition, the LiNi _{0 .6} prepared in Example 1 and Comparative Example 1, Mn _{0 .2 .2 0} Co ₂ O was carried out a pH titration (titration) to determine the amount of impurities on lithium, is also the result 2 Respectively. The pH meter was metrohm 794 and the pH was recorded by titration with 0.02 ml.

Figure 2 compares the amount of LiNi _{0 .6} of Example 1 and Comparative Example 1 according to the _{pH Mn 0 .2 Co 0 .2 O} 2 10 g 0.1 M HCl per.

Referring to FIG. 2, the amount of hydrochloric acid used in the titration of hydrochloric acid was about 7.3 ml, about 12 ml for Example 1, and about 38% less for Comparative Example 1 than Example 1 .

Experimental Example  3: Electrochemical evaluation experiment

The electrochemical evaluation experiments were carried out as follows to investigate the charging / discharging capacity, rate characteristics and lifetime characteristics of the lithium secondary batteries obtained in Example 2 and Comparative Example 2.

The lithium secondary batteries obtained in Example 2 and Comparative Example 2 were charged at 25 ° C until the constant current (CC) of 4.2 V was reached at 0.1 C and then charged at a constant voltage (CV) of 4.25 V to obtain a charging current of 0.05 mAh The first charging was performed. Thereafter, the battery was allowed to stand for 20 minutes, and discharged at a constant current of 0.1 C until it reached 3.0 V to measure the discharge capacity of the first cycle.

The lithium secondary batteries obtained in Example 2 and Comparative Example 2 were charged up to 4.4 V and charged conditions were varied. The constant current values were set at 0.1 C, 1.0 C, and 2.0 C, And life characteristics were measured. The results are shown in Table 2 below.

Example 2 Comparative Example 2 Example 2 Comparative Example 2 Charging voltage (V) 4.25 V 4.4 V Charging capacity
(mAh / g) 193.6 193.4 213.0 212.6 Discharge capacity
(mAh / g) 176.0 174.1 194.9 193.2 efficiency (%) 90.9 90.0 91.5 90.9 0.1 C (mAh / g) 177.5 176.6 195.2 194.8 1.0 C (mAh / g) 164.5 163 181.7 181.3 2.0 C (mAh / g) 159.4 156.9 176.5 175.2 1.0 C (%) 92.4 92.3 93.1 93.1 2.0 C (%) 89.8 88.8 90.4 89.9 30th cycle
(%) 98.3 97.4 95.3 94.4

As can be seen from the above Table 2, when the battery was charged to a voltage of 4.25 V according to the measuring method of Experimental Example 3, the charging capacity and the discharging capacity of Example 2 were increased as compared with Comparative Example 2. Particularly, even when the conditions of 0.1 C, 1.0 C and 2.0 C were measured, the capacity of Example 2 was slightly improved as compared with that of Comparative Example 2, regardless of the conditions. In addition, it can be confirmed that the efficiency characteristics and the life characteristics are improved as compared with Comparative Example 2.

On the other hand, it can be confirmed that charging and discharging characteristics, rate characteristics and lifetime characteristics of Example 2 were slightly improved even when the battery was charged to a high voltage of 4.4 V as compared with Comparative Example 2. It was possible to prevent the deterioration of the characteristics even with citric acid treatment, and to obtain the effect that the electrochemical characteristics were slightly improved.

This is because the surface condition can be maintained more clean by washing with lithium impurity citric acid contained in the surface of the positive electrode active material, and the surface of the positive electrode active material is activated by weak acid, I think it is because it decreased. Also, the charge / discharge efficiency is increased by activating the surface of the cathode active material, so that the charge / discharge capacity is increased and the high temperature lifetime is also improved.

3 and 4 are graphs showing the discharge capacity characteristics of the lithium secondary batteries of Example 2 and Comparative Example 2 according to the voltages of 4.2 V and 4.4 V, respectively.

FIG. 3 shows a case where the voltage is raised to 4.2 V, and FIG. 4 shows a result obtained when the voltage is raised to 4.4 V.

As can be seen from Figs. 3 and 4, it was confirmed that the resistance of the lithium secondary battery of Example 2 did not increase even in the high voltage range of 4.2 V or 4.4 V, and the discharge capacity was slightly increased .

Experimental Example  4: Evaluation of life characteristics at high temperature and high voltage

In order to examine the life characteristics of the lithium secondary battery obtained in Example 2 and Comparative Example 2 at high temperature and high voltage, the lithium secondary battery obtained in Example 2 and Comparative Example 2 was subjected to a constant current (CC) of 4.35 V at 45 ° C , And then charged at a constant voltage (CV) of 4.35 V until the charge current reached 0.05 mAh. Thereafter, the resultant was allowed to stand for 20 minutes, discharged at a constant current of 2 C until it reached 3.0 V, and then repeatedly subjected to 1 to 100 cycles. The results are shown in Table 3 and FIG.

Example 2 Comparative Example 2 Charging voltage (V) 4.35 V / 45 ° C Life characteristics (%) 82.1 80.7

- Life characteristics: (100th cycle discharge capacity / first cycle discharge capacity) 100

As can be seen from Table 3 and FIG. 5, the lifetime characteristics of the lithium secondary battery of Example 2 were 82.1%, 80.7%, and about 2%, respectively, even under the high temperature and high voltage condition of 4.35 V / Or more.

Claims (15)

Mixing the cathode active material containing lithium impurities with a first organic solvent dissociated from the organic acid, and stirring to dissolve the lithium impurities to remove lithium impurities.
The method according to claim 1,
Wherein the organic acid comprises any one selected from the group consisting of citric acid, tartaric acid, and malic acid, or a mixture of two or more thereof.
The method according to claim 1,
Wherein the first organic solvent is any one selected from the group consisting of methanol, ethanol, propanol, butanol, isopropyl alcohol, and acetone, or a mixture of two or more thereof.
The method according to claim 1,
Wherein the mixing weight ratio of the first organic solvent to the organic acid is 100: 2 to 20.
The method according to claim 1,
Wherein the organic acid is 5 wt% to 30 wt% based on the total weight of the cathode active material including the lithium impurity.
The method according to claim 1,
The lithium impurities method of producing a positive electrode active material, characterized in that LiOH, Li ₂ CO ₃ or a mixture thereof.
The method according to claim 1,
Wherein the stirring is performed at room temperature for 1 minute to 1 hour.
The method according to claim 1,
Further comprising the step of dissolving the lithium impurity and then washing the cathode active material with a second organic solvent.
9. The method of claim 8,
Wherein the cleaning is performed by adding water to the second organic solvent.
9. The method of claim 8,
Wherein the second organic solvent is any one selected from the group consisting of methanol, ethanol, propanol, butanol, isopropyl alcohol, and acetone, or a mixture of two or more thereof.
The method according to claim 1,
Removing the lithium impurities, and then filtering and drying the cathode active material.
Wherein the lithium impurity is contained in an amount of less than 0.3% by weight based on the total weight of the cathode active material.
13. The method of claim 12,
The Li dopant was the positive electrode active material, characterized in that LiOH, Li ₂ CO ₃ or a mixture thereof.
An anode comprising the cathode active material of claim 12.
A lithium secondary battery comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the positive electrode is the positive electrode according to Claim 14.

Images