KR20130102827A - Fabrication method of nanocomposite for lithium secondary battery - Google Patents

Abstract

PURPOSE: A manufacturing method of a nanocomposite is provided to manufacture a nanocomposite with improved stability, electrode capacity, and cycle performance by modifying the surface of a LiMO2-based electrode active material in a lamellar structure. CONSTITUTION: A manufacturing method of a nanocomposite comprises a step of dispersing a LiMO2-based positive active material into a dispersing medium; a step of adding monomers of a conductive polymer into the dispersion; and a step of conducting a polymerization reaction by adding an oxidant into the dispersion and washing and drying the product. M is selected from Ni, Co, Mn, Al, Sr, Cu, Fe, Mg, B, or Ga. The dispersion medium is selected from ethanol, butanol, acetonitrile, or a mixture thereof.

Description

Manufacturing method of nanocomposite for lithium secondary battery {Fabrication method of nanocomposite for lithium secondary battery}

The present invention relates to a method for manufacturing a nanocomposite for a lithium secondary battery, and more particularly, to a method of manufacturing a nanocomposite by coating a surface of an electrode active material having a layered structure, and lithium having excellent stability and cycle life in a high voltage region employing the same. It relates to a secondary battery.

Representative layered metal oxide cathode active materials for lithium secondary batteries include LiCoO ₂ , LiNiO ₂ , LiNi _x Co _{1 - x} O ₂ (0 <x <1), LiNi _{1- xy} Co _x M _y O ₂ (0 <x < 1, 0 <y <1, 0 <x + y <1, M is a metal such as Al, Sr, Mg, Fe, Mn), etc. Among them, LiCoO ₂ has a high capacity, low self-discharge rate and excellent cycle Due to its lifetime, it is most widely used as a commercial lithium secondary battery than other cathode active materials. However, unlike the high theoretical capacity, when looking at the charge and discharge behavior of Li _{1 - x} CoO ₂ it was reported that the capacity decreases rapidly as the cycle proceeds at x> 0.5. Therefore, the theoretical capacity is 274 mAh / g, but the actual capacity represents 145 mAh / g which is slightly larger than 137 mAh / g, which is 50% of the theoretical capacity, and the charging voltage of the electrode corresponding to this actual capacity corresponds to 4.1 to 4.2V. do. LiMO ₂ When the layered oxides such as (M = metal) are charged to 4.3V or more, the reversible capacity is significantly reduced due to the dissolution of the transition metal and the exchange of lithium and transition metal ions. Li _x MO ₂ with lithium deintercalation The deterioration of the surface structure of (M = metal) and the exothermic reaction accompanied by rapid structural collapse are a major problem in the stability of the battery.

In order to overcome this problem, studies have been conducted to suppress the dissolution of metal ions by structurally stabilizing by adding a small amount of hetero elements or by modifying the surface of the active material. Korean Patent No. 10-0374010 discloses a method of surface modification of a metal oxide electrode material powder used as a cathode of a lithium secondary battery, and a method of modifying a metal or metal oxide on a particle by thermal decomposition and chemical vapor deposition. US patent application Ser. No. 10 / 602,251 also discloses a method of providing a surface modified material by coating a coagulant on a core to use as a cathode active material. However, when the cathode active material surface-modified in this manner is employed, there is a limit in securing stability in the high voltage region, and the manufacturing process is complicated and thus difficult to be practical.

Meanwhile, in the case of the conventional LiCoO _2, the surface of the electrode active material was coated with a metal oxide such as ZrO ₂ , Al ₂ O ₃ , and a metal composite oxide to increase stability against high voltage, thereby increasing reversible capacity. In other words, when the surface of the positive electrode active material is coated, high voltage charge and discharge can be obtained by suppressing the dissolution of transition metals generated from the surface or by enhancing the surface stability at high voltage to suppress side reactions on the surface, thereby obtaining a larger capacity than before. have. In addition, surface modification can improve cycle efficiency, thermal stability, high capacity and high output at high rate discharges, and at the same time, dramatically improve the lifetime. However, there are also problems associated with surface modification, for example, the addition of surface modification can reduce the specific capacity, and the use of surface modification with low ion conductivity prevents the movement of lithium ions during charging and discharging behavior, thereby controlling the rate characteristics. There is a possibility of lowering. In addition, the surface modification may reduce the intercalation / deintercalation reaction area of lithium on the surface of the positive electrode active material, thereby degrading high rate characteristics.

Current _{LiNi 0 .8 Co 0 .15 Al 0} .05 O 2, LiNi 0 .5 Co 0 .2 Mn 0 .3 O 2, 3 -component system positive electrode, such as _{LiNi 1/3 Co 1/3} Mn 1/3 O 2 In the case of active materials, many studies have been conducted to ensure high capacity and stability by deintercalating more lithium to a high voltage region through surface modification. Until now, surface modification is mainly an electrochemically inert metal oxide and metal complex oxide. This was used. However, these metal oxides and metal composite oxide surface modifiers may contribute to the improvement of electrochemical properties on the surface of the positive electrode active material and may also act as a kind of resistance on the surface of the positive electrode active material due to electrochemical inactivation. Since the resistance in the electrode acts as a limiting factor on the performance of the lithium secondary battery, the development of a new surface modification method is required.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method of manufacturing a nanocomposite for a lithium secondary battery by modifying the surface of a LiMO ₂ based electrode active material having a layered structure, thereby overcoming the limitations of the conventional LiMO ₂ based electrode active material, thereby ensuring stability in a high voltage region and an electrode. It is to effectively improve the capacity and cycle characteristics.

In order to solve the above technical problem, the present invention comprises the steps of 1) dispersing a LiMO _2- based (M = metal) cathode active material in a dispersion medium; 2) adding a monomer of the conductive polymer to the dispersion; And 3) it provides a method for producing a nanocomposite for lithium secondary battery comprising the step of washing and drying after the polymerization reaction by adding an oxidizing agent to the dispersion to which the monomer is added.

In this case, the metal M may be Ni, Co, Mn, Al, Sr, Cu, Fe, Mg, B or Ga, for example. In addition, the LiMO ₂ -based (M = metal) positive electrode active material used in the present invention is, for example, LiCoO ₂ , LiNiO ₂ , LiNi _x Co _{1 - x} O ₂ (0 <x <1), LiNi _{1 -xy} Co _x M _y 0 ₂ (0 <x <1, 0 <y <1, 0 <x + y <1, M may be selected from metals such as Al, Sr, Mg, Fe, and Mn).

In addition, the conductive polymers usable in the present invention are polyaniline, poly (3,4-ethylenedioxythiophene) (PEDOT), polypyrrole, poly (3,4-ethylenedioxythiophene) -poly (styrenesulfonate) ( PEDOT: PSS) or mixtures thereof, and the dispersion medium may be selected from distilled water, acetone, butanol, acetonitrile or a mixture thereof.

In addition, the oxidizing agent used for the polymerization reaction of the present invention include cerium-based oxidizing agents such as FeCl ₃ , ammonium persulfate (APS), Ce (SO ₄ ) ₂ or (NH ₄ ) ₂ Ce (NO ₃ ) ₆ , and the like. Can be.

When the nanocomposite in which the surface of the electrode active material according to the present invention is modified with a conductive polymer is employed in a lithium secondary battery, a high voltage region, which is a problem of an electrode active material having a conventional layered structure LiMO ₂ (M = Ni, Co, Mn, Al, etc.) Reversible capacity reduction and surface instability can be solved. In particular, resistance elements and non-uniform coating effects caused by metal oxides and metal complex oxides, which are used as surface modifiers, can be improved. It is possible to improve electrochemical properties such as lifetime.

1 is a scanning electron micrograph of a PEDOT-coated LiNi _0.8 Co _0.15 Al _0.05 O ₂ nanocomposite synthesized according to an embodiment of the present invention.
Figure 2 is a graph showing the thermogravimetric analysis of the PEDOT-coated LiNi _0.8 Co _0.15 Al _0.05 O ₂ nanocomposites synthesized according to an embodiment of the present invention.
3 is a transmission electron micrograph of a PEDOT-coated LiNi _0.8 Co _0.15 Al _0.05 O ₂ nanocomposite synthesized according to an embodiment of the present invention.
Figure 4 is a scanning photomicrograph of a transfer that is not coated with PEDOT _{LiNi 0 .8 Co 0 .15 Al 0} .05 O 2 electrode active material.
Figure 5 is a is synthesized by an embodiment of the present invention PEDOT coated LiNi _0.8 Co _0.15 Al _0.05 O ₂ nanocomposite with a PEDOT uncoated _{LiNi 0 .8 Co 0 .15 Al 0} .05 O 2 electrode active material It is a graph comparing cycle life by setting the terminal voltage of the included anode to 4.5V.
Figure 6 is a is synthesized by an embodiment of the present invention PEDOT coated LiNi _0.8 Co _0.15 Al _0.05 O ₂ nanocomposite with a PEDOT uncoated _{LiNi 0 .8 Co 0 .15 Al 0} .05 O 2 electrode active material It is a graph comparing the cycle life by setting the terminal voltage of the included anode to 4.6V.
Figure 7 is a is synthesized by an embodiment of the present invention PEDOT coated LiNi _0.8 Co _0.15 Al _0.05 O ₂ nanocomposite with a PEDOT uncoated _{LiNi 0 .8 Co 0 .15 Al 0} .05 O 2 electrode active material It is a graph comparing the cycle life by setting the terminal voltage of the included anode to 4.8V.

Hereinafter, the present invention will be described in more detail.

The present invention provides a conductive polymer to overcome the limitations of the electrode active material of LiMO ₂ (M = Ni, Co, Mn, Al, Sr, Cu, Fe, Mg, B or Ga) series having a conventional layered structure It is to provide a nanocomposite having more improved electrochemical properties by coating the surface of the electrode active material using the as a surface modifier.

Previously, most of the conductive polymers have been researched to improve conductivity characteristics of a cathode active material having low conductivity, mainly around a LiFePO ₄ cathode active material, but the conductive polymer was selected as a surface modifier in the present invention. Conductive polymers can be more uniformly coated on the surface of the positive electrode active material than metal oxides or metal composite oxides, and because they have excellent conductivity, Li _x MO ₂ from which lithium is deintercalated It can exhibit stability and high capacity and improved cycle characteristics in the high voltage region of the (M = metal) electrode active material.

Specifically, the lithium composite battery nanocomposite according to the present invention comprises the steps of 1) dispersing a LiMO ₂ based (M = metal) positive electrode active material in a dispersion medium; 2) adding a monomer of the conductive polymer to the dispersion; And 3) injecting an oxidizing agent into the dispersion to which the monomer is added to perform a polymerization reaction, followed by washing and drying.

In this case, the metal M may be Ni, Co, Mn, Al, Sr, Cu, Fe, Mg, B or Ga, for example. In addition, the LiMO ₂ -based (M = metal) positive electrode active material used in the present invention is, for example, LiCoO ₂ , LiNiO ₂ , LiNi _x Co _{1 - x} O ₂ (0 <x <1), LiNi _{1 -xy} Co _x M _y 0 ₂ (0 <x <1, 0 <y <1, 0 <x + y <1, M may be selected from metals such as Al, Sr, Mg, Fe, and Mn).

Also conductive polymers usable in the present invention are polyaniline, poly (3,4-ethylenedioxythiophene) (PEDOT), polypyrrole, poly (3,4-ethylenedioxythiophene) -poly (styrenesulfonate) ( PEDOT: PSS) or mixtures thereof, and the dispersion medium may be selected from distilled water, acetone, butanol, acetonitrile or a mixture thereof.

In addition, the oxidizing agents used for the polymerization reaction of the present invention include cerium-based oxidizing agents such as FeCl ₃ , ammonium persulfate (APS), Ce (SO ₄ ) ₂ or (NH ₄ ) ₂ Ce (NO ₃ ) ₆ , and the like. Can be.

Specific manufacturing method according to the present invention is as follows.

First, the present invention is a metal compound and metal as a surface modification of the surface of the electrode active material of LiMO ₂ (M = Ni, Co, Mn, Al, Cu, Fe, Mg, B or Ga) series having a conventional layer structure Instead of the composite oxide, a conductive polymer is used as a surface modifier to synthesize a nanocomposite coated with the conductive polymer on the surface of the electrode active material. Conventional conductive polymer synthesis and coating is completed by oxidizing the conductive polymer in the solution by using an electrochemical method or a chemical method by simply adding an oxidant to the conductive polymer present in the solution. When the electrochemical method is used, the polymer coating is performed on the electrode surface layer applied on the current collector rather than the particle state, and thus, in the present invention, the conductive polymer may be coated at a more uniform level than the electrochemical method, that is, at the electrode active material particle level. Using a chemical method to synthesize a nanocomposite coating the surface of the electrode active material with a conductive polymer such as PEDOT.

As a raw material, first, a monomer of a conductive polymer is added to a solvent containing an electrode active material to disperse it into a homogeneous mixture for uniform surface coating onto the surface of the electrode active material. Thereafter, when a solution in which an oxidizing agent such as FeCl ₃ is dissolved is slowly injected into the solvent, the monomer of the conductive polymer starts to oxidize and polymer polymerization occurs if the polymerization time is maintained sufficiently.

Since the polymerization of the monomer proceeds in a solution in which the electrode active material particles are uniformly dispersed, it can be expected that the surface of the individual electrode active material particles can be uniformly coated. In order to remove the oligomers or other impurities caused by the polymerization reaction does not occur completely during the polymerization process and to collect only the electrode active material coated with the conductive polymer, it is collected several times using ethanol and distilled water and then collected. The remaining moisture in the electrode active material coated with the conductive polymer is sufficiently dried in the air and removed to prepare a cathode for a lithium secondary battery. Surface modification using conductive polymers is determined by variables such as polymerization temperature, polymerization holding time and type of oxidizing agent. This can be confirmed by electron microscope after synthesis.

Hereinafter, the present invention will be described in detail through Examples and Comparative Examples. However, the embodiments are only examples of the present invention, and the scope of the present invention is not limited thereto.

Example  One

LiNO ₃ , Ni (NO ₃ ) ₂ .6H ₂ O, Co (NO ₃ ) ₂ .6H ₂ O, and Mn (NO ₃ ) ₂ .4H ₂ O are dissolved in 150 ml of ethanol. To make the active material of the desired composition, the molar ratio of the transition metals is set to 0.8: 0.15: 0.05 and the lithium salt is added in excess of 10% in consideration of volatilization generated during the synthesis process. 10.24 g of citric acid is dissolved in 120 ml of ethanol as a chelating agent and added to the solution to form a transition metal complex ion. Each solution was mixed with each other and mixed at 350 rpm for 1 hour and then placed in an 80 ° C. oven to evaporate the solvent. When the solvent is completely evaporated to a solid state, it is ground using a mortar. The crushed powder is transferred to an alumina crucible at 400 ~ 600 ℃ for 4-6 hours to conduct the first heat treatment and re-pulverized, subjected to secondary heat treatment for 15 to 24 hours at _{800 ~ 1000 ℃ LiNi 0 .8 Co} 0. the ₁₅ Al _{0 .05} O ₂ positive active material was prepared.

Then, after inserting the _{LiNi 0 .8 Co 0 .15 Al 0} .05 O 2 positive active material and 0.5g EDOT 0.1ml to 50ml of distilled water, by using an ultrasonic dispersed for 5 to 10 minutes. After maintaining the 400rpm for uniform polymerization of the positive electrode active material and EDOT in solution FeCl ₃ 1 g of oxidant is dissolved in 10 ml of distilled water and slowly injected into the solution. After the injection is completed, the polymerization reaction is carried out for 30 minutes, and then washed three times in alternation with ethanol and 1 liter of distilled water. When the cleaning is complete, grinding was sufficiently dried the moisture remaining placed in an oven to obtain a 50 ℃ _{LiNi 0 .8 Co 0 .15 Al 0} .05 O 2 positive active material nanocomposite coating a PEDOT. Scanning electron micrographs, thermogravimetric analysis, and transmission electron micrographs of the PEDOT / LiNi _0.8 Co _0.15 Al _0.05 O ₂ cathode active material nanocomposites are shown in FIGS. 1 to 3.

Then, when the _{PEDOT / LiNi 0 .8 Co 0 .15} Al 0 .05 O 2 positive active material nanocomposite 0.5g and 0.03g, and Denka Black by the addition of 0.04g NMP PVDF blend is obtained after a suitable viscosity on the thin aluminum plate by rolling after casting to drying _{PEDOT / LiNi 0 .8 Co 0 .15} Al 0 .05 O 2 Electrode.

_{PEDOT / LiNi 0 .8 Co 0 .15} Al 0 .05 O 2 A lithium secondary battery half cell was formed by using an electrode, a PP separator, and a lithium metal as a counter electrode, and an EC: DMC: EMC (1: 1: 1) solution in which 1M LiPF ₆ was dissolved was injected. The charging and discharging behavior and the cycle life were investigated in the 3.0 to 4.5V potential section with current densities of C and 1C, and the results are shown in FIG. 5.

Example  2

Under the same conditions as in Example _{1 PEDOT / LiNi 0 .8 Co 0} .15 Al 0 .05 O 2 Half battery was fabricated and the charge and discharge behavior was investigated at 3.0 to 4.6V. The results are shown in FIG. 6.

Example  3

Under the same conditions as in Example _{1 PEDOT / LiNi 0 .8 Co 0} .15 Al 0 .05 O 2 Half battery was fabricated and the charge and discharge behavior was investigated at 3.0 to 4.8V. The results are shown in FIG. 7.

Comparative example  One

In this comparative example, LiNO ₃ , Ni (NO ₃ ) ₂ · 6H ₂ O, Co (NO ₃ ) ₂ · 6H ₂ O, and Mn (NO ₃ ) ₂ · 4H ₂ O were added to 150 ml of ethanol using a conventional sol-gel method. Dissolve. To make an active material of a desired composition, the molar ratio of the transition metals is set to 0.8: 0.15: 0.05, and the lithium salt is added in excess of 10% in consideration of volatilization generated during the synthesis process. 10.24 g of citric acid is dissolved in 120 ml of ethanol as a chelating agent and added to the solution to form a transition metal complex ion. Each solution was mixed with each other and mixed at 350 rpm for 1 hour and then placed in an 80 ° C. oven to evaporate the solvent. When the solvent is evaporated to an appropriate amount, the solution becomes a gel, which is dried and then ground. The crushed powder is then pulverized by a moving heat treatment at 400 to 600 ℃ for 4-6 hours a difference in an alumina crucible subjected to secondary heat treatment for 15 to 24 hours at _{800 ~ 1000 ℃ LiNi 0 .8 Co} 0 .15 Al _{A 0.05} O ₂ positive electrode active material was prepared.

Since the process was performed as Example 1, the _{LiNi 0 .8 Co 0 .15 Al 0} .05 O shows a scanning electron micrograph of the positive electrode active material ₂ in FIG. The charging and discharging behavior was examined under the same conditions as in Example 1, and the results are shown in FIG. 5.

Comparative example  2

Example 1 and conditions as _{LiNi 0 .8 Co 0 .15 Al 0} .05 O 2 , such as Half battery was fabricated and the charge and discharge behavior was investigated at 3.0 to 4.6V. The results are shown in FIG. 6.

Comparative example  3

Example 1 and conditions as _{LiNi 0 .8 Co 0 .15 Al 0} .05 O 2 , such as Half battery was fabricated and the charge and discharge behavior was investigated at 3.0 to 4.8V. The results are shown in FIG. 7.

In the present invention, in order to solve the limitations of the LiMO ₂ (M = Ni, Co, Mn, Al, Cu, Fe, Mg, B or Ga, etc.) electrode active material having a layered structure in the high voltage region, the surface of the electrode active material Instead of conventional surface modifiers, metal oxides and metal complex oxides, PEDOT, a conductive polymer, was used as the surface modifier. This results in a more uniform coating effect at the particle level, as well as the removal of resistive elements that can be considered from existing surface modifications, resulting in stability in the high voltage region of the electrode active material, resulting in increased electrode capacity and cycle life characteristics. The effect of improving the electrochemical properties can be expected.

When the PEDOT-coated nanocomposite electrode according to the present invention is charged and discharged with 4.5 V as the termination voltage, the PEDOT shows a relatively stable cycle characteristic compared to the electrode without the PEDOT coating, but when the termination voltage is increased to 4.8 V, the PEDOT is applied. Compared to this uncoated electrode, it showed significantly higher capacity and stable cycle characteristics. Therefore, when PEDOT is used as a surface modifier to coat the surface of the electrode active material, high voltage charging and discharging is possible even at a high end voltage, thereby increasing capacity.

In particular, in the case of the electrode active material of the layered LiMO ₂ (M = Ni, Co, Mn, Al, Cu, Fe, Mg, B or Ga, etc.) series used in the present invention, elution of metal ions during charge and discharge in a high voltage region and Limitations such as surface instability and the like. Therefore, in the present invention, instead of overcoming these limitations with existing surface modifiers such as metal oxides and metal composite oxides, conductive polymers are used as surface modifiers, which can be used in terms of resistive elements and non-uniform coating effects considered by conventional surface modifiers. In addition, the electrode active material exhibiting greater electrochemical properties such as capacity and cycle life of the PEDOT-coated nanocomposite electrode when charging and discharging in a high voltage region can be expected.

Claims (6)

1) dispersing a LiMO ₂ based (M = metal) positive active material in a dispersion medium;
2) adding a monomer of the conductive polymer to the dispersion; And
3) adding an oxidizing agent to the dispersion to which the monomer is added, performing a polymerization reaction, and then washing and drying the lithium secondary battery nanocomposite. The method of claim 1,
Wherein M is Ni, Co, Mn, Al, Sr, Cu, Fe, Mg, B or Ga method for producing a nanocomposite for lithium secondary battery, characterized in that selected from. The method of claim 1,
The LiMO ₂ based (M = metal) positive electrode active material is LiCoO ₂ , LiNiO ₂ , LiNi _x Co _{1 - x} O ₂ (0 <x <1), LiNi _{1- xy} Co _x M _y O ₂ (0 <x <1 , 0 <y <1, 0 <x + y <1, M = Al, Sr, Mg, Fe, Mn). A method of manufacturing a nanocomposite for a lithium secondary battery, characterized in that the selected. The method of claim 1,
The conductive polymer may be polyaniline, poly (3,4-ethylenedioxythiophene) (PEDOT), polypyrrole, poly (3,4-ethylenedioxythiophene) -poly (styrenesulfonate) (PEDOT: PSS) or Method for producing a nanocomposite for a lithium secondary battery, characterized in that selected from a mixture of these. The method of claim 1,
The dispersion medium is a method for producing a nanocomposite for lithium secondary battery, characterized in that selected from distilled water, ethanol, butanol, acetonitrile or a mixture thereof. The method of claim 1,
The oxidant is FeCl ₃ , ammonium persulfate (APS), Ce (SO ₄ ) ₂ Or (NH ₄ ) ₂ Ce (NO ₃ ) _{6 The} method for producing a nanocomposite for lithium secondary battery, characterized in that.

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