KR20090126962A - Positive active material for lithium secondary battery, method of preparing same, and lithium secondary battery comprising same - Google Patents

Abstract

PURPOSE: A positive active material for a lithium secondary battery is provided to ensure excellent cycle lifetime characteristic, especially, high temperature cycle lifetime characteristic because vanadium present in concentration gradient thereinside suppresses the manganese elution. CONSTITUTION: A positive active material for a lithium secondary battery comprises: a manganese-based compound represented by chemical formula 1: Li_aMn_(1-b)M_bO_(2-c)D_c or chemical formula 2: Li_aMn_(2-b')M_b'O_(4-c)D_c which has a cluster form consisting of nanorods; and vanadium present in concentration gradient in the inside direction from the surface of the manganese-based compound. In chemical formulas, 0.90 <= a <= 1.1, 0 <= b <= 0.5, 0 <= b' <= 0.5, 0 <= c <= 0.05 and M is selected from the group consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, rare earth materials and combinations thereof.

Description

A positive electrode active material for a lithium secondary battery, a method of manufacturing the same, and a lithium secondary battery including the same TECHNICAL FIELD

The present invention relates to a positive electrode active material for a lithium secondary battery, a method of manufacturing the same, and a lithium secondary battery including the same. More particularly, the positive electrode active material for a lithium secondary battery having excellent structural stability, excellent cycle life characteristics and high temperature storage properties, and a method of manufacturing the same. And it relates to a lithium secondary battery comprising the same.

There is an increasing need for high performance and high capacity of batteries used as power sources for portable electronic devices and electric vehicles.

The battery generates power by using a material capable of electrochemical reactions at the positive and negative electrodes. A typical example of such a battery is a lithium secondary battery that generates electric energy by a change in chemical potential when lithium ions are intercalated / deintercalated at a positive electrode and a negative electrode.

The lithium secondary battery is manufactured by using a material capable of reversible intercalation / deintercalation of lithium ions as a positive electrode and a negative electrode active material, and filling an organic electrolyte or a polymer electrolyte between the positive electrode and the negative electrode.

As a cathode active material of a lithium secondary battery, a lithium composite metal compound is used. Examples thereof include a composite of LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _1-x Co _x O ₂ (0 <x <1), LiMnO _2, and the like. Metal oxides are being studied.

Among the cathode active materials, spinel Mn-based cathode active materials such as LiMn ₂ O ₄ and LiMnO ₂ are easy to synthesize, are relatively inexpensive, have the best thermal stability compared to other active materials when overcharged, and have low environmental pollution and are attractive. Although a substance, there is a problem in that the capacity is rather small. In addition, during charging and discharging, Mn ³⁺ is eluted from the surface of the manganese-based positive electrode active material, and a disproportionation reaction (2Mn ^{3 +} → Mn ⁴⁺ + Mn ²⁺ ) occurs at the particle surface, which is a positive electrode active material. Will cause a defect. This reaction occurs more actively when in contact with the electrolyte, especially at high temperatures. According to this reaction, the manganese is eluted to decay the active material, resulting in deterioration of structural stability, reduction in capacity, and particularly, a decrease in cycle life at high temperatures.

To solve this problem, that is, doping of metal ions such as Mg ²⁺ , Ga ³⁺ , Al ³⁺ , Cu ²⁺ or Cr ^{3+ has} been studied to stabilize the structure and improve the high temperature cycle characteristics. Such doping ions can suppress capacity reduction by substituting Mn ³⁺ in the manganese-based active material, but the structural stability cannot be effectively improved as the fraction of Mn 3+ ions still exists in the spinel structure. In another method, in order to improve cycle characteristics at high temperature, a method of coating manganese-based active material particles with Li ₂ OB ₂ O ₃ , ZnO, LiCoO _2, or CoO has been studied. However, this method only reduced the rate at which Mn was eluted, and completely prevented Mn from eluting, thereby improving cycle life characteristics. In addition, when the manganese-based active material particles are coated with the compound, low-temperature heat treatment is not possible, and thus the heat-treatment should be performed at a temperature of at least 600 ° C., so that the coated material penetrates deeply into the interior rather than near the surface of the manganese-based active material particles. There is a problem of deterioration of characteristics.

In another method, a study to improve the charge and discharge characteristics by adjusting the morphology, but did not reach a satisfactory level.

SUMMARY OF THE INVENTION In order to solve the above problems, an object of the present invention is to provide a cathode active material for a lithium secondary battery excellent in structural stability, cycle life characteristics and high temperature storage stability.

Another object of the present invention is to provide a method for producing the cathode active material.

Still another object of the present invention is to provide a lithium secondary battery including a cathode including the cathode active material.

Technical problems to be achieved by the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned will be clearly understood by those skilled in the art from the following description.

A first embodiment of the present invention for achieving the above object is a manganese compound represented by the following formula (1) or 2 having a cluster shape consisting of nanorods (nanorod) and the concentration in the inner direction from the surface of the manganese compound It is to provide a cathode active material for a lithium secondary battery containing vanadium present in a gradient.

[Formula 1]

Li _a Mn _1-b M _b O _2-c D _c

[Formula 2]

Li _a Mn _{2-b '} M _b' O _4-c D _c

Where 0.90 ≦ a ≦ 1.1, 0 ≦ b ≦ 0.5, 0 ≦ b ′ ≦ 0.5, 0 ≦ c ≦ 0.05,

M is selected from the group consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, rare earth elements and combinations thereof)

In addition, a second embodiment of the present invention is a method for producing the positive electrode active material, which method is prepared by mixing a manganese precursor and an oxidizing agent in a solvent to prepare a MnO ₂ having a stellar-like structure; Mixing the MnO ₂ with a lithium precursor; Heat treating the mixture to prepare a manganese compound represented by Chemical Formula 1 or 2 having a cluster shape composed of nanorods; Coating the manganese compound with a vanadium compound solution; And annealing the obtained mixture.

In addition, a third embodiment of the present invention is to provide a lithium secondary battery including the positive electrode active material.

In the positive electrode active material of the present invention, vanadium existing in a concentration gradient therein can suppress manganese elution, and is excellent in cycle life characteristics, particularly high temperature cycle life characteristics.

Hereinafter, embodiments of the present invention will be described in detail. However, this is presented as an example, by which the present invention is not limited and the present invention is only defined by the scope of the claims to be described later.

The cathode active material according to the first embodiment of the present invention includes a manganese compound represented by Chemical Formula 1 or 2 having a cluster shape composed of nanorods, and vanadium present in a concentration gradient inward from the surface of the manganese compound. .

[Formula 1]

Li _a Mn _1-b M _b O _2-c D _c

[Formula 2]

Li _a Mn _{2-b '} M _b' O _4-c D _c

Where 0.90 ≦ a ≦ 1.1, 0 ≦ b ≦ 0.5, 0 ≦ b ′ ≦ 0.5, 0 ≦ c ≦ 0.05,

M is selected from the group consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, rare earth elements and combinations thereof)

The positive electrode active material of the present invention includes a manganese compound represented by Chemical Formula 1 or 2 having a cluster shape composed of nanorods, and the shape has a high specific surface area compared to the particulate manganese compound and has very high properties. Have In addition, since vanadium is present at a concentration gradient in the inner direction of the surface of the manganese compound, the phenomenon in which manganese is eluted from the manganese compound at a high temperature can be suppressed. That is, in the present invention, vanadium is present in the form of a concentration gradient present in a concentration gradually decreasing in the inner direction from the surface of the manganese compound, the phenomenon that the manganese is eluted when the vanadium is present in a high concentration on the surface of the manganese compound Since the effect of suppressing vanadium is very excellent compared to the case where vanadium is present in the same concentration as a whole of a manganese compound, it is preferable.

When the vanadium is 100% in diameter or length in the cathode active material nanorod of the present invention, the vanadium is distributed up to 30% in the inner direction from the surface of the manganese compound, and more preferably, it is distributed up to 10% in depth. desirable. If vanadium exceeds 30% depth inwardly from the surface of the manganese compound, that is, penetrates very inwardly, the capacity is reduced, and there is a problem that the structure suddenly collapses when left at high temperature, which is not preferable.

At this time, the vanadium concentration on the surface is preferably 80 to 70% by weight, and preferably present at a concentration of 20 to 10% by weight at the depth of 30% inside. When vanadium is present on the surface and in the concentration in the above range, vanadium is present at a high concentration on the surface, it is preferable because it can more effectively suppress the problem caused by the elution of manganese from the manganese compound. That is, in the cathode active material of the present invention, the manganese compound is present at 20 to 30 wt% on the surface, and the manganese compound is present at 80 to 90 wt% at a depth of 30% inside.

As such, the vanadium in the positive electrode active material of the present invention is mainly present in a concentration gradient gradually decreasing in concentration from the surface of the manganese compound. Very few may also be present on the outer surface of the manganese compound.

As such, since vanadium is distributed in the concentration gradient from the surface of the manganese compound to the inside, charging and discharging proceeds, particularly at high temperature storage or high temperature, during charging and discharging of the battery including the cathode active material. The phenomenon in which manganese elutes from the manganese compound can be suppressed, thereby improving cycle life characteristics.

The total weight of vanadium contained in the cathode active material of the present invention is preferably 1 to 5% by weight based on the weight of the manganese compound. If the content of the vanadium is more than 5% by weight, the capacity is reduced is not preferable, if less than 1% by weight there is a problem that the coating does not become uniform, which is not preferable.

In the present invention, the manganese-based compound has a cluster shape consisting of nanorods, wherein the nanorods are preferably smaller diameter, and may preferably be used having a nanometer diameter, for example, nanorods of 100 nm or less Can be preferably used. In addition, the nanorods may be suitably used having a length of 2㎛ or less.

Since the positive electrode active material of the present invention has a high specific surface area of 10 to 60 m ² / g, it is preferable because it has many active sites and can exhibit high capacity. When the specific surface area of the positive electrode active material exceeds 60 m ² / g, the reaction area with the electrolyte at a high temperature is too large to increase the elution of Mn, which is undesirable, and when less than 10 m ² / g, the active site is small and the capacity is lowered. Not desirable

The positive electrode active material of the present invention has excellent capacity retention and can reduce Mn elution when stored at high temperature, and thus has excellent high temperature stability and uses a manganese compound having a cluster shape composed of nanorods. In comparison, the lithium ion intercalation movement can be made faster, and thus the discharge rate can be improved.

Such a cathode active material of the present invention may be manufactured as follows according to the manufacturing method according to the second embodiment.

The manganese precursor and the oxidant are mixed in a solvent and maintained at 150 to 200 ° C. As the oxidant, ammonium persulfate, ammonium nitrate or a combination thereof may be used. As the manganese precursor, manganese sulfate, manganese acetate, manganese hydroxide or a combination thereof may be used. As the solvent, water, ethanol or a combination thereof may be used. At this time, the mixing ratio of the manganese precursor and the oxidizing agent is preferably 1: 1 to 2: 1 by weight. If the mixing ratio of the manganese precursor and the oxidant is out of the above range, MnO ₂ is not formed, which is not preferable.

At this time, by the oxidizing agent to get up the manganese precursor oxidation reaction, modulus - the MnO ₂ having a similar structure (urchin-like structure) have been manufactured, a sea urchin - depending on the use of the MnO ₂ having a similar structure in the step of mixing a lithium precursor The final product is able to produce a cluster-shaped manganese-based active material consisting of nanorods.

The star-like structure MnO ₂ and a lithium precursor are mixed. Lithium nitrate, lithium acetate, lithium hydroxide and the like can be used as the lithium compound, but is not limited thereto. In this step, a metal compound may be further added. Further addition of the metal compound results in a form in which some of the manganese is replaced by metal in the final product. As the metal, Al, Ni, Co, Cr, Fe, Mg, Sr, or a rare earth element may be used. Examples of the metal compound may be an oxide, nitrate salt, acetate salt, or hydroxide of the metal. This mixing step may be carried out dry or may be carried out wet using an organic solvent as the mixing medium. Alcohol, such as ethanol, or acetone can be used as an organic solvent.

The mixing ratio of the MnO ₂ and the lithium precursor is preferably 1.6: 1 to 2: 1 by weight. If the mixing ratio of MnO ₂ and the lithium precursor is out of the above range, the problem that the spinel phase is not formed occurs, which is not preferable.

Subsequently, the obtained mixture is heat-treated and slow cooled to prepare a manganese-based active material represented by the following Chemical Formula 1 or 2 having a cluster shape composed of nanorods.

[Formula 1]

Li _a Mn _1-b M _b O _2-c D _c

[Formula 2]

Li _a Mn _{2-b '} M _b' O _4-c D _c

Where 0.90 ≦ a ≦ 1.1, 0 ≦ b ≦ 0.5, 0 ≦ b ′ ≦ 0.5, 0 ≦ c ≦ 0.05,

M is selected from the group consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, rare earth elements and combinations thereof)

The heat treatment is preferably carried out at 700 to 750 ° C. for 5 to 15 hours, and the heat treatment atmosphere is preferably an air atmosphere or an oxygen atmosphere. When the heat treatment is performed at a temperature lower than 700 ° C., the reaction between the compounds to be used is not sufficient, and when the temperature is higher than 750 ° C., there is a problem in that an unstable structure is formed by evaporation of Li in the crystal structure. . In addition, when the heat treatment process is performed for less than 5 hours, there is a problem that crystallization does not occur. If the heat treatment is performed for more than 15 hours, excessive crystallization occurs or the evaporation of Li in the crystal structure occurs. There is a problem that an unstable structure is formed.

Next, a manganese compound prepared according to the above process is coated with a vanadium compound solution. As the vanadium compound, any organic substance including OV (CH ₃ H ₇ ) ₃ , or V or a combination thereof may be used. The solvent in the solution may be water or alcohol such as methanol, ethanol, isopropyl alcohol and the like. At this time, the concentration of the vanadium compound solution is suitable 20 to 40% by weight, if the concentration of the vanadium compound solution is less than 20% by weight or more than 40% by weight is not preferable because there is a problem in the uniform coating.

The use ratio of the manganese compound and the vanadium compound is preferably 99 to 95% by weight of the manganese compound and 1 to 5% by weight of the vanadium compound. If the amount of the vanadium compound is less than 1% by weight or more than 5% by weight, there is a problem in capacity reduction, which is not preferable.

As the coating method, a general coating method such as sputtering, chemical vapor deposition (CVD), dip coating, which is an impregnation method, may be used, but powder, which is the simplest coating method, is simply dipped in the coating solution. It is preferable to use the dip coating method taken out.

The obtained mixture is annealed at 300 to 400 ° C to prepare a positive electrode active material of the present invention. According to the annealing process, the vanadium compound is converted to vanadium oxide, VO _x (x is 1.5-2), and when annealing is gradually progressed, VO _x (x is 1.5-2) and vanadium is Mn in the structure of the manganese compound. It is replaced by a site, and the concentration is high at the surface and low concentration gradient exists inside. In addition, at VO _x (x is 1.5-2), oxygen may enter the O site or be released as O ₂ gas in the structure of the manganese compound.

When the annealing process temperature is carried out at a temperature of less than 300 ℃ vanadium penetrates into the interior of the manganese compound there is a problem that does not exist as a concentration gradient, when carried out at a temperature exceeding 400 ℃ due to excessive reaction Vanadium is doped too deep into the manganese-based compound may cause a problem that the structure is rapidly collapsed when the capacity is reduced and left at high temperatures. In the annealing process, the above-described temperature conditions are important, and the time for performing the annealing process may be appropriately adjusted according to the temperature conditions so that the effect of the annealing can be obtained, for example, about 10 hours. do. Before carrying out the annealing step, a step of gradually increasing the obtained mixture may be further carried out.

The positive electrode active material of the present invention can be usefully used for the positive electrode of a lithium secondary battery. The lithium secondary battery includes a negative electrode and an electrolyte including a negative electrode active material together with a positive electrode.

The positive electrode is prepared by mixing a positive electrode active material according to the present invention, a conductive material, a binder and a solvent to prepare a positive electrode active material composition, and then coating and drying the aluminum active material directly. Alternatively, the cathode active material composition may be cast on a separate support, and then the film obtained by peeling from the support may be manufactured by laminating on an aluminum current collector.

The conductive material is carbon black, graphite, metal powder, the binder is vinylidene fluoride / hexafluoropropylene copolymer, polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polytetrafluoroethylene And mixtures thereof. In addition, N-methylpyrrolidone, acetone, tetrahydrofuran, decane, etc. are used as a solvent. In this case, the contents of the positive electrode active material, the conductive material, the binder, and the solvent are used at levels commonly used in lithium secondary batteries.

Like the positive electrode, the negative electrode is mixed with a negative electrode active material, a binder, and a solvent to prepare an anode active material composition, and the negative electrode active material film coated on the copper current collector or cast on a separate support and peeled from the support is coated on the copper current collector. It is prepared by lamination. At this time, the negative electrode active material composition may further contain a conductive material if necessary.

As the negative electrode active material, a material capable of intercalating / deintercalating lithium is used, and for example, lithium metal, lithium alloy, coke, artificial graphite, natural graphite, organic polymer compound combustor, carbon fiber, or the like is used. . In addition, a conductive material, a binder, and a solvent are used similarly to the case of the positive electrode mentioned above.

The separator may be used as long as it is commonly used in lithium secondary batteries. For example, polyethylene, polypropylene, polyvinylidene fluoride or two or more multilayer films thereof may be used, and a polyethylene / polypropylene two-layer separator, It goes without saying that a mixed multilayer film such as polyethylene / polypropylene / polyethylene three-layer separator, polypropylene / polyethylene / polypropylene three-layer separator and the like can be used.

As the electrolyte to be charged in the lithium secondary battery, a non-aqueous electrolyte or a known solid electrolyte may be used, and a lithium salt is used.

Although the solvent of the said non-aqueous electrolyte is not specifically limited, Chain carbonate methyl acetate, ethyl acetate, such as cyclic carbonate dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, such as ethylene carbonate, a propylene carbonate, butylene carbonate, vinylene carbonate, Esters such as propyl acetate, methyl propionate, ethyl propionate and γ-butyrolactone 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 1,2-dioxane, 2-methyltetrahydro Amides, such as nitriles, such as ether acetonitrile, such as furan, and dimethylformamide, etc. can be used. These can be used individually or in combination of two or more. In particular, a mixed solvent of a cyclic carbonate and a linear carbonate can be preferably used.

As the electrolyte, a gel polymer electrolyte in which an electrolyte solution is impregnated with a polymer electrolyte such as polyethylene oxide or polyacrylonitrile, or an inorganic solid electrolyte such as LiI or Li ₃ N can be used.

The lithium salt is LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiSbF ₆ , LiAlO ₄ , LiAlCl ₄ , One selected from the group consisting of LiCl and LiI is possible.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only preferred examples of the present invention and the present invention is not limited to the following examples.

(Example 1)

50 g of manganese sulfate (MnSO ₄ H ₂ O) and 50 g of ammonium persulfate ((NH ₄ ) ₂ S ₂ O ₈ ) were dissolved in 100 ml of distilled water in a teflon-lined stainless steel autoclave. . The autoclave was sealed and held at 150 ° C. for 20 hours.

At this time, the oxidation reaction of Mn ²⁺ by S ₂ O ₈ ^2- occurred as in Scheme 1 below, thereby preparing MnO ₂ having sea urchin-like nanostructures.

Scheme 1

MnSO ₄ + (NH ₄ ) ₂ S ₂ O ₈ + 2H ₂ O → MnO ₂ + (NH ₄ ) ₂ SO ₄ + 2H ₂ SO ₄

After the reaction was completed, the obtained solid product was filtered and washed several times with distilled water. Finally washed with ethanol and dried overnight at 120 ℃.

Lithium acetate (C ₂ H ₃ O ₂ Li.2H ₂ O, 12.25 g) was dissolved in 100 ml of distilled water, and MnO ₂ (20.6 g) prepared according to the above process was added. The mixture was dried at 120 ° C., then annealed at 700 ° C. for 12 hours in an air atmosphere and slowly cooled to room temperature. According to this process, a cluster-shaped LiMn ₂ O ₄ composed of nanorods was prepared, wherein the nanorod diameter was 100 nm, and the total average particle diameter of LiMn ₂ O ₄ was 5 μm.

4 g of OV (CH ₃ H ₇ ) ₃ was dissolved in isopropyl alcohol to prepare a gel solution of 20% by weight concentration of vanadium compound. Cluster-shaped LiMn ₂ O ₄ consisting of the prepared nanorod 10 g was added to the gel solution and the resulting mixture was mixed well with increasing temperature slowly to 80 ° C. The resulting mixture was annealed at 400 ° C. for 3 hours to prepare a LiMn ₂ O ₄ cathode active material in which vanadium was present at a concentration gradient from the surface to the inside. At this time, the total vanadium content was 5% by weight of LiMn ₂ O ₄ weight, vanadium present on the surface was 80% by weight, the amount of vanadium present in the interior was 20% by weight.

(Comparative Example 1)

LiMn ₂ O ₄ having a cluster shape consisting of nanorods was prepared in the same manner as in Example 1 except that the coating with OV (CH ₃ H ₇ ) ₃ was not performed.

(Comparative Example 2)

Lithium acetate, aluminum nitrate and manganese acetate were dissolved in ethanol in a stoichiometric amount of 1.05: 0.1: 1.85 weight ratio, homogenized for 2 hours and dried at 100 ° C. overnight. The dried powder was calcined at 700 ° C. for 15 hours to prepare a bulk Li _1.05 Mn _1.85 Al _0.1 O ₄ spinel powder.

(Comparative Example 3)

Bulk Li _1.05 Mn _1.85 Al _0.1 O ₄ spinel powder prepared according to Comparative Example 1 was added to a gel solution of 20 wt% VOx precursor prepared by dissolving OV (CH ₃ H ₇ ) ₃ 4g in isopropyl alcohol. It was. The resulting solution was mixed well while slowly increasing the temperature to 80 ° C. The resulting mixture was annealed at 400 ° C. for 3 hours to prepare a bulk Li _1.05 Mn _1.85 Al _0.1 O ₄ spinel powder positive electrode active material in which vanadium was present in a concentration gradient from the surface to the inside. At this time, the total vanadium content was 5% by weight of Li _1.05 Mn _1.85 Al _0.1 O ₄ weight, vanadium present on the surface was 80% by weight, the amount of vanadium present in the interior was 20% by weight.

* Cycle life characteristics

90% by weight of the positive electrode active material prepared according to Example 1 and Comparative Examples 1 to 3, 5% by weight of polyvinylidene fluoride binder, and 5% by weight of super P carbon black were mixed to prepare a positive electrode active material slurry, and the slurry Using the positive electrode was prepared by the conventional method. A coin type half cell (2016R type) was manufactured using this negative electrode and lithium metal as a counter electrode. At this time, ethylene carbonate-diethylene carbonate-ethylmethyl carbonate (EC-DEC-EMC = 30: 30: 40% by volume) in which 1M LiPF ₆ was dissolved was used.

After the battery was charged and discharged at 0.2C at 55 ° C. and 65 ° C. for 30 times, the capacity retention rate was measured, and the results are shown in Table 1 below. The capacity retention rate is expressed as a% value of the discharge capacity after 30 charge / discharge cycles to the discharge capacity.

55 degrees Celsius (after 30 cycles of capacity retention rates) 65 degrees Celsius (after 30 cycles of capacity retention rates) Example 1 95% 90% Comparative Example 1 50% 20% Comparative Example 2 85% 60% Comparative Example 3 60% 30%

As shown in Table 1, Example 1 subjected to the OV (CH ₃ H ₇ ) ₃ coating process was very excellent capacity retention at 55 ℃ and 65 ℃ compared to Comparative Example 1 without the coating process. In addition, compared to Comparative Example 2 using the bulk spinel powder having excellent capacity retention compared to the cluster shape composed of nanorods (Comparative Example 1), it was found that the capacity retention rate of Example 1, which was subjected to the coating process, was particularly excellent. It can be seen that very high capacity retention can be obtained at a high temperature. In addition, when the coating process is performed on the bulk spinel powder, a result of lowering the capacity retention ratio is obtained, as in Comparative Example 3, and it can be seen that the coating process is not appropriate for the bulk spinel powder.

* XRD measurement

X-ray diffraction of the cathode active materials of Example 1 and Comparative Examples 1 and 2 was measured, and the results are shown in FIG. 1. In addition, the XRD measurement results of MnO ₂ are also shown in FIG. 1. As shown in Fig. 1, MnO ₂ it can be seen that it comprises a mixed phase of α-MnO ₂ and ortho rombik MnO _2. In addition, it was found that the positive electrode active material of Comparative Example 1 had a cubic spinel structure having a space group of Fd-3m , and the lattice constant of the positive electrode active material of Comparative Example 1 was measured to be about 8.239 ± 0.005 μs. In addition, the lattice constant of the positive electrode active material of Example 1 was measured, and was found to be 8.237 ± 0.005 Å similarly to Comparative Example 1.

In FIG. 1, the positive electrode active material of Comparative Example 2 shows an advanced crystal peak, which is considered to be the result of calcining at 700 ° C. to increase the surface area.

* SEM and TEM photo

SEM photograph and Comparative Example 1 of the cathode active material of MnO ₂ (a and b), Comparative Example 1 (c and d) and Example 1 (g and h) of the star-like structure prepared in Example 1 in Figure 2 TEM photographs of (e and f) are shown. As shown in a and b of FIG. 2, it can be seen that MnO ₂ prepared in Example 1 is a cluster composed of nanorods having a diameter of about 50 nm and a length exceeding 1 μm.

In addition, as shown in c and d of Figure 2, after mixing the MnO ₂ and lithium acetate, and heat treatment at 700 ℃, the cluster shape consisting of nanorods was maintained, the diameter of each of the nanorods to about 100nm It can be seen that the increase.

As can be seen from the TEM photographs shown in FIGS. 2E and 2F, the cathode active material of Comparative Example 1 has a d-spacing value of 2.5 Å between neighboring lattice, which is a spinel phase (311). This is the value corresponding to). As shown in FIG. 2 g and h, the positive electrode active material of Example 1 has a similar morphology to that of the positive electrode active material of Comparative Example 1, which is a morphology before coating because the coating process is carried out at a low temperature in Example 1 It seems to keep it as it is.

* Fourier Transform Infra-Red Spectrum

The FT-IR spectra of the cathode active materials of Example 1 and Comparative Example 1 were measured, and the results are shown in FIG. 3. In addition, the FT-IR spectrum for V ₂ O ₅ was also measured and shown in FIG. 3. At this time, the sample for measuring the FT-IR spectrum was prepared by dropping the positive electrode active material of Example 1 and Comparative Example 1 to hexane colloid in a KBr single crystal plate at room temperature, and volatilized the solvent.

As shown in FIG. 3, the positive electrode active material of Comparative Example 1 showed a vibration band at about 630 cm ⁻¹ corresponding to lattice Mn-O vibration. Also, V ₂ O ₅ showed a vibration band at about 1022 cm ⁻¹ corresponding to V = O vibration.

On the contrary, since the vibration band does not appear at 1022 cm ⁻¹ , the VO _x coating layer does not exist separately. In addition, in the positive electrode active material of Example 1, the vibration band at 630 cm ⁻¹ corresponding to Mn-O vibration was shifted to about 700 cm ⁻¹ , which is considered to be due to the formation of MVO bonds. Accordingly, it can be seen from the results shown in FIG. 3 that the V ions are substituted at the Mn site as the positive electrode active material of Example 1 performs the coating process.

* Line-scan and expanded TEM images

A line-scan photograph of the cathode active material of Example 1 is shown in FIG. 4A, and the expanded TEM photograph of FIG. 4A is shown in FIG. 4B. As shown in FIG. 4A, it can be seen that a solid solution of V and Mn atoms may be formed in the cathode active material of Example 1. FIG. These results indicate that V atoms are actually present within a depth of 30 nm at the surface. It can also be seen that the content of vanadium gradually decreases in the inner direction of the particles, and thus, the strength of V decreases. In addition, the d-distance from the surface is 2.1 It was confirmed that the spinel phase was formed from the lattice fringe of the corresponding (400) plane (FIG. 4B).

* Specific surface area measurement

Specific surface areas of the cathode active materials prepared according to Example 1 and Comparative Example 2 were measured by Brunauer-Emmet-Teller (BET), which is a method of nitrogen adsorption at 77 K, and the results are shown in FIG. 5. As shown in FIG. 5, the specific surface area of the positive electrode active material of Example 1 was found to be 57 m 2 / g, and the specific surface area measurement result of Example 1 showed a typical IV-type isotherm curve. It can be seen that the nitrogen adsorption and desorption hysteresis loop gradually increased to about 0.4 relative pressure (P / Po), and then rapidly increased in the high pressure region where the relative pressure (P / Po) exceeded 0.8. This phenomenon is caused by capillary condensation, which is caused by the adsorption of nitrogen to nanopores in nanostructured materials. On the other hand, the BET surface area of the positive electrode active material of Comparative Example 1 is 10 m ² / g, which is a result of a significant decrease in the amount of N ₂ adsorption and desorption. These results indicate that almost no nano pores are formed in the positive electrode active material particles having an average particle diameter of about 0.5 μm as can be seen in the SEM photograph of the positive electrode active material of Comparative Example 1 shown in FIG. 5.

* Charging / discharging characteristics

The battery using the positive electrode active materials of Example 1 and Comparative Example 1 was measured at a temperature of 21 ° C., while increasing the discharge rate from 0.2C to 7C (= 1400 mAh / g), and measuring charge and discharge characteristics once, and are shown in FIG. 6. It was. At this time, the charging rate was fixed at 0.5C (= 60mAh / g). One time charge and discharge capacity of Comparative Example 1 was 133 and 130 mAh / g, respectively, and the coulombic efficiency was 98%. In contrast, it can be seen that the charge and discharge capacities of the positive electrode active material of Example 1 are the same at 128 and 125 mAh / g, respectively.

In addition, the discharge rate of the battery using the positive electrode active materials of Example 1 and Comparative Examples 1 and 2 was 0.2C (4 times), 0.5C (5 times), 1C (5 times), 3C (10 times), 5C (15). Times) and 7C (15 times), and charging and discharging were carried out six times again at 0.2C to measure the discharge capacity in each case, and the results are shown in FIG. 7. As a result, as the discharge rate increased to 0.5C, 1C, 3C, 5C and 7C, the discharge capacity of Comparative Example 1 was reduced to 127, 126, 124, 122 and 121mAh / g, the capacity retention was 94% at 7C . In contrast, since the cathode active material of Example 1 was reduced to 123, 121, 119, 117 and 116 mAh / g, it can be seen that the discharge capacity is slightly reduced compared to Comparative Example 1, but the capacity retention rate is the same. In addition, the capacity retention rate at 7C of the positive electrode active material of Example 1 was 93% as compared with 0.2C (0.2C: 125 mAh / g, 7C: 116 mAh / g).

In Example 1 and Comparative Example 1, after performing charge and discharge at 7C, the capacity recovery at 0.2C was 100%. It can be seen from this result that the shorter the diffusion path, the better the lithium ion intercalation kinetics at the surface of the particles, the higher the discharge rate, and the less structural deformation at the surface of the particles.

On the other hand, as shown in Figure 7, it can be seen that the positive electrode active material of Comparative Example 2 prepared by the sol-gel method at 700 ℃ significantly exceeds the capacity retention rate. The discharge capacities of Comparative Example 2 were 101, 85, and 65 mAh / g at 3C, 5C, and 7C, respectively, and the capacity retention after 7C cycles was 48% (62 mAh / g). These results indicate that the BET surface area of the positive electrode used in Comparative Example 2 is 10 m ² / g, and the specific surface area is small, and as a result, the contact area with the electrolyte is reduced and the capacity is reduced at high rate.

After 0.7C charging and discharging, the capacity recovery rate at 0.2C was 96% (110 mAh / g), and it is considered that structural degradation occurred during the high rate charging and discharging.

The reduction in capacity retention at the high rates of Comparative Examples 1 and 2 is due to the higher polarization compared to Example 1, and this high polarization degree means that the interfacial resistance is high, and thus, lithium ion diffusion is slow. Therefore, in the positive electrode active materials of Comparative Examples 1 and 2 showing high polarization degree, the BET specific surface area is considered to be low in lithium diffusion.

In addition, the positive electrode active material of Comparative Example 1 exhibits a capacity retention similar to that of the positive electrode active material of Example 1, but has higher stoichiometery and BET specific surface area of lithium and manganese in the positive electrode active material, It can be expected that structural degradation will occur at high temperatures.

* Structural stability

In order to examine the structural stability at high temperatures of the positive electrode active materials of Example 1 and Comparative Examples 1 and 2, a coin-type half cell was prepared using the positive electrode active materials of Example 1 and Comparative Examples 1 and 2, and the battery was prepared. After charging at 4.3 V, the electrodes were separated from the cell. After charging and discharging at 21 ° C., the Mn elution amount of the charged sample was 100 ppm or less (Example 1: 25 ppm, Comparative Example 1: 50 ppm, Comparative Example 2: 40 ppm). Thus, the electrode was placed in a container containing an electrolyte solution. Store at 24 ° C. for 24 hours.

After storing the positive electrode active materials of Example 1 and Comparative Examples 1 and 2 at 80 ° C., the amount of Mn eluted was measured. For elution amount measurement, Mn and V dissolved in an electrolyte solution were measured using ICP-MS (Inductively Coupled Plasma Mass Spectrometery). As a result, in Comparative Example 1, about 5000 ppm Mn was eluted, but in Example 1, 60 ppm Mn was eluted, and V ion elution was not detected. In the case of Comparative Example 2, Mn was eluted at 400 ppm. In other words, it can be seen that the more the amount of Mn elution in the structure, the faster the collapse of the structure. From this result, it can be seen that the positive electrode active material of Example 1 in which VO _x is present on the surface and the inside thereof is most structurally stable.

All simple modifications or changes of the present invention can be easily carried out by those skilled in the art, and all such modifications or changes can be seen to be included in the scope of the present invention.

1 is a graph showing the X-ray diffraction measurement of the negative electrode active material of Example 1 and Comparative Examples 1 to 2 of the present invention.

2 is a SEM photograph and comparison of the negative active material of MnO 2 (a and b), Comparative Example 1 (c and d) and Example 1 (g and h) of the star-like structure prepared in Example 1 of the present invention TEM photographs of example 1 (e and f).

3 is a graph showing the FT-IR spectrum of the negative active material of Example 1 and Comparative Example 1 of the present invention.

Figure 4a is a line-scan photograph of the negative electrode active material of Example 1 of the present invention.

4B is an expanded TEM photograph of FIG. 4A.

5 is a graph showing the measurement of the specific surface area of the negative electrode active material prepared according to Example 1 and Comparative Example 2 of the present invention.

6 is a graph showing charge and discharge characteristics of a battery using the positive electrode active materials of Example 1 and Comparative Example 1 of the present invention.

7 is a discharge rate of 0.2C (4 times), 0.5C (5 times), 1C (5 times), 3C (10 times) of the battery using the positive electrode active material of Example 1 and Comparative Examples 1 and 2 of the present invention , Graphs showing the discharge capacity measured in each case, increasing charges to 5C (15 times) and 7C (15 times), and again 6 charges and discharges at 0.2C.

8 is an XRD graph of the positive electrode active materials of Example 1 and Comparative Examples 1 and 2 of the present invention after storage at 80 ° C.

9 is a TEM photograph before and after storage of the negative electrode active material of Example 1 and Comparative Example 1 of the present invention at 80 ℃.

Claims (13)

Manganese compounds represented by the following Formula 1 or 2 having a cluster shape consisting of nanorods and Vanadium present in a concentration gradient inward from the surface of the manganese compound A cathode active material for a lithium secondary battery comprising a. [Formula 1] Li _a Mn _1-b M _b O _2-c D _c [Formula 2] Li _a Mn _{2-b '} M _b' O _4-c D _c Where 0.90 ≦ a ≦ 1.1, 0 ≦ b ≦ 0.5, 0 ≦ b ′ ≦ 0.5, 0 ≦ c ≦ 0.05, M is selected from the group consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, rare earth elements and combinations thereof) The method of claim 1, The positive electrode active material is a positive electrode active material for lithium secondary batteries in which vanadium is distributed to a depth of 30% in the inner direction from the surface of the manganese compound with respect to 100% of the diameter and length of the cluster-shaped manganese compound composed of the nanorods. . The method of claim 2, The positive electrode active material is a positive electrode active material for lithium secondary batteries in which vanadium is distributed to a depth of 10% in the inner direction from the surface of the manganese compound with respect to 100% of the diameter and length of the cluster-shaped manganese compound composed of the nanorods. . The method of claim 1, The vanadium is present in a higher concentration on the surface than the inside of the manganese-based compound positive electrode active material for a lithium secondary battery. The method of claim 1, The concentration of vanadium on the surface of the manganese-based compound is 80 to 70% by weight, internally 20 to 10% by weight of a positive electrode active material for a lithium secondary battery. The method of claim 1, The positive electrode active material is a positive electrode active material for lithium secondary batteries containing 1 to 5% by weight of vanadium based on the weight of the manganese compound. Manganese precursor and oxidant are mixed in a solvent to prepare MnO ₂ having a star-like structure; Mixing the MnO ₂ with a lithium precursor; Heat treating the mixture to prepare a manganese compound represented by Chemical Formula 1 or 2 having a cluster shape composed of nanorods; Coating the manganese compound with a vanadium compound solution; Annealing the obtained mixture The manufacturing method of the positive electrode active material for lithium secondary batteries containing a process. [Formula 1] Li _a Mn _1-b M _b O _2-c D _c [Formula 2] Li _a Mn _{2-b '} M _b' O _4-c D _c Where 0.90 ≦ a ≦ 1.1, 0 ≦ b ≦ 0.5, 0 ≦ b ′ ≦ 0.5, 0 ≦ c ≦ 0.05, M is selected from the group consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, rare earth elements and combinations thereof) The method of claim 7, wherein The oxidizing agent is selected from the group consisting of ammonium persulfate, ammonium nitrate, and combinations thereof. The method of claim 7, wherein The mixing ratio of the manganese precursor and the oxidizing agent is 1: 1 to 2: 1 weight ratio of the manufacturing method of the positive electrode active material for lithium secondary batteries. The method of claim 7, wherein In the step of mixing the MnO ₂ and the lithium precursor, a metal compound is further added to the manufacturing method of the positive electrode active material for lithium secondary batteries. The method of claim 7, wherein The heat treatment process is a method of manufacturing a positive electrode active material for a lithium secondary battery that is carried out at 700 to 750 ℃ for 5 to 15 hours. The method of claim 7, wherein The annealing step is a manufacturing method of a positive electrode active material for lithium secondary batteries that is carried out at 300 to 400 ℃. A positive electrode including a positive electrode active material; A negative electrode including a negative electrode active material; And Contains an electrolyte, The cathode secondary battery is the cathode active material according to any one of claims 1 to 6.

Images