CATHODE ACTIVE MATERIAL FOR LITHIUM SECONDARY BATTERY, MANUFACTURING METHOD THEREOF, AND LITHIUM SECONDARY BATTERY INCLUDING THE SAME

Abstract

A manufacturing method of a cathode active material for lithium secondary battery, including: preparing a first solution by mixing a metal oxide and a solvent; preparing a metal-mixed solution by adding an acidic solution to the first solution and then applying ultrasonic waves to the mixture; centrifuging the metal-mixed solution; preparing a second solution by mixing a supernatant of the centrifuged metal-mixed solution, a reductant, and a solvent and then applying ultrasonic waves to the mixture; obtaining powder by filtering and then drying the second solution; forming mesoporous spherical nanoparticles by mixing the powder, a metal, a lithium precursor, and a solvent, applying ultrasonic waves to the mixture and then drying the mixture; and performing a heat treatment to the spherical nanoparticles, and a cathode active material for a lithium secondary battery obtained by the manufacturing method. The cathode active material for lithium secondary battery is mesoporous spherical nanoparticles.

Claims

1. A manufacturing method of a cathode active material for lithium secondary battery, the method comprising: mixing a metal oxide and a solvent to prepare a first solution; adding an acidic solution to the first solution and then applying ultrasonic waves to the mixture to prepare a metal-mixed solution; centrifuging the metal-mixed solution; mixing a supernatant of the centrifuged metal-mixed solution, a reductant, and a solvent and then applying ultrasonic waves to the mixture to prepare a second solution; filtering and then drying the second solution to obtain powder; mixing the powder, a metal, a lithium precursor, and a solvent, applying ultrasonic waves to the mixture and then drying the mixture to form mesoporous spherical nanoparticles; and performing a heat treatment to the spherical nanoparticles.

2. The manufacturing method of a cathode active material for lithium secondary battery according to claim 1, wherein in the preparing of the metal-mixed solution, the preparing of the second solution, and the forming of the mesoporous spherical nanoparticles, ultrasonic waves with a frequency in the range of from 5 kHz to 50 kHz are applied for 1 minute to 60 minutes.

3. The manufacturing method of a cathode active material for lithium secondary battery according to claim 1, wherein the preparing of the metal-mixed solution, the preparing of the second solution, and the forming of the mesoporous spherical nanoparticles are performed at a temperature in the range of from 10 C. to 40 C.

4. The manufacturing method of a cathode active material for lithium secondary battery according to claim 1, wherein the heat treatment is performed in the range of from 600 C. to 900 C.

5. The manufacturing method of a cathode active material for lithium secondary battery according to claim 1, wherein the acidic solution includes at least one acid selected from the group consisting of sulfuric acid (H.sub.2SO.sub.4), hydrochloric acid (HCl), phosphoric acid (H.sub.3PO.sub.4), acetic acid (CH.sub.3COOH), and nitric acid (HNO.sub.3).

6. The manufacturing method of a cathode active material for lithium secondary battery according to claim 1, wherein the metal oxide includes a metal oxide based on at least one metal selected from the group consisting of Li, B, C, Na, Mg, Al, Si, P, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Sr, Y, Zr, Nb, Mo, Sn, Ba, Hf, and La.

7. The manufacturing method of a cathode active material for lithium secondary battery according to claim 1, wherein the solvent includes at least one solvent selected from the group consisting of ethanol, anhydrous ethanol, isopropyl alcohol, or combinations thereof.

8. The manufacturing method of a cathode active material for lithium secondary battery according to claim 1, wherein the reductant is manganese sulfate (MnSO.sub.4) or manganese chloride (MnCl.sub.2).

9. A cathode active material for lithium secondary battery which includes at least one compound selected from general formula (1) below and has a mesoporous spherical nanoparticle shape:
Li.sub.1+xMn.sub.2yM.sub.yO.sub.4 (1) wherein in general formula (1), M is at least one metal selected from the group consisting of Ni, Co, Mn, Al, V, Fe, P, and Cr, 0x0.1, and 0.3y0.7.

10. A lithium secondary battery comprising: the cathode active material of claim 1; an anode active material; and an electrolyte.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The above and other aspects, features and other advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

[0031] FIG. 1 is an EDS image of a cathode active material manufactured according to Example 1;

[0032] FIG. 2 is a TEM image of the cathode active material manufactured according to Example 1;

[0033] FIG. 3(a) illustrates a capacity depending on the charge or discharge of a battery including the cathode active material manufactured according to Example 1; and

[0034] FIG. 3(b) illustrates a capacity depending on the charge or discharge of a battery including a cathode active material manufactured according to Comparative Example 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0035] Hereinafter, the present disclosure will be described in more detail with reference to examples and the accompanying drawings.

[0036] A manufacturing method of a cathode active material for lithium secondary battery according to the present disclosure includes: preparing a first solution by mixing a metal oxide and a solvent; preparing a metal-mixed solution by adding an acidic solution to the first solution and then applying ultrasonic waves to the mixture; centrifuging the metal-mixed solution; preparing a second solution by mixing a supernatant of the centrifuged metal-mixed solution, a reductant, and a solvent and then applying ultrasonic waves to the mixture; obtaining powder by filtering and then drying the second solution; forming mesoporous spherical nanoparticles by mixing the powder, a metal, a lithium precursor, and a solvent, applying ultrasonic waves to the mixture and then drying the mixture; and performing a heat treatment to the spherical nanoparticles.

[0037] In the preparing of the metal-mixed solution, the metal-mixed solution is prepared by applying ultrasonic waves while adding the acidic solution to the first solution in an isothermal reactor.

[0038] The isothermal reactor refers to a reactor configured to maintain a reacting solution at a constant temperature during a synthesis reaction and maintains the reacting solution at a constant temperature during application of ultrasonic waves in the preparing of the cathode active material.

[0039] The addition of the acidic solution to the first solution results in maintaining a pH suitable for one-dimensional co-precipitation, and the centrifugation makes it possible to easily remove by-products generated during a synthesis reaction for manufacturing the cathode active material. Preferably, the centrifugation is performed at about 4000 rpm for 1 to 5 minutes. If the centrifugation is performed for less than 1 minute, the mixed solution cannot be centrifugated well. If the centrifugation is performed for more than 5 minutes, manufacturing lead time can be increased.

[0040] The cathode active material for lithium secondary battery manufactured by the above-described method is a mesoporous material formed as a physically laminated structure of single crystals. Its three-dimensional nanostructure shape results in maximizing a reaction area, i.e., an effective reaction area, with battery reactants such as lithium ions and an electrolyte. Further, the cathode active material for lithium secondary battery has a structural stability due to its mesoporous surface formed as a physically laminated structure.

[0041] Preferably, in the preparing of the metal-mixed solution, the preparing of the second solution, and the forming of the mesoporous spherical nanoparticles, ultrasonic waves with a frequency in the range of from 5 kHz to 50 kHz are applied for 1 minute to 60 minutes, but the present disclosure is not limited thereto. Further, the ultrasonic waves may have various frequencies and application times depending on the concentrations and manufacturing conditions of the metal-mixed solution and the second solution.

[0042] If the ultrasonic waves have a frequency of 5 kHz and an application time of less than 1 minute, the overall diameters of particles of the cathode active material for lithium secondary battery may be increased but the crystallinity and recovery rate may be decreased. If the ultrasonic waves have a frequency of 50 kHz and an application time of more than 60 minutes, the crystallinity and recovery rate of particles of the cathode active material for lithium secondary battery may be improved but the cathode active material may be formed into smaller particles or particles of the manufactured cathode active material for lithium secondary battery may be ground, and, thus, the active area may be sharply decreased.

[0043] Preferably, the preparing of the metal-mixed solution, the preparing of the second solution, and the forming of the mesoporous spherical nanoparticles are performed at a temperature in the range of from 10 C. to 40 C.

[0044] The above-described temperature range refers to a temperature of a reaction phase during application of ultrasonic waves. In order to suppress an increase in temperature of the reaction phase caused by ultrasonic energy, an ultrasonic pulse may be regulated at about 1-minute intervals.

[0045] If the temperature is maintained at less than 10 C., the amount of unreacted material may be increased. If the temperature is more than 40 C., a rapid reaction may occur, which may result in the generation of a cathode active material which has irregular shapes instead of a regular spherical nanoparticle shape.

[0046] The heat treatment of the present disclosure is performed to transform the mesoporous spherical nanoparticles generated during the above-described manufacturing process into nanoparticles with a high crystallinity through a phase change. Preferably, the heat treatment is performed in the range of from 600 C. to 900 C. Further, preferably, the heat treatment is performed for 1 to 3 hours.

[0047] If the heat treatment is performed at less than 600 C., a phase change may not be sufficient, and if the heat treatment is performed at more than 900 C., the aggregation of nanoparticles may occur. Further, if the heat treatment is performed for less than 1 hour, a uniform crystallinity may not be achieved, and if the heat treatment is performed for more than 3 hours, a phase separation of the nanoparticles may occur.

[0048] In the present disclosure, the acidic solution is used to remove by-products generated during a reaction. The acidic solution may include at least one acid selected from the group consisting of sulfuric acid (H.sub.2SO.sub.4), hydrochloric acid (HCl), phosphoric acid (H.sub.3PO.sub.4), acetic acid (CH.sub.3COOH), and nitric acid (HNO.sub.3) and preferably H.sub.2SO.sub.4, but is not limited thereto.

[0049] The metal oxide is a precursor including a metal constituting the cathode active material. The metal oxide may include a metal oxide based on at least one metal selected from the group consisting of Li, B, C, Na, Mg, Al, Si, P, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Sr, Y, Zr, Nb, Mo, Sn, Ba, Hf, and La.

[0050] In the present disclosure, any solvent capable of easily dissolving the above-described metal oxide can be used without particular limitation. The solvent may include at least one solvent selected from the group consisting of ethanol, anhydrous ethanol, isopropyl alcohol, or combinations thereof.

[0051] The reductant of the present disclosure forces the reduction of a manganese oxide (MnO.sub.4) generated during the above-described manufacturing process, and, thus, single crystals of manganese oxide (MnO.sub.2) are formed. The reductant may be manganese sulfate (MnSO.sub.4) or manganese chloride (MnCl.sub.2) and preferably manganese sulfate (MnSO.sub.4), but is not limited thereto.

[0052] A cathode active material for lithium secondary battery according to the present disclosure includes at least one compound selected from general formula (1) below and has a mesoporous spherical nanoparticle shape.

[0053] The mesoporous spherical nanoparticle shape of the cathode active material for lithium secondary battery results in maximizing an effective reaction area with battery reactants such as lithium ions and an electrolyte. Due to its mesoporous surface formed as a physically laminated structure, a secondary battery employing the cathode active material for lithium secondary battery has a high energy density during charges and discharges and a high structural stability without being structurally destroyed by intercalation and deintercalation of reversible lithium ions.

Li.sub.1+xMn.sub.2yM.sub.yO.sub.4 (1)

[0054] In general formula (1), M is at least one metal selected from the group consisting of Ni, Co, Mn, Al, V, Fe, P, and Cr, 0x0.1, and 0.3y0.7.

[0055] The cathode active material for lithium secondary battery doped with dissimilar metals supplements a structural weakness of the existing lithium manganese composite metal compounds and reduces elution of Mn.sup.3+ from a crystal structure to increase the structural stability and improve the charge and discharge cycle characteristics and lifespan.

[0056] Preferably, the mesoporous spherical nanoparticles may have an average particle diameter of from 150 nm to 400 nm.

[0057] Rod-like single crystals constituting the mesoporous spherical nanoparticles may have a size of from 10 nm to 50 nm, and these single crystals are formed into a physically laminated structure, and, thus, spherical nanoparticles with a mesoporous surface are formed.

[0058] If the mesoporous spherical nanoparticles have an average particle diameter of less than 150 nm, the aggregation of the cathode active material may occur, which may cause a decrease in dispersibility of the cathode active material within the active material layer and an increase in resistance within the electrode. If the mesoporous spherical nanoparticles have an average particle diameter of more than 400 nm, the dispersibility of the cathode active material and the capacity may be decreased.

[0059] The spherical nanoparticles formed by laminating the single crystals are laminated to form mesoporous spherical clusters. The clusters may have an average particle diameter of from 1 m to 3 m.

[0060] A lithium secondary battery according to the present disclosure includes the cathode active material for lithium secondary battery, an anode active material, and an electrolyte.

[0061] Specifically, the lithium secondary battery may include a cathode composed of a cathode active material and a current collector, an anode composed of an anode active material and a current collector, and an electrolyte for conduction of lithium ions between the cathode and the anode.

[0062] The anode active material of the present disclosure may employ any compound suitable for reversible intercalation and deintercalation of lithium without particular limitation. The anode active material may include one or more members selected from the group consisting of carbonaceous materials such as synthetic graphite, natural graphite, graphitized carbon fibers, amorphous carbon, etc., metallic compounds which can be alloyed with lithium (Li) such as silicon (Si), aluminum (Al), tin (Sn), lead (Pb), zinc (Zn), bismuth (Bi), indium (In), manganese (Mn), gallium (Ga), cadmium (Cd), silicon alloys, tin alloys, or aluminum alloys, and composites containing the metallic compounds and the carbonaceous materials.

[0063] The electrolyte may include one or more members selected from the group consisting of organic liquid electrolytes, inorganic liquid electrolytes, solid-type polymer electrolytes, gel-type polymer electrolytes, inorganic solid electrolytes, and molten-type inorganic electrolytes which can be used in manufacturing a lithium secondary battery, but is not limited thereto.

[0064] The electrolyte may include a binder, a lithium salt, and an organic solvent.

[0065] The binder may include one or more members selected from the group consisting of polyvinyl difluoride, polyethylene, polypropylene, etc., but is not limited thereto.

[0066] The organic solvent may employ any solvent capable of functioning as a medium through which ions involved in chemical reaction of a battery can move without particular limitation. Specifically, the organic solvent may include one or more organic members selected from the group consisting of N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, -butyrolactone, 1,2-dimethoxyethane, tetrahydroxy Franc, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triesters, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0067] The lithium salt may employ any compound capable of supplying lithium ions to be used in a secondary battery without particular limitation. The lithium salt may include one or more members selected from the group consisting of LiPF.sub.6, LiBF.sub.4, LiSbF.sub.6, LiAsF.sub.6, LiC.sub.4F.sub.9SO.sub.3, LiClO.sub.4, LiAlO.sub.2, LiAlCl.sub.4, LiCl, LiI, LiB(C.sub.2O.sub.4).sub.2, and LiTFSI.

[0068] Hereinafter, the present disclosure will be described based on more detailed examples.

[Example] Preparation of Cathode Active Material for Lithium Secondary Battery

Example 1

[0069] A first solution was prepared by mixing 0.1 M barium manganate (BaMnO.sub.4) with 100 ml of distilled water. Then, 2.8 ml of 4 M sulfuric acid(H.sub.2SO.sub.4) was slowly added to the first solution, ultrasonic waves with 750 W power, 50% amplitude, and 20 kHz frequency were applied for 5 minutes, and a thus prepared metal-mixed solution was centrifuged. A supernatant of the centrifuged metal-mixed solution, 400 ml of distilled water, and manganese sulfate (MnSO.sub.4) were mixed and kept in a stationary state at 1-minute intervals to suppress a temperature increase at 750 W power, 50% amplitude, and 20 kHz frequency, and the mixture was applied with ultrasonic waves for 30 minutes and stirred at room temperature for 3 hours.

[0070] Then, the mixture was filtered using distilled water and ethanol and repeatedly washed and dried in a 100 C. oven for 12 hours to obtain manganese dioxide (MnO.sub.2) powder.

[0071] Ni(NO.sub.3).sub.2.Math.6H.sub.2O, LiOH and manganese dioxide (MnO.sub.2) were mixed with 10 ml of ethanol at a molar ratio of 5:10.4:15 and the mixture was applied with ultrasonic waves for 5 minutes and then dried to prepare a cathode active material for lithium secondary battery.

[0072] The prepared cathode active material for lithium secondary battery was heat-treated at 800 C. for 10 hours under air atmosphere.

[0073] FIG. 1 illustrates the distribution of metals contained in the cathode active material prepared in Example 1 using EDS. EDS stands for Energy Dispersive Spectrometer and refers to a device for direct observation of the distribution of atoms contained in a sample.

[0074] FIG. 1 illustrates the distribution of constituent elements of the cathode active material as the basis of EDS analysis. It can be seen from FIG. 1 that nickel, manganese, and oxygen are evenly distributed in the prepared cathode active material for lithium secondary battery.

Comparative Example 1

[0075] A cathode active material was manufactured in the same manner as described in Example 1 except that Ni(NO.sub.3).sub.2.Math.6H.sub.2O and LiOH were excluded and LiI was used instead of LiOH.

Fabrication of Coin Cell

[0076] Batteries were fabricated by the following method in order to evaluate electrochemical properties of secondary batteries including the cathode active materials prepared in Example 1 and Comparative Example 1, respectively.

[0077] A 70 wt % cathode active material, 15 wt % conductive activated carbon, and a 15 wt % polyvinyl difluoride binder were mixed with NMP (N-methyl-2-pyrrolidine) to prepare slurry. The slurry was applied to aluminum foil and then dried and rolled to prepare a cathode. The cathode, lithium metal, and a separator were used to fabricate a coin cell. Herein, a solution including 1.0 M LiPF.sub.6 dissolved in a 1:1-mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) was used as an electrolyte.

Evaluation Result

1. Transmission Electron Microscopic Image

[0078] The shape of the cathode active material for lithium secondary battery manufactured in Example 1 of the present disclosure was observed with a transmission electron microscope (TEM) and the result was as illustrated in FIG. 2.

[0079] It can be seen from FIG. 2 that the cathode active material for lithium secondary battery manufactured in Example 1 included crystalline rod particles laminated into a spherical shape and had a mesoporous surface.

2. Characteristics of Battery

[0080] In order to analyze battery properties of a battery including the cathode active materials prepared in Example 1 and Comparative Example 1, respectively, initial charge and discharge characteristics were evaluated by performing a charge and a discharge of 0.1 C at a charge/discharge potential of from 3.0 V to 4.3 V and the result was as illustrated in FIG. 3A and FIG. 3B.

[0081] The charge/discharge curves show that a voltage of the battery including the cathode active materials prepared in Example 1 as illustrated in FIG. 3A was higher than that of the battery including the cathode active materials prepared in Comparative Example 1 as illustrated in FIG. 3B. Further, as a result of comparison between the batteries in discharge capacity after 100 charge and discharge cycles, the battery of FIG. 3A was 140 mAh/g or more and the battery of FIG. 3B was nearly 110 mAh/g.

[0082] This means that its mesoporous structure maximizes the reaction area and thus enables lithium ions to be easily moved and its stable structure with a high crystallinity suppresses elution of Mn.sup.3+ from a crystal structure, and, thus, if the cathode active material is applied to a secondary battery, the cycle and lifespan characteristics of the secondary battery can be improved.