Positive electrode active material for lithium secondary batteries

Abstract

To provide a cathode active material for a lithium secondary battery, and more particularly, a cathode active material for a lithium secondary battery, which includes a core portion and a shell portion surrounding the core portion, in which the total content of cobalt in the core portion and the shell portion is 5-12 mol%, and the content of cobalt in the core portion and the shell portion is adjusted to be within a predetermined range.SOLUTION: In a cathode active material precursor and a cathode active material for a secondary battery prepared using the same, optimal capacity of a lithium secondary battery may be increased by adjusting a cobalt content in particles, and life characteristics may be also enhanced by improving stability.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material for a lithium secondary battery, and more particularly to a positive electrode active material for a lithium secondary battery, which comprises a core portion and a shell portion surrounding the core portion, and the cobalt content in the core portion and the shell portion is adjusted to be within a predetermined range and the total cobalt content in the core portion and the shell portion is adjusted to be 5 mol % to 12 mol %. [Background technology]

[0002] The development of portable mobile electronic devices such as smartphones, MP3 players, and tablet PCs has led to an explosive increase in demand for secondary batteries that can store electrical energy. In particular, the emergence of electric vehicles, medium- to large-sized energy storage systems, and portable devices that require high energy density has led to an increase in demand for lithium secondary batteries.

[0003] The layered structure of LiCoO2 is widely used as a positive electrode active material for lithium secondary batteries. LiCoO2 is the most widely used material due to its excellent lifespan and charge / discharge efficiency, but its low structural stability limits its application to high-capacity battery technology.

[0004] Positive electrode active materials to replace this include LiNiO2, LiMnO2, LiMn2O4, LiFePO4, and Li(Ni x Co y Mn z Various lithium composite metal oxides such as LiNiO2 have been developed. Among these, LiNiO2 has the advantage of exhibiting high discharge capacity battery characteristics, but it is difficult to synthesize by a simple solid-state reaction, and it has problems such as low thermal stability and low cycle characteristics. Lithium manganese oxides such as LiMnO2 or LiMn2O4 have the advantages of excellent thermal safety and low cost, but they have problems such as low capacity and poor high-temperature characteristics. In particular, LiMn2O4 has been commercialized in some low-cost products, but Mn 3+Due to the structural deformation (Jahn-Teller distortion), the life characteristics are not good. Also, LiFePO4 has a low price and excellent safety, and currently, many studies are being conducted for use in hybrid electric vehicles (HEVs). However, due to its low conductivity, there is a situation where it is difficult to apply to other fields.

[0005] As a result, the substance that has received the most attention in recent years as an alternative cathode active material for LiCoO2 is lithium nickel manganese cobalt oxide, Li(Ni x Co y Mn z )O2 (where x, y, and z are atomic fractions of independent oxide composition elements, etc., and 0 < x ≤ 1, 0 < y ≤ 1, 0 < z ≤ 1, 0 < x + y + z ≤ 1). This material has the advantages of being less expensive than LiCoO2 and being able to be used for high capacity and high voltage, but it has the disadvantage of poor rate capability and life characteristics at high temperatures.

[0006] In order to solve such problems, lithium nickel manganese cobalt oxide with a metal composition representing a concentration gradient, composed of a core part with a high nickel content and a shell part with a low nickel content, has been researched and developed. This method is, in short, a method of synthesizing an internal substance with a predetermined composition, then coating the outside with a substance having another composition to manufacture a double layer, and then mixing with a lithium salt and performing heat treatment. For the internal substance, commercially available lithium transition metal oxides can also be used.

[0007] However, this method has the problem that the metal composition of the cathode active material changes discontinuously between the composition of the generated internal substance and the external substance and does not change continuously, so the internal structure is unstable. Also, the powder synthesized in this invention does not use ammonia, which is a chelating agent, so it has a low tap density and is not suitable for use as a cathode active material for lithium secondary batteries. Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention aims to improve the safety and efficiency of cathode active materials having a core-shell structure, which has been a problem in the prior art, by providing a cathode active material precursor in which the total cobalt content in the core and shell regions is controlled to a predetermined concentration, and a cathode active material prepared using the same. [Means for solving the problem]

[0009] According to one aspect of the present invention, there is provided a positive electrode active material for a lithium secondary battery, comprising a core portion and a shell portion surrounding the core portion, wherein the total cobalt content in the core portion and the shell portion is maintained within a predetermined range of 5 mol % to 12 mol %.

[0010] Conventional cathode active materials having a core-shell structure have had problems such as internal structural instability due to discontinuous changes in the metal composition, resulting in reduced efficiency of lithium secondary batteries. However, in the present invention, it has been confirmed that the cathode active material for lithium secondary batteries having a core-shell structure has excellent stability and efficiency when the total cobalt content in the core and shell is kept constant, particularly when the content is adjusted to 5 mol% to 12 mol%, thereby solving the above problems.

[0011] According to another aspect of the present invention, there is provided a positive electrode active material for a lithium secondary battery, represented by the following Chemical Formula 1, in which the total content of cobalt in the core and shell is controlled to a certain value (5 mol% to 12 mol%):

[0012] [ka]

[0013] (In the above chemical formula 1, 0.9≦a≦1.3, 0.7≦x<1.0, 0.05≦y≦0.12, 0.0≦z≦0.3, 0.0≦1-xyz≦0.3, M is one or more metal elements selected from B, Ba, Ce, Cr, F, Mg, Al, Cr, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, W, P, Sr, Ge, and Cu).

[0014] In the positive electrode active material for a lithium secondary battery according to the present invention, when the cobalt content in the whole particle of the positive electrode active material is W, the cobalt content in the shell portion is 0.2W to 1.0W.

[0015] According to one aspect of the present invention, when the diameter of the entire particle of the positive electrode active material according to the present invention is D, D is 1 μm to 25 μm, and the thickness of the shell portion is 0.01 D to 0.3 D. That is, the present invention is characterized in that the Co content in the entire particle and the Co content in the shell are adjusted within predetermined ranges, thereby changing the thickness of the shell portion.

[0016] According to another aspect of the present invention, there is provided a lithium secondary battery comprising the positive electrode active material according to the present invention.

[0017] The lithium secondary battery includes a positive electrode containing a positive electrode active material having the above-described structure, and a negative electrode active material. The battery includes a negative electrode and a separator therebetween. It also includes an electrolyte impregnated in the positive electrode, negative electrode, and separator. The negative electrode active material is preferably one that can reversibly absorb and release lithium ions. Examples of such materials include artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon. Metallic lithium can also be used as the negative electrode active material. The electrolyte may be a liquid electrolyte containing a lithium salt and a non-aqueous organic solvent, or a polymer gel electrolyte. [Effects of the Invention]

[0018] As described above, the cathode active material precursor according to the present invention and the cathode active material for a lithium secondary battery prepared using the same can increase the optimum capacity of the lithium secondary battery by adjusting the cobalt content in the particles within a predetermined range, and can also improve the stability and life characteristics of the lithium secondary battery. [Brief explanation of the drawings]

[0019] [Figure 1] 1 shows the results of measuring the size and internal metal concentration of a positive electrode active material prepared in an example of the present invention. [Figure 2] 1 shows the results of measuring the size and internal metal concentration of a positive electrode active material prepared in an example of the present invention. [Figure 3] 1 shows the results of measuring the characteristics of batteries containing positive electrode active materials according to an example of the present invention and a comparative example. [Figure 4] 1 shows the results of measuring the characteristics of batteries containing positive electrode active materials according to an example of the present invention and a comparative example. [Figure 5] 1 shows the results of measuring the characteristics of batteries containing positive electrode active materials according to an example of the present invention and a comparative example. [Figure 6] 1 shows the results of measuring the characteristics of batteries containing positive electrode active materials according to an example of the present invention and a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention will be described in more detail below with reference to some examples. These examples are merely for the purpose of illustrating the present invention, and therefore the scope of the present invention is not to be construed as being limited by these examples.

[0021] (Preparation of precursor in this example) To produce a positive electrode active material, nickel sulfate, cobalt sulfate, and manganese sulfate were prepared, and precursors 1 to 3 each consisting of a core and a shell were produced by coprecipitation reaction. The cobalt compositions of the core and the shell as a whole were 5 mol %, 9 mol %, and 12 mol %, respectively (Examples 1 to 3).

[0022] LiOH was added as a lithium compound, and the mixture was subjected to a primary heat treatment in the presence of N2, O2 (1 LPM to 100 LPM) at a temperature increase rate of 1°C / min to 20°C / min for 4 to 20 hours (maintenance period). Then, an Al-containing compound was added in an amount of 0 mol% to 10 mol% and the secondary heat treatment was carried out to prepare a positive electrode active material for a lithium secondary battery.

[0023] Next, distilled water was prepared and kept at a constant temperature of 5°C to 40°C. The prepared positive electrode active material for a lithium secondary battery was then placed in the distilled water and washed for 0.1 to 10 hours while maintaining the temperature.

[0024] The washed positive electrode active material was passed through a filter press and then dried in an oxygen atmosphere at 50° C. to 300° C. for 3 to 24 hours.

[0025] (Preparation of precursor in comparative example) A positive electrode active material was prepared in the same manner as in the above example, except that the total cobalt content in the core and shell was 3 mol %.

[0026] (particle size measurement) The particle size of the positive electrode active material in Example 1 was measured, and the results are shown in Figure 1. As shown in Figure 1, it can be seen that the particle size of the positive electrode active material produced according to the example of the present invention is 10 μm to 25 μm.

[0027] (Measurement of shell thickness for each example) The shell thickness of the particles of the positive electrode active material according to Example 1 was measured from the metal concentration from the surface to the inside of the particle, and the results are shown in FIG.

[0028] As shown in FIG. 2, the particles of the positive electrode active material prepared according to the embodiment of the present invention have a shell thickness of 1.6 μm.

[0029] (Manufacturing half-cells) The positive electrode active materials prepared in Examples 1 to 3 and Comparative Example were mixed in the ratio of 94 wt %, 3 wt % of conductive material (super-P), and 3 wt % of binder (PVDF) at 4.7 g:0.15 g:0.15 g, respectively, and mixed in a stirrer at 1900 rpm for 10 min. The mixture was then applied to aluminum foil using a microfilm applicator and dried in a dry oven at 135°C for 4 hours to prepare a positive electrode plate.

[0030] The negative electrode plate is made of lithium metal foil, the separator is made of W-Scope-20 μm polypropylene, and the electrolyte is made of 1.15M EC / EMC=7 / 3. LiPF was used to fabricate coin cells.

[0031] (Charge / discharge characteristics measurement) The charge-discharge characteristics of the particles of the positive electrode active material of Examples 1 to 3 and the particles of the positive electrode active material of the comparative example were measured, and the results are shown in FIG.

[0032] As shown in FIG. 3 and Table 1, it was confirmed that when the Co molar fraction of the entire core and shell was 9%, the charge-discharge characteristics were significantly improved compared to the comparative example.

[0033] [Table 1]

[0034] (output characteristics measurement) The output characteristics of the particles of the positive electrode active material of Examples 1 to 3 and the particles of the half cell according to the comparative example were measured, and the results are shown in FIG.

[0035] As shown in FIG. 4 and Table 1, it was confirmed that when the Co mole fraction in the entire core and shell was 9%, the output characteristics were significantly improved compared to the comparative example.

[0036] Also, it was confirmed from FIG. 4 and Table 1 that the high rate discharge characteristics were particularly improved in the case of the secondary battery containing the positive electrode active material according to the present invention.

[0037] (Electrochemical Impedance Spectroscopy (EIS) characteristic measurement) The EIS resistance characteristics of the particles of the positive electrode active material of Examples 1 to 3 and the particles of the positive electrode active material of the comparative example were measured, and the results are shown in FIG.

[0038] As a result, as shown in FIG. 5 and Table 1, it was confirmed that when the Co mole fraction in the entire core and shell was 9%, the EIS resistance characteristics were significantly improved compared to the comparative example.

[0039] (Life characteristics measurement) The life characteristics of the particles of the positive electrode active material of Examples 1 to 3 and the particles of the positive electrode active material of the comparative example were measured, and the results are shown in FIG.

[0040] As a result, as shown in FIG. 6 and Table 1, it was confirmed that when the Co molar fraction in the entire core and shell was 12%, the life characteristics were significantly improved compared to the comparative example.

Claims

[Claim 1] A core portion and a shell portion surrounding the core portion, The cobalt content of the core and shell relative to the total content of transition metals is 5 mol% or more and less than 9 mol%, When the content of cobalt in the core portion and the shell portion is W, the content of cobalt in the shell portion is 0.2W to 1.0W; When the diameter of the entire particle is D, D is 10 μm to 25 μm, and the thickness of the shell portion is 0.01D to 0.3D; the cobalt concentration in the core portion is not the same as the cobalt concentration in the shell portion; A positive electrode active material for a lithium secondary battery represented by the following chemical formula 1. Li ａ Ni ｘ Co ｙ Mn ｚ M １－ｘ－ｙ－ｚ O ２ (In the above chemical formula 1, 0.9≦a≦1.3, 0.7≦x<1.0, 0.05≦y<0.09, 0.0≦z≦0.3, 0.0≦1-x-y-z≦0.3, and M is B, Ba, Ce, Cr, F, Mg, Al, Cr, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, W, P, Sr, Ge, Cu, and one or more elements selected therefrom).

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