KR102152882B1 - Cathode active material and lithium secondary batteries comprising the same - Google Patents

Abstract

The present invention relates to a positive electrode active material containing lithium-overlithiated oxide (OLO) and a lithium secondary battery containing the same, wherein the lithium metal composite oxide is in an X-ray diffraction (XRD) pattern using CuKα rays. It is a positive electrode active material having peaks on the left and right sides based on 21° in the range of 2θ value of 20 to 22°. The positive electrode active material according to the present invention can manufacture a lithium secondary battery having a high capacity of 220 mAh/g or more.

Description

Cathode active material and lithium secondary batteries comprising the same

The present invention relates to a positive electrode active material and a lithium secondary battery including the same, and more particularly, a lithium-overlithiated oxide (OLO) having a specific X-ray diffraction (XRD) pattern is used. Thus, it relates to a positive electrode active material capable of expressing high capacity and a lithium secondary battery including the same.

Lithium secondary batteries are widely used in small electronic products such as electronic devices, portable computers, and mobile phones as secondary batteries with excellent performance such as high capacity, high power and long life.

In particular, lithium with high energy density and discharge voltage as a driving source for electric vehicles that can replace vehicles that use fossil fuels such as gasoline vehicles and diesel vehicles, which are one of the main causes of air pollution, as interest in environmental issues grows. Research on secondary batteries is being actively conducted.

Currently, as positive electrode active materials for lithium secondary batteries, lithium manganese-based composite oxides having a spinel structure, lithium nickel-based composite oxides having a layered structure, lithium cobalt-based composite oxides having a layered structure, and the like have been put into practical use. Any lithium secondary battery using these lithium-containing composite oxides has advantages and disadvantages in terms of characteristics.

Specifically, the lithium manganese composite oxide having a spinel structure is inexpensive, relatively easy to synthesize, and has excellent safety when used as a battery, while its capacity is low, and its cycle characteristics and storage at high temperatures are inferior. The lithium-nickel-based composite oxide having a layered structure has high capacity and excellent high-temperature characteristics, but is difficult to synthesize, has poor safety when used as a battery, and requires attention in storage. A lithium cobalt-based composite oxide having a layered structure is widely used as a power source for portable devices because it is easy to synthesize and has excellent battery performance balance, but has a large disadvantage of insufficient safety and high cost.

Lithium nickel manganese cobalt-based composite oxide (LiNixCoyMnzO2, hereinafter referred to as NCM) having a layered structure has been proposed as a potential candidate for a positive electrode active material that minimizes the above-described defects and has an excellent battery performance balance.

In particular, in recent years, research has been conducted on changing the composition ratio of NCM, such as increasing the manganese/nickel atomic ratio of about 1 or more or reducing the cobalt ratio as demands for lower cost, higher voltage and higher safety. However, in the case of a lithium secondary battery using NCM having such a composition range as a positive electrode active material, there is a problem that load characteristics such as rate/output characteristics and low-temperature output characteristics are deteriorated.

In order to solve such a problem, Patent Document 1 (PCT/JP2011/050210), under consideration that it is important to obtain porous particles with high crystallinity, has a diffraction angle (2θ) around 64.5° during X-ray diffraction measurement. A lithium transition metal-based composite oxide that satisfies the relationship of 0.01≦FWHM≦0.5 is disclosed when the half width of the diffraction peak is FWHM. When the composite oxide prepared according to this method is used as a cathode material for a lithium secondary battery, it is possible to reduce the cost, increase the safety, and improve the powder handling property by improving the bulk density, along with the characteristics of low cost, high safety, and high load, but the battery capacity and capacity In terms of retention rate, improvement was needed.

In addition, Patent Document 2 (Korean Patent Application Publication No. 2013-0078571) discloses that the intensity ratio I ₀₀₃ /I ₁₀₄ between 003 and 104 peaks of the XRD diffraction pattern is controlled to be 1.5 or more. Disclosed is a nickel-based metal oxide. However, according to this method, it can be seen that by controlling the intensity ratio (I ₀₀₃ /I ₁₀₄ ) between peaks, it is possible to improve the battery characteristics in a lithium secondary battery using this as an electrode active material. In the case of the lithium secondary battery manufactured accordingly, the discharge capacity was less than 200mAh/g, and there was still a need for improvement in battery capacity.

PCT/JP2011/050210 KR 2013-0078571 A

In the present invention, in order to solve the problems of the prior art as described above, high capacity expression is possible by using a lithium-overlithiated oxide (OLO) having a specific X-ray diffraction (XRD) pattern. Its purpose is to provide a positive electrode active material.

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

In order to solve the above problems, the present invention includes a lithium metal composite oxide represented by Formula 1 below, and the lithium metal composite oxide has a 2θ value of 20 to 22° in an X-ray diffraction (XRD) pattern. A positive electrode active material having peaks on the left and right sides based on 21° in the range is provided.

Formula 1

Li _{1 + x} Ni _a Co _b Mn _c O _{2 +d}

(Where, 0.1 <x <0.33, 0 <a <0.4, 0 <b <0.4, 0.4 <c <0.9, -0.5 <d <0.5, x + a + b + c = 1)

It is preferable that the full width at half maximum (FWHM) of the peak present in the range of the 2θ value of 20 to 21° is 0.2° <FWHM <0.3°, and the 2θ value is in the range of 21 to 22° It is preferable that the ratio of the peak intensity (I _L ) present in the range of 20 to 21° to the peak intensity (I _R ) is 1.5 <I _L /I _R <3.

In addition, the present invention is a positive electrode including the positive electrode active material; A negative electrode including a negative active material; And it provides a lithium secondary battery comprising an electrolyte present between the positive electrode and the negative electrode.

According to the present invention, a positive active material for a lithium secondary battery having a high capacity of 220 mAh/g or more can be provided.

1 is a graph showing XRD data of a positive active material for a lithium secondary battery prepared according to Example 1.
FIG. 2 is a graph showing an enlarged portion of the 2θ value of 19 to 23° in FIG. 1.
3 is a graph showing XRD data of a positive active material for a lithium secondary battery prepared according to Comparative Example 1.

<Anode active material>

The positive electrode active material of the present invention includes a lithium metal composite oxide represented by Formula 1 below, and the lithium metal composite oxide is based on 21° in a range of 20 to 22° in a 2θ value in an X-ray diffraction (XRD) pattern. Each has a peak on the left and right.

Formula 1

Li _{1 + x} Ni _a Co _b Mn _c O _{2 +d}

(Where, 0.1 <x <0.33, 0 <a <0.4, 0 <b <0.4, 0.4 <c <0.9, -0.5 <d <0.5, x + a + b + c = 1)

The lithium metal composite oxide is a lithium-excessive layered composite oxide having a layered crystal structure in which an excess of lithium is mixed in the cation layer of the transition metal by the number of moles corresponding to the range of x, and manganese is different among metals excluding lithium. Contains excess compared to metal. The lithium metal composite oxide includes two phases represented by LiMO ₂ (here, M is Ni, Co, and Mn), which is a layered structure lithium transition metal oxide due to an excess of lithium, and Li ₂ MnO ₃ in a layered structure.

As described above, when the Li ₂ MnO ₃ structure is present in the positive electrode active material, a small peak appears in a section in which the 2θ value is in the range of 20 to 22° in the XRD data, and the performance of the positive active material depends on the characteristics of the peak.

1, in the case of the positive electrode active material according to the present invention, it can be seen that a peak appears at a 2θ value of 20 to 22°, and the existence of the peak is more clearly seen by looking at FIG. 2, which is an enlarged graph. have.

In the case of the positive active material for a high-capacity lithium secondary battery to be implemented in the present invention, the peak intensity (I _L ) present in the range of 20 to 21° with a 2θ value of 20 to 21° is the peak intensity (I _R ) present in the range of 21 to 22° Bigger and sharper Therefore, since it is convenient to calculate the half width from a large and sharp peak, the target of the half width measurement was set to a peak of 21 degrees or less. X-ray diffraction analysis was performed using a CuKα ray as an X-ray diffraction light source, and the performance of the battery may be influenced by the value of the half width of the diffraction peak present near a specific 2θ value of the X-ray diffraction pattern.

It is preferable that the full width at half maximum (FWHM) of the peak existing in the range of the 2θ value of 20 to 21° is 0.2° <FWHM <0.3°. When the FWHM is 0.3° or more, it is difficult to improve the capacity of the secondary battery because it means that the content of Li ₂ MnO ₃ is small or the crystal size is small, whereas when the FWHM is less than 0.2°, the content of Li ₂ MnO ₃ Since it means that the crystal size is large or the crystal size is large, the impedance of the positive electrode active material increases, and there is a concern that the battery capacity and rate characteristics are reduced.

Incidentally, the ratio of "20 ~ (I _L) peak intensity existing in the 21 ° range" for "peak intensity (I _R) present in the 21 ~ 22 ° range" is the 2θ value, 1.5 <I _L It is preferred that /I _R <3. When the intensity ratio (I _L / I _R ) is 3 or more, it is difficult to improve the capacity of the secondary battery because the crystallinity is high and the activation of Li ₂ MnO ₃ does not occur. On the other hand, in the case of 1.5 or less, Li ₂ MnO ₃ Since it means that the content is large or the crystal size is large, the impedance of the positive electrode active material is high, so there is a concern that the battery capacity and rate characteristics are reduced.

In short, if the content of Li ₂ MnO ₃ is large or the crystal size increases, the half width decreases and I _{L /} I _R This decreases. Such a positive electrode active material has a high impedance, leading to a decrease in capacity and rate characteristics in battery performance. Conversely, when the content of Li ₂ MnO ₃ is small, peaks are not observed or appear insignificant in the range of 2θ value of 20 to 22°, and the half width increases to 0.3 or more. In this case, it leads to a decrease in the capacity of the battery due to insufficient content of the fundamental material for realizing high capacity.

<Method of manufacturing positive electrode active material>

The method for preparing a positive electrode active material according to the present invention comprises: synthesizing a transition metal compound precursor; And mixing the transition metal compound precursor and the Li source, followed by heat treatment at 600 to 1000°C.

The lithium source may be one or more selected from the group consisting of Li ₂ CO ₃ , LiOH, LiNO ₃ and LiCH ₃ COO, preferably Li ₂ CO ₃ And LiOH may be selected and used.

First, for the synthesis of a precursor in the form of a transition metal hydroxide, in the form of a salt soluble in water, one selected from the group consisting of nickel sulfate, nickel nitrate, and nickel carbonate; One selected from the group consisting of cobalt sulfate, cobalt nitrate and cobalt carbonate; And after preparing an aqueous solution by dissolving one selected from the group consisting of manganese sulfate, manganese nitrate and manganese carbonate at a certain molar concentration, pH 10 ~ using at least one base selected from the group consisting of NaOH, NH ₄ OH and KOH It precipitates in the form of hydroxide in the range of 12.

In this case, when the pH is lower than 10, the particle agglomeration rate is higher than the nucleation rate of the particles, so that the size of the particles can grow to 3 μm or more. Since it is larger than that of particles, there may be a problem that it is difficult to obtain a transition metal hydroxide in which each component of the transition metal such as Ni, Co, and Mn is homogeneously mixed.

That is thus adsorbed on the surface of the precipitated powder _{^{SO 4 2 -, NH 4 +}} , NO 3 -, washed several times with distilled water to Na ^+, K ^+, etc. to synthesize a high-purity transition metal hydroxide precursor. The thus synthesized transition metal hydroxide precursor is dried in an oven at 150° C. for 24 hours or more so that the moisture content is less than 0.1 wt%.

The transition metal compound precursor thus prepared is represented by the formula Ni _a Co _b Mn _c (OH) ₂ (0.1 ≤ a <0.5, 0 ≤ b <0.7, 0.2 ≤ c <0.9, a + b + c = 1) It is preferably in the form of a transition metal hydroxide.

After drying the transition metal hydroxide precursor and the Li source are homogeneously mixed, heat treatment at a temperature range of 600 to 1000° C. for 5 to 30 hours to obtain a lithium metal composite oxide positive electrode active material.

When the heat treatment temperature is less than 600° C., the reaction between the Li source and the transition metal hydroxide precursor is not well performed. On the other hand, when the heat treatment temperature exceeds 1000° C., the particle size of the active material increases too much, resulting in a decrease in battery characteristics.

<Lithium secondary battery containing positive electrode active material>

The positive electrode active material according to the present invention can be used as a positive electrode material for a lithium secondary battery, has the same structure as a known secondary battery, except for the composition and crystal structure of the positive electrode active material, and can be manufactured by the same known manufacturing method. I can.

Preferably, the lithium secondary battery is a conventional method widely known in the art, and can be manufactured by inserting a porous separator between a positive electrode and a negative electrode and introducing an electrolyte. Manufactured using a solution in which lithium metal is used for the cathode, the separator is made of porous PE, and the electrolyte is 1.3M LiPF ₆ EC (ethylene carbonate): DMC (dimethyl carbonate): EC in a weight ratio of 5:3:2 do.

Hereinafter, a method of manufacturing a positive electrode active material according to the present invention and a lithium secondary battery including the positive electrode active material manufactured thereby will be described in detail through preferred examples and comparative examples. However, these examples are only preferred examples of the present invention, and the present invention should not be construed as being limited by these examples.

Example One

① Synthesis of transition metal hydroxide precursor

Nickel sulfate (NiSO ₄ ), cobalt sulfate (CoSO ₄ ), and manganese sulfate (MnSO ₄ ) were mixed at a molar ratio of 2:2:6 to prepare a 2M aqueous metal salt solution. The prepared aqueous metal salt solution was added to a 10 L continuous reactor at a rate of 0.5 L/h.

2M aqueous ammonia (NH ₄ OH) is added through the ammonia water supply of the reactor at a rate of 0.5L/hr, and a 2M aqueous sodium hydroxide (NaOH) solution is automatically added through the supply of sodium hydroxide aqueous solution of the reactor. , pH 10.8 was maintained through a pH meter and a control unit. The temperature of the reactor was set at 50°C, the residence time (RT) was adjusted to 10 hours, and the mixture was stirred at a speed of 500 rpm.

So then the resultant reaction solution was filtered through a filter and purified by distilled water, and dried for 24 hours in an oven of 120 ℃ to synthesize a transition metal hydroxide precursor _{Ni 0 .2 Co 0 .2 Mn 0} .6 (OH) 2.

② Positive active material synthesis

Lithium carbonate (Li ₂ CO ₃ ), which is a Li supply source, was mixed with 0.8 mole equivalent of the transition metal hydroxide precursor synthesized in ① above so that the equivalent of Li was 1.2 moles, and then heat-treated at 900° C. for 10 hours to obtain a positive electrode active material powder.

Example 2

A cathode active material powder was obtained in the same manner as in Example 1, except that lithium carbonate (Li ₂ CO ₃ ) as a source of Li was mixed so that the transition metal hydroxide precursor was 0.85 molar equivalent and the Li equivalent was 1.15 mol.

Comparative example One

A cathode active material powder was obtained in the same manner as in Example 1, except that lithium carbonate (Li ₂ CO ₃ ) as a source of Li was mixed so that 1.0 mol equivalent of the transition metal hydroxide precursor was 1.0 mol equivalent of Li.

Comparative example 2

A cathode active material powder was obtained in the same manner as in Example 1, except that lithium carbonate (Li ₂ CO ₃ ) as a source of Li was mixed so that the transition metal hydroxide precursor was 0.95 mol equivalent and the Li equivalent was 1.05 mol.

Comparative example 3

A cathode active material powder was obtained in the same manner as in Example 1, except that lithium carbonate (Li ₂ CO ₃ ), which is a source of Li, was mixed so that the transition metal hydroxide precursor was 0.90 molar equivalent and the Li equivalent was 1.1 mol.

Comparative example 4

A cathode active material powder was obtained in the same manner as in Example 1, except that lithium carbonate (Li ₂ CO ₃ ), which is a source of Li, was mixed so that the transition metal hydroxide precursor was 0.70 molar equivalent and the Li equivalent was 1.3 molar.

<Evaluation of battery capacity and life characteristics>

A slurry was prepared by mixing the positive electrode active material synthesized in Examples 1 and 2 and Comparative Examples 1 to 4, Denka Black as a conductive material, and polyvinylidene fluoride (PVDF) as a binder in a weight ratio of 92:4:4. The slurry was uniformly coated on an aluminum (Al) foil to prepare a positive electrode plate.

Lithium metal as a negative electrode, a porous PE separator as an electrolyte, and a solution of 1.3M LiPF ₆ EC: DMC: EC in a weight ratio of 5: 3: 2 as an electrolyte, and a coin cell type lithium secondary battery Was produced.

Positive active material XRD analysis

X-ray diffraction analysis was performed using a Cu Kα ray as an X-ray diffraction light source. Conducted at a 2θ value range of 15° to 70°, a scan rate of 1°/min, and a 2θ value range of 19° to 23°, a scan rate of 0.2°/min I did.

Half width ( FWHM , Full Width at Half Maximum )

In the X-ray diffraction pattern, the half width of the peak appearing in the range of 20 to 21° with a 2θ value was measured.

I _L / I _R

In the X-ray diffraction pattern, the ratio of the peak intensity (I _L ) present in the range of 20 to 21° to the peak intensity (I _R ) present in the range of 21 to 22° in the 2θ value was calculated and shown.

Battery capacity

The produced coin cell is left at a constant temperature of 25℃ for 24 hours, and then a lithium secondary battery charging/discharging test device (Toyo System) is used, and the voltage range of the test cell is set to 3.0 ~ 4.6V, CC (Constant Current) In the /CV (Constant Voltage) mode, charging and discharging was performed at 0.2C and the discharge capacity was calculated.

According to the evaluation method, the discharge capacity and capacity retention rate of the lithium secondary batteries including the positive electrode active materials prepared in Examples 1 and 2 and Comparative Examples 1 to 4 are shown in Table 1 below.

Peak presence ^* FWHM I _L / I _R Capacity (mAh/g) Example 1 ○ 0.24 2.2 228 Example 2 ○ 0.28 1.7 242 Comparative Example 1 × - - 185 Comparative Example 2 × - - 197 Comparative Example 3 ○ 0.35 3 210 Comparative Example 4 ○ 0.2 1.1 125

(* In the X-ray diffraction pattern, the presence or absence of a peak in the range where the 2θ value is 20 to 22° is represented by ○/×.)

As can be seen from Table 1, in the case of a secondary battery manufactured using a positive electrode active material having an XRD pattern according to the present invention, it was found that the battery capacity was significantly improved compared to a secondary battery manufactured using the positive electrode active material according to Comparative Examples. I can.

1 to 3, in FIGS. 1 and 2, which are graphs showing XRD data of the positive electrode active material for a lithium secondary battery prepared according to Example 1, it can be seen that a peak is detected at a 2θ value of 20° to 22°. In FIG. 3, which is a graph showing the XRD data of the positive active material for a lithium secondary battery prepared according to Comparative Example 1, no peak was detected at the 2θ value in the above range, and in this case, the initial discharge capacity of the lithium secondary battery was also 185 mAh/g. It was the worst.

Claims (4)

It includes a lithium metal composite oxide represented by the following formula (1),
The lithium metal composite oxide has peaks on the left and right sides based on 21° in the range of 20 to 22° in the 2θ value in the X-ray diffraction (XRD) pattern,
The ratio of the peak intensity (I _L ) present in the range of 20 to 21° to the peak intensity (I _R ) present in the range of the 2θ value is 21 to 22° is 1.5 <I _L /I _R <3 A positive electrode active material characterized in that:
Formula 1
Li _1+x Ni _a Co _b Mn _c O _2+d
(Where, 0.1 <x <0.33, 0 <a <0.4, 0 <b <0.4, 0.4 <c <0.9, -0.5 <d <0.5, x + a + b + c = 1). The method of claim 1,
The positive electrode active material, wherein the 2θ value is a full width at half maximum (FWHM) in the range of 20 to 21° is 0.2° <FWHM <0.3°. delete A positive electrode comprising the positive active material of any one of claims 1 and 2;
A negative electrode including a negative active material; And
A lithium secondary battery comprising an electrolyte present between the positive electrode and the negative electrode.

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