Patent Description:
Demand for secondary batteries as an energy source has been significantly increased as technology development and demand with respect to mobile devices and electric vehicles have recently increased. Among these secondary batteries, lithium secondary batteries having high energy density, high voltage, long cycle life, and low self-discharging rate have been commercialized and widely used.

Lithium transition metal composite oxides have been used as a positive electrode active material of the lithium secondary battery, and, among these oxides, a lithium cobalt composite metal oxide, such as LiCoO<NUM>, having a high operating voltage and excellent capacity characteristics has been mainly used. However, the LiCoO<NUM> has poor thermal properties due to an unstable crystal structure caused by delithiation. Also, since the LiCoO<NUM> is expensive, there is a limitation in using a large amount of the LiCoO<NUM> as a power source for applications such as electric vehicles.

Lithium manganese composite metal oxides (LiMnO<NUM> or LiMn<NUM>O<NUM>), lithium iron phosphate compounds (LiFePO<NUM>, etc.), or lithium nickel composite metal oxides (LiNiO<NUM>, etc.) have been developed as materials for replacing the LiCoO<NUM>. Among these materials, research and development of the lithium nickel composite metal oxides, in which a large capacity battery may be easily achieved due to a high reversible capacity of about <NUM> mAh/g, have been more actively conducted. However, the LiNiO<NUM> has limitations in that the LiNiO<NUM> has poorer thermal stability than the LiCoO<NUM> and, when an internal short circuit occurs in a charged state due to an external pressure, the positive electrode active material itself is decomposed to cause rupture and ignition of the battery. Accordingly, as a method to improve low thermal stability while maintaining the excellent reversible capacity of the LiNiO<NUM>, a lithium transition metal oxide, in which a portion of nickel (Ni) is substituted with cobalt (Co), manganese (Mn), or aluminum (Al), has been developed. Such materials are described in the literature, for example, in <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT> and by<NPL>.

With respect to a lithium ion battery using the lithium transition metal oxide, particularly, a Ni-rich lithium transition metal oxide, as a positive electrode active material, since an amount of oxidation of nickel is increased in the same voltage range due to a high nickel content, an amount of lithium ion movement is increased, and, as a result, there is a limitation in that stability of a positive electrode is reduced and life characteristics of the secondary battery are degraded.

Thus, there is a need to develop a Ni-rich positive electrode active material having excellent life characteristics.

An aspect of the present invention provides a method of preparing a nickel-rich positive electrode active material which may improve life characteristics by controlling sintering conditions during the preparation of the Ni-rich positive electrode active material.

According to an aspect of the present invention, there is provided a method of preparing a positive electrode active material which includes the steps of: (<NUM>) Primary sintering step: forming a pre-sintered product by mixing a transition metal precursor represented by Formula <NUM> or Formula <NUM> having a nickel content of <NUM> atm% or more and a lithium raw material and performing primary sintering in a temperature range of <NUM> to <NUM>; and (<NUM>) Secondary sintering step: forming a lithium composite transition metal oxide by performing secondary sintering on the pre-sintered product at a temperature that is <NUM> to <NUM> higher than the primary sintering temperature, wherein the primary sintering is performed such that a ratio of a spinel phase of the pre-sintered product is in a range of <NUM>% to <NUM>%:.

wherein, in Formula <NUM> and Formula <NUM>, <NUM>≤a<<NUM>, <NUM>≤b<<NUM>, and <NUM>≤c<<NUM>.

If necessary, the method may further include a step of measuring crystalline phase information of the pre-sintered product and/or a step of grinding or classifying the pre-sintered product after the forming of the pre-sintered product.

The transition metal precursor and the lithium raw material may be mixed in amounts such that a molar ratio of lithium : transition metal is in a range of <NUM> : <NUM> to <NUM> : <NUM>.

Preferably, the lithium raw material may be LiOH·H<NUM>O.

The secondary sintering may be performed in a temperature range of <NUM> to <NUM>.

A M<NUM> raw material (where M<NUM> is at least one selected from the group consisting of aluminum (Al), silicon (Si), boron (B), tungsten (W), molybdenum (Mo), magnesium (Mg), vanadium (V), titanium (Ti), zinc (Zn), gallium (Ga), indium (In), ruthenium (Ru), niobium (Nb), tantalum (Ta), tin (Sn), strontium (Sr), lanthanum (La), cerium (Ce), praseodymium (Pr), and zirconium (Zr)) may be further mixed during the primary sintering, and, in this case, the M<NUM> raw material may be aluminum hydroxide.

The lithium composite transition metal oxide prepared according to the preparation method of the present invention may be a compound represented by the following Formula <NUM>.

[Formula <NUM>]     Li<NUM>+x[NiaCobMncM<NUM>d]O<NUM>.

In Formula <NUM>, <NUM>≤x≤<NUM>, <NUM>≤a<<NUM>, <NUM>≤b<<NUM>, <NUM>≤c<<NUM>, <NUM>≤d<<NUM>, and a+b+c+d=<NUM>, and M<NUM> is at least one selected from the group consisting of Al, Si, B, W, Mo, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Ta, Sn, Sr, La, Ce, Pr, and Zr.

Since a method of preparing a positive electrode active material according to the present invention allows a content of a spinel phase of a pre-sintered product to be within a specific range by adjusting primary sintering conditions, the method improves reactivity of lithium and a transition metal precursor and crystallinity of a final product, and thus, the method allows to prepare a nickel-rich positive electrode active material having excellent life characteristics.

Also, according to the method of preparing a positive electrode active material of the present invention, since secondary sintering is performed after moisture and unnecessary gases are removed during primary sintering, density of the positive electrode active material is increased and an effect of improving the crystallinity may be obtained.

As a result of a significant amount of research conducted to improve initial capacity and life characteristics of a nickel (Ni)-rich positive electrode active material, the present inventors have found that, in case in which secondary sintering is performed after a pre-sintered product is formed through primary sintering during preparation of the Ni-rich positive electrode active material, but the primary sintering is performed such that a ratio of a spinel phase among crystalline phases of the pre-sintered product satisfies a specific range, the initial capacity and life characteristics of the Ni-rich positive electrode active material may be improved, thereby leading to the completion of the present invention.

A method of preparing a positive electrode active material according to the present invention includes the steps of: (<NUM>) Primary sintering step: forming a pre-sintered product by mixing a transition metal precursor represented by Formula <NUM> or Formula <NUM> having a nickel content of <NUM> atm% or more and a lithium raw material and performing primary sintering in a temperature range of <NUM> to <NUM>; and (<NUM>) Secondary sintering step: forming a lithium composite transition metal oxide by performing secondary sintering on the pre-sintered product at a temperature that is <NUM> to <NUM> higher than the primary sintering temperature, and, in this case, the primary sintering is performed such that a ratio of a spinel phase of the pre-sintered product is in a range of <NUM>% to <NUM>%:.

After the forming of the pre-sintered product, if necessary, the method may further include a step of measuring crystalline phase information of the pre-sintered product and/or a step of grinding or classifying the pre-sintered product.

Hereinafter, each step of the method of preparing a positive electrode active material according to the present invention will be described in detail.

First, a transition metal precursor and a lithium raw material are mixed and primary sintering is performed to form a pre-sintered product.

In this case, the transition metal precursor is a hydroxide or oxyhydroxide containing nickel, manganese, and cobalt, wherein an amount of nickel among total transition metals is <NUM> atm% or more, preferably <NUM> atm% to <NUM> atm%, and more preferably <NUM> atm% to <NUM> atm%.

Specifically, the transition metal precursor is a compound represented by the following Formula <NUM> or Formula <NUM>.

In Formula <NUM> and Formula <NUM>, <NUM>≤a<<NUM>, <NUM>≤b<<NUM>, and <NUM>≤c<<NUM>.

As the lithium raw material, for example, lithium-containing carbonates (e.g., lithium carbonate), hydrates (e.g., lithium hydroxide hydrate (LiOH·H<NUM>O), etc.), hydroxides (e.g., lithium hydroxide, etc.), nitrates (e.g., lithium nitrate (LiNO<NUM>), etc.), or chlorides (e.g., lithium chloride (LiCl), etc.) may be used, but, among them, the lithium raw material may particularly be LiOH·H<NUM>O.

It is desirable that the transition metal precursor and the lithium raw material may be mixed in amounts such that a molar ratio of lithium : transition metal is in a range of <NUM> : <NUM> to <NUM> : <NUM>, preferably <NUM> : <NUM> to <NUM> : <NUM>, and more preferably <NUM> : <NUM> to <NUM> : <NUM>. In a case in which the molar ratio of lithium : transition metal is less than <NUM> : <NUM>, since a degree of cation mixing becomes severe and hexagonal ordering of a lattice structure is broken, electrochemical performance of the positive electrode active material may be degraded. In a case in which the molar ratio of lithium : transition metal is greater than <NUM> : <NUM>, since an excessive amount of lithium may not be inserted into the lattice structure of the positive electrode active material and remains on a surface to act as an impurity, gelation occurs during preparation of a positive electrode slurry to not only make it difficult to form a uniform slurry but also increase an amount of gas generated during battery operation, and thus, stability and long-term life capability may be reduced.

In the present invention, the primary sintering is performed such that a ratio of a spinel phase in the pre-sintered product is in a range of <NUM>% to <NUM>%, for example, <NUM>% to <NUM>%. According to the study of the present inventors, in a case in which the ratio of the spinel phase of the pre-sintered product satisfies the above range, an excellent effect of improving life characteristics may be obtained, and, particularly, an effect of improving resistance characteristics is significantly exhibited.

The ratio of the spinel phase of the pre-sintered product varies according to complex factors such as sintering temperature, sintering time, types of raw materials used, and a mixing ratio of the raw materials. Thus, in order to control the ratio of the spinel phase of the pre-sintered product within the above range, there is a need to appropriately adjust the sintering temperature or sintering time in consideration of the types and mixing ratio of the raw materials used.

Specifically, in the present invention, the primary sintering is performed in a temperature range of <NUM> to <NUM>, for example, <NUM> to <NUM> for <NUM> hour to <NUM> hours, for example, <NUM> hours to <NUM> hours. When the primary sintering temperature and sintering time satisfy the above ranges, the ratio of the spinel phase is formed in the desired range.

Also, the primary sintering may be performed in an air atmosphere or an oxygen atmosphere. In a case in which the primary sintering is performed in an air atmosphere or an oxygen atmosphere, there is an effect that an oxidation reaction of the precursor is promoted in comparison to a case where sintering is performed in an inert atmosphere.

Although not essential, in addition to the transition metal precursor and the lithium raw material, an M<NUM> raw material containing at least one element (M<NUM> element) selected from the group consisting of aluminum (Al), silicon (Si), boron (B), tungsten (W), molybdenum (Mo), magnesium (Mg), vanadium (V), titanium (Ti), zinc (Zn), gallium (Ga), indium (In), ruthenium (Ru), niobium (Nb), tantalum (Ta), tin (Sn), strontium (Sr), lanthanum (La), cerium (Ce), praseodymium (Pr), and zirconium (Zr) may be further mixed during the primary sintering. In a case in which the M<NUM> raw material is further mixed during the primary sintering, a positive electrode active material doped with the M<NUM> element contained in the M<NUM> raw material may be prepared. The M<NUM> raw material, for example, may be an acetic acid salt, nitrate, sulfate, halide, sulfide, hydroxide, oxide, or oxyhydroxide containing the M<NUM> element, that is, at least one metallic element selected from the group consisting of Al, Si, B, W, Mo, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Ta, Sn, Sr, La, Ce, Pr, and Zr. In a case in which M<NUM> is Al, the M<NUM> raw material may be aluminum hydroxide.

After forming the pre-sintered product through the above-described process, if necessary, a step of measuring crystalline phase information of the pre-sintered product may be additionally performed.

The step of measuring the crystalline phase information of the pre-sintered product, for example, may be performed in such a manner that, after a sample of the pre-sintered product is collected, a ratio of a spinel phase among crystalline phases is measured by performing X-ray diffraction (XRD) analysis. That is, the ratio of the spinel phase may be calculated by measuring intensity contribution of each phase using XRD data measured for the pre-sintered product together with a cubic phase (NiO/LiN<NUM>O<NUM>) and a layered phase (LiNiO<NUM>) through Rietveld refinement.

After measuring the crystalline phase information of the pre-sintered product as described above, a positive electrode active material having excellent initial capacity and life characteristics may be prepared by performing secondary sintering when the ratio of the spinel phase of the pre-sintered product is in a range of <NUM>% to <NUM>%.

Also, the preparation method of the present invention may further include a step of grinding and/or classifying the pre-sintered product after the primary sintering, if necessary. In a case in which the step of grinding and/or classifying the pre-sintered product is additionally performed, since moisture and gas generated during the primary sintering may be removed during a grinding and/or classification process and mixing of the pre-sintered product is more effectively performed, sintering uniformity is improved during the secondary sintering and an effect of improving quality uniformity of the finally-prepared positive electrode active material may be obtained.

Next, secondary sintering is performed on the pre-sintered product to form a lithium composite transition metal oxide.

The secondary sintering is to change the spinel phase of the pre-sintered product into a layered phase, wherein the secondary sintering is performed at a temperature that is <NUM> to <NUM> higher than the primary sintering temperature, for example, at a temperature that is <NUM> to <NUM> higher than the primary sintering temperature.

In a case in which a difference between the secondary sintering temperature and the primary sintering temperature is less than <NUM>, conversion of the spinel phase into the layered phase is not smoothly performed, and, in a case in which the difference between the secondary sintering temperature and the primary sintering temperature is greater than <NUM>, since positive electrode active material particles are severely agglomerated, dispersion is difficult during the preparation of the positive electrode slurry and it is difficult to apply the positive electrode slurry to a uniform thickness, and thus, processability may be reduced.

Specifically, the secondary sintering may be performed in a temperature range of <NUM> to <NUM>, preferably <NUM> to <NUM>, and more preferably <NUM> to <NUM>.

The secondary sintering may be performed for <NUM> hour to <NUM> hours, for example, <NUM> hours to <NUM> hours.

Also, the secondary sintering may be performed in an air atmosphere or an oxygen atmosphere. In a case in which the secondary sintering is performed in an air atmosphere or an oxygen atmosphere, there is an effect that the oxidation reaction of the precursor, crystal growth, and phase change are promoted in comparison to a case where sintering is performed in an inert atmosphere.

In Formula <NUM>, M<NUM> is at least one element selected from the group consisting of Al, Si, B, W, Mo, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Ta, Sn, Sr, La, Ce, Pr, and Zr.

<NUM>+x represents an atomic ratio of lithium to total transition metals, wherein x satisfies <NUM>≤x≤<NUM>, preferably <NUM>≤x≤<NUM>, and more preferably <NUM>≤x≤<NUM>, for example, <NUM>≤x≤<NUM>.

The positive electrode active material prepared according to the method of the present invention has better capacity retention and resistance characteristics than a conventional nickel-rich positive electrode active material.

Hereinafter, the present invention will be described in detail, according to specific examples. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these example embodiments are provided so that this description will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art.

After a transition metal precursor, Ni<NUM>Co<NUM>Mn<NUM>(OH)<NUM>, and LiOH·H<NUM>O were added such that a molar ratio of Li : transition metal was <NUM> : <NUM> and Al(OH)<NUM> was mixed, primary sintering was performed at <NUM> for <NUM> hours in an oxygen atmosphere to prepare a pre-sintered product and the pre-sintered product was ground using an air classifying mill (ACM).

A ratio of a spinel phase was measured by Rietveld refinement analysis of XRD data, which were measured by X-ray diffraction analysis (Bruker D4 Endeavor) on the pre-sintered product, using a HighScore software.

As a result of the measurement, the ratio of the spinel phase among crystalline phases of the pre-sintered product was found to be <NUM>%.

Next, a lithium composite transition metal oxide, LiNi<NUM>Co<NUM>Mn<NUM>Al<NUM>O<NUM>, was prepared by performing secondary sintering on the pre-sintered product at <NUM> for <NUM> hours in an oxygen atmosphere. After the lithium composite transition metal oxide was ground, washed, and dried, powder of the lithium composite transition metal oxide was mixed with <NUM> wt% of H<NUM>BO<NUM> and heat-treated at <NUM> for <NUM> hours in an air atmosphere to prepare B-coated positive electrode active material powder.

A pre-sintered product and positive electrode active material powder were prepared in the same manner as in Example <NUM> except that a primary sintering temperature during the preparation of the pre-sintered product was <NUM>. A ratio of a spinel phase among crystalline phases of the pre-sintered product was found to be <NUM>%.

A pre-sintered product and positive electrode active material powder were prepared in the same manner as in Example <NUM> except that a molar ratio of Li : transition metal during the preparation of the pre-sintered product was <NUM> : <NUM>. A ratio of a spinel phase among crystalline phases of the pre-sintered product was found to be <NUM>%.

Each of the positive electrode active materials prepared in Examples <NUM> to <NUM> and Comparative Examples <NUM> to <NUM>, a conductive agent (FX35), and a binder (mixture in which KF9700 and BM730H were mixed in a weight ratio of <NUM>:<NUM>) were mixed in an N-methyl-<NUM>-pyrrolidone (NMP) solvent at a weight ratio of <NUM>:<NUM>:<NUM> to prepare a positive electrode slurry. One surface of an aluminum current collector was coated with the positive electrode slurry, dried at <NUM>, and then rolled to have a porosity of <NUM>% to prepare each positive electrode.

A lithium (Li) metal disk was used as a negative electrode.

After an electrode assembly was prepared by disposing a separator between the positive electrode and the negative electrode, a lithium secondary battery was prepared by disposing the electrode assembly in a battery case, and then injecting an electrolyte solution into the case. In this case, as the electrolyte solution, an electrolyte solution, in which <NUM> LiPF<NUM> was dissolved in an organic solvent in which ethylene carbonate : ethyl methyl carbonate : diethyl carbonate were mixed in a volume ratio of <NUM>:<NUM>:<NUM>, was used.

Each lithium secondary battery prepared as described above was charged at a constant current of <NUM> C to <NUM> V in a constant current/constant voltage (CC/CV) mode at <NUM> (cutoff current <NUM> C) and then was discharged to <NUM> V in a CC mode to measure initial discharge capacity. In this case, it was set that <NUM> C = <NUM> mA/g.

Also, the charge and discharge cycle was repeated <NUM> times at a constant current of <NUM> C in a range of <NUM> V to <NUM> V at <NUM> to measure a capacity retention and a resistance increase rate with respect to resistance for <NUM> seconds after the start of discharge of each cycle. Measurement results are presented in Table <NUM> below.

Claim 1:
A method of preparing a positive electrode active material, the method comprising:
(<NUM>) Primary sintering step: forming a pre-sintered product by mixing a transition metal precursor represented by Formula <NUM> or Formula <NUM> having a nickel content of <NUM> atm% or more and a lithium raw material and performing primary sintering in a temperature range of <NUM> to <NUM>; and
(<NUM>) Secondary sintering step: forming a lithium composite transition metal oxide by performing secondary sintering on the pre-sintered product at a temperature that is <NUM> to <NUM> higher than the primary sintering temperature,
wherein the primary sintering is performed such that a ratio of a spinel phase of the pre-sintered product is in a range of <NUM>% to <NUM>%:

        [Formula <NUM>]     NiaCobMnc(OH)<NUM>

        [Formula <NUM>]     NiaCobMncOOH

wherein, in Formula <NUM> and Formula <NUM>, <NUM>≤a<<NUM>, <NUM>≤b<<NUM>, and <NUM>≤c<<NUM>.