KR20250023810A - Cathode Active Material and Lithium Secondary Battery Including the Same - Google Patents

Abstract

The present invention provides a cathode active material containing a lithium transition metal oxide having a form of secondary particles in which a plurality of primary particles are aggregated, characterized in that the cathode active material has a crystal grain size of 85 nm or less as determined by X-ray diffraction analysis and an amount of Ni ²⁺ occupying a lithium site in the crystal structure of 2.5 atomic% or more.

Description

Cathode Active Material and Lithium Secondary Battery Including the Same {Cathode Active Material and Lithium Secondary Battery Including the Same}

The present invention relates to a cathode active material and a lithium secondary battery including the same, and more specifically, to a cathode active material which exhibits excellent electrochemical characteristics through complementary interaction thereof by satisfying conditions in which a crystal grain size is within a predetermined range or less and a cation mixing amount is set above a predetermined range, and a lithium secondary battery including the same.

Lithium secondary batteries are widely used in various devices due to their characteristics such as high output and high energy density. Recently, due to environmental issues, the demand for energy storage devices for transportation such as hybrid cars, plug-in hybrid cars, and electric cars, as well as for eco-friendly energy generation and storage, is increasing.

In particular, lithium secondary batteries used in electric vehicles, etc., must have high energy density and be usable for long periods of time under conditions where charging and discharging by current is repeated in a short period of time. Therefore, they inevitably require long-term life characteristics that are far superior to those of existing small lithium secondary batteries.

Among lithium transition metal composite oxides, lithium cobalt composite metal oxides have been mainly used as cathode active materials for conventional lithium secondary batteries. However, they are expensive in terms of cost and have disadvantages in terms of energy density, so they are recently being replaced by lithium nickel composite metal oxides, and in particular, high-Ni cathode active materials that are advantageous in terms of energy density are being actively studied.

However, in the case of high-Ni cathode active materials, although they have excellent effects in terms of capacity implementation, there is a problem in that the structural stability and chemical stability of the active material decrease as the nickel content increases, the surface structure of the active material deteriorates due to repeated charge/discharge, an exothermic reaction accompanied by rapid structural collapse occurs, and the battery stability is lowered, or the life characteristics are rapidly reduced due to structural transformation.

To overcome the above problems, technologies for doping metal elements or coating surfaces are being actively studied. However, there are problems such as unwanted substances being generated on the surface of active materials due to side reactions, or capacity decreasing due to doping elements, and high-temperature life resistance increasing.

Therefore, there is a need for fundamental technological development of cathode active materials that can improve life characteristics without reducing capacity.

The present invention aims to solve the problems of the prior art as described above and the technical tasks requested from the past.

The inventors of the present invention have developed a novel cathode active material with improved structural stability through in-depth research and repeated various experiments, and have confirmed that such cathode active material can secure excellent life characteristics while suppressing capacity decrease even when cation mixing inevitably increases as the Ni content increases, thereby completing the present invention.

Accordingly, the cathode active material according to the present invention is a cathode active material containing a lithium transition metal oxide having a form of secondary particles in which a plurality of primary particles are aggregated, characterized in that the crystal grain size is 85 nm or less in X-ray diffraction analysis, and the amount of Ni ²⁺ occupying lithium sites in the crystal structure is 2.5 atomic% or more.

As defined above, the cathode active material according to the present invention enables the implementation of high capacity along with improved charge/discharge and cycle life characteristics by setting the crystal grain size and the amount of cation mixing to specific ranges, respectively, through their complementary interaction.

First, with regard to the grain size, the inventors of the present application have confirmed that in the case of a cathode active material of secondary particles in which a plurality of primary particles are aggregated, since the particle strength is relatively weak, cracks easily occur at the interface between the primary particles due to volume expansion and contraction that occurs during charge and discharge, thereby severely damaging the active material. This means that the area that can unintentionally react with the electrolyte increases. For this reason, side reactions with the electrolyte increase, structural collapse within the active material becomes severe, and there is a possibility that thermal stability may be adversely affected. From the perspective of a secondary battery, the volume of the cell may expand due to gas generated by the decomposition of the electrolyte, and in some cases, a disconnection between the separator and the cathode active material may occur due to the gas, causing the cell to not operate.

On the other hand, according to the present invention, by providing a cathode active material having fine crystal grains of 85 nm or less, which can suppress particle breakage and reduce side reactions with an electrolyte, structural instability inside the active material is alleviated, and the surface area per primary particle volume is relatively increased, so that lithium movement becomes smooth, thereby improving rate characteristics and charge/discharge characteristics.

In addition, the microcrystalline grain size defined in the present invention can reduce the rate of increase in impedance during charge and discharge, thereby maintaining capacity characteristics, and consequently improving electrochemical characteristics such as life characteristics to a desired level.

Meanwhile, with regard to the cation mixing, cation mixing refers to a phenomenon in which transition metal ions such as Ni2 ⁺ occupy lithium sites within the crystal structure of the positive electrode active material, and the amount of cation mixing refers to the amount of Ni cations present in the lithium layer. In other words, cation mixing refers to the fact that the ionic radii of Li cations and Ni cations are similar, so that some Li and Ni are substituted for each other, which is known to cause structural instability. Such cation mixing is known to reduce the crystallinity of the structure, thereby deteriorating the cycle characteristics of the battery, and therefore, the industry is focusing on reducing cation mixing.

On the other hand, the present invention proposes an approach that is completely different from the prior art by setting the amount of cation mixture to a predetermined value or more. This is to solve the problem that, when the amount of cation mixture is reduced, the life characteristics can be improved, but the capacity decreases. That is, by setting the amount of cation mixture to a predetermined value or more, the decrease in capacity is minimized, and in order to prevent the decrease in cycle characteristics (life characteristics) that is inevitably caused by the increase in the amount of cation mixture, the crystal grain size is set to a predetermined value or less, thereby attempting to improve the charge/discharge and life characteristics.

The differential effects resulting from these complementary interactions are completely different from those taught in the prior art and can be clearly confirmed through the experimental results described below.

In one specific example, the lithium transition metal oxide may comprise a composition represented by the following chemical formula 1.

Li _x Ni _1-y M _y D _z O _2-f Q _f (1)

In the above formula,

M is one or more transition metal elements stable in four- or six-coordinate configurations;

D is at least one element selected from alkaline earth metals, transition metals, and non-metals;

Q is one or more anions selected from F, S, and P;

0.3≤x≤1.1, 0≤y≤0.4, 0≤z≤0.1, 0≤f≤0.2.

Note that if D is a transition metal, the transition metals defined in M may be excluded from these transition metals.

M can be one or more elements selected from the group consisting of, for example, Co, Mn, and D can be one or more elements selected from the group consisting of, for example, Al, W, Si, V, B, Ba, Ca, Zr, Ti, Mg, Ta, Nb and Mo.

For example, it may be preferable that the dopant D includes at least one element selected from Al and Zr.

Al is an element having a similar ionic radius to 6-coordinated Ni ³⁺ in the transition metal layer of the active material. For example, the ionic radius of Al ³⁺ (6-coordination) is 0.535Å, which is similar to that of Ni ³⁺ (6-coordination) at 0.56Å, and thus it can be seen that substitution is easy. Since Ni has an unstable structure in terms of electron configuration when it exists in the oxidation state of 3+, when the Al ³⁺ is doped into the Ni ³⁺ site, it can be effective in improving the life characteristics.

In the case of Zr, an element having a similar ionic radius to 6-coordinated Li ⁺ in the transition metal layer of the active material, for example, Zr ⁴⁺ (6-coordinated) having an ionic radius of 0.72Å can be applied. Since the ion can be doped into the 3a Octahedral site of the Li layer to improve structural stability, it can be more effective in achieving the effects of the present invention.

In some cases, the chemical stability can be improved by coating the surface of the active material particles with a certain substance.

For example, the surface of the active material particle may be partially or completely coated with a material containing B (boron), and B may be added to the outer surface of the active material particle in the form of B ₂ O ₃ or H ₃ BO ₃ . H ₃ BO ₃ has a melting point of 170°C, which is lower than B ₂ O _{3 ,} which has a melting point of 450°C, and undergoes a dehydration reaction (H ₃ BO ₃ → HBO ₂ + H ₂ O: Can be converted and applied at 170℃, and can be converted into B 2 O 3 by dehydration reaction (2HBO ₂ → B ₂ O ₃ + H ₂ O: Can be converted into B ₂ O ₃ at 300℃ or higher. This B ₂ O ₃ or H ₃ BO ₃ is an ion conductor with a 3D network and improves the structural stability by the strong BO bond with high chemical stability.

In general, when the Ni content is increased for the purpose of increasing capacity, the cation mixing phenomenon is aggravated and structural instability increases. Therefore, the present invention can be applied to a cathode active material with a high Ni content in which cation mixing is a particular problem, for example, a cathode active material having a Ni content of 70 mol% or more, preferably 80 mol% or more, based on the total transition metal content.

In the cathode active material of the present invention, the crystal grain size of the lithium transition metal oxide is, as defined above, 85 nm or less. If the crystal grain size exceeds 85 nm, the structural instability within the active material increases and the surface area per particle volume decreases, so that not only the rate characteristics and charge/discharge characteristics deteriorate, but also the impedance increase rate during charge/discharge increases, making it difficult to maintain the electrical characteristics. However, if the crystal grain size is excessively small, the contact area with the electrolyte increases significantly, which may cause a side reaction and significantly deteriorate the life characteristics. Therefore, a more preferable crystal grain size may be in the range of 50 nm to 80 nm.

Another key element of the present invention, the amount of cation mixing, is, as defined above, the amount of Ni ²⁺ occupying the lithium site is 2.5 at% or more. If the amount of cation mixing is too small, the life characteristics may be improved, but the capacity decreases significantly, making it difficult to provide a secondary battery exhibiting balanced properties. As described above, the amount of cation mixing has a complementary correlation with the crystal grain size. Therefore, it can be confirmed from the experimental results described below that the size setting thereof should be determined by taking this into consideration. Therefore, the optimal amount of cation mixing based on the experimental results may be in the range of 2.7 at% to 4 at%.

The cathode active material of the present invention has a differential crystal structure due to the complementary correlation between the crystal grain size and the cation mixing amount ranges.

In one specific example, when measuring an X-ray diffraction pattern, the a-axis value may be 2.873Å or more, and more preferably, may be in the range of 2.873Å to 2.878Å. The lattice constant in the a-axis direction is determined by the function of the distance between transition metals and the distance between lithium and lithium and between oxygen and oxygen. When the above range is satisfied, it may have an advantage of a charge capacity of 240 mAh/g, so that stability is improved without a decrease in capacity, which may be preferable, and when the a-axis length (La) is less than 2.873Å, the capacity may decrease, so that it may not be preferable.

In another specific example, when measuring an X-ray diffraction pattern, the R-factor value defined by the following equation may be 0.55 or greater.

R-factor = {I(006)+I(102)}/I(101)

That is, the R-factor value is the value obtained by dividing the sum of the 102 peak value and the 006 peak value by the 101 peak value. When the above range is satisfied, the Ni atoms can act as pillars at the Li site during charge and discharge to minimize the capacity decrease. When the crystal grain size range defined in the present invention is satisfied and the R-factor value range is satisfied, the size of the primary particles decreases together, and the impedance increase rate is reduced, so that the capacity decrease due to the increase in the R-factor is minimized while improving the life characteristics, which is more effective. The preferable range of the R-factor value can be 0.6 to 0.8, and a more preferable range can be 0.60 to 0.75.

The present invention also provides a lithium secondary battery including the positive electrode active material.

Since the composition of a lithium secondary battery and its manufacturing method are known in the art, a detailed description thereof is omitted in this specification.

As described above, the cathode active material according to the present invention is composed of fine crystal grains of a specific size while containing a certain amount or more of a cation mixture, unlike the conditions generally pursued in the art, so that a lithium secondary battery having not only long life characteristics but also little capacity loss and excellent electrochemical characteristics can be provided.

Figure 1 is a SEM image of the positive active material of Example 1;
Figure 2 is a SEM image of the positive active material of Example 3;
Figure 3 is a SEM image of the positive electrode active material of Comparative Example 1;
Figure 4 is a SEM image of the positive electrode active material of Comparative Example 4.

Hereinafter, the present invention will be described in more detail with reference to embodiments of the present invention, but the scope of the present invention is not limited thereto.

[Example 1]

A precursor containing NiSO ₄ as a nickel precursor, CoSO ₄ as a cobalt precursor, and MnSO ₄ as a manganese precursor in a ratio of 92:04:04 and LiOH as a lithium raw material were mixed in a ratio of 1:1 (Li:precursor), and 0.003 mol of Zr raw material ZrO ₂ and 0.01 mol of Al raw material Al(OH) ₃ were added, placed in a 500 L cylindrical reactor, and uniformly mixed at 28 Hz for about 40 minutes. The mixture was filled into a sagger made of mulite in an amount of about 3 to 5 kg and fired in a box-type sintering furnace at a sintering temperature of 700°C, while continuously passing oxygen at a flow rate of 10 mL/min during the heating and maintenance for a total of 35 hours. After the sintering was completed, the material was naturally cooled to room temperature, and then crushed under crushing/classifying conditions of 1300 rpm/2000 rpm. Then, relatively large particles were filtered out using a sieve to manufacture an active material doped with Zr and Al inside the core.

[Example 2]

A cathode active material was manufactured in the same manner as in Example 1, except that the precursor and raw materials were uniformly mixed at 28 Hz for about 30 minutes and the mixture was calcined at 680° C. for a total of 35 hours.

[Example 3]

A positive electrode active material was manufactured in the same manner as in Example 1, except that the precursor and raw materials were uniformly mixed at 35 Hz for about 30 minutes and the mixture was sintered at a temperature of 680°C.

On the particle surface of the above-mentioned manufactured active material, lithium of less than 2 wt% exists in the form of by-products LiOH and Li ₂ CO _3. Since this residual lithium adversely affects electrode manufacturing and electrochemical characteristics, the residual lithium was removed through a cleaning process.

To this end, the active material particles manufactured above were stirred in distilled water having a solid content of 50 to 80% at about 15°C in a 5 L reactor for about 1 minute, the slurry after stirring was filtered through a filtration device, and the filtered product thus obtained was dried in a vacuum oven at 150°C for more than 6 hours.

The dried filter was coated with boron (B) on the surface through mixing and sintering. To this end, the dried filter was placed in a 10 L cylindrical reactor together with 500 to 1,000 ppm of boric acid, stirred at 35 Hz for about 25 minutes, and about 4 to 5 kg of the refractory was filled, and sintered in a sintering furnace in an oxygen (O ₂ ) atmosphere at a sintering temperature of 300°C for a total of 15 hours, including heating and cooling sections, to manufacture a cathode active material having B coated on the surface of the Zr and Al-doped active material particles manufactured as described above.

[Example 4]

Except that the sintering temperature was set to 700°C during the sintering of the above active material particles, cleaning and coating were completed in the same manner as in Example 3, thereby manufacturing a positive electrode active material in which B was coated on the surface of the active material particles doped with Zr and Al.

[Example 5]

Except that LiOH and nickel-cobalt-manganese precursors were used in a ratio of 1.02:1 (Li:precursor), the precursors and raw materials were uniformly mixed at 28 Hz for about 40 minutes, and the mixture was calcined at 710°C for a total of 40 hours, the same method as in Example 3 was used to complete washing and coating, thereby manufacturing a positive electrode active material having B coated on the surface of active material particles doped with Zr and Al.

[Example 6]

Except that LiOH and nickel-cobalt-manganese precursors were placed in a 1.02:1 (Li:precursor) ratio, 0.003 mol of Zr raw material ZrO ₂ , and 0.01 mol of Al raw material Al(OH) ₃ were placed in a 500 L cylindrical reactor and uniformly mixed at 35 Hz for about 40 minutes, the same method as in Example 3 was used to complete washing and coating, thereby manufacturing a cathode active material having B coated on the surface of active material particles doped with Zr and Al.

[Example 7]

Except that 0.003 mol of ZrO ₂ , the Zr raw material, and 0.01 mol of Al(OH) ₃ , the Al raw material, were placed in a 500 L cylindrical reactor and uniformly mixed at 28 Hz for about 30 minutes and the sintering temperature was set to 710°C, washing and coating were completed in the same manner as in Example 3, thereby manufacturing a positive electrode active material having B coated on the surface of active material particles doped with Zr and Al.

[Comparative Example 1]

A positive electrode active material doped with Zr and Al in the core was manufactured in the same manner as in Example 1, except that the mixture was calcined at 720°C for a total of 40 hours.

[Comparative Example 2]

Except that LiOH and nickel-cobalt-manganese precursors were placed in a 1.01:1 (Li:precursor) ratio, 0.003 mol of Zr raw material ZrO ₂ , and 0.01 mol of Al raw material Al(OH) ₃ were placed in a 500 L cylindrical reactor, and the mixture was calcined at 750°C for a total of 45 hours, a positive electrode active material doped with Zr and Al in the core was manufactured in the same manner as in Example 1.

[Comparative Example 3]

Except that the firing of the active material core was performed at 720°C for a total of 45 hours, the same method as in Example 3 was used to complete cleaning and coating, thereby manufacturing a positive electrode active material having B coated on the surface of active material particles doped with Zr and Al.

[Comparative Example 4]

Except that the precursor and raw materials were uniformly mixed at 35 Hz for about 40 minutes and the mixture was calcined at 720°C for a total of 35 hours, cleaning and coating were completed in the same manner as in Example 3 to manufacture a positive electrode active material having B coated on the surface of active material particles doped with Zr and Al.

[Example 8]

A precursor containing NiSO ₄ as a nickel precursor, CoSO ₄ as a cobalt precursor, and MnSO ₄ as a manganese precursor in a ratio of 83:10:07 and LiOH as a lithium raw material were mixed in a 1:1 (Li:precursor) ratio, and 0.003 mol of Zr raw material ZrO ₂ was added, placed in a 500 L cylindrical reactor, and uniformly mixed at 35 Hz for about 40 minutes, and about 3 to 5 kg of the mixture was filled into a molite refractory case and sintered using a box-type sintering furnace at a sintering temperature of 750°C, while continuously passing oxygen at a flow rate of 10 mL/min during the heating and maintenance for a total of 35 hours. After completion of the sintering, natural cooling to room temperature was performed, and the material was pulverized under the pulverizing/classifying conditions of 1300 rpm/2000 rpm, and then relatively large particles were filtered out using a sieve to manufacture a Zr-doped active material.

Thereafter, cleaning and coating were completed in the same manner as in Example 3, thereby manufacturing a positive electrode active material in which B was coated on the surface of Zr-doped active material particles.

[Example 9]

A positive electrode active material having B coated on the surface of active material particles doped with Zr and Al was manufactured in the same manner as in Example 8, except that 0.003 mol of ZrO ₂ as a Zr raw material and _0.01 mol of Al(OH) 3 as an Al raw material were used as dopants.

[Example 10]

Except that LiOH and nickel-cobalt-manganese precursors were mixed in a ratio of 1.01:1 (Li:precursor), 0.003 mol of Zr raw material ZrO ₂ and 0.01 mol of Al raw material Al(OH) ₃ were added, placed in a 500 L cylindrical reactor, and uniformly mixed at 35 Hz for about 30 minutes, and the mixture was calcined at 720°C for a total of 40 hours, a positive electrode active material having B coated on the surface of active material particles doped with Zr and Al was manufactured in the same manner as in Example 8.

[Comparative Example 5]

A positive electrode active material having B coated on the surface of active material particles doped with Zr and Al was manufactured in the same manner as in Example 8, except that the LiOH and nickel-cobalt-manganese precursors were set at a ratio of 1.02:1 (Li:precursor) and the mixture was calcined at 800°C for a total of 45 hours.

[Comparative Example 6]

Except that 0.003 mol of ZrO ₂ , a Zr raw material, and 0.01 mol of Al(OH) ₃ , a Al raw material, were placed in a 500 L cylindrical reactor and uniformly mixed at 28 Hz for about 30 minutes and the mixture was calcined at 780°C for a total of 40 hours, a positive electrode active material having B coated on the surface of active material particles doped with Zr and Al was manufactured in the same manner as in Example 8.

[Comparative Example 7]

Except that 0.003 mol of ZrO ₂ , a Zr raw material, and 0.01 mol of Al(OH) ₃ , a Al raw material, were placed in a 500 L cylindrical reactor and uniformly mixed at 28 Hz for about 30 minutes and the mixture was calcined at 750°C for a total of 40 hours, a positive electrode active material having B coated on the surface of active material particles doped with Zr and Al was manufactured in the same manner as in Example 8.

[Example 11]

A precursor containing NiSO ₄ as a nickel precursor, CoSO ₄ as a cobalt precursor, and MnSO ₄ as a manganese precursor in a ratio of 70:15:15 and LiOH as a lithium raw material were mixed in a ratio of 1:1 (Li:precursor), and 0.003 mol of Zr raw material ZrO ₂ was added, placed in a 500 L cylindrical reactor, and uniformly mixed at 35 Hz for about 40 minutes, and the mixture was filled into a refractory case made of mollite in an amount of about 3 to 5 kg and fired using a box-type sintering furnace at a sintering temperature of 790°C, while continuously passing oxygen at a flow rate of 10 mL/min during the heating and maintenance for a total of 30 hours. After completion of the sintering, natural cooling to room temperature was performed, and the material was pulverized under the pulverizing/classifying conditions of 1300 rpm/2000 rpm, and then relatively large particles were filtered out using a sieve to manufacture a Zr-doped active material.

Thereafter, cleaning and coating were completed in the same manner as in Example 3, thereby manufacturing a positive electrode active material in which B was coated on the surface of Zr-doped active material particles.

[Example 12]

Except that the firing of the active material core was performed at 810°C for a total of 30 hours, the same method as in Example 11 was used to complete cleaning and coating, thereby manufacturing a positive electrode active material having B coated on the surface of active material particles doped with Zr and Al.

[Comparative Example 8]

Except that the firing of the active material core was performed at 850°C for a total of 35 hours, the same method as in Example 11 was used to complete cleaning and coating, thereby manufacturing a positive electrode active material having B coated on the surface of active material particles doped with Zr and Al.

[Experimental Example 1] Analysis of the properties of positive electrode active material, XRD

In order to analyze the crystal structure of the positive electrode active materials manufactured in Examples 1 to 12 and Comparative Examples 1 to 8, respectively, a Rigaku Ultima ₃ measuring device was used, and measurements were made under the following conditions. At this time, the obtained XRD analysis values were analyzed using Rietveld refinement in the TOPAS program to calculate the grain size, Ni ²⁺ value, and R-factor value, and the results are shown in Table 1, and the a-axis, c-axis, and c/a values of Examples 3 to 7 are listed in Table 2.

[XRD measurement conditions]

- Power source: CuKα (pre-focus), wavelength: 1.541836Å

- Operating axis: 2θ/θ, Measurement method: Continuous, Counting unit: cps

- Start angle: 10.0°, End angle: [0220] 120.0°, Number of accumulations: 1

- Sampling width: 0.01°, Scan speed: 2°/min

- Voltage: 40kV, Current: 40mA

- Divergence slit: 0.2mm, Divergence species limit slit: 10mm

- Scatter slit: open, receiving slit: open

- Offset angle: 0°

- Goniometer radius: 285mm, optical system: focusing method

- Attachment: ASC-48

- Slit: Slit for D/teX Ultra

- Detector: D/teX Ultra

- Incident Monochrome: CBO

- Ni-Kβ filter: None

- Rotation speed: 30rpm

Referring to Table 1 above, the cathode active materials of Examples 1 to 12 simultaneously satisfy the conditions of having a crystal grain size of 85 nm or less and the conditions of having an amount of Ni ²⁺ occupying a lithium site of 2.5 atomic% or more. On the other hand, it can be seen that the cathode active materials of Comparative Examples 1 to 8 have a crystal grain size in the range of 90 to 164 nm. Although, in some Comparative Examples, the amount of Ni ²⁺ is in the range of 2.5 atomic% or more, they do not satisfy the condition of having a crystal grain size of 85 nm or less.

Meanwhile, referring to Table 2 above, it can be seen that the a-axis values of Examples 3 to 7 fall within the range of 2.873 Å to 2.878 Å.

[Experimental Example 2] SEM Analysis

SEM analysis of the positive electrode active materials manufactured in each of Example 1, Example 3, Comparative Example 1, and Comparative Example 4 was conducted under the following conditions, and the results are shown in Figures 1 to 4, respectively.

[SEM measurement conditions]

- Model: HITACHI (S-4800)

- Resolution: 1.0㎚ 15㎸, 1.5㎚ 1㎸

- Magnification: x10,000

- Electron gun: Cold-cathode field emission type electron gun

- Accelerating voltage: 15㎸

- Detector: SE (BSE)

By comparing the SEM images of Example 1, Example 3, Comparative Example 1, and Comparative Example 4 with the results of Table 1 of Experimental Example 1, it can be confirmed that the size of the primary particles also changed in proportion to the crystal grain size.

[Experimental Example 3] Evaluation of Lithium Secondary Battery

PVdF as a binder was dissolved in N-methylpyrrolidone (NMP), and the positive electrode active materials manufactured in Examples 1 to 12 and Comparative Examples 1 to 8 and Super-C as a conductive material were mixed in a weight ratio of 3:95:2 to manufacture a composition for forming a positive electrode, which was then evenly applied onto an aluminum current collector and dried in a hot air dryer at 120°C to evaporate the NMP. Then, a rolling process was performed and dried in a vacuum oven at 120°C for 12 hours to manufacture a positive electrode.

The above-mentioned manufactured positive electrode is prepared to fit the size of a CR2032 type coin cell, Li metal is used as the negative electrode, and a porous polyethylene separator is interposed between them to manufacture an electrode assembly.

After placing the above-mentioned manufactured electrode assembly inside a CR2032 type coin cell case, a lithium secondary battery was manufactured by injecting an electrolyte solution in which a lithium salt is dissolved in a carbonate-based solvent.

The lithium secondary battery manufactured in this manner was aged at room temperature for 10 hours to achieve electrolyte impregnation and electrochemical equilibrium.

The above lithium secondary battery was evaluated using a Toscat-3100 charger/discharger. The oxidation evaluation was conducted in a 25°C constant temperature chamber, and the rate and life evaluations were conducted in a 45°C constant temperature chamber.

The voltage range of the Mars evaluation was 4.3 to 2.5 V, and a current density of 0.1 C was applied for both charge and discharge, followed by a 0.05 C constant voltage section, which was repeated twice in total.

The voltage range for rate and life evaluation was 4.3 to 3 V, and after applying a current density of 0.5 C/1 C for charge/discharge, a constant voltage section identical to that for the chemistry evaluation was applied. After performing the rate evaluation once, the life evaluation was performed 50 times.

The measurement results are shown in Table 3 below.

Referring to Table 3 above, it can be seen that the secondary batteries manufactured with the positive electrode active materials of Examples 1 to 13 have an optimal balance of capacity and life characteristics.

Specifically, when comparing Example 1, which is a direct comparison target, with Comparative Examples 1 and 2, it can be seen that the secondary battery of Example 1 has a slightly lower capacity than Comparative Examples 1 and 2 (0.91 to 1.36% difference), but has significantly higher life characteristics at 50 cycles (7.98 to 9.52%). In other words, the difference in improved life characteristics is about 10 times the difference in reduced capacity. This is also evident in the relationship between Example 3, which is a direct comparison target, and Comparative Examples 3 and 4, and also in the relationship between Example 8, which is a direct comparison target, and Comparative Examples 5 to 7.

The present invention is not limited to the above embodiments, but can be manufactured in various different forms, and a person having ordinary skill in the art to which the present invention pertains will understand that the present invention can be implemented in other specific forms without changing the technical idea or essential features of the present invention. Therefore, it should be understood that the embodiments described above are exemplary in all respects and not restrictive.

Claims (11)

A cathode active material containing a lithium transition metal oxide having a form of secondary particles in which a plurality of primary particles are aggregated,
A cathode active material characterized by a crystal grain size of 85 nm or less as determined by X-ray diffraction analysis and an amount of Ni ²⁺ occupying lithium sites in the crystal structure of 2.5 atomic% or more. In the first paragraph, the lithium transition metal oxide is a cathode active material characterized in that it comprises a composition represented by the following chemical formula 1:
Li _x Ni _1-y M _y D _z O _2-f Q _f (1)
In the above formula,
M is one or more transition metal elements stable in four- or six-coordinate configurations;
D is at least one element selected from alkaline earth metals, transition metals, and non-metals;
Q is one or more anions selected from F, S, and P;
0.3≤x≤1.1, 0≤y≤0.4, 0≤z≤0.1, 0≤f≤0.2. In the second paragraph, the lithium transition metal oxide is a cathode active material characterized in that the Ni content is 80% or more based on the total transition metal. A cathode active material, characterized in that in the second paragraph, D in the chemical formula 1 includes at least one of Al and Zr. A cathode active material, characterized in that in claim 1, the particle surface of the cathode active material is coated with a material containing B. A cathode active material, characterized in that in claim 1, the crystal grain size of the lithium transition metal oxide is 50 nm to 80 nm. In the first paragraph, the lithium transition metal oxide is a cathode active material characterized in that the amount of Ni ²⁺ occupying the lithium site is 2.7 atomic% to 4 atomic%. In the first paragraph, the lithium transition metal oxide is a cathode active material characterized in that the a-axis value is 2.873Å to 2.878Å. A cathode active material according to any one of claims 1, 6 and 7, wherein the lithium transition metal oxide has an R-factor value of 0.55 or more. In claim 9, the lithium transition metal oxide is a cathode active material characterized in that the R-factor value is 0.6 to 0.8. A lithium secondary battery comprising a cathode active material according to any one of claims 1 to 10.