KR102430966B1 - Positive electrode active material and lithium secondary battery comprising the same - Google Patents

Abstract

The present invention relates to a positive electrode active material having improved electrical properties by controlling the aspect ratio gradient of primary particles included in secondary particles, a positive electrode including the positive electrode active material, and a lithium secondary battery using the positive electrode.

Description

A cathode active material and a lithium secondary battery comprising the same

The present invention relates to a positive electrode active material having improved electrical properties by controlling the aspect ratio gradient of primary particles included in secondary particles, a positive electrode including the positive electrode active material, and a lithium secondary battery using the positive electrode.

A battery stores electric power by using a material capable of electrochemical reaction between the anode and the cathode. A representative example of such a battery is a lithium secondary battery that stores electrical energy by a difference in chemical potential when lithium ions are intercalated/deintercalated in a positive electrode and a negative electrode.

The lithium secondary battery is manufactured by using a material capable of reversible intercalation/deintercalation of lithium ions as a positive electrode and a negative electrode active material, and filling an organic electrolyte solution or a polymer electrolyte solution between the positive electrode and the negative electrode.

A lithium composite oxide is used as a cathode active material for a lithium secondary battery. For example, composite oxides such as LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , and LiMnO ₂ have been studied.

Among the cathode active materials, LiCoO ₂ is used the most because of its excellent lifespan characteristics and charge/discharge efficiency, but it is expensive due to the resource limitations of cobalt used as a raw material, so it has a limitation in price competitiveness.

LiMnO ₂ , LiMn ₂ O ₄ Lithium manganese oxide has advantages of excellent thermal stability and low price, but has problems in that it has a small capacity and poor high-temperature characteristics. In addition, the LiNiO ₂ -based positive electrode active material exhibits high discharge capacity battery characteristics, but is difficult to synthesize due to a cation mixing problem between Li and a transition metal, and thus has a large problem in rate characteristics.

In addition, a large amount of Li by-products are generated according to the degree of intensification of such cation mixing, and most of these Li by-products are composed of LiOH and Li ₂ CO ₃ compounds. It causes gas generation due to charging and discharging after manufacturing. Residual Li ₂ CO ₃ increases cell swelling, which not only reduces the cycle, but also causes the battery to swell.

Korean Patent Publication No. 10-2015-0069334

In the lithium secondary battery market, while the growth of lithium secondary batteries for electric vehicles is playing a leading role in the market, the demand for cathode materials used in lithium secondary batteries is also constantly changing.

For example, in the prior art, lithium secondary batteries using LFP have been mainly used from the viewpoint of ensuring safety, but recently, the use of nickel-based lithium composite oxides having a large energy capacity per weight compared to LFP has been expanding.

An object of the present invention is to provide a positive electrode active material having a high energy density and improved lifespan and stability in accordance with such a positive electrode material trend.

In particular, the present invention relates to a cathode active material including primary particles capable of intercalation/deintercalation of lithium and secondary particles in which the primary particles are aggregated, An object of the present invention is to provide a positive electrode active material having improved electrical properties by controlling an aspect ratio gradient or a sphericity gradient.

Another object of the present invention is to provide a positive electrode comprising a positive electrode active material as defined herein.

In addition, another object of the present invention is to provide a lithium secondary battery using the positive electrode as defined herein.

The objects of the present invention are not limited to the above-mentioned objects (eg, for an electric vehicle), and other objects and advantages of the present invention not mentioned can be understood by the following description and can be understood by the embodiments of the present invention. will be understood more clearly. It will also be readily apparent that the objects and advantages of the present invention can be realized by the means and combinations thereof indicated in the claims.

According to one aspect of the present invention, there is provided a cathode active material including primary particles capable of intercalation/deintercalation of lithium and secondary particles in which the primary particles are aggregated.

Here, the primary particles included in the secondary particles may exhibit an aspect ratio gradient that increases from the center of the secondary particles toward the surface of the secondary particles. That is, the average aspect ratio of the primary particles may have a gradient that increases from the center of the secondary particles toward the surface of the secondary particles.

At this time, by controlling the gradient range of the average aspect ratio of the primary particles present in the center of the secondary particles and the average aspect ratio of the primary particles present in the surface portion of the secondary particles, niobium (Nb) in the positive electrode active material and It is possible to minimize deterioration of the lifespan characteristics of the positive electrode active material due to doping with the same metal element and improve other electrochemical characteristics (eg, charging capacity, efficiency, output, etc.).

In particular, according to the present invention, the mixing ratio of the metal dopant to the precursor is increased to increase the content of the metal dopant in the primary particles during the manufacturing process of the positive active material, or the mixture of the precursor and the metal dopant is fired. By specifying the (heat treatment) conditions, the difference between the average aspect ratio of all primary particles in the surface portion of the secondary particles and the average aspect ratio of all primary particles in the center of the secondary particles tends to decrease. .

Through this, the primary particles and the secondary particles included in the positive active material may improve the particle density in the positive active material by satisfying the following conditions. In addition, the electrochemical properties of the positive electrode active material may be improved.

In the positive active material, the distance from the center of the secondary particle to the surface of the secondary particle is R, and the distance from the center of the secondary particle is 4/5R to R, the first region (R _{1 )} ), when a region in which the distance from the center of the secondary particles is 0 to 2/5R is a second region (R ₂ ), the average aspect ratio (W ₁ ) of all primary particles in the first region and the second region It is preferable that the ratio (W ₁ /W ₂ ) of the average aspect ratio (W ₂ ) of all primary particles in region 2 is less than 2.217.

In addition, the ratio (W ₁ /W ₂ ) of the average aspect ratio (W ₁ ) to the average aspect ratio (W ₂ ) may be greater than 1.280.

The average aspect ratio (W ₁ ) of all primary particles in the first region and the average aspect ratio (W ₂ ) of all primary particles in the second region are included in the secondary particles to have a value within the above-described range. When the primary particles exhibit an increasing aspect ratio gradient from the center of the secondary particles toward the surface of the secondary particles, due to asymmetric volume expansion during charging/discharging of a lithium secondary battery manufactured using the positive active material It is possible to improve the electrochemical properties by effectively reducing the local strain.

Further, according to another aspect of the present invention, there is provided a positive electrode comprising the positive electrode active material as defined herein.

In addition, according to another aspect of the present invention, a lithium secondary battery using the positive electrode as defined herein is provided.

The positive electrode active material according to various embodiments of the present invention includes primary particles capable of intercalation/deintercalation of lithium and secondary particles in which the primary particles are aggregated, wherein the first particles included in the secondary particles The secondary particles improve the density of the primary particles in the secondary particles by controlling the range of the aspect ratio gradient increasing from the center of the secondary particles toward the surface of the secondary particles, and at the same time, electrochemical properties of the positive active material It is possible to improve

In particular, when a metal element such as niobium (Nb) is doped in the positive active material, there is a risk that the lifespan characteristics may be deteriorated, but the average aspect ratio of the primary particles present in the center of the secondary particles and the surface of the secondary particles When the gradient range of the average aspect ratio of the primary particles present in the part is adjusted, the deterioration of the lifespan characteristics of the positive electrode active material is minimized and other electrochemical properties (eg, charging capacity, efficiency, output, etc.) are improved. can

In addition, although the primary particles included in the positive electrode active material according to various embodiments of the present invention exhibit an aspect ratio gradient that increases from the center of the secondary particles toward the surface portion of the secondary particles, the size of the aspect ratio gradient is The sphericity of the primary particles present on the surface of the secondary particles may be determined within a range that does not significantly damage. Accordingly, the primary particles present in the surface portion of the secondary particles have a shape close to the shape (eg, spherical) of the primary particles present in the center of the secondary particles rather than a rod shape. will be.

Through this, it is possible to improve the density of the primary particles in the positive active material, and further, partial strain due to asymmetric volume expansion of the positive active material during charging/discharging of a lithium secondary battery using the positive active material. can be effectively reduced. Through this strain reduction, cracks may be reduced after the life of the positive active material. Accordingly, the lifespan and stability of the lithium secondary battery may be improved.

In addition to the above-described effects, the specific effects of the present invention will be described together while describing specific details for carrying out the invention below.

1 is a cross-sectional TEM photograph of a cathode active material according to Example 1 of the present invention, and FIGS. 2 to 5 are partially enlarged photographs of the region indicated in FIG. 1 .
6 is a cross-sectional TEM photograph showing the result of measuring the aspect ratio of primary particles in the surface portion (first region) of the positive electrode active material according to Example 1 of the present invention, and FIG. 7 is a positive electrode according to Example 1 of the present invention. It is a cross-sectional TEM photograph showing the result of measuring the aspect ratio of the primary particles in the center (second region) of the active material.
8 is a cross-sectional TEM photograph of the positive electrode active material according to Comparative Example 1, and FIGS. 9 to 12 are partially enlarged photographs of the region indicated in FIG. 8 .
13 is a cross-sectional TEM photograph showing the result of measuring the aspect ratio of primary particles in the surface portion (first region) of the positive active material according to Comparative Example 1, and FIG. 14 is a central portion (second part) of the positive active material according to Comparative Example 1. It is a cross-sectional TEM photograph showing the result of measuring the aspect ratio of the primary particles in the region).
15 is a cross-sectional TEM photograph of the positive electrode active material according to Comparative Example 2, and FIGS. 16 to 19 are partially enlarged photographs of the region indicated in FIG. 15 .
20 is a cross-sectional TEM photograph showing the result of measuring the aspect ratio of primary particles in the surface portion (first region) of the positive electrode active material according to Comparative Example 2, and FIG. 21 is a central portion of the positive electrode active material according to Comparative Example 2 (second It is a cross-sectional TEM photograph showing the result of measuring the aspect ratio of the primary particles in the region).

In order to better understand the present invention, certain terms are defined herein for convenience. Unless defined otherwise herein, scientific and technical terms used herein shall have the meanings commonly understood by one of ordinary skill in the art. Also, unless the context specifically dictates otherwise, it is to be understood that a term in the singular includes its plural form as well, and a term in the plural form also includes its singular form.

Hereinafter, a lithium secondary battery using a positive electrode active material according to the present invention and a positive electrode including the positive electrode active material will be described in more detail.

cathode active material

According to one aspect of the present invention, there is provided a cathode active material including primary particles capable of intercalation/deintercalation of lithium and secondary particles in which the primary particles are aggregated.

Here, the primary particle means one grain or crystallite, and the secondary particle means an aggregate formed by aggregation of a plurality of primary particles. A void and/or a grain boundary may exist between the primary particles constituting the secondary particles.

In addition, the shape of the primary particle is not particularly limited, but preferably, the primary particle may have a shape satisfying an aspect ratio gradient range to be described later.

In general, in the case of the positive active material represented by the following Chemical Formula 1, as the primary particle has an aspect ratio gradient that increases from the center of the secondary particle toward the surface of the secondary particle, the surface portion of the secondary particle A rod shape may develop toward the Accordingly, while the primary particles having a relatively spherical shape are mainly present in the center of the secondary particles, a relatively rod-shaped (elongated shape extending in one direction) is present on the surface of the secondary particles. ) may be predominantly primary particles having

[Formula 1]

Li _w Ni _1-(x+y+z+z') Co _x M1 _y M2 _z Nb _z' O ₂

(here,

M1 is at least one selected from Mn and Al,

M2 is P, Sr, Ba, B, Ti, Zr, Mn, Al, W, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, At least one selected from Ge, Nd, Gd and Cu,

M1 and M2 are different elements,

0.5≤w≤1.5, 0≤x≤0.50, 0≤y≤0.20, 0≤z≤0.20, 0<z'≤0.20)

However, according to the present invention, by controlling the aspect ratio gradient range of the primary particles in the positive electrode active material, the primary particles present on the surface of the secondary particles are also relatively present in the center of the secondary particles rather than the rod shape. It may have a shape similar to that of the primary particles, that is, a shape that is relatively close to a spherical shape.

More specifically, according to the present invention, the average aspect ratio of the primary particles has a gradient that increases from the center of the secondary particles toward the surface of the secondary particles, and the content of the metal dopant in the primary particles increases. Accordingly, the difference between the average aspect ratio of all primary particles in the surface portion of the secondary particles and the average aspect ratio of all primary particles in the center of the secondary particles may tend to decrease.

Here, the primary particles have an average particle diameter of 0.1 nm to 20 nm, preferably 0.2 nm to 15 nm, more preferably 0.3 nm to 10 nm, thereby manufacturing using the cathode active material according to various embodiments of the present invention. It is possible to realize the optimum density of the anode. In addition, the average particle diameter of the secondary particles may vary depending on the average particle diameter and number of aggregated primary particles, but may be present in the range of 0.1 μm to 25 μm, preferably 2 μm to 20 μm, more preferably 3 μm to 15 μm. have.

In an embodiment, the primary particles included in the secondary particles exhibit an aspect ratio gradient that increases from a central portion of the secondary particles toward a surface portion of the secondary particles.

As used herein, the term 'aspect ratio' refers to a ratio (Length/Width ratio) of a long axis (a axis) and a short axis (Width; c axis) of the primary particle, wherein the long axis is relatively of the primary particle. When indicating the direction of the long region, the short axis indicates the length of the relatively short region of the primary particle. In this case, the short axis may be a direction perpendicular to the long axis. The 'aspect ratio' of the primary particle may be calculated as a ratio of a major axis to a minor axis of the primary particle measured from a cross section of the primary particle.

Accordingly, the overall shape of the primary particle may be determined according to the lengths of the major axis (a-axis) and the minor axis (c-axis). For example, when the aspect ratio, which is the ratio of the major axis to the minor axis, of the primary particles exceeds 5, the shape of the primary particles may be relatively close to a rod shape rather than a spherical shape. On the other hand, as the aspect ratio of the primary particles is closer to 1, the shape of the primary particles will be closer to a spherical shape.

That is, the aspect ratio of the primary particles may be used as an index indicating the sphericity of the primary particles, and it will be understood that the closer the aspect ratio of the primary particles to 1, the greater the sphericity of the primary particles.

In addition, the primary particles present at the center of the secondary particles have a shape close to a spherical shape, and the size of the aspect ratio gradient of the primary particles increasing from the center of the secondary particles toward the surface of the secondary particles is When it is small, it may be understood that the size of the sphericity gradient of the primary particles that decreases from the center of the secondary particles toward the surface of the secondary particles is small.

According to an embodiment of the present invention, the primary particles have an aspect ratio gradient that increases from the center of the secondary particles toward the surface portion of the secondary particles, and the size of the aspect ratio gradient satisfies a specific numerical range to be described later. characterized in that In addition, as the primary particles having the above-described aspect ratio gradient pattern exist radially from the center of the secondary particles, it is possible to effectively relieve strain due to volume expansion of the primary particles during charging/discharging. do. Accordingly, it is possible to improve the lifespan and stability of the lithium secondary battery using the positive electrode active material.

In the embodiment, the primary particles may exhibit an aspect ratio gradient that continuously increases from the center of the secondary particles toward the surface of the secondary particles, but is not necessarily limited thereto.

That is, the aspect ratio of the primary particles increases from the center of the secondary particles toward the surface of the secondary particles, but the aspect ratio gradient of the primary particles continuously or discontinuously according to a method of defining a gradient. may increase.

In this case, the primary particles and the secondary particles included in the positive electrode active material have at least an average aspect ratio of the primary particles present in the center of the secondary particles and the primary particles present in the surface portion of the secondary particles. By controlling the gradient range of the average aspect ratio, deterioration of the lifespan characteristics of the positive active material due to doping of a metal element such as niobium (Nb) in the positive active material is minimized, and at the same time, other electrochemical characteristics (e.g., charging capacity, efficiency) , output, etc.) can be improved.

In particular, according to the present invention, the mixing ratio of the metal dopant to the precursor is increased to increase the content of the metal dopant in the primary particles during the manufacturing process of the positive active material, or the mixture of the precursor and the metal dopant is fired. By specifying the (heat treatment) conditions, it is possible to implement the above-described aspect ratio gradient of the primary particles.

Specifically, the average aspect ratio of the primary particles included in the positive electrode active material has a gradient increasing from the center of the secondary particles toward the surface of the secondary particles, and the content of the metal dopant in the primary particles increases. Accordingly, it is preferable that the difference between the average aspect ratio of all primary particles in the surface portion of the secondary particles and the average aspect ratio of all primary particles in the center of the secondary particles decreases.

That is, it is preferable that the difference in the average aspect ratio of the primary particles in the surface portion of the secondary particles to the average aspect ratio of the primary particles in the center of the secondary particles is not excessively large. On the other hand, during the manufacturing process of the positive electrode active material, the mixing ratio of the metal dopant to the precursor is excessively increased to increase the content of the metal dopant in the primary particles, or the mixture of the precursor and the metal dopant is fired (heat treatment) conditions In this unsuitable case (for example, when the temperature increase rate is excessively fast, the heat treatment temperature is excessively high, or the heat treatment time is excessively long), the average aspect ratio of the primary particles in the center of the secondary particles is 2 The difference in the average aspect ratio of the primary particles in the surface portion of the primary particles may be excessively small. In this case, there is a risk of lowering the reversible efficiency and lifespan characteristics of the positive electrode active material.

Here, the primary particles and the secondary particles included in the positive active material may improve the particle density in the positive active material by satisfying at least the following conditions. Accordingly, the electrochemical properties of the positive active material may be improved.

In the positive active material, the distance from the center of the secondary particle to the surface of the secondary particle is R, and the distance from the center of the secondary particle is 4/5R to R, the first region (R _{1 )} ), a region in which the distance from the center of the secondary particles is 0 to 2/5R is defined as a second region R ₂ .

For example, when the average particle diameter of the secondary particles is 10 μm, the first region represents a region in which the distance from the outermost surface of the secondary particles is 0 to 1 μm, and the second region is the secondary particle size. A region of 0 to 2 μm from the center may be represented.

In this case, the average aspect ratio (W ₁ ) of all the primary particles in the first region may be preferably less than 4.083, more preferably less than 4.0.

When the average aspect ratio (W ₁ ) of all the primary particles in the first region exceeds the numerical range defined above, the aspect ratio of the primary particles present on the surface of the secondary particles compared to the center of the secondary particles is excessively large, and accordingly, the shape of the primary particles present in the surface portion of the secondary particles will be relatively close to the rod shape.

In addition, in order to exceed the numerical range defined above, the aspect ratio of the primary particles present in the surface portion of the secondary particles increases from the center of the secondary particles toward the surface of the secondary particles. The size of the aspect ratio gradient should be excessively large.

In this case, not only it is difficult to improve the density of the primary particles in the positive active material, but also partial strain due to asymmetric volume expansion of the positive active material during charging/discharging of a lithium secondary battery using the positive active material. ) can also be difficult to effectively reduce. That is, the difference in the aspect ratio between the central portion of the secondary particle and the primary particle existing in the surface portion of the secondary particle becomes excessively large, making it difficult to suppress strain due to volume expansion of the primary particle during charging/discharging. The possibility of cracking the cathode active material may increase. This may act as a cause of deterioration of the electrochemical properties and/or stability of the positive electrode active material.

In addition, as the density of the primary particles in the positive active material decreases, the specific surface area of the positive active material may increase. When the specific surface area of the positive electrode active material increases, the possibility of a side reaction between the positive electrode active material and the electrolyte solution in the lithium secondary battery increases, thereby reducing the lifespan and/or stability of the lithium secondary battery.

On the other hand, the average aspect ratio (W ₂ ) of all the primary particles in the second region may be preferably less than 1.842, more preferably less than 1.8.

When the average aspect ratio (W ₂ ) of all primary particles in the second region exceeds the numerical range defined above, since the aspect ratio of the primary particles present in the center of the secondary particles is excessively large, this is It may act as a cause of lowering the density of the primary particles in the active material.

In addition, the ratio (W ₁ /W ₂ ) of the average aspect ratio (W ₁ ) of all the primary particles in the first region to the average aspect ratio (W ₂ ) of all the primary particles in the second region is preferably less than 2.217 . As described above, in the positive active material according to the present invention, the spherical shape of the primary particles from the center of the secondary particles toward the surface of the secondary particles by controlling the aspect ratio gradient of the primary particles included in the secondary particles. It is characterized in that the electrical properties of the positive electrode active material are improved by reducing the degree of decrease.

That is, the ratio (W ₁ /W ₂ ) of the average aspect ratio (W ₁ ) to the average aspect ratio (W ₂ ) is preferably less than 2.217, more preferably less than or equal to 2.2, so that It is possible to reduce the size of the gradient of the aspect ratio of the primary particles that increases toward the surface of the secondary particles, and accordingly, the sphericity of the primary particles decreases from the center of the secondary particles toward the surface of the secondary particles. The size of the gradient can be reduced.

Similarly, when the ratio (W ₁ /W ₂ ) of the average aspect ratio (W ₁ ) and the average aspect ratio (W ₂ ) exceeds 2.217, the central portion of the secondary particle and the surface portion of the secondary particle exist The difference in the aspect ratio of the primary particles may be excessively large, so that the possibility of cracking due to volume expansion of the primary particles during charging/discharging may increase.

Further, the ratio (W ₁ /W ₂ ) of the average aspect ratio (W ₁ ) to the average aspect ratio (W ₂ ) may be at least 1.280, which means that at least the average aspect ratio of the primary particles is a central portion of the secondary particles. It may mean representing a gradient that increases toward the surface of the secondary particles. As such, the primary particles in the secondary particles exhibit an aspect ratio gradient that increases from the center of the secondary particles toward the surface of the secondary particles, and the average aspect ratio (W ₁ ) of all primary particles in the first region. ) and the average aspect ratio (W ₂ ) of the total primary particles in the second region (W ₁ /W ₂ ) The size of the aspect ratio gradient represented by the ratio exists in the range of more than 1.280 to less than 2.217, various trade-offs (trade-offs) -off), the electrical properties of the positive active material present in the relationship may be optimized.

Meanwhile, a ratio of primary particles having an aspect ratio smaller than the average aspect ratio (W ₁ ) among the primary particles in the first region may be 45% or more. That is, the ratio of the primary particles defined above is adjusted by adjusting the ratio of the primary particles having an aspect ratio greater than the average aspect ratio (W ₁ ) among the primary particles in the first region, so that the total number of primary particles in the first region is This is to prevent the average aspect ratio (W ₁ ) from having a value of 4.083 or more.

In addition, the ratio of the average aspect ratio (W ₃ ) to the average aspect ratio (W ₂ ) of the primary particles having an aspect ratio smaller than the average aspect ratio (W ₁ ) among the primary particles in the first region (W ₃ /W ₂ ) may be greater than 0.984 and less than 1.465. That is, in the positive active material having an aspect ratio gradient or a sphericity gradient as intended herein, the aspect ratio of the primary particles present in the first region is increased compared to the aspect ratio of the primary particles present in the second region This means that the primary particles present in the first region may have a relatively spherical shape.

On the other hand, the ratio of the average aspect ratio (W ₄ ) to the average aspect ratio (W ₂ ) of the primary particles having an aspect ratio greater than the average aspect ratio (W ₁ ) among the primary particles in the first region (W ₄ /W ₂ ) may be greater than 1.793 and less than 3.076. This is, in the positive active material having an aspect ratio gradient or a sphericity gradient as intended herein, a large increase compared to the aspect ratio of the primary particles present in the second region among the primary particles present in the first region This means that even a primary particle does not have a complete rod shape with an extremely large aspect ratio.

The primary particles satisfying the various conditions described above may be defined as a lithium composite oxide represented by Formula 1 below.

[Formula 1]

Li _w Ni _1-(x+y+z+z') Co _x M1 _y M2 _z Nb _z' O ₂

(here,

M1 is at least one selected from Mn and Al,

M2 is P, Sr, Ba, B, Ti, Zr, Mn, Al, W, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, At least one selected from Ge, Nd, Gd and Cu,

M1 and M2 are different elements,

0.5≤w≤1.5, 0≤x≤0.50, 0≤y≤0.20, 0≤z≤0.20, 0<z'≤0.20)

At this time, the difference between the average aspect ratio of all primary particles in the surface portion of the secondary particles and the average aspect ratio of all primary particles in the center of the secondary particles, in particular, the average aspect ratio (W ₁ ) and the average aspect ratio ( The ratio (W ₁ /W ₂ ) of W ₂ ) may decrease as the z′ increases. In addition, the ratio (W ₁ /W ₂ ) of the average aspect ratio (W ₁ ) and the average aspect ratio (W ₂ ) is not only z′, but also the precursor of the lithium composite oxide represented by Formula 1 and the metal dopant (Nb containing It can also be reduced by specifying the firing (heat treatment) conditions for the mixture of raw materials).

In addition, M2 and/or niobium (Nb) present in the surface portion of the secondary particles may exhibit a concentration gradient that decreases toward the center of the secondary particles. That is, the direction of the concentration gradient of M2 and/or niobium (Nb) may be a direction from a surface portion of the secondary particle toward a central portion of the secondary particle.

In particular, the lithium ion diffusion path existing in the primary particle is the same as the direction of the concentration gradient of M2 and/or niobium (Nb), that is, from the center of the secondary particle to the secondary particle. It may be formed in a direction toward the surface of the. The lithium ion diffusion path is the same as the direction of the concentration gradient of M2 and/or niobium (Nb) (or the direction from the center of the secondary particle toward the surface of the secondary particle), or at least the center of the secondary particle. It may exist to form an angle within ±40° with respect to an imaginary straight line connecting the surface portions of the secondary particles.

As such, as the lithium ion diffusion path in the primary particle is formed in a direction from the center of the secondary particle toward the surface of the secondary particle, the diffusion of lithium ions in the positive active material can be improved, and further, the positive electrode It can contribute to the improvement of the electrical properties of the active material.

In addition, in another embodiment, the positive active material according to the present invention is at least among the surfaces of the primary particles (eg, the interface between the primary particles) and/or the secondary particles formed by aggregation of the primary particles. It may include a coating layer covering a part.

For example, the coating layer may be present to cover at least a portion of the exposed surface of the primary particle. In particular, the coating layer may be present to cover at least a portion of the exposed surface of the primary particles present in the outermost portion of the secondary particles.

Accordingly, the coating layer may exist as a layer for continuously or discontinuously coating the surface of the primary particles and/or the secondary particles formed by agglomeration of the primary particles. When the coating layer is discontinuous, it may exist in the form of an island.

The coating layer present as described above may contribute to the improvement of physical and electrochemical properties of the positive electrode active material.

In addition, the coating layer may exist in the form of a solid solution that does not form a boundary with the primary particles and/or the secondary particles formed by agglomeration of the primary particles.

The coating layer may include at least one oxide represented by Chemical Formula 2 below. That is, the coating layer may be defined as a region in which an oxide represented by the following Chemical Formula 2 exists.

[Formula 2]

Li _a M3 _b O _c

(here,

M3 is Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, W, Ce, At least one selected from V, Ba, Ta, Sn, Hf, Ce, Gd and Nd,

0≤a≤10, 0≤b≤8, 2≤c≤13)

In addition, the coating layer may have a form in which different kinds of oxides are present in one layer at the same time, or different kinds of oxides represented by Chemical Formula 2 are present in separate layers.

The oxide represented by Formula 2 may be physically and/or chemically bonded to the primary particle represented by Formula 1 above. In addition, the oxide may exist in a state in which a solid solution is formed with the primary particle represented by Chemical Formula 1 above.

The positive active material according to this embodiment includes a coating layer covering at least a portion of the surface of the primary particles (eg, the interface between the primary particles) and/or the secondary particles formed by aggregation of the primary particles By doing so, structural stability can be increased. In addition, when such a positive active material is used in a lithium secondary battery, high temperature storage stability and lifespan characteristics of the positive active material may be improved. In addition, the oxide reduces residual lithium in the positive electrode active material and at the same time acts as a movement path of lithium ions, thereby having an effect on improving the efficiency characteristics of the lithium secondary battery.

In addition, in some cases, the oxide may exist not only in at least a portion of an interface between the primary particles and a surface of the secondary particles, but also in internal pores formed inside the secondary particles.

The oxide is an oxide in which lithium and an element represented by A are complexed, or an oxide of A, wherein the oxide is, for example, Li _a W _b O _c , Li _a Zr _b O _c , Li _a Ti _b O _c , Li _a Ni _b O _c , Li _a B _b O _c , Li _a Co _b O _c , Li _a Al _b O _c , Co _b O _c , Al _b O _c , W _b O _c , Zr _b O _c , Ti _b O It may be _c or B _b O _c , but the above-described examples are merely described for convenience and the oxide defined herein is not limited to the above-described examples.

In another embodiment, the oxide may further include an oxide in which lithium and at least two elements represented by A are complexed, or an oxide in which lithium and at least two elements represented by A are complexed. The oxide in which lithium and at least two elements represented by A are complexed is, for example, Li _a (W/Ti) _b O _c , Li _a (W/Zr) _b O _c , Li _a (W/Ti/Zr ) _b O _c , Li _a (W/Ti/B) _b O _c , etc., but is not necessarily limited thereto.

Here, the oxide may exhibit a concentration gradient that decreases from the surface portion of the secondary particle toward the center of the secondary particle. Accordingly, the concentration of the oxide may decrease from the outermost surface of the secondary particle toward the center of the secondary particle.

As described above, the oxide exhibits a concentration gradient that decreases from the surface portion of the secondary particle toward the center of the secondary particle, thereby effectively reducing the residual lithium present on the surface of the positive electrode active material to the unreacted residual lithium. side reactions can be prevented in advance. In addition, it is possible to prevent the crystallinity from being lowered in the region inside the surface of the positive electrode active material by the oxide. In addition, it is possible to prevent the overall structure of the positive electrode active material from being collapsed by the oxide during the electrochemical reaction.

Additionally, the coating layer includes a first oxide layer including at least one oxide represented by Chemical Formula 2 and at least one oxide represented by Chemical Formula 2, and is different from the oxide included in the first oxide layer. A second oxide layer including an oxide may be included.

For example, the first oxide layer may be present to cover at least a portion of the exposed surfaces of the primary particles present in the outermost portion of the secondary particles, and the second oxide layer is on the first oxide layer. It may be present so as to cover at least a portion of the exposed surface of the primary particle and the surface of the first oxide layer that are not covered by the layer.

lithium secondary battery

According to another aspect of the present invention, a positive electrode including a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector may be provided. Here, the positive active material layer may include the positive active material according to various embodiments of the present invention. Therefore, since the positive active material is the same as described above, a detailed description thereof will be omitted for convenience, and only the remaining components not described above will be described below.

The positive electrode current collector is not particularly limited as long as it has conductivity without causing a chemical change in the battery, and for example, stainless steel, aluminum, nickel, titanium, fired carbon, or carbon, nickel, titanium on the surface of aluminum or stainless steel. , silver or the like surface-treated may be used. In addition, the positive electrode current collector may typically have a thickness of 3 to 500 μm, and may increase the adhesion of the positive electrode active material by forming fine irregularities on the surface of the current collector. For example, it may be used in various forms, such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven body.

The positive electrode active material layer may be prepared by applying a positive electrode slurry composition including a conductive material and optionally a binder together with the positive electrode active material to the positive electrode current collector.

In this case, the positive active material may be included in an amount of 80 to 99 wt%, more specifically, 85 to 98.5 wt%, based on the total weight of the positive active material layer. It may exhibit excellent capacity characteristics when included in the above content range, but is not necessarily limited thereto.

The conductive material is used to impart conductivity to the electrode, and in the configured battery, it can be used without any particular limitation as long as it has electronic conductivity without causing chemical change. Specific examples include graphite such as natural graphite and artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, summer black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskeys such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or a conductive polymer such as a polyphenylene derivative, and the like, and one or a mixture of two or more thereof may be used. The conductive material may be included in an amount of 0.1 to 15 wt% based on the total weight of the positive active material layer.

The binder serves to improve adhesion between the positive active material particles and adhesion between the positive active material and the current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC) ), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and any one of them or a mixture of two or more thereof may be used. The binder may be included in an amount of 0.1 to 15 wt% based on the total weight of the positive active material layer.

The positive electrode may be manufactured according to a conventional positive electrode manufacturing method except for using the above positive electrode active material. Specifically, the positive electrode active material and, optionally, a positive electrode slurry composition prepared by dissolving or dispersing a binder and a conductive material in a solvent may be coated on a positive electrode current collector, and then dried and rolled.

The solvent may be a solvent generally used in the art, dimethyl sulfoxide (DMSO), isopropyl alcohol (isopropyl alcohol), N-methylpyrrolidone (NMP), acetone (acetone) or water and the like, and any one of them or a mixture of two or more thereof may be used. The amount of the solvent used is enough to dissolve or disperse the positive electrode active material, the conductive material and the binder in consideration of the application thickness of the slurry and the production yield, and to have a viscosity capable of exhibiting excellent thickness uniformity when applied for the production of the positive electrode thereafter. do.

Further, in another embodiment, the positive electrode may be prepared by casting the positive electrode slurry composition on a separate support and then laminating a film obtained by peeling the positive electrode slurry composition from the support on a positive electrode current collector.

In addition, according to another aspect of the present invention, an electrochemical device including the above-described positive electrode may be provided. The electrochemical device may specifically be a battery, a capacitor, or the like, and more specifically, a lithium secondary battery.

Specifically, the lithium secondary battery may include a positive electrode, a negative electrode positioned opposite to the positive electrode, and a separator and an electrolyte interposed between the positive electrode and the negative electrode. Here, since the 　 anode is the same as described above, a detailed description will be omitted for convenience, and only the remaining components not described above will be described in detail below.

The lithium secondary battery may optionally further include a battery container accommodating the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member sealing the battery container.

The negative electrode may include a negative electrode current collector and an anode 　 active material layer positioned on the negative electrode current collector.

The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical change in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface. Carbon, nickel, titanium, silver, etc. surface-treated, aluminum-cadmium alloy, etc. may be used. In addition, the negative electrode current collector may have a thickness of typically 3 μm to 500 μm, and similarly to the positive electrode current collector, fine irregularities may be formed on the surface of the current collector to strengthen the bonding force of the negative electrode active material. For example, it may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam body, a nonwoven body, and the like.

The negative electrode active material layer may be prepared by applying a negative electrode slurry composition including a conductive material and optionally a binder along with the negative electrode active material to the negative electrode current collector.

As the anode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metal compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy; metal oxides capable of doping and dedoping lithium, such as SiO _β (0 < β < 2), SnO ₂ , vanadium oxide, and lithium vanadium oxide; Alternatively, a composite including the metallic compound and a carbonaceous material such as a Si-C composite or a Sn-C composite may be used, and any one or a mixture of two or more thereof may be used. In addition, a metal lithium thin film may be used as the negative electrode active material. In addition, as the carbon material, both low crystalline carbon and high crystalline carbon may be used. As low crystalline carbon, soft carbon and hard carbon are representative, and as high crystalline carbon, natural or artificial graphite of amorphous, plate-like, scale-like, spherical or fibrous shape, and Kish graphite (Kish) graphite), pyrolytic carbon, mesophase pitch based carbon fiber, meso-carbon microbeads, liquid crystal pitches (Mesophase pitches), and petroleum and coal tar pitch (petroleum or coal tar pitch) High-temperature calcined carbon such as derived cokes) is a representative example.

The negative active material may be included in an amount of 80 to 99 wt% based on the total weight of the negative electrode and the active material layer.

The binder is a component that assists in bonding between the conductive material, the active material, and the current collector, and may be typically added in an amount of 0.1 to 10 wt% based on the total weight of the negative electrode and the active material layer. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoro and roethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

The conductive material is a component for further improving the conductivity of the negative active material, and may be added in an amount of 10 wt% or less, preferably 5 wt% or less, based on the total weight of the negative electrode active material layer. Such a conductive material is not particularly limited as long as it has conductivity without causing a chemical change in the battery. For example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskeys such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives may be used.

In one embodiment, the negative electrode 　 active material layer is prepared by applying and drying a negative electrode 　 slurry composition prepared by dissolving or dispersing the negative electrode 　 active material, and optionally a binder and a conductive material in a solvent on the negative electrode current collector and drying, or the negative electrode 　 slurry It can be prepared by casting the composition on a separate support and then laminating the film obtained by peeling it off the support on the negative electrode current collector.

In addition, in another embodiment, the negative electrode 　 active material layer is coated with a negative electrode 　 slurry composition prepared by dissolving or dispersing the negative electrode 　 active material, and optionally a binder and a conductive material in a solvent on the negative electrode current collector and drying, or the negative electrode 　 slurry It can also be prepared by casting the composition on a separate support and then laminating a film obtained by peeling it from the support onto a negative electrode current collector.

On the other hand, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a passage for lithium ions to move, and as long as it is used as a separator in a lithium secondary battery, it can be used without any particular limitation, especially for the movement of ions in the electrolyte It is preferable to have a low resistance to and excellent electrolyte moisture content. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, and an ethylene/methacrylate copolymer, or these A laminate structure of two or more layers of may be used. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of high melting point glass fiber, polyethylene terephthalate fiber, etc. may be used. In addition, a coated separator including a ceramic component or a polymer material may be used to secure heat resistance or mechanical strength, and may optionally be used in a single-layer or multi-layer structure.

In addition, examples of the electrolyte used in the present invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes, which can be used in the manufacture of lithium secondary batteries, and are limited to these. it's not going to be

Specifically, the electrolyte may include an organic solvent and a lithium salt.

The organic solvent may be used without particular limitation as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, as the organic solvent, ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, ε-caprolactone; ether-based solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), propylene carbonate, carbonate-based solvents such as PC); alcohol solvents such as ethyl alcohol and isopropyl alcohol; Nitriles, such as R-CN (R is a C2-C20 linear, branched or cyclic hydrocarbon group, and may contain a double bond aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; Or sulfolanes and the like may be used. Among these, a carbonate-based solvent is preferable, and a cyclic carbonate (eg, ethylene carbonate or propylene carbonate, etc.) having high ionic conductivity and high dielectric constant capable of increasing the charge/discharge performance of the battery, and a low-viscosity linear carbonate-based compound ( For example, a mixture of ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate) is more preferable. In this case, when the cyclic carbonate and the chain carbonate are mixed in a volume ratio of about 1:1 to about 1:9, the electrolyte may exhibit excellent performance.

The lithium salt may be used without particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the lithium salt is LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAl0 ₄ , LiAlCl ₄ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN(C ₂ F ₅ SO ₃ ) ₂ , LiN(C ₂ F ₅ SO ₂ ) ₂ , LiN(CF ₃ SO ₂ ) _2. LiCl, LiI, or LiB(C ₂ O ₄ ) ₂ , etc. may be used. The concentration of the lithium salt is preferably used within the range of 0.1 to 2.0 M. When the concentration of the lithium salt is included in the above range, since the electrolyte has an appropriate conductivity and viscosity, excellent electrolyte performance may be exhibited, and lithium ions may move effectively.

In addition to the electrolyte components, the electrolyte includes, for example, haloalkylene carbonate-based compounds such as difluoroethylene carbonate, pyridine, tri Ethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imida One or more additives such as jolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol or aluminum trichloride may be further included. In this case, the additive may be included in an amount of 0.1 to 5 wt% based on the total weight of the electrolyte.

As described above, the lithium secondary battery containing the positive electrode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics and lifespan characteristics, so portable devices such as mobile phones, notebook computers, digital cameras, and hybrid electric vehicles ( It is useful in the field of electric vehicles such as hybrid electric vehicle, HEV).

The external shape of the lithium secondary battery according to the present invention is not particularly limited, but may be a cylindrical shape using a can, a prismatic shape, a pouch type, or a coin type. In addition, the lithium secondary battery may be used not only in a battery cell used as a power source for a small device, but may also be preferably used as a unit cell in a medium or large battery module including a plurality of battery cells.

According to another aspect of the present invention, a battery module including the lithium secondary battery as a unit cell and/or a battery pack including the same may be provided.

The battery module or the battery pack is a power tool (Power Tool); electric vehicles, including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); Alternatively, it may be used as a power source for any one or more medium and large-sized devices in a system for power storage.

Hereinafter, the present invention will be described in more detail through examples. However, these Examples are only for illustrating the present invention, and it will not be construed that the scope of the present invention is limited by these Examples.

Preparation Example 1. Preparation of positive active material

(1) Example 1

A spherical Ni _0.80 Co _0.10 Mn _0.10 (OH) ₂ hydroxide precursor was synthesized by a co-precipitation method. Specifically, in a 90L reactor, 25 wt% of NaOH and 30 wt% of NH ₄ OH were added to a 1.5 M complex transition metal sulfuric acid aqueous solution in which nickel sulfate, cobalt sulfate and manganese sulfate were mixed in a molar ratio of 80:1:1. was put in. The pH in the reactor was maintained at 11.5, and the reactor temperature at this time was maintained at 60° C., and N ₂ , an inert gas, was introduced into the reactor to prevent oxidation of the prepared precursor. After completion of the synthesis stirring, washing and dehydration were performed using a filter press (F/P) equipment, to obtain a Ni _0.8 Co _0.1 Mn _0.1 (OH) ₂ hydroxide precursor.

Then, LiOH and Nb-containing raw materials (Nb ₂ O ₅ ) were added to the synthesized precursor and then calcined to prepare a lithium composite oxide. Specifically, after mixing the precursor with LiOH and Nb-containing raw material (Nb ₂ O ₅ ), maintaining the O ₂ atmosphere in the kiln, raising the temperature to 830° C. at 1° C. per minute for 15 hours, followed by natural cooling to lithium composite oxide got The Nb-containing raw material (Nb ₂ O ₅ ) was mixed so as to be 0.5 mol% of the total composition before firing.

Thereafter, the obtained lithium composite oxide was heated to 700° C. at 2° C. per minute while maintaining an O ₂ atmosphere in the kiln, heat-treated for 20 hours, and then cooled naturally.

(2) Example 2

After mixing the Nb-containing raw material (Nb ₂ O ₅ ) to 1.0 mol% of the total composition before firing, the O ₂ atmosphere was maintained in the firing furnace and the temperature was raised at 1° C. per minute to 860° C. Except for heat treatment, Example A positive electrode active material was prepared in the same manner as in 1 .

(2) Example 3

After synthesizing a spherical Ni _0.94 Co _0.03 Mn _0.03 (OH) ₂ hydroxide precursor by the co-precipitation method, LiOH and Nb-containing raw materials (Nb ₂ O ₅ ) are added thereto, and calcined to lithium A composite oxide was prepared, but the Nb-containing raw material (Nb ₂ O ₅ ) was mixed so that it became 1.0 mol% of the total composition before firing, and then heat-treated by maintaining the O ₂ atmosphere in the firing furnace and raising the temperature to 780° C. per minute at 1° C. Except that, a positive active material was prepared in the same manner as in Example 1.

(2) Comparative Example 1

The cathode active material in the same manner as in Example 1, except that the Nb-containing raw material (Nb ₂ O ₅ ) was not mixed before firing, and the temperature was increased to 1° C. per minute to 810° C. while maintaining the O ₂ atmosphere in the firing furnace. was prepared.

(3) Comparative Example 2

After mixing the Nb-containing raw material (Nb ₂ O ₅ ) to 2.0 mol% of the total composition before firing, maintaining the O ₂ atmosphere in the firing furnace and heating it up to 860° C. A positive electrode active material was prepared in the same manner as in 1 .

(4) Comparative Example 3

After mixing the Nb-containing raw material (Nb ₂ O ₅ ) to 0.5 mol% of the total composition before firing, the O ₂ atmosphere is maintained in the firing furnace and the temperature is raised at 1° C. per minute to 760° C. Except for heat treatment, Example A positive electrode active material was prepared in the same manner as in 3 .

(5) Comparative Example 4

After mixing so that the Nb-containing raw material (Nb ₂ O ₅ ) is 2.0 mol% of the total composition before firing, the O ₂ atmosphere is maintained in the firing furnace and the temperature is raised at 1° C. per minute to 820° C. Except for heat treatment, Example A positive electrode active material was prepared in the same manner as in 3 .

(6) Comparative Example 5

After mixing the synthesized precursor with LiOH and Nb-containing raw material (Nb ₂ O ₅ ) to 0.5 mol% of the total composition, the O ₂ atmosphere was maintained in the kiln, and the temperature was raised to 830° C. at 5° C. per minute and heat-treated for 15 hours. Then, a positive active material was prepared in the same manner as in Example 1, except that the lithium composite oxide was obtained by natural cooling.

(7) Comparative Example 6

After mixing LiOH and Nb-containing raw material (Nb ₂ O ₅ ) in the synthesized precursor to 0.5 mol% of the total composition, the O ₂ atmosphere was maintained in the kiln, and the temperature was increased to 830° C. at 1° C. per minute and heat-treated for 20 hours. Then, a positive active material was prepared in the same manner as in Example 1, except that the lithium composite oxide was obtained by natural cooling.

Preparation Example 2. Preparation of a lithium secondary battery

A positive electrode slurry was prepared by dispersing 94 wt% of each of the positive active material prepared according to Preparation Example 1, 3 wt% of carbon black, and 3 wt% of PVDF binder in 30 g of N-methyl-2 pyrrolidone (NMP). did. The positive electrode slurry was applied and dried on an aluminum (Al) thin film, which is a positive electrode current collector, having a thickness of 15 μm, followed by roll press to prepare a positive electrode. The loading level of the positive electrode was 10 mg/cm ² , and the electrode density was 3.2 g/cm ³ .

For the positive electrode, metallic lithium was used as a counter electrode, and as an electrolyte, ethylene carbonate (EC), ethylmethyl carbonate (EMC) and dimethyl carbonate (DMC) were mixed in a volume ratio of 2:4:4 in a solvent of 1.15. M LiPF ₆ was added.

A battery assembly was formed by interposing a separator made of a porous polyethylene (PE) film between the positive electrode and the negative electrode, and the electrolyte was injected to prepare a lithium secondary battery (coin cell).

Experimental Example 1. TEM analysis of positive electrode active material

After cross-sectioning each of the positive active materials (secondary particles) according to Example 1, Comparative Example 1, and Comparative Example 2 with a cross-section polisher (accelerated voltage 5.0 kV, milling for 4 hours), cross-sectional TEM images were obtained. Here, the average particle diameter of the positive active material is 10 μm, and a region having a distance of 0 to 1 μm from the outermost surface of the positive active material on a cross-sectional TEM photograph is defined as a surface portion (first region), and 0 from the center of the positive active material A region of 2 μm to 2 μm was defined as the central region (second region).

1 is a cross-sectional TEM photograph of a cathode active material according to Example 1 of the present invention, and FIGS. 2 to 5 are partially enlarged photographs of the region indicated in FIG. 1 .

6 is a cross-sectional TEM photograph showing the result of measuring the aspect ratio of primary particles in the surface portion (first region) of the positive electrode active material according to Example 1 of the present invention, and FIG. 7 is a positive electrode according to Example 1 of the present invention. It is a cross-sectional TEM photograph showing the result of measuring the aspect ratio of the primary particles in the center (second region) of the active material.

1 to 7, the positive electrode active material according to Example 1 has an aspect ratio gradient that increases from the center of the secondary particles toward the surface of the secondary particles, and the first region (Section 1, 2 It can be seen that the shape of the primary particles present in the section No. and No. 4) is relatively close to a spherical shape. In particular, looking at the aspect ratio measurement results shown in FIGS. 6 and 7 , the increase in the aspect ratio of the primary particles present in the first region compared to the primary particles present in the second region is not excessively large, so that the first It can be seen that the aspect ratio or sphericity of the primary particles present in the region is larger than that of Comparative Example 1.

8 is a cross-sectional TEM photograph of the positive electrode active material according to Comparative Example 1, and FIGS. 9 to 12 are partially enlarged photographs of the region indicated in FIG. 8 .

13 is a cross-sectional TEM photograph showing the result of measuring the aspect ratio of primary particles in the surface portion (first region) of the positive electrode active material according to Comparative Example 1, and FIG. 14 is a central portion (second portion) of the positive active material according to Comparative Example 1. It is a cross-sectional TEM photograph showing the result of measuring the aspect ratio of the primary particles in the region).

8 to 12 , the positive active material according to Comparative Example 1 has an aspect ratio gradient that increases from the center of the secondary particle toward the surface of the secondary particle, and in particular, the first region (Section 1, It can be seen that the shape of the primary particles present in the second compartment and the fourth compartment) is a rod shape. In addition, looking at the aspect ratio measurement results shown in FIGS. 13 and 14 , the increase in the aspect ratio of the primary particles present in the first region compared to the primary particles present in the second region is excessively large, and the first It can be seen that the aspect ratio or sphericity of the primary particles present in the region is lower than in Example 1.

15 is a cross-sectional TEM photograph of the positive electrode active material according to Comparative Example 2, and FIGS. 16 to 19 are partially enlarged photographs of the region indicated in FIG. 15 .

20 is a cross-sectional TEM photograph showing the result of measuring the aspect ratio of primary particles in the surface portion (first region) of the positive active material according to Comparative Example 2, and FIG. 21 is a central portion (second part) of the positive active material according to Comparative Example 2. It is a cross-sectional TEM photograph showing the result of measuring the aspect ratio of the primary particles in the region).

15 to 19 , the positive active material according to Comparative Example 2 hardly exhibits an aspect ratio gradient that increases from the center of the secondary particle toward the surface of the secondary particle, and the first region (Section 1, It can be seen that the shapes of the primary particles present in the second and fourth compartments are also substantially similar to the primary particles present in the second region (the third compartment). These results can also be confirmed in the aspect ratio measurement results shown in FIGS. 20 and 21 .

Tables 1 and 2 below show the average aspect ratio (W ₁ ) in the first region, the average aspect ratio (W ₂ ) in the second region, and the average of the primary particles in the first region of the positive active material prepared according to Preparation Example. The average aspect ratio (W 3 ) of the primary particles having an aspect ratio smaller than the aspect ratio (W ₁ ) and the average aspect ratio (W ₃ ) of the primary particles having an aspect ratio greater than the average aspect ratio (W ₁ ) among the primary particles in the first region ₄ ) measurement results are shown respectively. The average values are average values of aspect ratios of at least 100 primary particles in each region.

division Example 1 Example 2 Example 3 W ₁ 2.222 1.898 2.775 W ₂ 1.478 1.401 1.642 W ₁ /W ₂ 1.504 1.355 1.690 W ₃ 1.596 1.400 1.998 W ₃ /W ₂ 1.080 0.999 1.217 W ₄ 3.043 2.717 3.897 W ₄ /W ₂ 2.059 1.939 2.373 P 56.2% 58.9% 57.9%

W ₁ : Average aspect ratio of all primary particles in the first region (region in which the distance from the outermost surface of the positive electrode active material is 0 to 4 μm)

W ₂ : Average aspect ratio of all primary particles in the second region (region 0 to 2 μm from the center of the positive electrode active material)

W ₃ : The average aspect ratio of the primary particles having an aspect ratio smaller than W ₁ among the primary particles in the first region

W ₄ : The average aspect ratio of the primary particles having an aspect ratio greater than W ₁ among the primary particles in the first region

P: ratio of primary particles having an aspect ratio smaller than W ₁ among primary particles in the first region

division Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Comparative Example 6 W ₁ 4.083 1.569 3.855 1.912 1.746 1.618 W ₂ 1.842 1.226 1.716 1.512 1.410 1.375 W ₁ /W ₂ 2.217 1.280 2.247 1.265 1.238 1.177 W ₃ 2.699 1.206 2.519 1.471 1.211 1.195 W ₃ /W ₂ 1.465 0.984 1.468 0.973 0.859 0.869 W ₄ 5.666 2.146 5.415 2.711 2.251 2.103 W ₄ /W ₂ 3.076 1.750 3.156 1.793 1.596 1.529 P 44.7% 61.3% 42.9% 64.7% 59.2% 60.5%

Experimental Example 2. Evaluation of battery capacity and lifespan characteristics of lithium secondary batteries

The lithium secondary battery prepared according to Preparation Example 2 was subjected to a charge/discharge experiment using an electrochemical analyzer (Toyo, Toscat-3100) by applying a discharge rate of 25°C, a voltage range of 3.0V to 4.3V, and a discharge rate of 0.2C to 5.0C. By carrying out, the initial charge capacity, the initial discharge capacity, the initial reversible efficiency and rate characteristics were measured.

In addition, the lithium secondary battery manufactured by the above method was charged/discharged 50 times under the conditions of 1C/1C within the driving voltage range of 3.0V to 4.4V at a temperature of 25°C, and then the discharge capacity at the 50th cycle compared to the initial capacity The ratio of (cycle capacity retention rate; capacity retention) was measured.

Among the measured battery capacity and lifespan characteristics, the measurement results for the positive active material having a precursor composition of Ni _0.8 Co _0.1 Mn _0.1 are shown in Tables 3 and 4 below, and the positive electrode active material having a precursor composition of Ni _0.94 Co _0.03 Mn _0.03 Measurement results for is shown in Tables 5 and 6 below.

division initial charge capacity
(mAh/g) Initial discharge capacity
(mAh/g) Initial reversible efficiency
(%) Example 1 224.9 213.6 95.0 Example 2 225.0 213.3 94.8 Comparative Example 1 225.1 210.4 93.5 Comparative Example 2 225.2 211.5 93.9 Comparative Example 5 225.3 211.1 93.7 Comparative Example 6 225.2 210.6 93.5

division Rate Characteristics (%) Life characteristics (%) 0.2C 0.5C 1.0C 1.5C 2.0 5.0C 50cy Example 1 97.2 93.1 89.6 87.7 86.0 81.4 97.3 Example 2 97.3 93.2 89.8 88.0 86.3 81.2 97.2 Comparative Example 1 97.4 93.4 90.1 88.1 86.6 80.6 97.3 Comparative Example 2 97.7 93.8 90.2 88.3 86.7 80.9 92.0 Comparative Example 5 97.6 93.8 90.1 88.9 86.5 80.8 93.3 Comparative Example 6 97.5 93.6 90.1 88.0 86.4 80.3 94.7

Referring to the results of Table 3, Example 1 in which the gradient range of the average aspect ratio of the primary particles present in the center of the secondary particles and the average aspect ratio of the primary particles present in the surface portion of the secondary particles is adjusted And it can be seen that the positive electrode active material according to Example 2 has excellent initial reversible efficiency compared to the positive electrode active materials according to Comparative Examples 1, 2, 5 and 6, respectively.

In addition, referring to the results of Table 4, the positive active materials according to Examples 1 and 2 were positive active materials according to Comparative Examples 2, 5, and 6 in which Nb-containing raw materials were used in the positive active material manufacturing process. In contrast, it can be seen that the Nb-containing raw material exhibits a similar level of lifespan characteristics to the cathode active material according to Comparative Example 1 in which the Nb-containing raw material is not used. That is, the positive active material according to the embodiment of the present invention is doped with a metal element such as niobium (Nb) in the positive active material by adjusting the gradient of the average aspect ratio of the primary particles in the secondary particles, thereby maintaining the capacity while maintaining the lifespan characteristics characteristics can be improved.

division initial charge capacity
(mAh/g) Initial discharge capacity
(mAh/g) Initial reversible efficiency
(%) Example 3 242.8 226.5 93.3 Comparative Example 3 243.7 222.3 91.2 Comparative Example 4 243.3 221.4 91.0

division Rate characteristics (%) Life characteristics (%) 0.2C 0.5C 1.0C 1.5C 2.0 5.0C 50cy Example 3 96.6 91.4 88.5 87.2 86.2 81.9 94.1 Comparative Example 3 96.6 91.5 88.8 87.4 86.3 80.1 89.3 Comparative Example 4 96.8 91.3 88.5 87.1 86.0 80.4 90.6

Overall, the positive active material according to Example 3 uses a precursor having a composition of Ni _0.94 Co _0.03 Mn _0.03 , so it can be seen that the initial charge capacity of the positive active material according to Example 1 is high. However, since the positive active material according to Example 3 had a larger initial irreversible capacity than the positive active material according to Example 1, it was measured that the initial reversible efficiency was low.

On the other hand, referring to the results of Tables 5 and 6, it can be seen that the positive active material according to Example 3 has superior initial reversible efficiency and lifespan characteristics compared to the positive active materials according to Comparative Examples 3 and 4 using precursors of the same composition. can The results are similar to the results of Tables 3 and 4, even if the positive active material according to an embodiment of the present invention is doped with a metal element such as niobium (Nb) in the positive active material, the average aspect ratio of the primary particles in the secondary particles is This is because, by adjusting the gradient, the capacity characteristics can be improved while maintaining the lifespan characteristics.

As mentioned above, although embodiments of the present invention have been described, those of ordinary skill in the art may add, change, delete or add components within the scope that does not depart from the spirit of the present invention described in the claims. The present invention may be variously modified and changed by the present invention, and this will also be included within the scope of the present invention.

Claims (14)

Lithium intercalation/deintercalation possible primary particles and secondary particles in which the primary particles are aggregated,
The average aspect ratio of the primary particles has a gradient that increases from the center of the secondary particles toward the surface of the secondary particles,
As the content of the metal dopant in the primary particles increases, the difference between the average aspect ratio of all primary particles in the surface portion of the secondary particles and the average aspect ratio of all primary particles in the center of the secondary particles decreases,
The distance from the center of the secondary particle to the surface of the secondary particle is R, the distance from the center of the secondary particle is 4/5R to R, the first region (R ₁ ), the secondary particle When a region in which the distance from the center of is 0 to 2/5R is called a second region (R ₂ ),
The ratio (W ₁ /W ₂ ) of the average aspect ratio (W ₁ ) of all the primary particles in the first region to the average aspect ratio (W ₂ ) of all the primary particles in the second region is less than 2.217,
The metal dopant comprises niobium (Nb),
positive electrode active material.
The method of claim 1
wherein the ratio (W ₁ /W ₂ ) of the average aspect ratio (W ₁ ) to the average aspect ratio (W ₂ ) is greater than 1.280,
positive electrode active material.
The method of claim 1
Among the primary particles in the first region, the ratio of the primary particles having an aspect ratio smaller than the average aspect ratio (W ₁ ) is 45% or more,
positive electrode active material.
4. The method of claim 3,
The ratio (W ₃ /W ₂ ) of the average aspect ratio (W ₃ ) of the primary particles having an aspect ratio smaller than the average aspect ratio (W ₁ ) to the average aspect ratio (W ₂ ) among the primary particles in the first region is 0.984 greater than 1.465;
positive electrode active material.
4. The method of claim 3,
The ratio (W ₄ /W ₂ ) of the average aspect ratio (W ₄ ) of the primary particles having an aspect ratio greater than the average aspect ratio (W ₁ ) to the average aspect ratio (W ₂ ) among the primary particles in the first region is 1.793 greater than 3.076;
positive electrode active material.
The method of claim 1
wherein the average aspect ratio (W ₁ ) is less than 4.083;
positive electrode active material.
The method of claim 1
wherein the average aspect ratio (W ₂ ) is less than 1.842,
positive electrode active material.
The method of claim 1,
The primary particles are represented by the following formula (1),
positive electrode active material.
[Formula 1]
Li _w Ni _1-(x+y+z+z') Co _x M1 _y M2 _z Nb _z' O ₂
(here,
M1 is at least one selected from Mn and Al,
M2 is P, Sr, Ba, B, Ti, Zr, Mn, Al, W, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, At least one selected from Ge, Nd, Gd and Cu,
M1 and M2 are different elements,
0.5≤w≤1.5, 0≤x≤0.50, 0≤y≤0.20, 0≤z≤0.20, 0<z'≤0.20)
9. The method of claim 8,
The ratio (W ₁ /W ₂ ) of the average aspect ratio (W ₁ ) to the average aspect ratio (W ₂ ) decreases as the z' increases,
positive electrode active material.
The method of claim 1,
A coating layer covering at least a portion of an area selected from the interface between the primary particles and the surface of the secondary particles,
The coating layer comprises at least one oxide represented by the following Chemical Formula 2,
positive electrode active material.
[Formula 2]
Li _a M3 _b O _c
(where M3 is Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, W , Ce, V, Ba, Ta, Sn, Hf, Ce, Gd, and at least one selected from Nd, 0≤a≤10, 0<b≤8, 2≤c≤13)
A positive electrode comprising the positive active material according to any one of claims 1 to 10.
A lithium secondary battery using the positive electrode according to claim 11. delete delete

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