Positive electrode active material, positive electrode, and lithium secondary battery using the same

Abstract

The present invention relates to a positive electrode active material and a lithium secondary battery using the same, and more specifically to a bimodal type positive electrode active material including a first lithium composite oxide as small particles and a second lithium composite oxide as large particles, in which the shape and crystal structure of the small particles contained in the positive electrode active material are controlled to improve lithium ion transport capacity, and the particle stability of the small particles is increased to mitigate or prevent early deterioration and shortened lifespan of the bimodal type positive electrode active material caused by the small particles, and a lithium secondary battery using the same.

Description

Technical Field

[0001] The present invention relates to a positive electrode active material and a lithium secondary battery using the same. More specifically, the present invention relates to a bimodal type positive electrode active material containing a first lithium composite oxide as small particles and a second lithium composite oxide as large particles, in which the lithium ion transport ability is improved by controlling the shape and crystal structure of the small particles contained in the positive electrode active material, and further, by enhancing the particle stability of the small particles, early deterioration and shortening of the life of the bimodal type positive electrode active material caused by the small particles can be alleviated or prevented. The present invention also relates to a positive electrode and a lithium secondary battery using the same.

Background Art

[0002] A battery stores electric power by using substances capable of electrochemical reactions for a positive electrode and a negative electrode. As a typical example of such a battery, there is a lithium secondary battery that stores electric energy by the difference in chemical potential when lithium ions are intercalated / deintercalated in a positive electrode and a negative electrode.

[0003] The lithium secondary battery is manufactured by using substances capable of reversible intercalation / deintercalation of lithium ions as a positive electrode and a negative electrode active material, and filling an organic electrolyte or a polymer electrolyte between the positive electrode and the negative electrode.

[0004] As the positive electrode active material of the lithium secondary battery, a lithium composite oxide is used. Examples thereof include composite oxides such as LiCoO2, LiMn2O4, LiNiO2, and LiMnO2, which are being studied.

[0005] Among the positive electrode active materials, LiCoO2 is excellent in life characteristics and charge / discharge efficiency and is the most widely used. However, it is expensive due to the resource limitation of cobalt used as a raw material, and thus has a disadvantage of limited price competitiveness.

[0006] Lithium manganese oxides such as LiMnO2 and LiMn2O4 have the advantages of excellent thermal safety and low cost, but have problems such as small capacity and poor high-temperature characteristics. In addition, although LiNiO2-based cathode active materials exhibit battery characteristics with high discharge capacity, synthesis is difficult due to the cation mixing problem between Li and transition metals, and thus there are significant problems with rate characteristics.

[0007] In addition, the lithium composite oxide contained in the cathode active material undergoes volume changes due to the intercalation / deintercalation of lithium ions into / from the lithium composite oxide during charge and discharge. Usually, the lithium composite oxide is in the form of secondary particles aggregated from a plurality of unit particles (primary particles). However, when rapid volume changes of the primary particles occur during charge and discharge, or when stress due to repeated charge and discharge accumulates, there are problems such as the occurrence of cracks in the primary particles and / or within the secondary particles, or the collapse of the crystal structure or a change in the crystal structure (phase transition).

[0008] Such problems ultimately act as a cause for reducing the stability and reliability of the cathode active material. Therefore, various studies have been continuously conducted to alleviate the volume change of the lithium composite oxide during charge and discharge, or to minimize the stress generation due to volume change to prevent particle damage.

[0009] Recently, for the purpose of increasing the capacity of lithium secondary batteries, a bimodal type of cathode active material in which small particles and large particles with different average particle diameters are mixed is often used. When mixing small particles and large particles, by filling the voids between the large particles with small particles having a relatively small average particle diameter, the integration density of the lithium composite oxide in the unit volume can be improved, and the energy density per unit volume can be increased.

[0010] However, such a bimodal-type positive electrode active material contains small particles and large particles with different average particle sizes and particle size distributions. If the stability of either the small particles or the large particles decreases, there is a problem that early deterioration and shortening of the life of the bimodal-type positive electrode active material can be caused.

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0012] Conventionally, in order to alleviate the volume change of the lithium composite oxide constituting the positive electrode active material during charge and discharge, or to minimize the stress generation due to the volume change and prevent damage to the particles, a predetermined gap is provided between adjacent primary particles, thereby dispersing the stress due to the volume change of the primary particles. However, such a lithium composite oxide has a limitation in that the energy density per unit volume is low.

[0013] In the case of a bimodal-type positive electrode active material, since the stress applied to the small particles and the large particles by repeated charge and discharge and the resulting particle volume change levels are different, it can be said that the balance of particle stability between the small particles and the large particles is important.

[0014] In order to enhance the particle stability of the small particles among the bimodal-type positive electrode active materials, it is possible to consider reducing the number of unit particles constituting the small particles compared to the number of unit particles constituting the large particles.

[0015] The fewer the number of unit particles, the greater the influence on the electrochemical properties and particle stability of each unit particle. Therefore, if only the number of unit particles constituting the small particles is reduced without considering the electrochemical properties and particle stability of the unit particles constituting the small particles, such small particles merely serve to fill the gaps between the large particles, or rather act as a cause for the early deterioration and shortened lifespan of the bimodal-type positive electrode active material.

[0016] Under such various circumstances, the inventors of the present invention confirmed that not only does the lithium intercalation / deintercalation efficiency of the small particles vary significantly depending on the shape and crystal structure of the small particles among the bimodal-type positive electrode active materials, but also the degree of polarization phenomenon changes during charge and discharge.

[0017] Accordingly, the present invention provides a bimodal-type positive electrode active material including a first lithium composite oxide as small particles and a second lithium composite oxide as large particles, and by controlling the shape and crystal structure of the small particles contained in the positive electrode active material, the lithium ion transport ability is improved, and through this, the transport and diffusion ability of lithium ions within, between, and between the small particles and the large particles is enhanced to alleviate or prevent an unnecessary increase in resistance of the positive electrode active material.

[0018] Furthermore, the present invention provides a bimodal-type positive electrode active material including a first lithium composite oxide as small particles and a second lithium composite oxide as large particles, and by controlling the shape and crystal structure of the small particles contained in the positive electrode active material, the particle stability of the small particles is enhanced to alleviate or prevent the occurrence of cracks, the collapse of the crystal structure, and / or the change in the crystal structure (phase transition) within the small particles.

[0019] Another object of the present invention is to provide a positive electrode including the positive electrode active material defined in the present application.

[0020] Another object of the present invention is to provide a lithium secondary battery using a positive electrode defined in the present application.

[0021] The object of the present invention is not limited to the objects mentioned above (for example, for electric vehicles), and other objects and advantages of the present invention not mentioned can be understood from the following description and can be more clearly understood from the embodiments of the present invention. Also, it can be easily known that the objects and advantages of the present invention can be realized by the means shown in the claims and combinations thereof.

Means for Solving the Problems

[0022] According to one aspect of the present invention, a bimodal type positive electrode active material containing a first lithium composite oxide as small particles and a second lithium composite oxide as large particles is provided.

[0023] Here, the first lithium composite oxide and the second lithium composite oxide each independently contain at least one unit particle, and the number of unit particles constituting the first lithium composite oxide is made smaller than that of the second lithium composite oxide, whereby the lithium intercalation / deintercalation efficiency by the first lithium composite oxide can be improved to the same or a similar level as that of the second lithium composite oxide.

[0024] In one embodiment, the first lithium composite oxide may exist as a single non-aggregated unit particle, and the second lithium composite oxide may exist as a secondary particle in a form in which a plurality of unit particles are aggregated.

[0025] At this time, by setting the ratio of the major axis length r1 to the minor axis length r2 of the unit particles constituting the first lithium composite oxide within the range of 1.3 to 2.1, the lithium ion transport ability by the first lithium composite oxide can be improved.

[0026] Further, by setting the ratio of the major axis length r1 to the minor axis length r2 of the unit particles constituting the first lithium composite oxide within the range of 1.3 to 2.1, the polarization phenomenon within the unit particles during charge and discharge can be reduced, thereby alleviating or preventing the occurrence of cracks within the unit particles.

[0027] In one embodiment, a lithium ion diffusion path may be formed along the major axis direction of the unit particles within the unit particles constituting the first lithium composite oxide.

[0028] At this time, the major axis length r1 of the unit particles constituting the first lithium composite oxide may correspond to the major axis length of the first crystal plane corresponding to the (003) plane in the crystal structure of the unit particles, and the minor axis length r2 of the unit particles constituting the first lithium composite oxide may correspond to the minor axis length perpendicular to the major axis length r1 of the first crystal plane. At this time, a lithium ion diffusion path may be formed along the major axis direction of the first crystal plane within the unit particles constituting the first lithium composite oxide.

[0029] The lithium ion diffusion path formed within the unit particles constituting the first lithium composite oxide can be directed to other crystal planes where lithium ion release is relatively easy by being formed along the major axis direction of the (003) plane, instead of being directed to the (003) plane where lithium ion release is relatively blocked.

[0030] At this time, when the major axis length r1 of the first crystal plane becomes excessively long, the diffusion ability of lithium ions within the unit particles decreases as the lithium ion diffusion path within the unit particles becomes excessively long, and further, the resistance of the unit particles increases.

[0031] The first lithium composite oxide and the second lithium composite oxide may each independently be represented by the following Chemical Formula 1.

[0032] [Chemical Formula 1] Li w Ni 1-(x+y+z) Cox M1 y M2 z O2 (Here, M1 is at least one selected from Mn and Al, M2 is at least one selected from P, Sr, Ba, B, Ce, Cr, Mn, Mo, Na, Ca, K, Ti, Zr, Al, Hf, Ta, Mg, V, Zn, Si, Y, Sn, Ge, Nb, W, and Cu, M1 and M2 are different from each other, 0.5 ≦ w ≦ 1.5, 0 ≦ x ≦ 0.20, 0 ≦ y ≦ 0.20, and 0 ≦ z ≦ 0.20.)

[0033] Furthermore, it further includes a first coating layer that covers at least a part of the surface of the unit particles constituting the first lithium composite oxide, and the first coating layer may include at least one metal oxide represented by the following Chemical Formula 2.

[0034] [Chemical Formula 2] Li a A b O c (Here, A is at least one selected from Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, W, V, Ba, Ta, Sn, Hf, Ce, Gd, and Nd, 0 ≦ a ≦ 10, 0 ≦ b ≦ 8, 2 ≦ c ≦ 15, and a and b are not both 0 at the same time.)

[0035] Furthermore, it further includes a second coating layer that covers at least a part of the surface of the unit particles constituting the second lithium composite oxide, and the second coating layer may include at least one metal oxide represented by the following Chemical Formula 2.

[0036] [Chemical Formula 2] Li a A b O c (Here, A is at least one selected from Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, W, V, Ba, Ta, Sn, Hf, Ce, Gd, and Nd, 0 ≦ a ≦ 10, 0 ≦ b ≦ 8, 2 ≦ c ≦ 15, and a and b are not both 0 at the same time.)

[0037] The metal oxides present in the first coating layer and the second coating layer may be the same as or different from each other.

[0038] Also, according to another aspect of the present invention, a positive electrode including the positive electrode active material defined in the present application is provided.

[0039] Also, according to still another aspect of the present invention, a lithium secondary battery using the positive electrode defined in the present application is provided.

Advantages of the Invention

[0040] The positive electrode active materials according to various embodiments of the present invention are bimodal-type positive electrode active materials including a first lithium composite oxide as small particles having different average particle diameters from each other and a second lithium composite oxide as large particles, and by filling the voids between the large particles with small particles having a relatively small average particle diameter, the integration density of the lithium composite oxide in a unit volume is improved, and the energy density per unit volume can be increased.

[0041] Also, according to the present invention, by controlling the shape and crystal structure of the small particles contained in the bimodal-type positive electrode active material, the lithium ion transport ability is improved, and through this, the transport and diffusion ability of lithium ions within the small particles, between the small particles, and between the small particles and the large particles can be enhanced, and an unnecessary increase in resistance of the positive electrode active material can be alleviated or prevented.

[0042] In addition, according to the present invention, by controlling the shape and crystal structure of the small particles contained in the bimodal type cathode active material, the particle stability of the small particles can be enhanced, and the generation of cracks, the collapse of the crystal structure, and / or the change (phase transition) of the crystal structure within the small particles can be alleviated or prevented.

[0043] Together with the above-described effects, the specific effects of the present invention will be described while explaining the specific matters for implementing the following invention.

Mode for Carrying Out the Invention

[0044] For easier understanding of the present invention, for convenience, specific terms are defined in this application. Unless otherwise specifically defined in this application, the scientific and technical terms used in the present invention have meanings generally understood by those having ordinary knowledge in the relevant technical field. Also, unless otherwise specified in the context, terms in the singular form are to be understood as including their plural forms, and terms in the plural form are to be understood as including their singular forms.

[0045] Hereinafter, the cathode active material according to the present invention and the lithium secondary battery including the cathode active material will be described in more detail.

[0046] Positive electrode active material According to one aspect of the present invention, there is provided a bimodal type cathode active material including a first lithium composite oxide as small particles and a second lithium composite oxide as large particles.

[0047] Here, the first lithium composite oxide and the second lithium composite oxide each independently contain at least one unit particle, and by making the number of unit particles constituting the first lithium composite oxide less than that of the second lithium composite oxide, it is possible to improve the lithium intercalation / deintercalation efficiency of the first lithium composite oxide to a level equivalent to or similar to that of the second lithium composite oxide. The term "unit particle" used in the present application is used in the same meaning as primary particle.

[0048] In the present application, as the small particles, a lithium composite oxide having an average particle size (D50) of 8.5 μm or less, preferably 2.0 μm to 6.0 μm, more preferably 2.5 μm to 5.0 μm can be used.

[0049] Also, in the present application, the large particles mean a lithium composite oxide having an average particle size (D50) exceeding 8.5 μm, preferably 10 μm to 18 μm, more preferably 12 μm to 16 μm.

[0050] In one embodiment, the first lithium composite oxide may exist as a non-aggregated single unit particle, and the second lithium composite oxide may exist as a secondary particle in a form in which a plurality of unit particles are aggregated.

[0051] At this time, the average particle size of the unit particles constituting the second lithium composite oxide is 0.1 μm to 5.0 μm. At this time, the average particle size of the unit particles constituting the second lithium composite oxide may be smaller than the average particle size of the first lithium composite oxide which is the unit particle itself.

[0052] In the bimodal type cathode active material according to various embodiments of the present invention, when the weight% of the first lithium composite oxide is w1 and the weight% of the second lithium composite oxide is w2, w2 / w1 may be in the range of 1.5 to 9.0.

[0053] The first lithium composite oxide may be present in a form filled in the voids between the second lithium composite oxides, may adhere to the surface of the second lithium composite oxide, or may exist in a form in which the first lithium composite oxides aggregate together.

[0054] In the positive electrode active material, when the proportion of the first lithium composite oxide is excessively small compared to the second lithium composite oxide, it is difficult for the first lithium composite oxide to be sufficiently filled in the voids formed by the second lithium composite oxide.

[0055] On the other hand, in the positive electrode active material, when the proportion of the first lithium composite oxide is excessively large compared to the second lithium composite oxide, the proportion of aggregation of the first lithium composite oxides in the positive electrode active material increases, which may reduce the stability of the positive electrode active material, such as a decrease in the press density of the positive electrode active material.

[0056] The first lithium composite oxide and the second lithium composite oxide are each independently represented by the following Chemical Formula 1.

[0057] [Chemical Formula 1] Li w Ni 1-(x+y+z) Co x M1 y M2 z O2 (Here, M1 is at least one selected from Mn and Al, M2 is at least one selected from P, Sr, Ba, B, Ce, Cr, Mn, Mo, Na, Ca, K, Ti, Zr, Al, Hf, Ta, Mg, V, Zn, Si, Y, Sn, Ge, Nb, W, and Cu, M1 and M2 are different from each other, and 0.5 ≦ w ≦ 1.5, 0 ≦ x ≦ 0.20, 0 ≦ y ≦ 0.20, 0 ≦ z ≦ 0.20.)

[0058] The molar ratio of nickel calculated by the following Formula 1 for the first lithium composite oxide and the second lithium composite oxide may be 0.6 or more, preferably 0.8 or more.

[0059] [Formula 1] Ni (molar ratio) = Ni (mol%) / (Ni (mol%) + Co (mol%) + M1 (mol%) + M2 (mol%))

[0060] Further, the molar ratio of cobalt calculated by the following formula 2 for the first lithium composite oxide and the second lithium composite oxide may be 0.2 or less, preferably 0.15 or less.

[0061] [Formula 2] Co (molar ratio) = Co (mol%) / (Ni (mol%) + Co (mol%) + M1 (mol%) + M2 (mol%))

[0062] Generally, in a lithium composite oxide containing at least nickel and cobalt, it is known that the higher the content of Ni, the more likely it is to cause structural instability of the lithium composite oxide due to Li / Ni cation mixing. Also, in a lithium composite oxide containing at least Ni and Co, it has been reported that the lower the content of Co, the higher the initial overvoltage (resistance), and thus a decrease in rate performance is inevitable.

[0063] However, the first lithium composite oxide and the second lithium composite oxide contained in the positive electrode active material according to various embodiments of the present invention can improve the structural stability and rate performance of high-Ni type or high-Ni / low-Co type lithium composite oxides by having a coating layer formed on at least a part of the surface or having a concentration gradient in which the concentration of metal elements decreases from the surface portion toward the central portion.

[0064] In one embodiment, by setting the ratio of the major axis length r1 to the minor axis length r2 of the unit particles constituting the first lithium composite oxide within the range of 1.3 to 2.1, preferably 1.5 to 2.0, the lithium ion transport ability by the first lithium composite oxide can be improved.

[0065] Further, by setting the ratio of the major axis length r1 to the minor axis length r2 of the unit particles constituting the first lithium composite oxide within the range of 1.3 to 2.1, the polarization phenomenon within the unit particles during charge and discharge can be reduced, thereby alleviating or preventing the occurrence of cracks within the unit particles.

[0066] Within the unit particles constituting the first lithium composite oxide, a lithium ion diffusion path along the major axis direction of the unit particles may be formed for lithium ions to be transported / diffused. The lithium ion diffusion path refers to the main one-dimensional and / or two-dimensional paths for lithium ions to be transported / diffused within the primary particles by the vacancy hopping mechanism.

[0067] At this time, the major axis length r1 of the unit particles constituting the first lithium composite oxide corresponds to the major axis length of the first crystal plane corresponding to the (003) plane in the crystal structure of the unit particles, and the minor axis length r2 of the unit particles constituting the first lithium composite oxide corresponds to the minor axis length perpendicular to the major axis length r1 of the first crystal plane.

[0068] The crystal structure of the unit particles and the types of crystal planes included in the crystal structure can be confirmed through X-ray diffraction analysis using Cu-Kα rays for the unit particles.

[0069] That is, within the unit particles constituting the first lithium composite oxide, a lithium ion diffusion path may be formed along the major axis direction of the first crystal plane. Thereby, the lithium ion diffusion path formed within the unit particles constituting the first lithium composite oxide can be directed along the major axis direction of the (003) plane, rather than pointing to the (003) plane where the release of lithium ions is relatively blocked, so that it can point to other crystal planes where the release of lithium ions is relatively easy.

[0070] Within the unit particles constituting the first lithium composite oxide, there may be at least one second crystal plane different from the (003) plane. At this time, the lithium ion diffusion path may be formed so as to be directed to the second crystal plane. The second crystal plane to which the lithium ion diffusion path is directed may be the (012) plane and / or the (104) plane, or may be another crystal plane not defined in the present application. However, in this case, it is preferable that the second crystal plane is a crystal plane in which the release of lithium ions is relatively less blocked compared to the (003) plane.

[0071] That the ratio of the major axis length r1 to the minor axis length r2 of the unit particles constituting the first lithium composite oxide is greater than 2.1 means that the major axis length r1 of the first crystal plane is excessively long or the minor axis length r2 of the first crystal plane is excessively short.

[0072] When the major axis length of the first crystal plane within the unit particles becomes excessively long, the diffusion efficiency of lithium ions within the unit particles decreases, and further, the resistance of the unit particles increases. In addition, the possibility of the occurrence of a polarization phenomenon within the unit particles becomes high. As the polarization phenomenon within the unit particles deepens, the possibility of the occurrence of cracks within the unit particles can increase.

[0073] When the minor axis length of the first crystal plane within the unit particles becomes excessively short, the number of lithium ion diffusion paths within the unit particles is insufficiently formed, so that not only the amount of lithium ions transported and diffused per unit length decreases, but also the amount of lithium ions released to the outside of the unit particles decreases.

[0074] On the other hand, that the ratio of the major axis length r1 to the minor axis length r2 of the unit particles constituting the first lithium composite oxide is less than 1.3 means that the major axis length r1 of the first crystal plane is excessively short or the minor axis length r2 of the first crystal plane is excessively long.

[0075] When the major axis length of the first crystal plane within the unit particle is excessively short or the minor axis length of the first crystal plane within the unit particle is excessively long, the length of the lithium ion diffusion path within the unit particle is not sufficiently formed, resulting in a decrease in the diffusion efficiency of lithium ions within the unit particle, or it is difficult to achieve a sufficient press density of the bimodal type cathode active material using the small particles.

[0076] In order to predict the shape and crystal structure of the unit particles constituting the first lithium composite oxide, the peak intensity for any crystal plane obtained by X-ray diffraction analysis using Cu-Kα rays for the unit particles can be used. For example, when any crystal plane in the crystal structure of the unit particles develops, the intensity of the diffraction peak attributed to that crystal plane becomes stronger.

[0077] Therefore, the shape and crystal structure of the unit particles can be predicted through the ratio of the peak intensities of the (003) plane where lithium ion release is relatively blocked and the (012) plane and (104) plane where lithium ion release is relatively easy in the crystal structure of the unit particles.

[0078] The ratio of the peak intensities attributed to the (003) plane and (012) plane obtained by X-ray diffraction analysis using Cu-Kα rays for the unit particles constituting the first lithium composite oxide defined in the present application can satisfy the following relational expression 1.

[0079] [Relational Expression 1] 0.090 ≦ I(012) / I(003) ≦ 0.120

[0080] Also, preferably, I(012) / I(003) calculated by relational expression 1 may be 0.095 or more and 0.118 or less.

[0081] The ratio of the peak intensities attributed to the (003) plane and (104) plane obtained by X-ray diffraction analysis using Cu-Kα rays for the unit particles constituting the first lithium composite oxide can satisfy the following relational expression 2.

[0082] [Relational Expression 2] 0.420 ≦ I(104) / I(003) ≦ 0.540

[0083] Also, preferably, I(104) / I(003) calculated by Relational Expression 2 may be 0.450 or more and 0.535 or less.

[0084] The greater the degree of development of the (003) plane compared to the (012) plane or the (104) plane in the crystal structure of the unit particle, the smaller I(012) / I(003) calculated by Relational Expression 1 or I(104) / I(003) calculated by Relational Expression 2. On the other hand, the greater the degree of development of the (012) plane or the (104) plane compared to the (003) plane in the crystal structure of the unit particle, the greater I(012) / I(003) calculated by Relational Expression 1 or I(104) / I(003) calculated by Relational Expression 2.

[0085] When the (003) plane is excessively developed compared to the (012) plane or the (104) plane in the crystal structure of the unit particle, the diffusion efficiency of lithium ions in the unit particle decreases, and further, the resistance of the unit particle increases. Also, the possibility of the occurrence of the polarization phenomenon in the unit particle becomes high.

[0086] On the other hand, when the (012) plane or the (104) plane is developed more than necessary compared to the (003) plane in the crystal structure of the unit particle, the length of the lithium ion diffusion path in the unit particle is not sufficiently formed, so that the diffusion efficiency of lithium ions in the unit particle decreases, or it is difficult to realize a sufficient press density of the bimodal - type positive electrode active material using the small particles.

[0087] It further includes a first coating layer covering at least a part of the surface of the unit particle constituting the first lithium composite oxide, and the first coating layer may include at least one metal oxide represented by the following Chemical Formula 2.

[0088] [Chemical Formula 2] Lia A b O c (Here, A is at least one selected from Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, W, V, Ba, Ta, Sn, Hf, Ce, Gd, and Nd, 0 ≦ a ≦ 10, 0 ≦ b ≦ 8, 2 ≦ c ≦ 15, and a and b are not both 0 at the same time)

[0089] At this time, there may be a concentration gradient in which the concentration of element A in Chemical Formula 2 decreases from the surface portion to the central portion of the first lithium composite oxide (unit particle) existing as non-aggregated single unit particles. The concentration gradient of element A existing within the unit particle can appear due to the doping and diffusion of element A constituting the metal oxide represented by Chemical Formula 2 within the unit particle.

[0090] Furthermore, it further includes a second coating layer that covers at least a part of the surface of the unit particles constituting the second lithium composite oxide, and the second coating layer may include at least one metal oxide represented by the following Chemical Formula 2.

[0091] [Chemical Formula 2] Li a A b O c (Here, A is at least one selected from Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, W, V, Ba, Ta, Sn, Hf, Ce, Gd, and Nd, 0 ≦ a ≦ 10, 0 ≦ b ≦ 8, 2 ≦ c ≦ 15, and a and b are not both 0 at the same time)

[0092] At this time, at least a part of the unit particles constituting the second lithium composite oxide may have a concentration gradient in which the concentration of element A of Chemical Formula 2 decreases from the surface portion to the central portion of the unit particle. The concentration gradient of element A present in the unit particle can appear by doping and diffusing element A constituting the metal oxide represented by Chemical Formula 2 into the unit particle.

[0093] Further, the metal oxide may exist in a state of being diffused along the grain boundary defined between adjacent unit particles from the surface portion to the central portion of the second lithium composite oxide existing as secondary particles in a form in which a plurality of unit particles are aggregated.

[0094] The concentration of the metal oxide can exhibit a concentration gradient that decreases from the surface portion of the secondary particle toward the central portion of the secondary particle.

[0095] As described above, the first lithium composite oxide and the second lithium composite oxide contained in the bimodal type cathode active material defined in the present application have a coating layer formed on at least a part of the surface, or a concentration gradient in which the concentration of the metal element decreases from the surface portion to the central portion, thereby improving the structural stability and rate characteristics of the high-Ni type or high-Ni / low-Co type lithium composite oxide.

[0096] The concentration of any element in the first lithium composite oxide and the second lithium composite oxide can be measured by various known methods. For example, after cross-sectionally treating the first lithium composite oxide and the second lithium composite oxide respectively, the concentration of the target element can be analyzed by the line scanning method through EDS mapping for the concentration of the target element. In this case, the change in the concentration of the target element can be confirmed in the direction from the surface portion to the central portion of the first lithium composite oxide and the second lithium composite oxide.

[0097] Also, the acceleration voltage V of the electron beam irradiated onto the surfaces of the first lithium composite oxide and the second lithium composite oxide acc There is an Energy Profiling-Energy Dispersive X-ray Spectroscopy (EP-EDS) method for measuring the concentration of a target element accumulated up to a specific depth at which the electron beam of each acceleration voltage penetrates from the surface of the lithium composite oxide by changing

[0098] For example, after separately selecting the first lithium composite oxide (small particles) and the second lithium composite oxide (large particles) contained in the positive electrode active material according to the present application, the acceleration voltage of the electron beam irradiated onto the surfaces of the selected first lithium composite oxide (small particles) and the second lithium composite oxide (large particles) is changed from 1 kV to 30 kV (1 kV, 3 kV, 5 kV, 7.5 kV, 10 kV, 12.5 kV, 15 kV, 20 kV, 30 kV), and the cumulative concentration (at%) of the transition metal up to a specific depth at which the electron beam of each acceleration voltage penetrates from the surfaces of the first lithium composite oxide (small particles) and the second lithium composite oxide (large particles) can be analyzed.

[0099] For example, when the surfaces of the first lithium composite oxide and the second lithium composite oxide are irradiated with an electron beam having an acceleration voltage of 10 kV and the electron beam can penetrate from the surfaces of the first lithium composite oxide and the second lithium composite oxide to a depth of about 300 nm, the concentration of the target element present in the region at a depth of 300 nm from the surfaces of the first lithium composite oxide and the second lithium composite oxide can be measured through the EP-EDS analysis. Also, when the surfaces of the first lithium composite oxide and the second lithium composite oxide are irradiated with an electron beam having an acceleration voltage of 20 kV and the electron beam can penetrate from the surfaces of the first lithium composite oxide and the second lithium composite oxide to a depth of about 800 nm, the concentration of the target element present in the region at a depth of 800 nm from the surfaces of the first lithium composite oxide and the second lithium composite oxide can be measured through the EP-EDS analysis.

[0100] That is, when the cumulative concentration of the target element measured decreases as the acceleration voltage of the electron beam irradiated on the surfaces of the first lithium composite oxide and the second lithium composite oxide increases, it can be interpreted that the concentration of the target element has a gradient of decreasing from the surface portion to the central portion of the first lithium composite oxide and the second lithium composite oxide.

[0101] Lithium secondary battery According to still 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 can be provided. Here, the positive electrode active material layer may include a positive electrode active material according to various embodiments of the present invention. Therefore, since the positive electrode active material is the same as those described above, for the sake of convenience, specific descriptions are omitted, and hereinafter, only the remaining configurations not described above will be described.

[0102] The positive electrode current collector is not particularly limited as long as it has conductivity without inducing a chemical change in the battery. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or a material obtained by surface treatment of the surface of aluminum or stainless steel with carbon, nickel, titanium, silver, etc. may be used. Further, the positive electrode current collector may usually have a thickness of 3 to 500 μm, and fine irregularities may be formed on the surface of the current collector to enhance the adhesion of the positive 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, and a non-woven fabric.

[0103] The positive electrode active material layer may be manufactured by applying a positive electrode slurry composition containing a conductive material and optionally a binder together with the positive electrode active material to the positive electrode current collector.

[0104] At this time, the positive electrode active material may be contained in an amount of 80 to 99 wt%, more specifically 85 to 98.5 wt%, based on the total weight of the positive electrode active material layer. When included in this content range, excellent capacity characteristics can be exhibited, but it is not necessarily limited thereto.

[0105] The conductive material is used to impart conductivity to the electrode, and in the battery being configured, it can be used without particular limitation as long as it has electron conductivity without causing chemical changes. Specific examples include graphite such as natural graphite and artificial graphite, carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon-based substances such as carbon fiber, metal powders or metal fibers such as copper, nickel, aluminum, silver, conductive whiskers such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, or conductive polymers such as polyphenylene derivatives. Among these, one kind alone or a mixture of two or more kinds may be used. The conductive material may be contained in an amount of 0.1 to 15% by weight based on the total weight of the positive electrode active material layer.

[0106] The binder plays a role in improving the adhesion between the positive electrode active material particles and the adhesive force between the positive electrode 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, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene - propylene - diene polymer (EPDM), sulfonated - EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof. Among these, one kind alone or a mixture of two or more kinds may be used. The binder may be contained in an amount of 0.1 to 15% by weight based on the total weight of the positive electrode active material layer.

[0107] The positive electrode may be manufactured by a normal positive electrode manufacturing method, except for using the positive electrode active material. Specifically, after applying a positive electrode slurry composition, which is prepared by dissolving or dispersing the positive electrode active material and optionally a binder and a conductive material in a solvent, onto a positive electrode current collector, it may be manufactured by drying and rolling.

[0108] The solvent may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and one of these alone or a mixture of two or more thereof may be used. The amount of the solvent used is sufficient as long as it can dissolve or disperse the positive electrode active material, the conductive material, and the binder and give a viscosity that can exhibit excellent thickness uniformity during coating for positive electrode manufacturing, taking into account the coating thickness of the slurry and the manufacturing yield.

[0109] In another embodiment, the positive electrode may be manufactured by casting the positive electrode slurry composition onto a separate support and then laminating the film obtained by peeling it from the support onto a positive electrode current collector.

[0110] According to still another aspect of the present invention, an electrochemical device including the above-described positive electrode may be provided. Specifically, the electrochemical device may be a battery, a capacitor, etc., and more specifically, it may be a lithium secondary battery.

[0111] The lithium secondary battery may specifically 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 positive electrode is as described above, for the sake of convenience, a specific description thereof is omitted, and hereinafter, only the remaining configurations not described above will be specifically described.

[0112] The lithium secondary battery may further selectively include a battery container that houses the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

[0113] The negative electrode may include a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0114] The negative electrode current collector is not particularly limited as long as it has high conductivity without inducing a chemical change in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, those surface-treated with carbon, nickel, titanium, silver, etc. on the surface of copper or stainless steel, aluminum-cadmium alloy, etc. may be used. Further, the negative electrode current collector may usually have a thickness of 3 μm to 500 μm, and similar to the positive electrode current collector, fine irregularities may be formed on the surface of the current collector to strengthen the binding force of the negative electrode active material. For example, it may be used in various forms such as films, sheets, foils, nets, porous bodies, foams, non-woven fabrics, etc.

[0115] The negative electrode active material layer may be manufactured by applying a negative electrode slurry composition containing the negative electrode active material, a conductive material, and optionally a binder, to the negative electrode current collector.

[0116] As the negative electrode 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 fibers, amorphous carbon, etc., metal compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys, SiO β(0 < β < 2), metal oxides such as SnO2, vanadium oxides, lithium vanadium oxides that can be doped and de-doped with lithium, or composites containing the metal compound and a carbonaceous material such as a Si-C composite or a Sn-C composite, etc. may be mentioned, and a mixture of any one or two or more of these may be used. Further, a thin film of metallic lithium may be used as the negative electrode active material. Further, as the carbon material, all of low-crystalline carbon and highly crystalline carbon may be used. Representative examples of low-crystalline carbon are soft carbon and hard carbon, and representative examples of highly crystalline carbon are amorphous, plate-like, flaky, spherical or fibrous natural graphite or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch derived cokes.

[0117] The negative electrode active material may be contained at 80 to 99 wt% based on the total weight of the negative electrode active material layer.

[0118] The binder is usually added at 0.1 to 10 wt% based on the total weight of the negative electrode active material layer as a component that assists in the bonding between the conductive material, the active material, and the current collector. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0119] The conductive material may be added as a component for further improving the conductivity of the negative electrode active material, in an amount of 10% by weight or less, preferably 5% by weight 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 inducing a chemical change in the battery. For example, graphite such as natural graphite and artificial graphite, carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, conductive fibers such as carbon fibers and metal fibers, metal powders such as carbon fluoride, aluminum, and nickel powder, conductive whiskers such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, and conductive materials such as polyphenylene derivatives may be used.

[0120] In one embodiment, the negative electrode active material layer is manufactured by applying and drying a negative electrode slurry composition prepared by dissolving or dispersing a negative electrode active material, and optionally a binder and a conductive material, in a solvent on a negative electrode current collector, or by casting the negative electrode slurry composition on a separate support and then laminating the film obtained by peeling the support on the negative electrode current collector.

[0121] Also, in another embodiment, the negative electrode active material layer is manufactured by applying and drying a negative electrode slurry composition prepared by dissolving or dispersing a negative electrode active material, and optionally a binder and a conductive material, in a solvent on a negative electrode current collector, or by casting the negative electrode slurry composition on a separate support and then laminating the film obtained by peeling the support on the negative electrode current collector.

[0122] On the one hand, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a migration path for lithium ions. Generally, any separator that can be used in a lithium secondary battery can be used without particular limitation, and it is particularly preferable that it has low resistance to the ion migration of the electrolyte and excellent electrolyte moisture retention ability. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as a polyethylene homopolymer, a polypropylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof may be used. Also, a normal porous non-woven fabric, for example, a non-woven fabric made of high-melting glass fibers, polyethylene terephthalate fibers, etc. may be used. Further, in order to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer substance may be used, and it may be selectively used as a single-layer or multi-layer structure.

[0123] In addition, examples of the electrolyte used in the present invention include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used during the manufacture of lithium secondary batteries.

[0124] Specifically, the electrolyte may contain an organic solvent and a lithium salt.

[0125] As the organic solvent, any solvent may be used without particular limitation as long as it serves as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether and tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; carbonate solvents such as dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (where R is a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may contain a double bond aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes. Among these, carbonate solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant, which can enhance the charge and discharge performance of the battery, and a linear carbonate compound having low viscosity (e.g., ethylmethylcarbonate, dimethylcarbonate, or diethylcarbonate) is more preferred. In this case, when the cyclic carbonate and the chain carbonate are mixed and used at a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte can be excellent.

[0126] 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 may be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3), LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2, etc. 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 within the above range, the electrolyte has appropriate conductivity and viscosity, so it can exhibit excellent electrolyte performance and lithium ions can move effectively.

[0127] In addition to the above-described electrolyte constituent components, the electrolyte may further contain one or more additives such as haloalkylene carbonate-based compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride for the purpose of improving the life characteristics of the battery, suppressing the decrease in battery capacity, and improving the discharge capacity of the battery. At this time, the additive may be contained in an amount of 0.1 to 5% by weight based on the total weight of the electrolyte.

[0128] 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 life characteristics, and thus is useful in portable devices such as mobile phones, notebook personal computers, digital cameras, and the field of electric vehicles such as hybrid electric vehicles (HEV).

[0129] The outer shape of the lithium secondary battery according to the present invention is not particularly limited, and may be, for example, a cylindrical shape, a square shape, a pouch shape, a coin shape, etc. using a can. Further, the lithium secondary battery can be used not only for a battery cell used as a power source for a small device, but also preferably used as a unit cell for a medium- to large-sized battery module including a plurality of battery cells.

[0130] According to still 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 can be provided.

[0131] The battery module or the battery pack can be used as a power source for any one or more medium- to large-sized devices, such as a power tool, an electric vehicle (including an electric vehicle (EV), a hybrid electric vehicle, and a plug-in hybrid electric vehicle (PHEV)), or a power storage system.

[0132] Hereinafter, the present invention will be described in more detail by way of examples. However, these examples are for illustrative purposes only and are not to be construed as limiting the scope of the present invention.

[0133] Production Example 1. Production of bimodal type positive electrode active material Example 1 (a) Using a known co-precipitation method using nickel sulfate, cobalt sulfate, and aluminum sulfate, NiCoAl(OH)2 hydroxide precursors of a first lithium composite oxide as small particles and a second lithium composite oxide as large particles designed at an atomic ratio of Ni:Co:Al = 95:4:1 (at%) were synthesized respectively.

[0134] The synthesized NiCoAl(OH)2 hydroxide precursor was heated to 400 °C at a rate of 2 °C per minute and calcined at 400 °C for 6 hours to convert it into an oxide precursor.

[0135] The average particle size (D50) of the oxide precursor (first oxide precursor) of the first lithium composite oxide was 3.0 μm, and the average particle size (D50) of the oxide precursor (second oxide precursor) of the second lithium composite oxide was 15.0 μm.

[0136] (b) After weighing so that the weight ratio of the first oxide precursor and the second oxide precursor produced in step (a) was 30:70, LiOH (Li / (Ni+Co+Al) mol ratio = 1.05) and NaOH (Na / (Ni+Co+Al) mol ratio = 0.005) were added and mixed. Then, while maintaining an O2 atmosphere in a firing furnace, the temperature was raised to 850 °C at a rate of 2 °C per minute and heat-treated for 12 hours to obtain an intermediate product.

[0137] (c) Distilled water was added to the intermediate product, and an aqueous cobalt sulfate solution of 7.0 wt% was added while maintaining the cobalt from the aqueous cobalt sulfate solution at 3.0 mol% with respect to the metal elements (Ni+Co+Al) excluding lithium in the intermediate product, and the mixture was stirred at 300 rpm for 1 hour to coat the surfaces of the first oxide precursor and the second oxide precursor in the intermediate product with cobalt. After completion of the reaction, it was dried at 120 °C for 12 hours.

[0138] (d) While maintaining an O2 atmosphere in a firing furnace, the intermediate product was heated to 700 °C at a rate of 2 °C per minute and heat-treated at 700 °C for 12 hours to obtain a bimodal type cathode active material in which the first lithium composite oxide as small particles and the second lithium composite oxide as large particles were mixed at a predetermined ratio.

[0139] It was confirmed that in the cathode active material obtained in step (d), the first lithium composite oxide existed as non-aggregated single unit particles, and the second lithium composite oxide existed as secondary particles in which a plurality of unit particles were aggregated.

[0140] Example 2 The positive electrode active material was obtained in the same manner as in Example 1, except that KOH (K / (Ni + Co + Al) mol ratio = 0.005) was used instead of NaOH in step (b).

[0141] It was confirmed that the positive electrode active material obtained in Example 2 was in a bimodal form in which small particles existing as single unit particles in a non-aggregated form and large particles existing as secondary particles in a form in which a plurality of unit particles were aggregated were mixed, the same as in Example 1.

[0142] Example 3 The positive electrode active material was obtained in the same manner as in Example 1, except that Ca(OH)2 (Ca / (Ni + Co + Al) mol ratio = 0.005) was used instead of NaOH in step (b).

[0143] It was confirmed that the positive electrode active material obtained in Example 3 was in a bimodal form in which small particles existing as single unit particles in a non-aggregated form and large particles existing as secondary particles in a form in which a plurality of unit particles were aggregated were mixed, the same as in Example 1.

[0144] Example 4 The positive electrode active material was obtained in the same manner as in Example 1, except that NaOH (Na / (Ni + Co + Al) mol ratio = 0.003) and Zr(OH)4 (Zr / (Ni + Co + Al) mol ratio = 0.002) were mixed and used in step (b).

[0145] It was confirmed that the positive electrode active material obtained in Example 4 was in a bimodal form in which small particles existing as single unit particles in a non-aggregated form and large particles existing as secondary particles in a form in which a plurality of unit particles were aggregated were mixed, the same as in Example 1.

[0146] Example 5 The positive electrode active material was obtained in the same manner as in Example 1, except that in step (b), KOH (K / (Ni + Co + Al) mol ratio = 0.003) and Zr(OH)4 (Zr / (Ni + Co + Al) mol ratio = 0.002) were mixed and used instead of NaOH.

[0147] It was confirmed that the positive electrode active material obtained in Example 5 was in the same bimodal form as in Example 1, in which small particles existing as single unit particles in a non-aggregated form and large particles existing as secondary particles in a form in which a plurality of unit particles were aggregated were mixed.

[0148] Example 6 The positive electrode active material was obtained in the same manner as in Example 1, except that in step (b), Ca(OH)2 (Ca / (Ni + Co + Al) mol ratio = 0.003) and Zr(OH)4 (Zr / (Ni + Co + Al) mol ratio = 0.002) were mixed and used instead of NaOH.

[0149] It was confirmed that the positive electrode active material obtained in Example 6 was in the same bimodal form as in Example 1, in which small particles existing as single unit particles in a non-aggregated form and large particles existing as secondary particles in a form in which a plurality of unit particles were aggregated were mixed.

[0150] Comparative Example 1 The positive electrode active material was obtained in the same manner as in Example 1, except that in step (b), Zr(OH)4 (Zr / (Ni + Co + Al) mol ratio = 0.005) was used instead of NaOH.

[0151] It was confirmed that the positive electrode active material obtained in Comparative Example 1 was in the same bimodal form as in Example 1, in which small particles existing as single unit particles in a non-aggregated form and large particles existing as secondary particles in a form in which a plurality of unit particles were aggregated were mixed.

[0152] Comparative Example 2 A positive electrode active material was obtained in the same manner as in Example 1, except that NaOH was not used in step (b).

[0153] It was confirmed that both the small particles and the large particles contained in the positive electrode active material obtained in Comparative Example 2 exist as secondary particles in a form in which a plurality of unit particles are aggregated.

[0154] Comparative Example 3 A positive electrode active material was obtained in the same manner as in Example 1, except that the content of NaOH used in step (b) was 0.003 mol% (Na / (Ni + Co + Al) mol ratio = 0.003).

[0155] It was confirmed that the positive electrode active material obtained in Comparative Example 3 has a bimodal form in which small particles existing as single non-aggregated unit particles and large particles existing as secondary particles in a form in which a plurality of unit particles are aggregated are mixed, the same as in Example 1.

[0156] Comparative Example 4 A positive electrode active material was obtained in the same manner as in Example 1, except that the content of NaOH used in step (b) was 0.003 mol% (Na / (Ni + Co + Al) mol ratio = 0.003), and after mixing the intermediate product and NaOH (Na / (Ni + Co + Al) mol ratio = 0.002) in step (d), heat treatment was performed.

[0157] It was confirmed that the positive electrode active material obtained in Comparative Example 4 has a bimodal form in which small particles existing as single non-aggregated unit particles and large particles existing as secondary particles in a form in which a plurality of unit particles are aggregated are mixed, the same as in Example 1.

[0158] Production Example 2. Production of lithium secondary battery 92 wt% of the cathode active material produced by Production Example 1, 4 wt% of artificial graphite, and 4 wt% of a PVDF binder were dispersed in 30 g of N-methyl-2-pyrrolidone (NMP) to produce a cathode slurry. The cathode slurry was uniformly coated on an aluminum thin film with a thickness of 15 μm and vacuum dried at 135°C to produce a cathode for a lithium secondary battery.

[0159] Using a lithium foil as a counter electrode with respect to the cathode, a porous polyethylene film (Celgard 2300, thickness: 25 μm) as a separator, and an electrolytic solution in which LiPF6 was present at a concentration of 1.15 M in a solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 3:7, a coin cell was produced.

[0160] Experimental Example 1. EP-EDS analysis of small particles / large particles In Production Example 1, a step of forming a metal oxide containing cobalt on at least a part of the surface of the unit particles constituting the first lithium composite oxide (small particles) and the second lithium composite oxide (large particles) was performed.

[0161] Thereby, EP-EDS analysis was carried out to measure the cobalt content in the first lithium composite oxide (small particles) and the second lithium composite oxide (large particles) contained in the cathode active material produced by Production Example 1.

[0162] For the EP-EDS analysis, after separately selecting the first lithium composite oxide (small particles) and the second lithium composite oxide (large particles) contained in the cathode active materials of Example 1, Example 3, and Example 5, the acceleration voltage of the electron beam irradiated on the surfaces of the selected first lithium composite oxide (small particles) and the second lithium composite oxide (large particles) was changed from 1 kV to 30 kV (1 kV, 3 kV, 5 kV, 7.5 kV, 10 kV, 12.5 kV, 15 kV, 22.5 kV, 30 kV), and the cumulative concentration (at%) of cobalt up to a specific depth penetrated by the electron beam at each acceleration voltage was analyzed.

[0163] The absolute value of the slope of the cobalt concentration gradient measured in the region where the acceleration voltage of the electron beam irradiated on the surface of the first lithium composite oxide (small particles) ranges from 1 kV to 30 kV is referred to as s1, and the absolute value of the slope of the cobalt concentration gradient measured in the region where the acceleration voltage of the electron beam irradiated on the surface of the second lithium composite oxide (large particles) ranges from 1 kV to 30 kV is referred to as s2.

[0164] S1 and s2 indicating the slope of the cobalt concentration gradient were calculated with the change amount of cobalt concentration (△at%) on the y-axis and the change amount of EDS acceleration voltage (△kV%) on the x-axis, and were the average values calculated by summing up the slopes of each measurement region. The EP-EDS analysis results are shown in Table 1 below.

[0165]

Table 1

[0166] Referring to the results in Table 1 above, it can be confirmed that there is a concentration gradient in which the cobalt concentration decreases from the surface portion to the central portion of the first lithium composite oxide (small particles) existing as single unit particles in a non-aggregated form. The above results are due to the cobalt coated on the surface of the first oxide precursor of the first lithium composite oxide (small particles) being doped and diffused into the unit particles at stage (c) of Production Example 1.

[0167] Also, referring to the results in Table 1 above, it can be confirmed that there is a concentration gradient in which the cobalt concentration decreases from the surface portion to the central portion of the second lithium composite oxide (large particles) existing as secondary particles in a form in which a plurality of unit particles are aggregated. The above results are due to the cobalt coated on the surface of the second oxide precursor of the second lithium composite oxide (large particles) at stage (c) of Production Example 1 existing in a state where the cobalt-containing metal oxide is diffused along the grain boundaries defined between adjacent unit particles from the surface portion to the central portion of the secondary particles.

[0168] Experimental Example 2. SEM analysis of the first lithium composite oxide (small particles) After separately selecting the first lithium composite oxide (small particles) and the second lithium composite oxide (large particles) contained in the positive electrode active material produced according to Production Example 1, FE-SEM (Bruker) was used to take cross-sectional SEM images of the selected first lithium composite oxide (small particles) and the second lithium composite oxide (large particles), and then the average particle sizes of the first lithium composite oxide (small particles) and the second lithium composite oxide (large particles) were measured.

[0169] [Table 2]

[0170] Referring to the results of Comparative Example 2 described in Table 1 above, it can be confirmed that even when the small particles exist as secondary particles in a form in which a plurality of unit particles are aggregated, the average particle size is almost similar to those of other Examples and Comparative Examples.

[0171] Next, the ratio of the major axis length r1 to the minor axis length r2 of the first lithium composite oxide (unit particles) existing as a single non-aggregated unit particle was calculated from the cross-sectional SEM image. Table 3 below shows the average value of r1 / r2 calculated from 100 small particles selected from each positive electrode active material.

[0172] [Table 3]

[0173] Experimental Example 3. XRD analysis of the first lithium composite oxide (small particles) After selecting the first lithium composite oxide (small particles) contained in the positive electrode active material produced according to Production Example 1, X-ray diffraction (XRD) analysis was performed on the first lithium composite oxide to detect peaks attributed to crystal planes contained in the crystal structure of the first lithium composite oxide.

[0174] XRD analysis was performed using a Bruker D8 ENDEAVOR diffractometer with Cu-Kα radiation (1.540598 Å), and the intensity ratios between the peaks attributed to specific crystal planes are shown in Table 4 below.

[0175]

Table 4

[0176] The change in the ratio between the peaks attributed to each crystal plane can be used as an indicator to confirm the presence or absence of growth of any crystal plane. For example, the ratio of the (104) / (003) peak intensities can be used as an indicator to predict the degree of growth of the (104) plane compared to the (003) plane or the degree of growth of the (003) plane compared to the (104) plane.

[0177] Referring to the results in Table 4 above, it can be confirmed that the ratios of the (104) / (003) peak intensities and the (012) / (003) peak intensities measured from the first lithium composite oxide (small particles) contained in the cathode active materials according to Comparative Examples 1 to 3 are smaller than those measured from the first lithium composite oxide (small particles) contained in the cathode active materials according to Examples 1 to 6. The above results are due to the fact that the growth of the (104) plane and the (012) plane is induced compared to the (003) plane of the small particles contained in the cathode active materials according to Examples 1 to 6 compared to the small particles contained in the cathode active materials according to Comparative Examples 1 to 3.

[0178] When considering the ratio of the major axis length r1 to the minor axis length r2 described in Table 3 above, it can be confirmed that there is a proportional relationship between the degree of growth of the (003) plane and r1 / r2. In this case, when the growth of the (104) plane and the (012) plane is induced compared to the growth of the (003) plane, r1 / r2 has a relatively small value.

[0179] Therefore, referring to the results in Table 3 and Table 4 above, it can be predicted that the major axis direction of the small particles included in the positive electrode active materials according to Examples 1 to 6 corresponds to the major axis direction of the (003) plane. Since the lithium ion diffusion path within the small particles is formed along the major axis direction of the (003) plane (i.e., formed along a direction parallel to the major axis direction of the (003) plane), the lithium ion diffusion path within the small particles is formed so as to be directed to the (104) plane and / or the (012) plane where lithium ions can be relatively easily released.

[0180] Experimental Example 4. Evaluation of physical properties of positive electrode active material (1) After weighing 3 g each of the respective positive electrode active materials produced according to Production Example 1 using a pelletizer, they were pressed at 4.5 tons for 5 seconds, and then the press density was measured.

[0181] (2) With respect to the lithium secondary battery (coin cell) produced in Production Example 2, after performing 50 charge / discharge cycles under the condition of 1C / 1C within the driving voltage range of 25 °C and 3.0 V to 4.25 V, the positive electrode was separated, and after recovering the positive electrode active material from the separated positive electrode, a cross-sectional SEM image was taken. In order to quantify the crack area in the cross-sectional SEM image, the outer contours of a plurality of particles confirmed from the (binary) cross-sectional SEM image were set, the dark regions within the outer contours were regarded as cracks, and the ratio of the area of the dark regions to the total area within the outer contours was defined as the crack occurrence rate (%).

[0182] The measurement results are shown in Table 5 below.

[0183]

Table 5

[0184] Referring to the results in Table 5 above, it can be confirmed that although the press densities of the positive electrode active materials according to Examples 1 to 6 and the positive electrode active materials according to Comparative Examples 1 to 4 are almost similar, the crack occurrence rate of the positive electrode active materials according to Examples 1 to 6 has decreased.

[0185] The cracks generated in the positive electrode active material due to repeated charge and discharge are likely to be caused by the polarization phenomenon induced during charge and discharge. As the internal particle polarization phenomenon increases, stress accumulates against the volume change of the particles due to repeated charge and discharge, resulting in the generation of internal cracks in the particles, which may consequently cause premature degradation and shortened lifespan of the positive electrode active material.

[0186] Experimental Example 5. Evaluation of electrochemical characteristics of lithium secondary battery (1) For the lithium secondary battery (coin cell) manufactured in Production Example 2, an electrochemical analyzer (Toyo, Toscat-3100) was used to measure the C-rate efficiency of 5.0C / 0.1C through a charge-discharge experiment at 25°C with a discharge rate applied in the voltage range of 3.0V to 4.3V and 0.1C to 5.0C.

[0187] (2) Also, for the same lithium secondary battery, after performing 50 charge-discharges under the condition of 1C / 1C within the driving voltage range of 25°C and 3.0V to 4.4V, the ratio of the discharge capacity at the 50th cycle to the initial capacity (cycle capacity retention) was measured.

[0188] (3) Also, for the same lithium secondary battery, after charging for 1 cycle at 1C condition (SOC 100%) at 25°C and then performing 50 charge-discharges under the conditions of DC-IR and 1C / 1C, the DC-IR at the charged state (SOC 100%) was measured to confirm the increase in DC-IR before and after the lifespan.

[0189] The measurement results are shown in Table 6 below.

[0190]

Table 6

[0191] Referring to the results in Table 6 above, it can be confirmed that the lithium secondary batteries using the positive electrode active materials according to Examples 1 to 6 and the lithium secondary batteries using the positive electrode active materials according to Comparative Examples 1 to 4 exhibit substantially similar charge-discharge efficiencies.

[0192] However, it can be confirmed that the DC-IR before and after the life of the lithium secondary battery using the cathode active material according to Examples 1 to 6 is lower than that of the lithium secondary battery using the cathode active material according to Comparative Examples 1 to 4.

[0193] The above results conform to the gist of the present invention, which aims to improve the lithium ion transport / diffusion ability by controlling the shape and crystal structure of the small particles contained in the bimodal type cathode active material and thereby prevent an unnecessary increase in resistance.

[0194] In addition, it can be confirmed that the ratio of the discharge capacity at the 50th cycle (cycle capacity retention rate) of the lithium secondary battery using the cathode active material according to Examples 1 to 6 is also higher than that of the lithium secondary battery using the cathode active material according to Comparative Examples 1 to 4.

[0195] There can be various factors affecting the cycle capacity retention rate of the lithium secondary battery. However, when comprehensively considering the results in Tables 5 and 6, it can be predicted that the above results are due to the fact that the crack generation of the cathode active material according to Examples 1 to 6 is suppressed compared to the cathode active material according to Comparative Examples 1 to 4, and as a result, the early deterioration and shortening of the life of the cathode active material are suppressed.

[0196] As described above, the embodiments of the present invention have been explained. However, those with ordinary knowledge in the technical field can variously modify and change the present invention by adding, changing, deleting, or adding components without departing from the idea of the present invention described in the claims, and it can also be said that this is included within the scope of the rights of the present invention.

Claims

1. A bimodal type cathode active material comprising a first lithium composite oxide as small particles and a second lithium composite oxide as large particles, wherein the first lithium composite oxide and the second lithium composite oxide each independently contain at least one unit particle, the number of unit particles constituting the first lithium composite oxide is less than that of the second lithium composite oxide, the ratio of the major axis length r1 to the minor axis length r2 of the unit particles constituting the first lithium composite oxide is 1.3 to 2.1, the major axis length r1 of the unit particles constituting the first lithium composite oxide corresponds to the major axis length of the first crystal plane corresponding to the (003) plane in the crystal structure of the unit particles, the minor axis length r2 of the unit particles constituting the first lithium composite oxide corresponds to the minor axis length perpendicular to the major axis length r1 of the first crystal plane, within the unit particles constituting the first lithium composite oxide, a lithium ion diffusion path is formed along the major axis direction of the first crystal plane, the ratio of the peak intensities attributed to the (003) plane and the (104) plane obtained by X-ray diffraction analysis using Cu-Kα rays for the unit particles constituting the first lithium composite oxide satisfies the following relational expression 2, [Relational expression 2] 0.420 ≤ I(104) / I(003) ≤ 0.540 Cathode active material.

2. the first lithium composite oxide exists as a single non-aggregated unit particle, the second lithium composite oxide exists as secondary particles in a form in which a plurality of unit particles are aggregated, the cathode active material according to Claim 1.

3. within the unit particles constituting the first lithium composite oxide, a lithium ion diffusion path is formed along the major axis direction of the unit particles, the cathode active material according to Claim 1.

4. within the unit particles constituting the first lithium composite oxide, there is at least one second crystal plane different from the (003) plane, the lithium ion diffusion path is formed so as to direct to the second crystal plane, the cathode active material according to Claim 1.

5. the second crystal plane includes at least one crystal plane selected from the (012) plane and the (104) plane, the cathode active material according to Claim 4.

6. the first lithium composite oxide and the second lithium composite oxide are each independently represented by the following chemical formula 1, the cathode active material according to Claim 1. [Chemical formula 1] Li ｗ Ni １－（ｘ＋ｙ＋ｚ） Co ｘ M1 ｙ M2 ｚ O ２ (wherein, M1 is at least one selected from Mn and Al, M2 is at least one selected from P, Sr, Ba, B, Ce, Cr, Mn, Mo, Na, Ca, K, Ti, Zr, Al, Hf, Ta, Mg, V, Zn, Si, Y, Sn, Ge, Nb, W, and Cu, M1 and M2 are different from each other, 0.5 ≦ w ≦ 1.5, 0 ≦ x ≦ 0.20, 0 ≦ y ≦ 0.20, 0 ≦ z ≦ 0.20)

7. The cathode active material according to claim 6, wherein a molar ratio of nickel calculated by the following formula (1) with respect to the first lithium composite oxide and the second lithium composite oxide is 0.6 or more. [Formula (1)] Ni (molar ratio) = Ni (mol%) / (Ni (mol%) + Co (mol%) + M1 (mol%) + M2 (mol%))

8. The cathode active material according to claim 1, wherein an average particle diameter of the first lithium composite oxide is 2.5 μm to 5.0 μm.

9. The cathode active material according to claim 1, wherein an average particle diameter of the second lithium composite oxide is 10 μm to 18 μm.

10. The cathode active material according to claim 9, wherein an average particle diameter of the unit particles constituting the second lithium composite oxide is 0.1 μm to 5.0 μm.

11. The cathode active material according to claim 1, wherein a ratio of peak intensities attributed to (003) plane and (012) plane obtained by X-ray diffraction analysis using Cu-Kα ray with respect to the unit particles constituting the first lithium composite oxide satisfies the following relational expression (1). [Relational expression (1)] 0.090 ≦ I(012) / I(003) ≦ 0.120

12. The cathode active material according to claim 1, further including a first coating layer covering at least a part of a surface of the unit particles constituting the first lithium composite oxide, The first coating layer includes at least one metal oxide represented by the following chemical formula (2). [Chemical formula (2)] Li ａ A ｂ O ｃ (where A is at least one selected from Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, W, V, Ba, Ta, Sn, Hf, Ce, Gd, and Nd, 0 ≦ a ≦ 10, 0 ≦ b ≦ 8, 2 ≦ c ≦ 15, and a and b are not 0 at the same time)

13. The positive electrode active material according to claim 12, wherein there is a concentration gradient in which the concentration of element A of Chemical Formula 2 decreases from the surface portion of the unit particle toward the center portion of the unit particle.

14. Further comprising a second coating layer covering at least a part of the surface of the unit particle constituting the second lithium composite oxide, The positive electrode active material according to claim 1, wherein the second coating layer contains at least one metal oxide represented by the following Chemical Formula 2. [Chemical Formula 2] Li ａ A ｂ O ｃ (where A is at least one selected from Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, W, V, Ba, Ta, Sn, Hf, Ce, Gd, and Nd, 0 ≦ a ≦ 10, 0 ≦ b ≦ 8, 2 ≦ c ≦ 15, and a and b are not simultaneously 0)

15. The positive electrode active material according to claim 14, wherein there is a concentration gradient in which the concentration of element A of Chemical Formula 2 decreases from the surface portion of the unit particle toward the center portion of the unit particle.

16. The second lithium composite oxide exists as secondary particles in a form in which a plurality of unit particles are aggregated, The positive electrode active material according to claim 14, wherein the metal oxide exists in a state of being diffused along a crystal grain boundary defined between adjacent unit particles from the surface portion of the secondary particle toward the center portion of the secondary particle.

17. In the positive electrode active material, when the weight percentage of the first lithium composite oxide is w1 and the weight percentage of the second lithium composite oxide is w2, w2 / w1 is 1.5 to 9.

0. The positive electrode active material according to claim 1.

18. A positive electrode comprising the positive electrode active material according to any one of claims 1 to 17.

19. A lithium secondary battery using the positive electrode according to claim 18.

20. A bimodal type positive electrode active material containing a first lithium composite oxide as small particles and a second lithium composite oxide as large particles, The first lithium composite oxide and the second lithium composite oxide each independently contain at least one unit particle, The number of unit particles constituting the first lithium composite oxide is less than that of the second lithium composite oxide, The average particle diameter of the unit particle constituting the second lithium composite oxide is smaller than the average particle diameter of the unit particle constituting the first lithium composite oxide, The ratio of the major axis length r1 to the minor axis length r2 of the unit particles constituting the first lithium composite oxide is 1.3 to 2.1, The major axis length r1 of the unit particles constituting the first lithium composite oxide corresponds to the major axis length of the first crystal plane corresponding to the (003) plane in the crystal structure of the unit particles, The minor axis length r2 of the unit particles constituting the first lithium composite oxide corresponds to the minor axis length perpendicular to the major axis length r1 of the first crystal plane, In the unit particles constituting the first lithium composite oxide, a lithium ion diffusion path is formed along the major axis direction of the first crystal plane, The ratio of the peak intensities attributed to the (003) plane and the (104) plane obtained by X-ray diffraction analysis using Cu-Kα rays for the unit particles constituting the first lithium composite oxide satisfies the following relational expression 2, [Relational Expression 2] 0.420 ≤ I(104) / I(003) ≤ 0.540 Positive electrode active material.

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