Positive electrode active material, positive electrode, and lithium secondary battery

Abstract

The present invention relates to a positive electrode active material, a positive electrode, and a lithium secondary battery, and more specifically, to a positive electrode active material for reducing the amount of residual lithium remaining on the surface of the positive electrode active material, controlling the content of lithium impurities remaining on the surface without a water washing step, and reducing the amount of residual lithium remaining on the surface of the positive electrode active material. The positive electrode active material and the lithium secondary battery using the positive electrode including the positive electrode active material can prevent degradation of electrochemical performance and stability of the positive electrode active material caused by the lithium impurity in advance. The positive electrode active material includes: a lithium composite oxide having a layered crystal structure including at least nickel and cobalt; and a coating layer present on at least a part of a surface of the lithium composite oxide, the coating layer including a first oxide represented by Chemical Formula 3 and a second oxide represented by Chemical Formula 4.

Description

[0001] This application is a divisional application of application number 202110718827.6 filed on June 28, 2021, entitled "Positive electrode active material and lithium secondary battery including the same". Technical Field

[0002] This invention relates to a positive electrode active material with improved electrochemical performance and stability, and a lithium secondary battery using a positive electrode comprising the above-mentioned positive electrode active material. More specifically, this invention relates to a positive electrode active material that controls the content of lithium impurities remaining on the surface of the positive electrode active material without a water washing process in order to reduce the amount of residual lithium remaining on the surface of the positive electrode active material, so as to prevent the decline in the electrochemical performance and stability of the positive electrode active material caused by the above-mentioned lithium impurities in advance, and a lithium secondary battery using a positive electrode comprising the above-mentioned positive electrode active material. Background Technology

[0003] Batteries store electricity using materials that can undergo electrochemical reactions at the positive and negative electrodes. A representative example of such a battery is the lithium secondary battery, which stores electrical energy through changes in chemical potential during the insertion / extraction of lithium ions at the positive and negative electrodes.

[0004] The aforementioned lithium secondary battery uses materials that can reversibly insert or deintercalate lithium ions as positive and negative electrode active materials, and fills the space between the positive and negative electrodes with an organic electrolyte or a polymer electrolyte.

[0005] Lithium composite oxides have been used as positive electrode active materials for lithium secondary batteries. For example, composite oxides of LiCoO2, LiMn2O4, LiNiO2, and LiMnO2 are under investigation.

[0006] Among the aforementioned positive electrode active materials, although LiCoO2, which has excellent lifetime and charge / discharge efficiency, is widely used, its price competitiveness is limited due to the limited availability and high price of cobalt, which is used as a raw material.

[0007] While lithium manganese oxides such as LiMnO2 and LiMn2O4 have advantages such as good thermal stability and low price, they also suffer from problems such as small capacity and poor high-temperature performance. Furthermore, although LiNiO2-based cathode active materials exhibit high discharge capacity, their synthesis is extremely difficult due to the cation mixing problem between lithium and transition metals, resulting in significant issues with rate performance.

[0008] Furthermore, depending on the degree of cation mixing described above, a large amount of lithium byproducts are generated. Since most of these byproducts consist of compounds of LiOH and Li₂CO₃, problems arise during the manufacture of the cathode slurry, including gelation, and gas generation during charge-discharge operations after electrode fabrication. Residual Li₂CO₃ increases battery expansion, reducing cycle life and causing the battery to swell.

[0009] On the other hand, the content of lithium impurities present on the surface of the positive electrode active material tends to increase proportionally to the content of nickel in the aforementioned positive electrode active material.

[0010] Therefore, in the case of high-nickel cathode active materials recently introduced to improve the capacity characteristics of cathode active materials, there is an excessive amount of lithium impurities on the surface, so a water washing process is necessary to remove these impurities.

[0011] However, while the water washing process described above can reduce the residual lithium on the surface of the positive electrode active material, it also has the disadvantage of potentially damaging the surface of the positive electrode active material. When the surface of the positive electrode active material is damaged by the water washing process, the electrochemical performance and stability of the positive electrode active material may deteriorate. Summary of the Invention

[0012] Technical issues

[0013] To address various problems with existing cathode active materials used in lithium-ion batteries, the present invention aims to provide a cathode active material with improved electrochemical performance and stability. In particular, the present invention aims to provide a cathode active material that controls the content of residual lithium impurities on the surface of the cathode active material without a water washing process, thereby preventing the degradation of the electrochemical performance and stability of the cathode active material caused by the aforementioned lithium impurities.

[0014] Furthermore, another object of the present invention is to provide a positive electrode comprising a positive active material as defined herein.

[0015] Another object of the present invention is to provide a lithium secondary battery using a positive electrode as defined herein.

[0016] The objectives of this invention are not limited to those stated above. Other objectives and advantages of this invention not mentioned above can be understood through the following description and through embodiments of this invention. Furthermore, it will be readily understood that the objectives and advantages of this invention can be achieved by the means and combinations thereof described in the claims.

[0017] Solution to the problem

[0018] According to one aspect of the present invention, a positive electrode active material is provided, comprising: a lithium composite oxide having a layered crystal structure containing at least nickel and cobalt; and a coating present in at least a portion of the surface of the lithium composite oxide, the coating comprising a first oxide represented by chemical formula 3 and a second oxide represented by chemical formula 4.

[0019] [Chemical Formula 3]

[0020] Li a' Co b' M3' c' (P β' O γ' ) d'

[0021] Wherein, M3' is at least one selected from Ni, Mn, 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, Gd, and Nd.

[0022] 0 <a'≤10,0≤b'≤8,0≤c'≤8,0<d'≤13,0≤β'≤4,0<γ'≤10,

[0023] [Chemical Formula 4]

[0024] Co b" M3" c" (P β" O γ" ) d"

[0025] Wherein, M3" is selected from at least one of Ni, Mn, 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, Gd, and Nd.

[0026] 0≤b"≤8,0≤c"≤8,0 <d"≤13,0≤β"≤4,0<γ"≤10。

[0027] According to one aspect of the present invention, a positive electrode active material is provided, the positive electrode active material comprising: a first compound capable of lithium intercalation and deintercalation; and a second compound present in at least a portion of the surface of the first compound.

[0028] At this time, the second compound may be an oxide comprising at least one first element selected from Group 1A, Group 3A and Group 5A elements and at least one second element selected from Group 8 elements.

[0029] The first compound mentioned above can be represented by the following chemical formula 1.

[0030] [Chemical Formula 1]

[0031] Li w Ni 1-(x+y+z) Co x M1 y M2 z O 2+α

[0032] (wherein, M1 is at least one selected from Mn or Al,)

[0033] M2 is selected from at least one of Mn, P, Sr, Ba, B, Ti, Zr, Al, Hf, Ta, Mg, V, Zn, Si, Y, Sn, Ge, Nb, W, and Cu.

[0034] M1 and M2 are different elements.

[0035] 0.5≤w≤1.5, 0≤x≤0.50, 0≤y≤0.20, 0≤z≤0.20, 0≤α≤0.02)

[0036] The second compound mentioned above can be represented by the following chemical formula 2.

[0037] [Chemical Formula 2]

[0038] Li a Co b M3 c (P β O γ ) d

[0039] (Where M3 is selected from at least one of Ni, Mn, 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 Nd,)

[0040] 0≤a≤10, 0≤b≤8, 0≤c≤8, 0 <d≤13,0<β≤4,0<γ≤10)

[0041] At this time, the second compound may include a first oxide represented by the following chemical formula 3 and a second oxide represented by the following chemical formula 4.

[0042] [Chemical Formula 3]

[0043] Li a' Co b' M3' c' (Pβ' O γ' ) d'

[0044] (Where M3' is selected from at least one of Ni, Mn, 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 Nd,)

[0045] 0 <a'≤10,0≤b'≤8,0≤c'≤8,0<d'≤13,0≤β'≤4,0<γ'≤10)

[0046] [Chemical Formula 4]

[0047] Co b" M3" c" (P β" O γ" ) d"

[0048] (Where M3" is selected from at least one of Ni, Mn, 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 Nd.)

[0049] 0≤b"≤8,0≤c"≤8,0 <d"≤13,0≤β"≤4,0<γ"≤10)

[0050] In another embodiment, at least a portion of the surface of the first compound may further include a third compound represented by the following chemical formula 5.

[0051] [Chemical Formula 5]

[0052] Li e W f M4 g O h

[0053] (Where M4 is selected from at least one of Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, Ce, V, Ba, Ta, Sn, Hf, Ce, Gd, and Nd,)

[0054] 0≤e≤10,0 <f≤8,0≤g≤8,2≤h≤13)

[0055] Furthermore, according to another aspect of the present invention, a positive electrode comprising the above-described positive electrode active material is provided.

[0056] In addition, according to another aspect of the present invention, a lithium secondary battery using the above-described positive electrode is provided.

[0057] The effects of the invention

[0058] According to various embodiments of the present invention, the positive electrode active material does not require a water washing process to reduce the amount of residual lithium present on the surface of the positive electrode active material. Instead, the content of lithium impurities remaining on the surface can be controlled by forming lithium metal oxide and / or lithium metal phosphate within the surface.

[0059] Therefore, it is possible to prevent the decline in the electrochemical performance and stability of the positive electrode active material caused by lithium impurities remaining on the surface of the positive electrode active material.

[0060] Therefore, by using the positive electrode active material according to various embodiments of the present invention, the various electrochemical characteristics such as capacity characteristics, lifetime characteristics, and rate characteristics, which are important indicators for evaluating the performance of lithium secondary batteries, can be improved.

[0061] The specific details for implementing the invention are described below, along with the specific effects of the invention and the effects described above. Detailed Implementation

[0062] To facilitate understanding of this invention, specific terms have been appropriately defined herein. Unless otherwise defined herein, scientific and technical terms used in this invention have the meanings commonly understood by one of ordinary skill in the art to which this invention pertains. Furthermore, unless the context otherwise requires, singular terms shall include plural forms and plural terms shall include singular forms.

[0063] The positive electrode active material according to the present invention, the positive electrode including the above-described positive electrode active material, and the lithium secondary battery using the above-described positive electrode will be described in further detail below.

[0064] Positive electrode active material

[0065] According to one aspect of the present invention, a positive electrode active material is provided comprising a first compound and a coating, wherein the first compound is capable of lithium insertion and extraction, and the coating is present in at least a portion of the surface of the first compound.

[0066] The first compound described above can be a lithium composite oxide in single-crystal or polycrystalline form, but is preferably a lithium composite oxide in polycrystalline form. A lithium composite oxide in polycrystalline form refers to an aggregate comprising primary particles and secondary particles formed by agglomerating multiple of the primary particles.

[0067] The aforementioned primary particle refers to a single grain (or crystallite), while secondary particles refer to aggregates formed by the aggregation of multiple primary particles. Voids and / or grain boundaries may exist between the primary particles constituting the secondary particles.

[0068] For example, the aforementioned primary particles can be spaced apart from adjacent primary particles within the aforementioned secondary particles to form internal voids. Alternatively, the aforementioned primary particles can form a surface existing within the aforementioned secondary particles by contacting the internal voids, rather than forming grain boundaries by contacting adjacent primary particles.

[0069] On the other hand, the primary particles that exist on the outermost surface of the secondary particles are exposed to the surface of the external air to form the surface of the secondary particles.

[0070] The average particle size of the primary particles ranges from 0.01 μm to 5 μm, preferably from 0.01 μm to 3 μm, thereby achieving the optimal density of the cathode prepared using the cathode active material of various embodiments of the present invention. Furthermore, the average particle size of the secondary particles can vary depending on the number of primary particles aggregated, but can be from 3 μm to 20 μm.

[0071] Furthermore, the aforementioned primary particles and / or secondary particles may have rod-shaped, elliptical, and / or irregular shapes.

[0072] The first compound is a lithium composite oxide represented by the following chemical formula 1.

[0073] [Chemical Formula 1]

[0074] Li w Ni 1-(x+y+z) Co x M1 y M2 z O 2+α

[0075] (wherein, M1 is at least one selected from Mn or Al,)

[0076] M2 is selected from at least one of Mn, P, Sr, Ba, B, Ti, Zr, Al, Hf, Ta, Mg, V, Zn, Si, Y, Sn, Ge, Nb, W, and Cu.

[0077] M1 and M2 are different elements.

[0078] 0.5≤w≤1.5, 0≤x≤0.50, 0≤y≤0.20, 0≤z≤0.20, 0≤α≤0.02)

[0079] In this case, the first compound can be a lithium composite oxide having a layered crystal structure containing at least nickel and cobalt. Furthermore, the first compound is preferably a high-nickel lithium composite oxide in which x+y+z is 0.20 or less in the above chemical formula 1.

[0080] As described above, in the case of lithium composite oxides containing nickel, as the mixing of lithium and nickel cations becomes more severe, a large amount of residual lithium, i.e., lithium impurities, may form on the surface of the aforementioned lithium composite oxides. These lithium impurities mainly include LiOH and Li₂CO₃, and they can cause gelation during the preparation of the paste used to manufacture the cathode or contribute to battery swelling.

[0081] As the nickel content in the aforementioned lithium composite oxide increases, the content of lithium impurities also increases proportionally. Typically, in the case of high-nickel cathode active materials (lithium composite oxides) with a nickel content of 80 mol% or higher, a water washing process is necessary to remove surface lithium impurities. However, this water washing process can actually cause some surface damage to the aforementioned lithium composite oxides, which may reduce their electrochemical performance and stability.

[0082] On the other hand, the lithium composite oxide and the positive electrode active material containing the lithium composite oxide according to various embodiments of the present invention can effectively reduce the lithium impurity content remaining on the surface of the lithium composite oxide by forming lithium metal oxide and / or lithium metal phosphate as described below, without the need for a water washing process.

[0083] In addition, the first compound may be doped with a metal element represented by M1 as shown in the above chemical formula 1. Preferably, M1 may include tungsten (W).

[0084] In this case, the aforementioned tungsten can exist in the crystal lattice of the first compound. That is, the aforementioned tungsten can exist in a state in which nickel is substituted for at least one of the Li 3a and 3b sites in the aforementioned first compound.

[0085] On the other hand, when the aforementioned tungsten is doped into the aforementioned first compound, the proportion of nickel inserted into the Li 3a site as determined by Rietveld X-ray diffraction analysis can be increased (Ni occ This can improve the electrochemical performance and stability of the aforementioned positive electrode active material. At this time, the ratio of nickel inserted into the Li 3a site (Ni...) occThe increase could be due to an increase in the amount of nickel inserted into the Li 3a site through tungsten doping, or it could be due to tungsten inserting into the Li 3a site. Preferably, when tungsten is doped into the first compound, at least a portion of the tungsten doped into the first compound inserts into the Li 3a site, thereby contributing to improving the electrochemical performance and stability of the first compound.

[0086] In this case, the first compound can be represented by the following chemical formula 1-1.

[0087] [Chemical Formula 1-1]

[0088] Li w Ni 1-(x+y+z) Co x M1 y M2 z W z' O 2+δ

[0089] (Where M1 is selected from at least one of Mn or Al, and M2 is selected from at least one of Mn, P, Sr, Ba, B, Ti, Zr, Al, Hf, Ta, Mg, V, Zn, Si, Y, Sn, Ge, Nb, and Cu. M1 and M2 are distinct elements, and the following conditions apply: 0.5 ≤ w ≤ 1.5, 0 ≤ x ≤ 0.50, 0 ≤ y ≤ 0.20, 0 ≤ z ≤ 0.20, 0 ≤ z' ≤ 0.20, 0 ≤ δ ≤ 0.02)

[0090] Furthermore, M1 present in the aforementioned lithium composite oxide can exhibit a concentration gradient that decreases from the surface portion of the secondary particle toward the center portion of the secondary particle.

[0091] The concentration gradient mentioned above refers to the negative (-) slope between the concentration of M1 at any point on the surface of the secondary particle and the concentration of M1 at any point in the center of the secondary particle.

[0092] As described above, a concentration gradient of M1 exists in the secondary particles, preferably a concentration gradient of tungsten (W), so that the movement path (diffusion path of lithium ions) of the secondary particles and the primary particles constituting the secondary particles can be formed from the surface portion of the secondary particles toward the center portion.

[0093] The positive electrode active material according to various embodiments of the present invention is characterized in that, in order to reduce the content of residual lithium, i.e. lithium impurities, present on the surface of the first compound, a coating containing a second compound is formed on the surface.

[0094] At this time, the second compound may exist at least in the interface between the primary particles of the first compound and on the surface of the secondary particles formed by agglomeration of the primary particles. Furthermore, the concentration of the second compound may exhibit a concentration gradient that decreases from the surface portion of the secondary particles toward the center portion of the secondary particles.

[0095] The coating may include the second compound, which is an oxide comprising at least one first element selected from Group 1A, Group 3A and Group 5A elements and at least one second element selected from Group 8 elements.

[0096] The second compound mentioned above can be represented by the following chemical formula 2.

[0097] [Chemical Formula 2]

[0098] Li a Co b M2 c (P β O γ ) d

[0099] (Where M2 is selected from at least one of Ni, Mn, 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 Nd,)

[0100] 0≤a≤10, 0≤b≤8, 0≤c≤8, 0 <d≤13,0<β≤4,0<γ≤10)

[0101] At this point, the second compound can be a single crystal structure or an amorphous oxide, but is not limited thereto, and can be a collection of at least one crystal structure and / or amorphous heterooxides.

[0102] If at least one of the aforementioned second compounds is an oxide with a crystalline structure, then the aforementioned second compound may have a crystalline structure belonging to space group Fd-3m, Pnma, P* / n, R3c, or R-3m. In this case, the aforementioned crystalline structure may be a monoclinic crystal, a cubic crystal, an orthorhombic crystal, or a rhombohedral crystal structure.

[0103] In a preferred embodiment, to effectively remove residual lithium present on the surface of the first compound, the proportion of the second compound having a crystal structure belonging to space group Fd-3m, R3c, or R-3m is 13 mol% or less, more preferably 10 mol% or less, and even more preferably 5 mol% or less. The crystal structure belonging to space group Fd-3m, R3c, or R-3m in the second compound can be cubic or rhombic.

[0104] Representative examples of oxides having crystal structures belonging to space groups Fd-3m, R3c, or R-3m in the second compound are Co3O4, LiCoO2, and P2O5. In addition, oxides that act as impurities similar to residual lithium due to their presence on the surface of the first compound may also be included.

[0105] When the proportion of compounds with crystal structures belonging to space groups Fd-3m, R3c, or R-3m in the second compound is greater than 13 mol% in order to effectively remove residual lithium present on the surface of the first compound, the improvement effect of the second compound on the electrochemical performance and stability of the positive electrode active material is minimal, or may even reduce the electrochemical performance and stability of the positive electrode active material.

[0106] Furthermore, the aforementioned second compound may include a first oxide represented by the following chemical formula 3 and a second oxide represented by the following chemical formula 4.

[0107] [Chemical Formula 3]

[0108] Li a' Co b' M3' c' (P β' O γ' ) d'

[0109] (Where M3' is selected from at least one of Ni, Mn, 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 Nd,)

[0110] 0 <a'≤10,0≤b'≤8,0≤c'≤8,0<d'≤13,0≤β'≤4,0<γ'≤10)

[0111] [Chemical Formula 4]

[0112] Co b" M3" c" (P β" Oγ" ) d"

[0113] (Where M3" is selected from at least one of Ni, Mn, 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 Nd.)

[0114] 0≤b"≤8,0≤c"≤8,0 <d"≤13,0≤β"≤4,0<γ"≤10)

[0115] For example, the first oxide may be lithium phosphate, lithium cobalt phosphate, lithium metal phosphate (excluding cobalt), lithium metal phosphate (excluding cobalt)-cobalt, lithium cobalt oxide and / or lithium metal (excluding cobalt) oxide, and the second oxide may be phosphorus pentoxide, cobalt phosphate, metal phosphate (excluding cobalt), metal phosphate (excluding cobalt)-cobalt, cobalt oxide and / or metal (excluding cobalt) oxide.

[0116] Furthermore, the ratio (first oxide / second oxide) of the first oxide to the second oxide as defined above in the second compound present in the coating is preferably 0.87 or more.

[0117] When the ratio of the first oxide to the second oxide (first oxide / second oxide) in the second compound present in the coating is less than 0.87 (i.e., when the proportion of metal oxides relative to lithium phosphate increases), the effect of the second compound on improving the electrochemical performance and stability of the positive electrode active material is minimal, or may even reduce the electrochemical performance and stability of the positive electrode active material.

[0118] On the other hand, in the second compound, the ratio r1 / r2 (e.g., LiCoO2) of the first oxide (e.g., LiCo(PO4), LiPO3 and LiCoO2) having a crystal structure belonging to space group Fd-3m, R3c or R-3m and the ratio r2 (e.g., LiCo(PO4) and LiPO3) having a crystal structure belonging to a space group other than Fd-3m, R3c or R-3m) is preferably 0.03 or less.

[0119] Furthermore, the ratio s1 / s2 (e.g., Co3O4 and P2O5) of the second oxides (e.g., Co3(PO4)2, Co3O4 and P2O5) having a crystal structure belonging to space group Fd-3m, R3c or R-3m and the ratio s2 (e.g., Co3(PO4)2) having a crystal structure belonging to a space group other than Fd-3m, R3c or R-3m) of the second oxides in the second compound is preferably 0.24 or less.

[0120] When the ratio of compounds containing crystal structures belonging to space groups Fd-3m, R3c, or R-3m in the first oxide and the second oxide, and compounds having crystal structures belonging to space groups other than Fd-3m, R3c, or R-3m, in the second compound is within the above-mentioned range, the reduction in the electrochemical performance and stability of the positive electrode active material can be minimized.

[0121] Furthermore, as described above, the first oxide and the second oxide can exhibit a concentration gradient that decreases from the surface portion of the secondary particle toward the center portion of the secondary particle. Consequently, the cobalt concentration in the lithium composite oxide can also decrease from the surface portion of the secondary particle toward the center portion of the secondary particle.

[0122] In another embodiment, at least a portion of the surface of the first compound may further include a third compound represented by the following chemical formula 5.

[0123] [Chemical Formula 5]

[0124] Li e W f M4 g O h

[0125] (Where M4 is selected from at least one of Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, Ce, V, Ba, Ta, Sn, Hf, Ce, Gd, and Nd,)

[0126] 0≤e≤10,0 <f≤8,0≤g≤8,2≤h≤13)

[0127] The third compound may be present in the coating in which the second compound is present or may exist independently of the coating. Furthermore, the third compound may be present at least in a portion of the interface between the primary particles of the first compound and on the surface of the secondary particles formed by agglomeration of the primary particles. Moreover, the concentration of the third compound may exhibit a concentration gradient decreasing from the surface portion of the secondary particles towards the center portion of the secondary particles.

[0128] In another embodiment, the positive electrode active material may further include a shell covering at least a portion of the surface of the first compound (the surface not covered by the coating) and the surface of the coating.

[0129] At this point, the aforementioned shell may include a fourth compound represented by the following chemical formula 4. That is, the aforementioned shell can be defined as the region where the fourth compound represented by the following chemical formula 4 is present.

[0130] [Chemical Formula 4]

[0131] Li j M4 k O l

[0132] (Where M4 is selected from at least one of 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 Nd.)

[0133] 0≤j≤10, 0≤k≤8, 2≤l≤13)

[0134] In addition, the shell can have the following forms: different types of fourth compounds exist simultaneously in the same layer, or different types of fourth compounds represented by the above chemical formula 4 exist in separate layers.

[0135] The fourth compound represented by the above chemical formula 4 can be in a state of physical and / or chemical bonding with the above first compound, the above second compound, and / or the above third compound. Furthermore, the fourth compound can exist in a state of forming a solid solution with the above first compound, the above second compound, and / or the above third compound.

[0136] The fourth compound mentioned above is an oxide or an oxide of M4 formed by combining lithium with an element represented by M4. An example of such an oxide is Li. a W b O c Li a Zr b O cLi a Ti b O c Li a Ni b O c Li a B b O c W b O c Zr b O c Ti b O c Or B b O c However, the examples above are only for ease of understanding, and the oxides defined herein are not limited to the examples above.

[0137] In another embodiment, the fourth compound may be an oxide formed by combining lithium with at least two elements represented by M4, or further may include an oxide formed by combining lithium with at least two elements represented by M4. An example of an oxide formed by combining lithium with at least two elements represented by M4 may be 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 However, the present invention is not limited thereto.

[0138] The fourth compound can exhibit a concentration gradient that decreases from the surface of the secondary particle toward the center of the secondary particle. Therefore, the concentration of the fourth compound can decrease from the outermost surface of the secondary particle toward the center of the secondary particle.

[0139] As described above, the fourth compound exhibits a concentration gradient that decreases from the surface of the secondary particles towards the center, thereby further reducing residual lithium on the surface of the first compound. Furthermore, it prevents a decrease in crystallinity in the inner surface region of the first compound due to the fourth compound. Additionally, it prevents the overall structure of the positive electrode active material from being destroyed by the fourth compound during the electrochemical reaction.

[0140] Furthermore, the shell may include a first shell and a second shell, wherein the first shell includes at least one fourth compound represented by the above chemical formula 4, and the second shell includes at least one fourth compound represented by the above chemical formula 4 and includes an oxide different from the oxide contained in the first shell.

[0141] Lithium secondary batteries

[0142] According to another aspect of the present invention, a positive electrode comprising a positive current collector and a positive active material layer formed on the positive current collector can be provided. The positive active material layer may comprise positive active materials according to various embodiments of the present invention. Therefore, since the content regarding the positive active material is the same as described above, detailed descriptions will be omitted for convenience, and only the remaining components not described above will be described below.

[0143] There are no particular limitations on the aforementioned positive electrode current collector, as long as it is conductive and does not cause chemical changes in the battery. Examples of such materials include stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel with a surface treated with carbon, nickel, titanium, silver, etc. Furthermore, the aforementioned positive electrode current collector can typically have a thickness of 3 μm to 500 μm, or the adhesion of the positive electrode active material can be increased by forming minute irregularities on the surface of the current collector. For example, various forms such as films, sheets, boxes, meshes, porous materials, foams, and nonwoven fabrics can be used.

[0144] The aforementioned positive electrode active material layer can be prepared by coating the aforementioned positive electrode slurry composition onto the aforementioned positive electrode current collector. The aforementioned positive electrode slurry composition includes the aforementioned positive electrode active material and conductive material, and optionally includes an adhesive as needed.

[0145] At this point, relative to the total weight of the positive electrode active material layer, the content of the positive electrode active material can be from 80% to 99% by weight, and more specifically, from 85% to 98.5% by weight. When the content of the positive electrode active material is within the above range, it can have excellent capacity characteristics, but the present invention is not necessarily limited thereto.

[0146] The aforementioned conductive materials are used to provide conductivity to the electrodes and can be used without restriction, as long as they possess electronic conductivity without causing chemical changes in the assembled battery. Specific examples may include: graphite such as natural or artificial graphite; carbon materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermally cracked carbon black, or carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, or silver; conductive whiskers such as zinc oxide whiskers or potassium titanate whiskers; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, etc., and these substances can be used alone or as a mixture of two or more. The content of the aforementioned conductive materials relative to the total weight of the positive electrode active material layer can be from 0.1% by weight to 15% by weight.

[0147] The aforementioned adhesive serves to adhere the positive electrode active material particles to each other and improve the adhesion between the positive electrode active material and the current collector. Specific examples may include: polyvinylidene fluoride (PVDF), PVDF-co-HFP copolymer, polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and these substances may be used alone or as a mixture of two or more. The content of the aforementioned adhesive relative to the total weight of the positive electrode active material layer may be from 0.1% by weight to 15% by weight.

[0148] In addition to using the aforementioned positive electrode active material, the aforementioned positive electrode can be prepared using conventional positive electrode preparation methods. Specifically, the aforementioned positive electrode can be prepared by coating a positive electrode slurry composition onto a positive electrode current collector, and then drying and calendering the resulting material. The positive electrode slurry composition is prepared by dissolving or dispersing the aforementioned positive electrode active material, along with selective binders and conductive materials, in a solvent.

[0149] The solvents described above can be solvents commonly used in the art, and may include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, water, etc., and these substances can be used alone or as a mixture of two or more. Considering the coating thickness and preparation yield of the slurry, the amount of solvent used is sufficient as long as it can dissolve or disperse the positive electrode active material, conductive material, and binder and has a viscosity sufficient to obtain excellent thickness uniformity when subsequently used for coating to prepare the positive electrode.

[0150] Furthermore, in another embodiment, the positive electrode can also be prepared by casting the above-mentioned positive electrode paste composition onto a separate carrier and then pressing the film layer obtained by peeling it off from the carrier onto the positive electrode current collector.

[0151] Furthermore, according to another aspect of the present invention, an electrochemical device comprising the aforementioned positive electrode can be provided. Specifically, the aforementioned electrochemical device can be a battery, a capacitor, or the like, and more specifically, it can be a lithium secondary battery.

[0152] Specifically, the aforementioned lithium secondary battery may include a positive electrode, a negative electrode positioned opposite to the positive electrode, and a separator and an electrolyte situated between the positive and negative electrodes. Since the positive electrode described above is the same as the one described above, a detailed description will be omitted for convenience, and only the remaining components not described above will be specifically described below.

[0153] The aforementioned lithium secondary battery may optionally further include a battery container that houses the electrode assembly comprising the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

[0154] The aforementioned negative electrode may include a negative electrode current collector and a layer of negative electrode active material located on the aforementioned negative electrode current collector.

[0155] There are no particular limitations on the aforementioned negative electrode current collector, as long as it has high conductivity without causing chemical changes in the battery. Examples of such materials include copper, stainless steel, aluminum, nickel, titanium, sintered carbon, or copper or stainless steel, aluminum-cadmium alloys with surfaces treated with carbon, nickel, titanium, silver, etc. Furthermore, the aforementioned negative electrode current collector can typically have a thickness of 3 μm to 500 μm, or, similar to the positive electrode current collector, the adhesion of the negative electrode active material can be increased by forming minute irregularities on the surface of the current collector. For example, various forms such as films, sheets, boxes, meshes, porous materials, foams, and nonwoven fabrics can be used.

[0156] The aforementioned negative electrode active material layer can be prepared by coating the aforementioned negative electrode current collector with a negative electrode paste composition, the aforementioned negative electrode active material and conductive material, and selectively including an adhesive as needed.

[0157] The aforementioned negative electrode active materials can use compounds capable of reversibly inserting and de-intercalating lithium. Specific examples may include: carbon materials such as artificial graphite, natural graphite, graphitized carbon fibers, and amorphous carbon; 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; and metal oxides capable of doping and de-doping lithium, such as SiO₂. β(0<β<2), SnO2, vanadium oxide, and lithium vanadium oxide; or composites containing metal compounds and carbon materials such as Si-C composites or Sn-C composites, and any one or a mixture of two or more of these substances can be used. Additionally, a thin film of metallic lithium can be used as the aforementioned negative electrode active material. Furthermore, both low-crystallinity carbon and high-crystallinity carbon can be used as the carbon material. Low-crystallinity carbon typically includes soft carbon and hard carbon, while high-crystallinity carbon typically includes: amorphous, plate-like, scaly, spherical, or fibrous natural or artificial graphite, as well as high-temperature sintered carbon such as kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and petroleum or coal tar pitch-derived cokes.

[0158] Based on the total weight of the negative electrode active material layer, the content of the aforementioned negative electrode active material can be from 80% to 99% by weight.

[0159] The aforementioned adhesive is a component that facilitates the bonding between conductive materials, active materials, and current collectors, and its content is typically from 0.1% to 10% by weight based on the total weight of the negative electrode active material layer. Examples of the aforementioned adhesive may include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, acrylonitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0160] The aforementioned conductive material is a component used to further improve the conductivity of the negative electrode active material. Based on the total weight of the negative electrode active material layer, the content of the conductive material can be 10% by weight or less, preferably 5% by weight or less. The aforementioned conductive material is not limited in any way, as long as it is conductive and does not cause chemical changes in the battery. Conductive materials such as graphite (natural or artificial); carbon black (acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermally cracked carbon black); conductive fibers (carbon fiber or metal fiber); metal powders (fluorocarbons, aluminum or nickel powder); conductive whiskers (zinc oxide whiskers, potassium titanate whiskers); conductive metal oxides (titanium oxide); and polyphenylene derivatives can be used.

[0161] In one embodiment, the above-mentioned negative electrode active material layer can be prepared by: coating a negative electrode paste composition prepared by dissolving or dispersing the negative electrode active material, as well as selective binders and conductive materials in a solvent, onto a negative electrode current collector and drying the result; or by casting the above-mentioned negative electrode paste composition onto a separate carrier and then pressing the film layer obtained by peeling it off from the carrier onto the above-mentioned negative electrode current collector.

[0162] On the other hand, in the aforementioned lithium secondary battery, the separator separates the negative and positive electrodes and provides a migration channel for lithium ions. There are no particular limitations, as long as it is commonly used as a separator in lithium secondary batteries. In particular, separators with low resistance to electrolyte ion migration and excellent ability to contain electrolyte liquid moisture are preferred. Specifically, porous polymer membranes can be used, such as porous polymer membranes prepared from polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer; or laminates of two or more layers thereof. Alternatively, commonly used porous nonwoven fabrics such as nonwoven fabrics made from high-melting-point glass fibers or polyethylene terephthalate fibers can be used. Furthermore, to ensure heat resistance and mechanical strength, separators coated with ceramic components or polymer materials can also be used, and can be selectively used in single-layer or multi-layer structures.

[0163] In addition, the electrolyte used in this invention may include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc., which can be used in the preparation of lithium secondary batteries, but this invention is not limited thereto.

[0164] Specifically, the electrolyte may contain organic solvents and lithium salts.

[0165] The aforementioned organic solvents can be used without particular restrictions, as long as they function as a medium for the migration of ions participating in the electrochemical reaction of the battery. Specific examples of the aforementioned organic solvents may include: ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, or ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene or fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methylethyl carbonate (MEC), ethylmethyl carbonate (EMC), ethylene carbonate (EC), or propylene carbonate. Carbonate (PC); alcohol solvents such as ethanol or isopropanol; nitriles such as R-CN (R is a straight-chain, branched, or cyclic hydrocarbon group of C2 to C20, and may contain double-bonded aromatic rings or ether bonds); amides such as dimethylformamide; dioxolane such as 1,3-dioxolane; sulfolane, etc. Among these, carbonate solvents are preferred, and a mixture of cyclic carbonates (e.g., ethylene carbonate, propylene carbonate, etc.) with high ionic conductivity and high dielectric constant that can enhance the charging and discharging performance of the battery and low viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, etc.) is even more preferred. In this case, mixing cyclic carbonates and linear carbonates in a volume ratio of about 1:1 to 1:9 can produce excellent electrolyte performance.

[0166] The lithium salts mentioned above can be used without particular restrictions, as long as they are compounds capable of providing lithium ions in lithium secondary batteries. Specific examples of the lithium salts mentioned above may include LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, etc. Lithium salts can be advantageously used in a concentration range of 0.1M to 2.0M. When the lithium salt concentration is within the above range, the electrolyte has suitable conductivity and viscosity, thereby exhibiting excellent electrolyte performance, and lithium ions can migrate efficiently.

[0167] In the aforementioned electrolyte, to enhance battery life, suppress battery capacity degradation, and improve battery discharge capacity, in addition to the electrolyte-forming components, one or more additives may be included, such as: alkylene carbonate compounds like difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glycine dimethyl ether, triammonium hexaphosphate, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidinanes, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride. The content of these additives relative to the total weight of the electrolyte may be from 0.1% to 5% by weight.

[0168] Lithium secondary batteries containing the positive electrode active material according to the present invention as described above stably exhibit excellent discharge capacity, output characteristics, and lifespan characteristics, and are therefore used in the following fields: portable devices such as mobile phones, laptop computers, and digital cameras; and electric vehicles such as hybrid electric vehicles (HEVs).

[0169] The shape of the lithium secondary battery according to the present invention is not particularly limited, and it can be cylindrical, triangular, pouch-shaped, or coin-shaped, similar to a can. Furthermore, preferably, the lithium secondary battery can be used as a battery cell for powering small devices, or as a unit battery in medium and large battery modules containing multiple battery cells.

[0170] According to another aspect of the present invention, a battery module comprising the above-described lithium secondary battery as a unit cell and / or a battery pack comprising the same can be provided.

[0171] The aforementioned battery module or battery pack can be used as a power source for one or more medium and large devices in a power tool; electric vehicles, including electric vehicles (EVs), hybrid vehicles, and plug-in hybrid electric vehicles (PHEVs); or for storing electricity in a system.

[0172] The invention will be described in more detail below with reference to embodiments. However, it should be understood that these embodiments are for illustrative purposes only and should not be construed as limiting the scope of the invention.

[0173] Experimental Example 1.

[0174] Preparation Example 1. Preparation of Positive Electrode Active Material

[0175] (1) Example 1

[0176] Spherical Ni was synthesized by co-precipitation method. 0.91 Co 0.08 Mn 0.01 (OH)₂ hydroxide precursor. Specifically, in a 90L reactor, 25 wt% NaOH and 30 wt% NH₄OH were added to a 1.5M aqueous solution of a complex transition metal sulfuric acid, which was prepared by mixing nickel sulfate, cobalt sulfate, and manganese sulfate in a molar ratio of 91:8:1. The pH of the reactor was maintained at 11.5, the reactor temperature was maintained at 60°C, and N₂ was introduced into the reactor as an inert gas to prevent oxidation of the prepared precursor. After stirring during synthesis, the precursor was washed and dehydrated using a filter press (F / P) to obtain Ni. 0.91 Co 0.08 Mn 0.01 (OH)2 hydroxide precursor.

[0177] Next, LiOH (Li / (Ni+Co+Mn) molar ratio = 1.01) was mixed into the synthesized precursor, and then the mixture was heat-treated in a sintering furnace under an O2 atmosphere at a rate of 2°C per minute to 700°C for 10 hours to obtain lithium composite oxide.

[0178] Next, the lithium composite oxide, cobalt-containing raw material (Co3(PO4)2), and tungsten-containing raw material (WO3) are mixed and then sintered to finally prepare the positive electrode active material. Specifically, after mixing the lithium composite oxide, cobalt-containing raw material (Co3(PO4)2), and tungsten-containing raw material (WO3), the mixture is kept in an O2 atmosphere in a sintering furnace, heated to 400°C at a rate of 2°C per minute, and heat-treated for 5 hours, followed by natural cooling to obtain the positive electrode active material.

[0179] The materials were mixed such that the contents of the cobalt-containing raw material (Co3(PO4)2) and the tungsten-containing raw material (WO3) were 0.3 mol% relative to the mixture of the lithium composite oxide, the cobalt-containing raw material (Co3(PO4)2) and the tungsten-containing raw material (WO3).

[0180] The ICP analysis results of the above positive electrode active materials are shown in Table 1 below.

[0181] Table 1

[0182] element Ni Co Mn W P Content (mol%) 90.35 8.22 0.99 0.31 0.13

[0183] (2) Example 2

[0184] Except for mixing the cobalt-containing raw material (Co3(PO4)2) relative to the mixture of the lithium composite oxide, cobalt-containing raw material (Co3(PO4)2), and tungsten-containing raw material (WO3) at a ratio of 0.5 mol%, the positive electrode active material was prepared in the same manner as in Example 1. The ICP analysis results of the composition of the above positive electrode active material are shown in Table 2 below.

[0185] Table 2

[0186] element Ni Co Mn W P Content (mol%) 90.08 8.21 0.99 0.50 0.22

[0187] (3) Example 3

[0188] Except for mixing the above-mentioned tungsten-containing raw material (WO3) in a manner that makes the mixture of the above-mentioned lithium composite oxide, cobalt-containing raw material (Co3(PO4)2) and tungsten-containing raw material (WO3) 1.0 mol%, the positive electrode active material was prepared in the same manner as in Example 1. The ICP analysis results of the composition of the above-mentioned positive electrode active material are shown in Table 3 below.

[0189] Table 3

[0190] element Ni Co Mn W P Content (mol%) 89.33 8.31 0.98 0.97 0.41

[0191] (4) Example 4

[0192] Except for further mixing 0.05 mol% of a zirconium-containing compound (ZrO2) into the synthesized precursor, the positive electrode active material was prepared in the same manner as in Example 1. The ICP analysis results of the composition of the above positive electrode active material are shown in Table 4 below.

[0193] Table 4

[0194] element Ni Co Mn Zr W P Content (mol%) 90.34 8.20 0.99 0.05 0.30 0.12

[0195] (5) Example 5

[0196] Except for the following steps: after mixing the aforementioned lithium composite oxide, cobalt-containing raw material (Co3(PO4)2), and tungsten-containing raw material (WO3), heat-treating the mixture, and then further mixing 0.2 mol% of a titanium-containing compound (TiO2) in the same sintering furnace while maintaining an O2 atmosphere in the sintering furnace, heating to 400°C at a rate of 2°C per minute, and heat-treating for 5 hours, the positive electrode active material was prepared in the same manner as in Example 1. The ICP analysis results of the composition of the above positive electrode active material are shown in Table 5 below.

[0197] Table 5

[0198] element Ni Co Mn Ti W P Content (mol%) 90.28 8.18 0.99 0.18 0.25 0.12

[0199] (6) Comparative Example 1

[0200] Except that the cobalt-containing raw material (Co3(PO4)2) and the tungsten-containing raw material (WO3) were not mixed in the above-mentioned lithium composite oxide, and the temperature was increased to 400°C at a rate of 2°C per minute and heat-treated for 5 hours, the positive electrode active material was prepared in the same manner as in Example 1. The ICP analysis results of the composition of the above-mentioned positive electrode active material are shown in Table 6 below.

[0201] Table 6

[0202] element Ni Co Mn W P Content (mol%) 91.01 7.93 1.06 - -

[0203] (7) Comparative Example 2

[0204] Except for the calcination process after mixing only tungsten-containing raw material (WO3) into the above-mentioned lithium composite oxide, the positive electrode active material was prepared in the same manner as in Example 1. The ICP analysis results of the composition of the above-mentioned positive electrode active material are shown in Table 7 below.

[0205] Table 7

[0206] element Ni Co Mn W P Content (mol%) 90.78 7.90 1.01 0.31 -

[0207] (8) Comparative Example 3

[0208] Except for the fact that only cobalt-containing raw material (Co3(PO4)2) was mixed into the above-mentioned lithium composite oxide before calcination, the positive electrode active material was prepared in the same manner as in Example 1. The ICP analysis results of the composition of the above-mentioned positive electrode active material are shown in Table 8 below.

[0209] Table 8

[0210] element Ni Co Mn W P Content (mol%) 90.57 8.28 1.01 - 0.14

[0211] (9) Comparative Example 4

[0212] Except for the absence of cobalt-containing raw material (Co3(PO4)2) and tungsten-containing raw material (WO3) in the above-mentioned lithium composite oxide, and the heating to 700°C at a rate of 2°C per minute followed by heat treatment for 5 hours, the positive electrode active material was prepared in the same manner as in Example 1. The ICP analysis results of the composition of the above-mentioned positive electrode active material are shown in Table 9 below.

[0213] Table 9

[0214] element Ni Co Mn W P Content (mol%) 91.09 7.91 1.00 - -

[0215] (10) Comparative Example 5

[0216] Except for the process of mixing only tungsten-containing raw material (WO3) into the above-mentioned lithium composite oxide, followed by heating to 700°C at a rate of 2°C per minute and heat treatment for 5 hours, the positive electrode active material was prepared in the same manner as in Example 1. The ICP analysis results of the composition of the above-mentioned positive electrode active material are shown in Table 10 below.

[0217] Table 10

[0218] element Ni Co Mn W P Content (mol%) 90.74 7.96 1.00 0.30 -

[0219] (11) Comparative Example 6

[0220] Except for mixing only cobalt-containing raw material (Co3(PO4)2) into the above-mentioned lithium composite oxide, followed by heating to 700°C at a rate of 2°C per minute and heat treatment for 5 hours, the positive electrode active material was prepared in the same manner as in Example 1. The ICP analysis results of the composition of the above-mentioned positive electrode active material are shown in Table 11 below.

[0221] Table 11

[0222] element Ni Co Mn W P Content (mol%) 90.60 8.24 1.01 - 0.15

[0223] (12) Comparative Example 7

[0224] Except for the process of mixing cobalt-containing raw material (Co3(PO4)2) and tungsten-containing raw material (WO3) into the above-mentioned lithium composite oxide, followed by heating to 700°C at a rate of 2°C per minute and heat treatment for 5 hours, the positive electrode active material was prepared in the same manner as in Example 1. The ICP analysis results of the composition of the above-mentioned positive electrode active material are shown in Table 12 below.

[0225] Table 12

[0226] element Ni Co Mn W P Content (mol%) 90.27 8.34 0.94 0.30 0.15

[0227] Preparation Example 2. Preparation of Lithium Secondary Batteries

[0228] A positive electrode slurry was prepared by dispersing 92% by weight of the positive electrode active material prepared according to Preparation Example 1, 4% by weight of artificial carbon black, and 4% by weight of PVDF binder in 30g of N-methyl-2-pyrrolidone (NMP). The above positive electrode slurry was uniformly coated on an aluminum film with a thickness of 15μm and vacuum dried at 135°C to prepare a positive electrode for lithium secondary batteries.

[0229] A button cell was prepared using lithium foil as the counter electrode of the positive electrode and a porous polyethylene membrane (Celgard 2300, thickness: 25 μm) as the separator. The electrolyte containing LiPF6 at a concentration of 1.15 M was used in a solvent consisting of ethylene carbonate and ethyl methyl carbonate mixed in a volume ratio of 3:7.

[0230] Experimental Example 1. XRD Analysis of Positive Electrode Active Materials

[0231] X-ray diffraction (XRD) analysis was performed on the positive electrode active material prepared according to Preparation Example 1 to confirm the Ni content of the positive electrode active material. occ And the coating on the surface of the aforementioned positive electrode active material. Through the use of Cu Kαradiation XRD analysis was performed using a Bruker D8 Advance diffractometer.

[0232] (1) Ni of the positive electrode active material occ Measurement

[0233] The occupancy (or content) of nickel metal inserted into the Li 3a site of the positive electrode active material prepared according to Preparation Example 1 was determined by Reitveld analysis of the X-ray diffraction pattern. The results are shown in Table 13 below.

[0234] Table 13

[0235] Classification c / a ratio <![CDATA[Ni at the 3a position occ (%)]]> Example 1 4.940 2.41 Example 2 4.940 2.47 Example 3 4.940 2.52 Example 4 4.940 2.23 Example 5 4.940 2.10 Comparative Example 1 4.940 2.27 Comparative Example 2 4.940 2.39 Comparative Example 3 4.940 2.30 Comparative Example 4 4.940 1.62 Comparative Example 5 4.940 2.09 Comparative Example 6 4.940 1.68 Comparative Example 7 4.940 2.29

[0236] Referring to the results in Table 13 above, it can be confirmed that when cobalt-containing raw material (Co3(PO4)2) and tungsten-containing raw material (WO3) are mixed in the above-mentioned lithium composite oxide, the nickel occupancy rate inserted into the Li 3a site increases compared to the case where only cobalt-containing raw material (Co3(PO4)2) or tungsten-containing raw material (WO3) is mixed in the above-mentioned lithium composite oxide. Furthermore, referring to Example 1 and Comparative Example 7, after mixing cobalt-containing raw material (Co3(PO4)2) and tungsten-containing raw material (WO3) in the above-mentioned lithium composite oxide, heat treatment is performed at a relatively low temperature, thereby increasing the nickel occupancy rate inserted into the above-mentioned Li 3a site.

[0237] As described above, after mixing cobalt-containing raw material (Co3(PO4)2) and tungsten-containing raw material (WO3) into the lithium composite oxide, heat treatment is performed at a relatively low temperature. Therefore, compared with the case where cobalt-containing raw material (Co3(PO4)2) or tungsten-containing raw material (WO3) is mixed alone into the lithium composite oxide and then heat treatment is performed, or where cobalt-containing raw material (Co3(PO4)2) and tungsten-containing raw material (WO3) are mixed and then heat treatment is performed at a relatively high temperature, the increase in nickel occupancy at the Li 3a site may be because tungsten is inserted into the Li 3a site when tungsten is incorporated into the lattice of the lithium composite oxide.

[0238] (2) Analysis of coating content on the surface of positive electrode active material

[0239] The oxides shown in Tables 14 and 15 below were screened using raw XRD data determined using Bruker's EVA program, thereby quantitatively analyzing the content of the coating (i.e., the second compound) on the surface of the positive electrode active material prepared according to Preparation Example 1.

[0240] The analytical results of the coating on the surface of the positive electrode active material analyzed according to the above method are shown in Tables 14 to 16 below.

[0241] Table 14

[0242] Classification Space Group Crystal structure <![CDATA[Co3(PO4)2]]> <![CDATA[P21 / c]]> monoclinic <![CDATA[LiCo(PO4)]]> Pnma orthogonal <![CDATA[Co3O4]]> Fd-3m cube <![CDATA[LiPO3]]> P* / n monoclinic <![CDATA[P2O5]]> R3c diamond <![CDATA[LiCoO2]]> R-3m diamond

[0243] Table 15

[0244]

[0245]

[0246] Table 16

[0247]

[0248] Referring to the results in Table 14 above, it can be confirmed that the second compound, which is present as a coating in at least a portion of the surface of the first compound as a lithium composite oxide, has a crystal structure belonging to space group Fd-3m, R3c or R-3m and has a cubic or rhombohedral crystal structure.

[0249] Referring to the results in Tables 14 to 16 above, it can be confirmed that, unlike the positive electrode active materials according to Comparative Examples 6 and 7, in the case of the positive electrode active materials according to Examples 1 to 5, after mixing cobalt-containing raw material (Co3(PO4)2) and tungsten-containing raw material (WO3) into the above-mentioned lithium composite oxide, heat treatment was performed at a relatively low temperature, so that the ratio of compounds (Co3O4, LiCoO2, and P2O5) having crystal structures belonging to space groups Fd-3m, R3c, or R-3m in the oxide defined herein as the second compound is 13 mol% or less.

[0250] Furthermore, unlike the positive electrode active materials according to Comparative Examples 6 and 7, in the case of the positive electrode active materials according to Examples 1 to 5, it can be confirmed that the first oxides in the above-mentioned second compound, namely LiCo(PO4), LiPO3 and LiCoO2, have a crystal structure belonging to space group Fd-3m, R3c or R-3m, namely compound r1, that is, LiCoO2 and compound r2 having a crystal structure belonging to space group other than space group Fd-3m, R3c or R-3m, that is, the ratio of LiCo(PO4) and LiPO3 (r1 / r2 = LiCoO2 (mol%) / LiCo(PO4) (mol%) + LiPO3 (mol%)) is 0.03 or less.

[0251] Furthermore, unlike the positive electrode active materials according to Comparative Examples 6 and 7, in the case of the positive electrode active materials according to Examples 1 to 5, it can be confirmed that the second oxides in the above-mentioned second compound, namely Co3(PO4)2, Co3O4 and P2O5, have a crystal structure belonging to space group Fd-3m, R3c or R-3m, namely compound s1, that is, Co3O4 and P2O5 and compound s2 having a crystal structure belonging to space group other than space group Fd-3m, R3c or R-3m, that is, the ratio of Co3(PO4)2 (s1 / s2 = Co3O4 (mol%) + P2O5 (mol%) / Co3(PO4)2 (mol%)) is 0.24 or less.

[0252] On the other hand, it can be confirmed that in the case of the positive electrode active material according to Comparative Example 3, except that the heat treatment is performed without mixing with tungsten-containing raw material (WO3), the rest is prepared in the same way as the positive electrode active material according to Example 1, so that it exists in a composition similar to the oxide defined as the second compound.

[0253] Experimental Example 2. Determination of Unreacted Lithium in Positive Electrode Active Materials

[0254] The unreacted lithium in the positive electrode active material prepared according to Preparation Example 1 was determined by pH titration using 0.1M HCl up to pH 4. First, 5g of the positive electrode active material prepared according to Preparation Example 1 was added to 100ml of DIW. After stirring for 15 minutes, the solution was filtered. 50ml of the filtrate was taken, and 0.1M HCl was added to it. The amount of HCl consumed according to the pH change was measured to determine Q1 and Q2, thereby calculating the content of unreacted LiOH.

[0255] M1 = 23.95 (Molecular weight of LiOH)

[0256] M2 = 73.89 (Li2CO3 molecular weight) SPL size = (sample weight × solution weight) / water weight

[0257] LiOH (wt%) = [(Q1-Q2)×C×M1×100] / (SPL size×1000)

[0258] The results of the determination of the lithium impurity content in the above positive electrode active material by the above calculation formula are shown in Table 17 below.

[0259] Table 17

[0260]

[0261]

[0262] (2) Electrochemical performance evaluation of lithium secondary batteries

[0263] Using an electrochemical analysis apparatus (Toyo, Toscat-3100), the initial charge capacity, initial discharge capacity, initial reversible efficiency, and discharge capacity ratio (C-rate) of the lithium secondary battery prepared according to Preparation Example 2 were determined by conducting charge / discharge experiments on the battery at 25°C with discharge rates of 3.0V to 4.3V and 0.1C to 5.0C.

[0264] Furthermore, the lithium secondary battery prepared by the above method was subjected to 50 charge / discharge cycles at a driving voltage range of 3.0V to 4.4V at a temperature of 25°C and at a 1C / 1C condition. Then, the ratio of the discharge capacity at the 50th cycle to the initial capacity was measured (cycle capacity retention).

[0265] On the other hand, the initial impedance of the lithium secondary battery prepared according to Preparation Example 2 was determined using electrochemical impedance spectroscopy (EIS) in the frequency range of 10 kHz to 0.01 Hz.

[0266] The results of the above measurements are shown in Table 18 below.

[0267] Table 18

[0268]

[0269] While the embodiments of the present invention have been described above, those skilled in the art will understand that various modifications and alterations can be made to the present invention by adding, modifying, deleting, or supplementing the constituent elements without departing from the spirit of the present invention as described in the claims, and these modifications and alterations also fall within the scope of the present invention.

Claims

1. A positive electrode active material, characterized by, Comprising: a lithium complex oxide having a layered crystal structure containing at least nickel and cobalt; and a coating layer present in at least a portion of the surface of the above lithium complex oxide, the above coating layer comprising a first oxide represented by the following Chemical Formula 3 and a second oxide represented by the following Chemical Formula 4, [Chemical Formula 3] Li a' Co b' M3' c' (P β' O γ' ) d' wherein, M3' is at least one selected from the group consisting of Ni, Mn, 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, Gd, and Nd, 0 < a' ≤ 10, 0 ≤ b' ≤ 8, 0 ≤ c' ≤ 8, 0 < d' ≤ 13, 0 ≤ β' ≤ 4, 0 < γ' ≤ 10, [Chemical Formula 4] Co b" M3" c" (P β" O γ" ) d" wherein, M3" is at least one selected from the group consisting of Ni, Mn, 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, Gd, and Nd, 0 ≤ b" ≤ 8, 0 ≤ c" ≤ 8, 0 < d" ≤ 13, 0 ≤ β" ≤ 4, 0 < γ" ≤ 10.

2. The positive electrode active material according to claim 1, wherein the lithium complex oxide is a single crystal or a polycrystal lithium complex oxide.

3. The positive electrode active material according to claim 1, wherein the lithium complex oxide is a secondary particle formed by agglomeration of a plurality of primary particles.

4. The positive electrode active material according to claim 3, wherein the coating layer is present in at least a portion of the interface between the primary particles and the surface of the secondary particle formed by agglomeration of the primary particles.

5. The positive electrode active material according to claim 3, wherein the first oxide and the second oxide exhibit a concentration gradient that decreases from the surface portion of the secondary particle toward the center portion of the secondary particle.

6. The positive electrode active material according to claim 1, wherein the first oxide and the second oxide have a crystal structure belonging to a space group other than space groups Fd-3m, R3c, or R-3m.

7. The positive electrode active material according to claim 1, wherein the first oxide contains LiCo(PO4) and LiPO3.

8. The positive electrode active material according to claim 1, wherein the second oxide contains Co3(PO4)2.

9. The positive electrode active material according to claim 1, wherein the first oxide and the second oxide each independently have a crystal structure selected from monoclinic and orthorhombic.

10. The positive electrode active material according to claim 9, wherein at least one selected from the first oxide and the second oxide contains an oxide having a monoclinic crystal structure and an oxide having an orthorhombic crystal structure.

11. The positive electrode active material according to claim 1, wherein at least a portion of the surface of the lithium complex oxide contains a third compound represented by the following Chemical Formula 5, [Chemical Formula 5] ​ ​ ​ ​ ​ ​ ​ ​ ​ [Chemical Formula 5] Li e W f M4 g O h wherein, M4 is at least one selected from the group consisting of Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, Ce, V, Ba, Ta, Sn, Hf, Gd, and Nd, 0 ≤ e ≤ 10, 0 < f ≤ 8, 0 ≤ g ≤ 8, 2 ≤ h ≤ 13.

12. The positive electrode active material according to claim 11, wherein The third compound is present in the coating or independently of the coating.

13. The positive electrode active material according to claim 11, wherein The lithium complex oxide is a secondary particle in which a plurality of primary particles are agglomerated, The third compound is present at least in part in an interface between the primary particles and a surface of the secondary particle.

14. The positive electrode active material according to claim 13, wherein The third compound exhibits a concentration gradient that decreases from a surface portion of the secondary particle toward a center portion of the secondary particle.

15. The positive electrode active material according to claim 1, wherein The lithium complex oxide is represented by the following Chemical Formula 1, [Chemical Formula 1] Li w Ni 1-(x+y+z) Co x M1 y M2 z O 2+ α wherein, M1 is at least one selected from the group consisting of Mn and Al, M2 is at least one selected from the group consisting of Mn, P, Sr, Ba, B, Ti, Zr, Al, Hf, Ta, Mg, V, Zn, Si, Y, Sn, Ge, Nb, W, 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 ≤ α ≤ 0.

02.

16. A positive electrode, characterized by comprising: The positive electrode active material according to any one of claims 1 to 15.

17. A lithium secondary battery, characterized by comprising: The positive electrode according to claim 16. The positive electrode according to claim 16.

Images