Positive Electrode Active Material for Lithium Secondary Battery and Method for Producing the Same

Abstract

The present invention relates to a positive electrode active material for a lithium secondary battery and a method for producing the same. In a layered metal oxide crystal structure including a lithium layer and a metal layer, the crystal structure includes a first metal layer in which a part of the metal in the metal layer is substituted with lithium, the crystal structure includes a first lithium layer in which a part of the lithium in the lithium layer is substituted with a metal, the substitution amount of lithium in the first metal layer is 1.5 mol% or more, and a positive electrode active material having a nickel content of 80 mol% or more based on 100 mol% of the metal in the positive electrode active material is provided.

Description

Technical Field

[0001] The present invention relates to a positive electrode active material for a lithium secondary battery and a method for producing the same.

Background Art

[0002] Recently, supported by the expanding demand for electric vehicles, the demand for lithium secondary batteries for driving them has been rapidly increasing. As such a positive electrode active material for a lithium secondary battery, layered lithium transition metal oxides (LiMO2, M = Ni, Co, Mn, etc.) are mainly used, and research on increasing the capacity thereof has been actively conducted. Among the existing layered positive electrode active materials, the material with the highest capacity is LiNiO2 and high-nickel layered positive electrode materials. However, due to the instability of Ni(III), there are difficulties in synthesizing stoichiometric materials, and there is a problem that even a small change in lithium in the synthesis process causes a large change in electrochemical characteristics.

Summary of the Invention

Problems to be Solved by the Invention

[0003] Provided are a lithium-excess high-nickel positive electrode active material having high price competitiveness, high stability, and high energy density, a method for producing the same, and a lithium secondary battery including the same.

Means for Solving the Problems

[0004] In one embodiment of the present invention, in a layered metal oxide crystal structure including a lithium layer and a metal layer, the crystal structure includes a first metal layer in which a part of the metal in the metal layer is substituted with lithium, the crystal structure includes a first lithium layer in which a part of the lithium in the lithium layer is substituted with a metal, the substitution amount of lithium in the first metal layer is 1.5 mol% or more, and a positive electrode active material having a nickel content of 80 mol% or more based on 100 mol% of the metal in the positive electrode active material is provided.

[0005] The substitution amount of lithium in the first metal layer may be 1.5 mol% or more and 15 mol% or less.

[0006] The substitution amount of the metal in the first lithium layer may be 0.5 mol% or more and 5 mol% or less.

[0007] The substitution amount of lithium in the first metal layer may be more than the substitution amount of the metal in the first lithium layer.

[0008] The difference between the substitution amount of lithium in the first metal layer and the substitution amount of the metal in the first lithium layer may be 1 mol% or more and 15 mol% or less.

[0009] The molar ratio of lithium to the total metal in the positive electrode active material may be 1.01 or more and 1.2 or less.

[0010] The positive electrode active material may have a c-axis lattice constant change rate of 2.5% or less due to charge and discharge in the range of 2.5 V to 4.25 V for the R-3m structure.

[0011] During charge and discharge in the range of 2.5 V to 4.25 V, the maximum value of the c-axis lattice constant of the R-3m structure may be 14.410 Å or less.

[0012] During charge and discharge in the range of 2.5 V to 4.25 V, the c-axis lattice constant of the R-3m structure can change within the range of 13.900 Å to 14.410 Å.

[0013] The bond length between the metal and oxygen in the positive electrode active material may be 1.92 Å or less.

[0014] The content of Li2CO3 present in the positive electrode active material measured through X-ray diffraction analysis may be less than 0.5 wt%.

[0015] The content of Li2O present in the positive electrode active material measured through X-ray diffraction analysis may be less than 1.0 wt%.

[0016] Based on 100 mol% of the metal in the positive electrode active material, the nickel content may be 90 mol% or more.

[0017] In another embodiment of the present invention, in a layered metal oxide crystal structure including a lithium layer and a metal layer, the crystal structure includes a first metal layer in which a part of the metal in the metal layer is substituted with lithium, the crystal structure includes a first lithium layer in which a part of the lithium in the lithium layer is substituted with a metal, and a cathode active material is provided in which the substitution amount of lithium in the first metal layer is larger than the substitution amount of the metal in the first lithium layer.

[0018] In still another embodiment of the present invention, a method for manufacturing a cathode active material includes: preparing a metal hydroxide precursor and a lithium raw material substance; and mixing and firing the metal hydroxide precursor and the lithium raw material substance, wherein the firing temperature is t, the molar ratio of lithium in the lithium raw material substance to the metal in the metal oxide precursor is l / m, t is 780 ° C or lower, and the method satisfies the following formula 1.

[0019] [Formula 1] 500 ≦ t / (l / m) ≦ 710

[0020] More specifically, 530 ≦ t / (l / m) ≦ 710, or 600 ≦ t / (l / m) ≦ 710 may be satisfied.

Advantages of the Invention

[0021] The cathode active material according to one embodiment and the lithium secondary battery including the same achieve high price competitiveness, high stability, high energy density, high capacity, and high life characteristics.

Brief Description of the Drawings

[0022]

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Embodiments for Carrying Out the Invention

[0023] Hereinafter, embodiments of the present invention will be described in detail. However, this is presented as an example and does not limit the present invention, which is defined only by the scope of the claims described below.

[0024] In one embodiment of the present invention, in a layered metal oxide crystal structure including a lithium layer and a metal layer, the crystal structure includes a first metal layer in which a part of the metal in the metal layer is substituted with lithium, and the crystal structure includes a first lithium layer in which a part of the lithium in the lithium layer is substituted with a metal. The substitution amount of lithium in the first metal layer is 1.5 mol% or more, and a positive electrode active material is provided in which the nickel content is 80 mol% or more based on 100 mol% of the metal in the positive electrode active material.

[0025] The positive electrode active material may be a layered positive electrode active material in which a transition metal layer and a lithium layer repeatedly exist.

[0026] At this time, cation exchange may occur through appropriate control, and a part of the lithium in the lithium layer may be mixed into the transition metal layer. Conversely, the metal in the transition metal layer may be mixed into the lithium layer.

[0027] At this time, the metal of a typical transition metal layer may be nickel.

[0028] The substitution amount of lithium in the first metal layer may be 1.5 mol% or more and 15 mol% or less.

[0029] More specifically, it may be 4.7 mol% or more and 15 mol% or less. Or it may be 6.2 mol% or more, 7.0 mol% or more, 10.7 mol% or more. When such a range is satisfied, improved life characteristics can be obtained.

[0030] The substitution amount of the metal in the first lithium layer may be 0.5 mol% or more and 5 mol% or less. That is, the substitution amount of the metal may be less than the substitution amount of lithium. More specifically, it may be 4.7 mol% or less, 3.3 mol% or less.

[0031] The difference between the substitution amount of lithium in the first metal layer and the substitution amount of metal in the first lithium layer may be 1 mol% or more and 15 mol% or less.

[0032] The positive electrode active material for a lithium secondary battery according to one embodiment is a lithium-excess nickel-based layered positive electrode active material containing a compound represented by the following Chemical Formula 1.

[0033] [Chemical Formula 1] Li 1+a (Ni b M 1 1-b ) 1-a O2

[0034] In Chemical Formula 1 above, 0 < a ≤ 0.3, 0.8 < b < 1, and M 1 is one or more elements selected from Co, Mn, Al, Mg, Ca, Ti, V, Cr, Zr, Nb, Mo, and W (more specifically, 0 < a < 0.2).

[0035] Here, lithium excess means that excess lithium enters the active material structure and lithium occupies a part of the transition metal sites. FIG. 1 is a diagram showing the chemical structure of the positive electrode active material according to one embodiment, showing a structure in which excess lithium enters a part of the transition metal sites such as Ni, Co, and / or Mn. According to one embodiment, the molar content of lithium present in the structure of the positive electrode active material is 1.02 to 1.15 with respect to 1 mol of the positive electrode active material.

[0036] The molar content of the lithium can be measured, for example, through neutron diffraction analysis. The molar content of lithium present in the positive electrode active material structure can also be expressed as 1.02 to 1.30 with respect to 1 mol of the compound represented by Chemical Formula 1. In Chemical Formula 1, the range of a in (1 + a) indicating the lithium content in the active material structure may be, for example, 0.005 ≤ a ≤ 0.19, 0.01 ≤ a ≤ 0.17, or 0.02 ≤ a ≤ 0.15.

[0037] In addition, the high-nickel type means that the content of nickel in the active material is high. Specifically, it can be meant that the content of nickel exceeds 80 mol% based on the total content of transition metals excluding lithium. For example, the content of nickel may be 81 mol% or more, 85 mol% or more, 89 mol% or more, 90 mol% or more, or 92 mol% or more. In Chemical Formula 1, the b value indicating the content of nickel may be, for example, 0.81 ≤ b ≤ 0.99, 0.83 ≤ b ≤ 0.99, 0.85 ≤ b ≤ 0.99, 0.87 ≤ b ≤ 0.99, 0.89 ≤ b ≤ 0.99, 0.90 ≤ b ≤ 0.99, 0.91 ≤ b ≤ 0.99, 0.92 ≤ b ≤ 0.99, or 0.81 ≤ b ≤ 0.98.

[0038] That is, the cathode active material according to one embodiment is a lithium-excess active material in which lithium with a content of 1.02 to 1.15 mol has entered the active material structure while being a high-nickel-based material with a nickel content exceeding 80 mol%.

[0039] Generally, high-nickel cathode active materials achieve high capacity. However, first, the synthesis itself is difficult and it is difficult to ensure structural stability. Even if synthesized, the problem that the cation mixing phenomenon in which Ni 2+ ions occupy lithium sites increases and rather the capacity decreases frequently occurs, making it difficult to ensure battery safety. When an excessive amount of lithium raw material is added during synthesis to reduce cation mixing and increase the capacity, lithium often does not enter the active material structure and remains in the form of impurities such as Li2CO3 and Li2O. Such impurities can cause a decrease in the battery capacity and stability problems.

[0040] Therefore, the inventors of the present invention have found that the electrochemical properties of the lithium-excess high-nickel layered cathode active material change significantly with minute changes in the lithium composition and the synthesis temperature. When synthesis is carried out within a specific temperature range within a specific lithium content range, a structure is formed in which a certain amount of lithium enters the active material structure while the nickel content is very high, and a stable structure cathode active material with a change rate of the c-axis lattice constant of the R-3m structure of less than 2.3% due to charge and discharge in the range of 2.5V to 4.25V can be successfully synthesized. It was also confirmed that the synthesized cathode active material achieved high capacity and high energy density while improving the life characteristics and stability of the battery.

[0041] When the lithium-excess high-nickel layered cathode active material according to one embodiment is charged and discharged in the range of 2.5V to 4.25V, the change in the c-axis lattice constant is less than 2.3%. Specifically, while the lithium secondary battery applying the cathode active material is charged and discharged in the range of 2.5V to 4.25V, real-time X-ray diffraction pattern analysis is performed to analyze the change in the lattice constant thereby. According to this, the cathode active material according to one embodiment has a very small change in the X-ray diffraction peak, a small change in the a-axis lattice parameter and the c-axis lattice parameter, and in particular, the change rate of the c-axis lattice constant satisfies 2.5% or less. This is understood to be because the phase transition is suppressed by the lithium present in the transition metal layer, thereby reducing the change in the lattice constant. When the lattice constant change rate satisfies the above range, the occurrence of strain and crack in the cathode active material during charge and discharge is suppressed, the phenomenon of the cathode active material cracking or peeling off is reduced, and the life characteristics of the lithium secondary battery are remarkably improved.

[0042] The positive electrode active material can be expressed as having a change in the c-axis lattice constant of the layered structure of the compound represented by Chemical Formula 1 measured through real-time X-ray diffraction analysis of 2.5% or less during charge and discharge between 2.5 V and 4.25 V. Alternatively, the lithium secondary battery including the positive electrode active material can be expressed as having a change in the c-axis lattice constant of the positive electrode active material measured through real-time X-ray diffraction analysis of 2.5% or less during charge and discharge in the range of 2.5 V to 4.25 V.

[0043] Further, the c-axis lattice constant may be a value measured by an R-3m structure model, or may be a value measured by a C2 / m structure model. Alternatively, it may be a value measured by a mixture model of the R-3m structure and the C2 / m structure. No matter which structure model is used for measurement, the change rate of the c-axis lattice constant due to charge and discharge can satisfy 2.5% or less.

[0044] The change in the c-axis lattice constant of the positive electrode active material due to charge and discharge may be, for example, 2.2% or less, 2.0% or less, 1.8% or less, 1.7% or less, 1.5% or less, or 1.0% or less, and may also be 0.1% or more, 0.2% or more, 0.3% or more, 0.4% or more, or 0.5% or more. The positive electrode active material satisfying the above range is structurally very stable and can show excellent life characteristics without collapsing or cracking even during repeated charge and discharge, and can achieve high capacity.

[0045] Here, the change rate (%) of the c-axis lattice constant of the positive electrode active material due to charge and discharge may be derived through the calculation formula {(MAX - MIN) / MAX×100}. In the above calculation formula, MAX is the maximum value of the c-axis lattice during charge and discharge in the range of 2.5 V to 4.25 V, and MIN means the minimum value of the c-axis lattice.

[0046] Further, during charge and discharge in the range of 2.5V to 4.25V, the minimum value of the c-axis lattice constant of the positive electrode active material may be 99.5% or more, for example, 99.6% or more, 99.7% or more, 99.8% or more, or 99.9% or more with respect to the initial lattice constant. In this case, the positive electrode active material can maintain structural stability even during repeated charge and discharge and can exhibit excellent life characteristics.

[0047] During charge and discharge in the range of 2.5V to 4.25V, the c-axis lattice constant of the R-3m structure of the positive electrode active material may vary within the range of 13.90 Å to 14.46 Å, for example, within the range of 13.90 Å to 14.40 Å, or 14.00 Å to 14.30 Å, or 14.13 Å to 14.22 Å, 14.16 Å to 14.41 Å. When the c-axis lattice constant varies within such a narrow range, the positive electrode active material can achieve structural stability even during repeated charge and discharge, and thereby can exhibit high life characteristics.

[0048] Further, during charge and discharge in the range of 2.5V to 4.25V, the minimum value of the c-axis lattice constant of the positive electrode active material may be 99.5% or more, for example, 99.6% or more, 99.7% or more, 99.8% or more, or 99.9% or more with respect to the initial lattice constant. In this case, the positive electrode active material can maintain structural stability even during repeated charge and discharge and can exhibit excellent life characteristics.

[0049] During charge and discharge in the range of 2.5V to 4.25V, the c-axis lattice constant of the R-3m structure of the positive electrode active material may vary within the range of 13.90 Å to 14.46 Å, for example, within the range of 13.90 Å to 14.40 Å, or 14.00 Å to 14.30 Å, or 14.13 Å to 14.22 Å, 14.16 Å to 14.41 Å. When the c-axis lattice constant varies within such a narrow range, the positive electrode active material can achieve structural stability even during repeated charge and discharge, and thereby can exhibit high life characteristics.

[0050] Here, the change in the c-axis lattice constant can mean not only the value during the initial charge-discharge, but also the values during repeated charge-discharges such as the second and third charges. That is, the cathode active material according to one embodiment is structurally stable and can exhibit a very low change in the c-axis lattice constant even during repeated charge-discharges.

[0051] The cathode active material according to one embodiment has a cation mixing that means the nickel content in the lithium site is less than 5 atomic %. In the case of a high-nickel cathode active material with a nickel content exceeding 80 mol %, there is a problem that the Ni 2+ ion occupies the lithium site and the cation mixing phenomenon occurs excessively, resulting in a decrease in capacity. On the contrary, in the cathode active material according to one embodiment, excessive lithium comes to occupy a part of the transition metal sites, and the phenomenon that the average oxidation number of the transition metal increases appears, and it has been confirmed that this reduces the cation mixing. For example, it has been confirmed that due to excess lithium, the average oxidation number of nickel increases, the formation of the rock salt phase of the Ni(II)-O bond on the surface of the cathode active material is suppressed, the cation mixing is reduced, and the elution of nickel is suppressed. The cation mixing may be, for example, less than 4.5 atomic %, or may be less than 4.0 atomic % or less than 3.5 atomic %. When the cation mixing satisfies the above range, the cathode active material can achieve sufficient capacity and ensure battery stability.

[0052] Since the positive electrode active material has an excessive amount of lithium successfully incorporated into the active material structure, the content of impurities such as Li2CO3 and Li2O remaining in the active material is very low. Specifically, the content of Li2CO3 present in the positive electrode active material may be less than 0.5% by weight, for example, less than 0.4% by weight. Also, the content of Li2O present in the positive electrode active material may be less than 1.0% by weight, for example, less than 0.8% by weight or less than 0.5% by weight. The content of Li2CO3 and Li2O may be measured, for example, through X-ray diffraction analysis. When the content of impurities such as Li2CO3 and Li2O satisfies the above range, it is also proven that the lithium-excess positive electrode active material structure has been successfully synthesized, and a positive electrode active material that satisfies this can exhibit excellent battery characteristics such as high capacity and high life characteristics.

[0053] As an example, the positive electrode active material may contain a compound represented by the following Chemical Formula 2. [Chemical Formula 2] Li 1+a2 (Ni b2 Mn c2 M 2 1-b2-c2 ) 1-a2 O2

[0054] In the above Chemical Formula 2, 0 < a2 < 0.2, 0.8 < b2 < 1, 0 < c2 < 0.2, 0 ≤ 1 - b2 - c2 < 0.2, M 2 is one or more elements selected from Co, Al, Mg, Ca, Ti, V, Cr, Zr, Nb, Mo, and W. The positive electrode active material containing the compound represented by Chemical Formula 2 can exhibit excellent battery characteristics such as high capacity and high life characteristics.

[0055] As another example, the positive electrode active material may contain a compound represented by the following Chemical Formula 3.

[0056] [Chemical Formula 3] Li 1+a3 (Ni b3 Co c3 M 3 1-b3-c3 )1-a3 O2

[0057] In the above chemical formula 3, 0 < a3 < 0.2, 0.8 < b3 < 1, 0 < c3 < 0.2, 0 ≤ 1 - b3 - c3 < 0.2, M 3 is one or more elements selected from Mn, Al, Mg, Ca, Ti, V, Cr, Zr, Nb, Mo, and W. The positive electrode active material containing the compound represented by the above chemical formula 3 can exhibit excellent battery characteristics such as high capacity and high life characteristics while realizing high capacity.

[0058] As another example, the positive electrode active material may contain a compound represented by the following chemical formula 4.

[0059] [Chemical formula 4] Li 1+a4 (Ni b4 Co c4 Mn d4 M 4 1-b4-c4-d4 ) 1-a4 O2

[0060] In the above chemical formula 4, 0 < a4 < 0.2, 0.8 < b4 < 1, 0 < c4 < 0.1, 0 < d4 < 0.1, 0 ≤ 1 - b3 - c3 - d4 < 0.1, M 4 is one or more elements selected from Al, Mg, Ca, Ti, V, Cr, Zr, Nb, Mo, and W. The positive electrode active material containing the compound represented by the above chemical formula 4 can exhibit excellent battery characteristics such as high capacity and high life characteristics while realizing high capacity.

[0061] The average particle size of the positive electrode active material may be about 2 μm to 25 μm, for example, 5 μm to 25 μm, 10 μm to 25 μm, or 10 μm to 20 μm. When the average particle size of the positive electrode active material satisfies the above range, a positive electrode active material with a high tap density and a high energy density per unit volume can be realized.

[0062] In one embodiment, a method for manufacturing the above-described positive electrode active material is provided.

[0063] More specifically, it includes the steps of preparing a metal hydroxide precursor and a lithium raw material substance; and mixing and firing the metal hydroxide precursor and the lithium raw material substance; the firing temperature is t, the molar ratio of lithium in the lithium raw material substance to the metal in the metal oxide precursor is l / m, t is 780 °C or lower, and it can be a method for producing a positive electrode active material that satisfies the following formula 1.

[0064] [Formula 1] 500 ≦ t / (l / m) ≦ 710

[0065] Taking a specific example, it may be 530 ≦ t / (l / m) ≦ 710, or 600 ≦ t / (l / m) ≦ 710.

[0066] When satisfying such a range, an appropriate mixing effect of the metal and lithium in the aforementioned positive electrode material can be obtained, and thereby a positive electrode active material with improved life characteristics can be obtained.

[0067] The manufacturing method includes the step of mixing a precursor containing a compound represented by the following chemical formula 5 and a lithium raw material in a molar ratio of 1:1.06 to 1:1.3 and performing heat treatment in a temperature range of 650 °C to 780 °C.

[0068] [Chemical formula 5] Ni b11 M 11 1-b11 (OH)2

[0069] In the above chemical formula 11, 0.8 < b11 < 1, M 11 is one or more elements selected from Co, Mn, Al, Mg, Ca, Ti, V, Cr, Zr, Nb, Mo, W.

[0070] According to such a manufacturing method, it is possible to successfully synthesize a layered cathode active material which is a high-nickel-based material with a nickel content exceeding 80 mol% and has an excessive amount of lithium incorporated into the structure, and it is possible to synthesize a cathode active material with a change in the c-axis lattice constant of 2.5% or less due to charge and discharge. The synthesized cathode active material can exhibit excellent battery characteristics such as high capacity and high energy density while having high life characteristics.

[0071] The compound represented by Chemical Formula 5 is a transition metal hydroxide containing nickel and is a precursor of the cathode active material. In Chemical Formula 11, b11 represents the molar content of nickel with respect to the total content of the transition metal, and for example, 0.81 ≦ b11 ≦ 0.99, 0.83 ≦ b11 ≦ 0.99, 0.85 ≦ b11 ≦ 0.99, 0.87 ≦ b11 ≦ 0.99, 0.89 ≦ b11 ≦ 0.99, 0.90 ≦ b11 ≦ 0.99, 0.91 ≦ b11 ≦ 0.99, 0.92 ≦ b11 ≦ 0.99, or 0.81 ≦ b11 ≦ 0.98 may be used.

[0072] Synthesizing a lithium-excess high-nickel cathode active material has the characteristic that the success or failure of the synthesis can be determined by even a slight change in the lithium content to be added, and the electrochemical properties of the synthesized active material change significantly. In one embodiment, a stable structure is maintained while achieving a high capacity by mixing a transition metal hydroxide precursor and a lithium raw material at a molar ratio of 1:1.06 to 1:1.3 and heat-treating in a temperature range of 680°C to 780°C, and a lithium-excess high-nickel cathode active material can be successfully synthesized. The mixing ratio of the transition metal hydroxide precursor and the lithium raw material may be, for example, a molar ratio of 1:1.06 to 1:1.25, or 1:1.06 to 1:1.2. Also, the heat treatment temperature may be, for example, 680°C to 750°C, 680°C to 740°C, 680°C to 730°C, 680°C to 710°C, 680°C to 700°C, or 690°C to 780°C, or 700°C to 750°C. When the mixing ratio of the transition metal hydroxide precursor and the lithium raw material satisfies the above range, and when the heat treatment temperature satisfies the above range, the target lithium-excess high-nickel cathode active material can be successfully synthesized. That is, a lithium-excess cathode active material can be successfully synthesized from a material with a nickel content exceeding 80 mol% and a molar content of lithium present in the structure satisfying 1.02 to 1.15. The synthesized cathode active material can exhibit excellent battery characteristics such as high capacity, high energy density, and high cycle life characteristics.

[0073] The transition metal hydroxide, that is, the precursor of the cathode active material, can be produced by a general coprecipitation method. For example, the precursor can be produced by introducing an aqueous metal salt solution containing a nickel raw material such as a nickel salt, an aqueous ammonia solution as a chelating agent, and an alkaline aqueous solution such as NaOH for pH adjustment into a coprecipitation reactor and performing a coprecipitation reaction while injecting N2 to prevent oxidation of metal ions.

[0074] The nickel salt may be nickel sulfate, nickel nitrate, nickel chloride, nickel fluoride, or a combination thereof. The aqueous metal salt solution may further contain a cobalt salt, a manganese salt, an aluminum salt, etc. in addition to the nickel salt. The cobalt salt may be, for example, cobalt sulfate, cobalt nitrate, cobalt chloride, cobalt fluoride, or a combination thereof. The manganese salt may be, for example, manganese sulfate, manganese nitrate, manganese chloride, manganese fluoride, or a combination thereof, and the aluminum salt may be, for example, aluminum sulfate, aluminum nitrate, aluminum chloride, aluminum fluoride, or a combination thereof.

[0075] The lithium raw material may contain, for example, Li2CO3, LiOH, or a combination thereof.

[0076] Another embodiment provides a lithium secondary battery including a positive electrode containing the aforementioned positive electrode active material, a negative electrode, and a non-aqueous electrolyte.

[0077] The positive electrode includes a current collector and a positive electrode active material layer located on the current collector. The positive electrode active material layer contains a positive electrode active material, and the positive electrode active material may contain a positive electrode active material for a lithium secondary battery according to the aforementioned embodiment. In the positive electrode active material layer, the content of the positive electrode active material may be 90% to 99% by weight based on the total weight of the positive electrode active material layer.

[0078] The positive electrode active material layer may further contain a binder and / or a conductive material. At this time, the contents of the binder and the conductive material may each be 1% to 5% by weight based on the total weight of the positive electrode active material layer.

[0079] The binder serves to well adhere the positive electrode active material particles to each other and also well adhere the positive electrode active material to the current collector. As typical examples of the binder, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene butadiene rubber, acrylated styrene butadiene rubber, epoxy resin, nylon, etc. can be used, but it is not limited thereto.

[0080] The conductive material is used to impart conductivity to the electrode, and in the battery to be configured, any material can be used as long as it is an electron conductive material that does not cause a chemical change. Examples of the conductive material include carbon-based substances such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, and carbon fiber; metal-based substances such as metal powders or metal fibers of copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or conductive materials containing a mixture thereof.

[0081] As the positive electrode current collector, aluminum foil, nickel foil, or a combination thereof can be used, but it is not limited thereto.

[0082] The negative electrode includes a current collector and a negative electrode active material layer formed on the current collector, and the negative electrode active material layer contains a negative electrode active material.

[0083] Examples of the negative electrode active material include substances that can reversibly intercalate / deintercalate lithium ions, lithium metal, alloys of lithium metal, substances that can dope and undope lithium, or transition metal oxides.

[0084] As the substance capable of reversibly intercalating / deintercalating the lithium ions, a carbonaceous substance can be used, and any of the carbonaceous negative electrode active materials generally used in lithium secondary batteries can be used. Representative examples thereof include crystalline carbon, amorphous carbon, or these can be used together.

[0085] As the alloy of the lithium metal, an alloy of lithium and a metal selected from the group consisting of Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn can be used.

[0086] As the substance capable of doping and undoping the lithium, Si, SiO x (0 < x < 2), Si - Y alloy (wherein Y is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a transition metal, a rare earth element, and combinations thereof, and is not Si), Sn, SnO2, Sn - Y (wherein Y is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a transition metal, a rare earth element, and combinations thereof, and is not Sn), etc. can be mentioned.

[0087] Examples of the transition metal oxide include vanadium oxide, lithium vanadium oxide, etc. The negative electrode active material layer also contains a binder and may further selectively contain a conductive material.

[0088] The binder serves to well adhere the negative electrode active material particles to each other and also to well adhere the negative electrode active material to the current collector.

[0089] The conductive material is used to impart conductivity to the electrode, and in the constructed battery, any electron conductive material that does not cause a chemical change can be used.

[0090] As the current collector, those selected from the group consisting of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof can be used.

[0091] The negative electrode and the positive electrode are manufactured by mixing an active material, a conductive material, and a binder in a solvent to produce an active material composition, and applying this composition to a current collector. Since such an electrode manufacturing method is well-known in the art, detailed description thereof is omitted herein. As the solvent, N-methylpyrrolidone or the like can be used, but is not limited thereto.

[0092] The electrolyte contains a non-aqueous organic solvent and a lithium salt.

[0093] The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move.

[0094] The lithium salt is dissolved in an organic solvent and acts as a source of lithium ions in the battery to enable the operation of a basic lithium secondary battery, and plays a role in promoting the movement of lithium ions between the positive electrode and the negative electrode.

[0095] Depending on the type of lithium secondary battery, a separator may be present between the positive electrode and the negative electrode. As such a separator, polyethylene, polypropylene, polyvinylidene fluoride, or a multilayer film of two or more layers thereof can be used, and mixed multilayer films such as a two-layer separator of polyethylene / polypropylene, a three-layer separator of polyethylene / polypropylene / polyethylene, and a three-layer separator of polypropylene / polyethylene / polypropylene can of course be used.

[0096] Lithium secondary batteries can be classified into lithium-ion batteries, lithium-ion polymer batteries, and lithium polymer batteries depending on the type of separator and electrolyte used, can be classified into cylindrical, square, coin-type, pouch-type, etc. depending on the form, and can be divided into bulk type and thin film type depending on the size. Since the structures and manufacturing methods of these batteries are widely known in this field, detailed descriptions will be omitted.

[0097] Hereinafter, preferred embodiments and comparative examples of the present invention will be described. However, the following examples are merely preferred examples of the present invention, and the present invention is not limited to the following examples.

Example

[0098] 「Example」 (1) Production of cathode active material precursor Ni 0.92 Co 0.04 Mn 0.04 A precursor having a composition of (OH)2 is produced by a general coprecipitation method. Specifically, NiSO4·6H2O, CoSO4·7H2O, and MnSO4·H2O are dissolved in distilled water to produce an aqueous metal salt solution. After preparing a coprecipitation reactor, N2 is injected to prevent oxidation of metal ions during the coprecipitation reaction, and the reactor temperature is maintained at 50°C. NH4(OH) is introduced into the coprecipitation reactor as a chelating agent, and NaOH is introduced for pH adjustment. The precipitate obtained by the coprecipitation process is filtered, washed with distilled water, and then dried in an oven at 100°C for 24 hours to produce a cathode active material precursor having an average diameter of about 14.8 μm.

[0099] (2) Production of cathode active material After mixing the produced precursor and LiOH·H2O in various molar ratios, it is placed in a tube furnace and fired while flowing oxygen at 50 mL / min. After firing under the conditions shown in the following table, the product is then naturally cooled to 25°C.

[0100] (3) Production of battery 92% of the manufactured cathode active material, 4% of Super P as the conductive material, and 4% of PVDF as the binder were put into an NMP solution, mixed to produce a slurry, and then coated on an aluminum current collector to manufacture a cathode. After that, a coin half-cell with lithium metal as the anode was fabricated.

[0101]

Table 1

[0102] “Evaluation Example: X-ray Diffraction Pattern Analysis” X-ray diffraction analysis was performed on the cathode active materials manufactured in the examples and comparative examples, and the results are shown in FIGS. 2 to 4. FIG. 2 is the X-ray diffraction pattern of the active material of the comparative example, FIG. 3 is the X-ray diffraction pattern of the active material of the example, and FIG. 4 is the X-ray diffraction pattern of the active material of the example.

[0103] In the X-ray diffraction patterns of FIGS. 2 to 4, the broad peak near 21 degrees on the horizontal axis corresponds to the peak due to the C2 / m phase. In the case of the examples, it can be confirmed that as the lithium content increases, the phase corresponding to the C2 / m space group is increasingly formed, thereby indirectly confirming that excessive lithium has penetrated into the active material. On the contrary, it can be seen that in the comparative example, lithium is not contained in the active material structure even though the lithium content increases.

[0104] “Evaluation Example: Analysis of the Content of Residual Impurities” Through the Rietveld analysis method using X-ray diffraction analysis on the cathode active materials manufactured in the examples and comparative examples, the contents of the residual impurities Li2CO3 and Li2O were measured.

[0105] The content of impurities in the cathode active material of the comparative example is shown in FIG. 5, the content of impurities in the cathode active material of the example is shown in FIG. 6, and the content of impurities in the cathode active material of the example is shown in FIG. 7.

[0106] In Figure 5, in the case of the comparative example, it can be confirmed that excess lithium does not enter the active material structure and remains in the form of impurities such as Li2CO3 and Li2O. On the contrary, in Figure 6, in the case of the example, the impurity content is very much reduced, and in Figure 7, in the case of the example, it can be confirmed that there is almost no impurity and excess lithium has penetrated into the active material structure.

[0107] "Evaluation Example: Analysis of the Oxidation Number of Nickel" Through X-ray Absorption Near-Edge Structure (XANES) analysis of the cathode active materials produced in Examples 1 to 12 and Comparative Examples 1 to 6, the oxidation number of nickel was analyzed. The change in the oxidation number of nickel for the cathode active material of the comparative example is shown in Figure 8, and the change in the oxidation number of nickel for the cathode active material of the example is shown in Figures 9 and 10.

[0108] Generally, if lithium penetrates into the active material structure to form a lithium-excess structure, the oxidation number of the remaining transition metal, where lithium occupies the site of the transition metal, increases. Referring to Figures 8 to 10, in the case of the comparative example, there is almost no change in the oxidation number of nickel as the lithium content increases, but in the case of the example, it can be confirmed that the increase in the oxidation number of nickel is clearly seen as the lithium content increases. Thus, it can be confirmed that excess lithium has successfully entered the active material structure and the synthesis of the target cathode active material has been smoothly carried out.

[0109] "Evaluation Example: Analysis of Lithium Content in Cathode Active Material Structure through Neutron Diffraction Analysis" Neutron diffraction analysis was performed on the cathode active materials produced in the examples and comparative examples using the HANARO reactor of the Korea Atomic Energy Research Institute, and Rietveld refinement was performed using this, thereby analyzing the lithium content in the cathode active material structure, and the results are shown in Figure 11.

[0110] Referring to Fig. 11, in the case of the example, it can be confirmed that the molar content of lithium in the active material structure ranges from 1.01 to 1.15 with respect to 1 mole of the active material, and it can be confirmed once again that lithium enters the active material structure better than in the comparative example.

[0111] Table 2 below shows the results of analyzing the lithium molar concentration in the transition metal layer and the transition metal molar concentration in the lithium layer.

[0112] It can be confirmed that the life characteristics can be improved only when the substitution amount of lithium in the first metal layer reaches 1.5 mol% or more.

[0113] Also, it can be confirmed that the life characteristics can be ensured only when the substitution amount of lithium in the first metal layer is more than the substitution amount of the metal in the first lithium layer and the difference is 1 mol% or more.

[0114]

Table 2

[0115] "Evaluation Example 3: PDF (Pair Distribution Function) Analysis of Fourier-Transformed Neutron Diffraction Analysis Results" For understanding the local structure change of the M-O bond due to the substitution of transition metals with excess lithium, a two-body distribution function (PDF; Pair Distribution Function) analysis of the neutron diffraction analysis results was performed by Fourier transformation. Fig. 19 shows the PDF analysis results for the positive electrode active material manufactured in the comparative example, and Fig. 20 shows the PDF analysis results for the positive electrode active material manufactured in the comparative example and the example.

[0116] Comparing FIGS. 19 and 20, in the case of the comparative example synthesized at 800°C, there is no significant difference in the Ni-O bond distance. This means that the lithium / transition metal ratio within the crystal structure is maintained almost constant regardless of the lithium / transition metal ratio input during synthesis. On the contrary, in the case of one example of synthesis at 700°C, the phenomenon that the Ni-O bond distance decreases as the lithium becomes excessive is shown. This phenomenon occurs because at a low synthesis temperature, the ratio of lithium in the structure increases and the ratio of transition metals decreases. As a result, the oxidation number of Ni increases, the Ni-O bond strength increases, and thus the Ni-O bond distance decreases. This means that the excessively input lithium is contained in the structure at 700°C.

[0117] Tables 3 and 4 below are data obtained by manufacturing more diverse cathode materials and evaluating up to the battery characteristics according to the length of M-O.

[0118] It can be confirmed that the examples with a reduced M-O length ensure excellent life characteristics.

[0119]

Table 3

[0120]

Table 4

[0121] 「Evaluation Example: Cation Mixed Analysis」 Through Rietveld analysis using neutron diffraction analysis on the cathode active materials produced in the examples and comparative examples, the ratio occupied by nickel in the lithium site, that is, cation mixing, was analyzed, and the results are shown in FIG. 12. Looking at the graph of the examples in FIG. 12, it can be confirmed that as the lithium content increases and the amount of excess lithium entering the active material structure increases, the oxidation number of the transition metal increases, and thereby cation mixing decreases. Also, it can be confirmed that the cation mixing of the cathode active material of the examples is less than 5 atomic %. It is confirmed that the cation mixing in the examples is more than that in the comparative examples because the synthesis temperature in the examples was relatively lower than that in the comparative examples, resulting in more cation mixing being shown.

[0122] "Evaluation Example: Real-Time X-Ray Diffraction Analysis during Charge and Discharge" For the batteries produced in the examples and comparative examples where the molar ratio of the lithium raw material to the precursor is 1.30, charge and discharge are carried out in the range of 2.5 V to 4.25 V to analyze real-time X-ray diffraction analysis and the lattice constant change of the R-3m structure model thereby. The X-ray diffraction analysis (XRD) was carried out with an X-ray diffractometer (Empyrean, Malvern Panalytical) equipped with CuKα radiation (λ = 1.540598 Å).

[0123] In the case of the examples, it can be seen that the change in the lattice constant is even smaller compared to the comparative examples. From this, it can be understood that the active material produced in the examples suppresses structural collapse and cracks due to charge and discharge, and thereby the life characteristics of the battery are improved.

[0124] For the batteries produced in the examples and comparative examples, a second charge and discharge was carried out in the range of 2.5 V to 4.25 V, and the lattice constant change by real-time X-ray diffraction analysis was confirmed.

[0125] In the case of the examples, it can be confirmed that the change in the lattice constant is even smaller compared to the comparative examples. Through this, it can be understood that the change in the lattice constant is a reversible characteristic and not a characteristic only of the first cycle.

[0126] The change in the c-axis lattice constant is specifically shown in Tables 5 and 6 below.

[0127]

Table 5

[0128]

Table 6

[0129] Referring to the above tables, in the case of the examples, in the range of 2.5 V to 4.25 V, the change rate of the c-axis lattice constant of the R-3m structure at the initial and second charge and discharge is 2.5% or less, the maximum value of the c-axis lattice constant is less than 14.41 Å, and the change value of the c-axis lattice constant is 0.35 Å or less. It can be confirmed that the positive electrode active material of the example is very stable structurally and exhibits high life characteristics without collapsing even under repeated charge and discharge.

[0130] It can also be seen that the change in the lattice constant is a characteristic that is controlled not only by the firing temperature but also by the lithium addition amount.

[0131] “Evaluation Example: Electrochemical Characteristics of Batteries at High Voltages” Charging was performed on the half cells manufactured in the examples and comparative examples with a cut-off at 4.9 V. The electrochemical characteristics of the batteries of the comparative examples are shown in FIG. 13, the electrochemical characteristics of the batteries of the examples are shown in FIG. 14, and the electrochemical characteristics of the batteries of the examples are shown in FIG. 15.

[0132] Generally, in the case of a lithium-excess material, that is, an active material in which excess lithium is present in the active material structure, it is known to show an oxidation / reduction reaction of oxygen at high voltages. In the case of the batteries of the examples shown in FIGS. 14 and 15, it can be confirmed that an irreversible reaction occurred at around 4.6 V during the first charge, and this reaction is generally known to be an oxidation / reduction reaction of oxygen. Therefore, it can be confirmed once again that the target lithium-excess positive electrode active material was smoothly synthesized in the examples.

[0133] "Evaluation Example: Evaluation of Battery Life Characteristics" The batteries manufactured in the examples and comparative examples were charged and discharged up to about 50 cycles to evaluate their life characteristics.

[0134] The life characteristics of the batteries in the comparative examples are shown in Fig. 16, and the life characteristics of the batteries in the examples are shown in Figs. 17 and 18.

[0135] Referring to Figs. 16 to 18, it can be confirmed that the batteries of the examples achieve high capacity and high life characteristics.

[0136] As described above in detail with respect to the preferred embodiments, the scope of the rights of the present invention is not limited thereto, and various modifications and improvements by those skilled in the art using the basic concepts defined in the following claims also belong to the scope of the rights of the present invention.

Claims

1. In a layered metal oxide crystal structure including a lithium layer and a metal layer, the crystal structure includes a first metal layer in which part of the metal in the metal layer is substituted with lithium, the crystal structure includes a first lithium layer in which part of the lithium in the lithium layer is substituted with a metal, the substitution amount of lithium in the first metal layer is 1.5 mol% or more, A positive electrode active material having a nickel content of 80 mol% or more based on 100 mol% of the metal in the positive electrode active material.

2. The positive electrode active material according to claim 1, wherein the substitution amount of lithium in the first metal layer is 1.5 mol% or more and 15 mol% or less.

3. The positive electrode active material according to claim 1, wherein the substitution amount of the metal in the first lithium layer is 0.5 mol% or more and 5 mol% or less.

4. The positive electrode active material according to claim 1, wherein the substitution amount of lithium in the first metal layer is more than the substitution amount of the metal in the first lithium layer.

5. The positive electrode active material according to claim 4, wherein the difference between the substitution amount of lithium in the first metal layer and the substitution amount of the metal in the first lithium layer is 1 mol% or more and 15 mol% or less.

6. The positive electrode active material according to claim 1, wherein the molar ratio of lithium to the total metal in the positive electrode active material is 1.01 or more and 1.2 or less.

7. The positive electrode active material according to claim 1, wherein the c-axis lattice constant change rate of the R-3m structure during charge and discharge in the range of 2.5 V to 4.25 V is 2.5% or less.

8. The positive electrode active material for a lithium secondary battery according to claim 7, wherein the maximum value of the c-axis lattice constant of the R-3m structure during charge and discharge in the range of 2.5 V to 4.25 V is 14.410 Å or less.

9. The positive electrode active material for a lithium secondary battery according to claim 8, wherein the c-axis lattice constant of the R-3m structure changes within the range of 13.900 Å to 14.410 Å during charge and discharge in the range of 2.5 V to 4.25 V.

10. The positive electrode active material according to claim 1, wherein the bond length between the metal and oxygen in the positive electrode active material is 1.92 Å or less.

11. Li present in the positive electrode active material measured through X-ray diffraction analysis ２ CO ３ The content of is less than 0.5% by weight, and the positive electrode active material for a lithium secondary battery according to claim 1.

12. Li present in the positive electrode active material measured through X-ray diffraction analysis ２ The positive electrode active material for a lithium secondary battery according to claim 1, wherein the content of Li ２ O is less than 1.0% by weight.

13. The positive electrode active material for a lithium secondary battery according to any one of claims 1 to 12, wherein the nickel content is 90 mol% or more based on 100 mol% of the metal in the positive electrode active material.

14. In a layered metal oxide crystal structure including a lithium layer and a metal layer, the crystal structure includes a first metal layer in which part of the metal in the metal layer is substituted with lithium, the crystal structure includes a first lithium layer in which part of the lithium in the lithium layer is substituted with a metal, A positive electrode active material in which the substitution amount of lithium in the first metal layer is greater than the substitution amount of metal in the first lithium layer.

15. Preparing a metal hydroxide precursor and a lithium raw material substance; and Mixing and firing the metal hydroxide precursor and the lithium raw material substance; comprising the firing temperature is t, and the molar ratio of lithium in the lithium raw material substance to the metal in the metal oxide precursor is l / m, the t is 780 °C or lower, A method for producing a positive electrode active material that satisfies the following Formula 1. [Formula 1] 500 ≤ t / (l / m) ≤ 710

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