Positive electrode active material for lithium secondary batteries, method for manufacturing the same, and lithium secondary battery

Abstract

The present invention relates to a positive electrode active material for a lithium secondary battery, the positive electrode active material including a compound represented by Chemical Formula 1, the molar content of lithium present in the structure of the positive electrode active material being 1.02 to 1.15 per mole of the positive electrode active material as measured by neutron diffraction analysis, and the rate of change in c-axis lattice constant of the R-3m structure upon charging and discharging in a range of 2.5 V to 4.25 V is less than 2.3%. The present invention also relates to a positive electrode active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery including the same. [Chemical formula 1] Li 1+a (Ni b M 1 1-b ) 1-a O 2 In the above Chemical Formula 1, 0 <a<0.2、0.8<b<1、M 1 is one or more elements selected from Co, Mn, Al, Mg, Ca, Ti, V, Cr, Zr, Nb, Mo, and W.

Description

[Technical Field]

[0001] This disclosure relates to a positive electrode active material for lithium secondary batteries, a method for manufacturing the same, and a lithium secondary battery containing the same. [Background technology]

[0002] Recently, with the expanding demand for electric vehicles, the demand for lithium-ion batteries to power them has increased dramatically. Layered lithium transition metal oxides (LiMO2, M=Ni, Co, Mn, etc.) are mainly used as cathode active materials for such lithium-ion batteries, and research to increase their capacity is actively underway. Among existing layered cathode active materials, the materials with the highest capacity are LiNiO2 and high-nickel layered cathode materials. However, the instability of Ni(III) makes it difficult to synthesize the material stoichiometrically, and even small changes in lithium during the synthesis process can significantly alter the electrochemical properties. [Overview of the Initiative] [Problems that the invention aims to solve]

[0003] This disclosure provides a lithium-rich high-nickel cathode active material having high price competitiveness, high stability, and high energy density, a method for producing the same, and a lithium secondary battery containing the same. [Means for solving the problem]

[0004] In one embodiment, a positive electrode active material for a lithium secondary battery is provided, comprising a compound represented by chemical formula 1, wherein the molar content of lithium present in the structure of the positive electrode active material, as measured by neutron diffraction analysis, is 1.02 to 1.15 per mole of the positive electrode active material, and the change in the c-axis lattice parameter of the R-3m structure due to charging and discharging in the range of 2.5V to 4.25V is less than 2.3%.

[0005] [Chemical formula 1] Li 1+a(Ni b M 1 1-b ) 1-a O2 In Chemical Formula 1, 0 < a < 0.2, 0.8 < b < 1, M 1 is one or more elements selected from Co, Mn, Al, Mg, Ca, Ti, V, Cr, Zr, Nb, Mo, and W.

[0006] In another embodiment, there is provided a method for producing a positive electrode active material for a lithium secondary battery, including mixing a precursor containing a compound represented by Chemical Formula 11 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 680 °C to 780 °C.

[0007] [Chemical Formula 11] Ni b11 M 11 1-b11 (OH)2

[0008] In 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, and W.

[0009] In yet another embodiment, there is provided a lithium secondary battery including a positive electrode containing the positive electrode active material, a negative electrode, and an electrolyte. [Advantages of the Invention]

[0010] The positive electrode active material according to one embodiment and the lithium secondary battery including the same embody high price competitiveness, high stability, high energy density, high capacity, and high life characteristics. [Brief Description of the Drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram showing the chemical structure of a high-nickel-based positive electrode active material with excess lithium. [Figure 2] FIG. 2 is a graph analyzing the lithium content in the positive electrode active material structures of Examples 1 to 8 and Comparative Examples 1 to 5, 11, and 12. [Figure 3] Figure 3 is a real-time X-ray diffraction pattern analysis graph for the positive electrode active material of Example 4. [Figure 4] Figure 4 is a real-time X-ray diffraction pattern analysis graph for the positive electrode active material of Example 8. [Figure 5] Figure 5 shows the real-time X-ray diffraction pattern analysis graph for the positive electrode active material of Comparative Example 5. [Figure 6] Figure 6 is a graph showing the change in lattice constants obtained by real-time X-ray diffraction analysis of the positive electrode active material in Example 4. [Figure 7] Figure 7 is a graph showing the change in lattice constants obtained by real-time X-ray diffraction analysis of the positive electrode active material in Example 8. [Figure 8] Figure 8 is a graph showing the change in lattice constants of the positive electrode active material of Comparative Example 5, obtained by real-time X-ray diffraction analysis. [Figure 9] Figure 9 is a graph showing the change in lattice constants of the positive electrode active material in Example 4 as determined by real-time X-ray diffraction analysis during the second charge-discharge cycle. [Figure 10] Figure 10 is a graph showing the change in lattice constants of the positive electrode active material in Example 8 as determined by real-time X-ray diffraction analysis during the second charge-discharge cycle. [Figure 11] Figure 11 is a graph showing the change in lattice constants of the positive electrode active material of Comparative Example 5 as determined by real-time X-ray diffraction analysis during the second charge-discharge cycle. [Figure 12] Figure 12 shows the pair distribution function (PDF) analysis graph for the cathode active materials produced in Comparative Examples 1, 11, and 12. [Figure 13] Figure 13 shows the pair distribution function (PDF) analysis graph for the cathode active materials produced in Example 4, Example 8, and Comparative Example 5. [Figure 14] Figure 14 is a graph analyzing the cation mixing of the positive electrode active materials in Examples 1 to 8 and Comparative Examples 1 to 5, 11, and 12. [Figure 15] Figure 15 is a graph showing the content of Li2CO3 and Li2O impurities remaining in the positive electrode active materials of Comparative Examples 1 to 5. [Figure 16] Figure 16 is a graph showing the content of Li2CO3 and Li2O impurities remaining in the positive electrode active materials of Comparative Example 12 and Examples 5 to 8. [Figure 17] Figure 17 is a graph showing the content of Li2CO3 and Li2O impurities remaining in the positive electrode active materials of Comparative Example 11 and Examples 1 to 4. [Figure 18] Figure 18 is a graph showing the life characteristics of batteries for Comparative Example 12 and Examples 5, 6, 7, and 8. [Figure 19] Figure 19 is a graph showing the life characteristics of batteries for Comparative Example 11 and Examples 1, 2, 3, and 4. [Figure 20] Figure 20 is a graph showing the lifespan characteristics of batteries for Comparative Examples 1, 2, 3, 4, and 5. [Modes for carrying out the invention]

[0012] The embodiments of this disclosure are described below in detail so as to be easily implemented by a person with ordinary skill in the art. However, this disclosure can be embodied in a variety of different forms and is not limited to the embodiments described herein.

[0013] The terms used herein are for illustrative purposes only and are not intended to limit the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0014] Here, "these combinations" refers to mixtures of components, laminates, composites, copolymers, alloys, blends, reaction products, etc.

[0015] Here, terms such as “include,” “equip,” or “possess” are intended to specify the presence of implemented features, figures, steps, components, or combinations thereof, and should be understood not to preemptively exclude the presence or possibility of adding one or more other features, figures, steps, components, or combinations thereof.

[0016] To clearly represent various layers and regions in the drawings, thicknesses are shown enlarged, and similar parts are denoted by the same reference numerals throughout the specification. When a layer, film, region, plate, or other part is said to be "on top of" or "above" another part, this includes not only when it is "directly on top of" another part, but also when there is another part in between. Conversely, when a part is said to be "directly on top of" another part, it means that there is no other part in between.

[0017] Furthermore, the term "layer" here includes not only the shapes formed on the entire surface when observed in a plan view, but also the shapes formed on some of the surfaces.

[0018] According to one embodiment, the positive electrode active material for a lithium secondary battery is a lithium-rich, high-nickel layered positive electrode active material containing a compound represented by the following chemical formula 1.

[0019] [Chemical formula 1] Li 1+a (Ni b M 1 1-b ) 1-a O2

[0020] In the above chemical formula 1, 0 <a<0.2、0.8<b<1、M 1 is one or more elements selected from Co, Mn, Al, Mg, Ca, Ti, V, Cr, Zr, Nb, Mo, and W.

[0021] Here, lithium excess means that excess lithium is present in the active material structure, occupying a portion of the transition metal space. Figure 1 shows the chemical structure of a positive electrode active material according to one embodiment, and shows a structure in which excess lithium is present in a portion of the transition metal space 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 per mole of positive electrode active material. The molar content of lithium may be measured, for example, by neutron diffraction analysis. The molar content of lithium present in the structure of the positive electrode active material can also be expressed as 1.02 to 1.15 per mole of the compound represented by chemical formula 1. In chemical formula 1, (1+a) indicates the lithium content in the active material structure, and the range of a may be, for example, 0.005≦a≦0.19, 0.01≦a≦0.17, or 0.02≦a≦0.15.

[0022] Furthermore, the term "high-nickel system" means that the active material has a high nickel content, specifically that the nickel content exceeds 80 mol% based on the total content of transition metals excluding lithium. For example, the nickel content may be 81 mol% or more, 85 mol% or more, 89 mol% or more, 90 mol% or more, or 92 mol% or more. In the chemical formula 1, the b value indicating the nickel content 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.

[0023] In other words, the positive electrode active material according to one embodiment is a high-nickel material with a nickel content exceeding 80 mol%, and is a lithium-rich active material in which 1.02 to 1.15 mol of lithium is incorporated into the active material structure.

[0024] Generally, high-nickel cathode active materials achieve high capacity, but firstly, synthesis itself is difficult, and ensuring structural stability is also difficult, and even if synthesized, Ni 2+ A frequent problem arises where cation mixing, where ions occupy lithium sites, increases, actually reduces capacity, making it difficult to ensure battery safety. When excess lithium raw material is added during synthesis to reduce cation mixing and increase 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 reduce battery capacity and cause stability problems.

[0025] Therefore, the inventors discovered that minute changes in lithium composition and synthesis temperature significantly alter the electrochemical properties of lithium-rich, high-nickel layered cathode active materials. They confirmed that by synthesizing within a specific lithium content range and temperature range, it is possible to successfully synthesize a cathode active material with a very high nickel content, a constant lithium content within the active material structure, and a stable structure in which the c-axis lattice constant change rate of the R-3m structure is less than 2.3% during charge-discharge in the 2.5V to 4.25V range. Furthermore, they confirmed that the synthesized cathode active material exhibited high capacity and high energy density while improving battery life characteristics and stability.

[0026] In one embodiment, a lithium-rich high-nickel layered positive electrode active material exhibits a change in the c-axis lattice constant of less than 2.3% when charged and discharged in the 2.5V to 4.25V range. Specifically, a lithium secondary battery to which the positive electrode active material is applied is subjected to real-time X-ray diffraction pattern analysis while being charged and discharged in the 2.5V to 4.25V range, and the resulting change in the lattice constant is analyzed. According to this analysis, the positive electrode active material according to one embodiment exhibits very small changes in the X-ray diffraction peak, small changes in the a-axis lattice constant and c-axis lattice constant, and in particular, the rate of change in the c-axis lattice constant satisfies less than 2.3%. 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 rate of change of the lattice constant satisfies the above range, the occurrence of strain and cracks in the positive electrode active material during charging and discharging is suppressed, reducing the phenomenon of the positive electrode active material breaking down or falling off, and dramatically improving the life characteristics of the lithium secondary battery.

[0027] The positive electrode active material may be described as having a change in the c-axis lattice constant of the layered structure of the compound represented by chemical formula 1, measured by real-time X-ray diffraction analysis during charging and discharging at 2.5V to 4.25V, of less than 2.3%. Alternatively, a lithium secondary battery containing the positive electrode active material may be described as having a change in the c-axis lattice constant of the positive electrode active material, measured by real-time X-ray diffraction analysis during charging and discharging in the 2.5V to 4.25V range, of less than 2.3%.

[0028] Furthermore, the c-axis lattice constant may be a value measured using the R-3m structure model, or a value measured using the C2 / m structure model. Alternatively, it may be a value measured using a mixed model of the R-3m and C2 / m structures. Regardless of which structure model is used for measurement, the rate of change of the c-axis lattice constant due to charging and discharging can satisfy the requirement of less than 2.3%.

[0029] The change in the c-axis lattice constant of the positive electrode active material due to charging and discharging 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, or 0.1% or more, 0.2% or more, 0.3% or more, 0.4% or more, or 0.5% or more. A positive electrode active material satisfying the above range is structurally very stable and can exhibit excellent life characteristics without collapsing or breaking down even after repeated charging and discharging, and can realize high capacity.

[0030] Here, the rate of change (%) of the c-axis lattice constant of the positive electrode active material due to charging and discharging 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 charging and discharging in the range of 2.5V to 4.25V, and MIN is the minimum value of the c-axis lattice.

[0031] The positive electrode active material may have a change in the c-axis lattice constant of its R-3m structure due to charging and discharging in the range of 2.5V to 4.25V, for example, 0.33 Å or less, 0.32 Å or less, 0.31 Å or less, 0.30 Å or less, less than 0.30 Å, 0.28 Å or less, 0.26 Å or less, or 0.25 Å or less, or 0.01 Å or more, 0.05 Å or more, or 0.08 Å or more. When such a range is satisfied, the positive electrode active material can exhibit excellent lifetime characteristics, as the phenomenon of structural collapse or breakdown is suppressed even after repeated charging and discharging.

[0032] The change in the c-axis grid constant refers to the value obtained by subtracting the minimum value (MIN) from the maximum value (MAX) of the c-axis grid during charging and discharging in the range of 2.5V to 4.25V.

[0033] Furthermore, the positive electrode active material may have a minimum c-axis lattice constant of 99.5% or more of the initial lattice constant during charging and discharging in the 2.5V to 4.25V range, for example, 99.6% or more, 99.7% or more, 99.8% or more, or 99.9% or more. In this case, the positive electrode active material can maintain structural stability even after repeated charging and discharging, and can exhibit excellent life characteristics.

[0034] The positive electrode active material may also have a c-axis lattice constant of the R-3m structure that changes within the range of 13.90 Å to 14.46 Å during charging and discharging in the 2.5 V to 4.25 V range. For example, it may change within the range of 13.90 Å to 14.40 Å, 14.00 Å to 14.30 Å, 14.13 Å to 14.22 Å, or 14.16 Å to 14.41 Å. When the c-axis lattice constant changes within such a narrow range, the positive electrode active material can exhibit structural stability even after repeated charging and discharging, thereby demonstrating high lifetime characteristics.

[0035] Here, the change in the c-axis lattice constant can refer not only to the value during the initial charge and discharge, but also to the value during repeated charge and discharge cycles, such as the second and third charges. In other words, the positive electrode active material according to one embodiment is structurally stable and can exhibit a very small change in the c-axis lattice constant even after repeated charge and discharge cycles.

[0036] In one embodiment, the positive electrode active material has a cation mixture of less than 5 atomic percent, which means the nickel content within the lithium site. In the case of a high-nickel positive electrode active material with a nickel content exceeding 80 mol%, Ni 2+ There is a problem in which excessive cation mixing occurs, where ions occupy lithium sites, leading to a decrease in capacity. In contrast, in one embodiment of the positive electrode active material, excess lithium occupies a portion of the transition metal space, resulting in an increase in the average oxidation state of the transition metal, which in turn reduces cation mixing. For example, it was confirmed that excess lithium increases the average oxidation state of nickel, suppressing the formation of the rock salt phase of Ni(II)-O bonds on the surface of the positive electrode active material, reducing cation mixing, and suppressing nickel elution. The cation mixing may be, for example, less than 4.5 atomic%, less than 4.0 atomic%, or less than 3.5 atomic%. When the cation mixing satisfies the above range, the positive electrode active material can realize sufficient capacity and ensure battery stability.

[0037] Since the positive electrode active material has successfully incorporated excess lithium into its active material structure, the content of impurities such as Li2CO-3 and Li2O remaining in the active material is very low. Specifically, the content of Li2CO-3 in the positive electrode active material may be less than 0.5% by weight, for example, less than 0.4% by weight. The content of Li2O 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 Li2CO-3 and Li2O may be measured, for example, by X-ray diffraction analysis. When the content of impurities such as Li2CO-3 and Li2O satisfies the above range, it proves that a lithium-rich positive electrode active material structure has been successfully synthesized, and a positive electrode active material satisfying this can exhibit excellent battery characteristics such as high capacity and long lifespan.

[0038] As an example, the positive electrode active material may include a compound represented by the following chemical formula 2.

[0039] [Chemical formula 2] Li 1+a2 (Ni b2 Mn c2 M 2 1-b2-c2 ) 1-a2 O2

[0040] In the aforementioned 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. A positive electrode active material containing the compound represented by the chemical formula 2 can exhibit excellent battery characteristics, such as high capacity and long lifespan.

[0041] As another example, the positive electrode active material may include a compound represented by the following chemical formula 3.

[0042] [Chemical formula 3] Li 1+a3 (Ni b3Co c3 M 3 1-b3-c3 ) 1-a3 O2

[0043] In the aforementioned chemical formula 3, 0 <a3<0.2、0.8<b3<1、0<c3<0.2、0≦1-b3-c3<0.2、M 3 This is one or more elements selected from Mn, Al, Mg, Ca, Ti, V, Cr, Zr, Nb, Mo, and W. A positive electrode active material containing the compound represented by the chemical formula 3 can exhibit excellent battery characteristics, such as high capacity and long lifespan.

[0044] As another example, the positive electrode active material may include a compound represented by the following chemical formula 4.

[0045] [Chemical formula 4] Li 1+a4 (Ni b4 Co c4 Mn d4 M 4 1-b4-c4-d4 ) 1-a4 O2

[0046] In the aforementioned 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. A positive electrode active material containing the compound represented by the chemical formula 4 can exhibit excellent battery characteristics, such as high capacity and long lifespan.

[0047] The average particle size of the positive electrode active material may be approximately 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 high tap density and high energy density per unit volume can be realized.

[0048] In one embodiment, a method for producing the positive electrode active material described above is provided. The method for producing the positive electrode active material includes mixing a precursor containing a compound represented by the following chemical formula 11 with a lithium raw material in a molar ratio of 1:1.06 to 1:1.3, and heat-treating it in a temperature range of 650°C to 780°C.

[0049] [Chemical formula 11] Ni b11 M 11 1-b11 (OH)2

[0050] In the aforementioned 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, and W.

[0051] This manufacturing method allows for the successful synthesis of a layered positive electrode active material with a nickel content exceeding 80 mol% and containing excess lithium within its structure. This enables the synthesis of a positive electrode active material with a c-axis lattice constant change of less than 2.3% due to charging and discharging. The synthesized positive electrode active material exhibits excellent battery characteristics, such as high capacity, high energy density, and long lifespan.

[0052] The compound represented by the chemical formula 11 is a nickel-containing transition metal hydroxide and is a precursor for the positive electrode active material. In the chemical formula 11, b11 represents the molar content of nickel relative to the total content of the transition metal, and may be, 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.

[0053] The success or failure of a lithium-rich high-nickel cathode active material synthesis depends on even slight changes in the lithium content added, and the electrochemical properties of the synthesized active material change significantly. In one embodiment, by mixing a transition metal hydroxide precursor and a lithium raw material in a molar ratio of 1:1.06 to 1:1.3 and heat-treating it in a temperature range of 680°C to 780°C, a lithium-rich high-nickel cathode active material that exhibits high capacity while maintaining a stable structure was successfully synthesized.

[0054] The mixing ratio of the transition metal hydroxide precursor to 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. The heat treatment temperature may also 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, 690°C to 780°C, or 700°C to 750°C.

[0055] When the mixing ratio of the transition metal hydroxide precursor and the lithium raw material satisfies the aforementioned range, and the heat treatment temperature also satisfies the aforementioned range, the desired lithium-rich high-nickel cathode active material can be successfully synthesized. In other words, a lithium-rich cathode active material can be successfully synthesized that has a nickel content exceeding 80 mol% and satisfies a molar content of lithium present in the structure of 1.02 to 1.15. The synthesized cathode active material can exhibit excellent battery characteristics, such as high capacity, high energy density, and long lifespan.

[0056] The transition metal hydroxide, i.e., the precursor of the positive electrode active material, can be produced by a general coprecipitation method. For example, the precursor can be produced by adding an aqueous metal salt solution containing the lead material, such as a nickel salt, and an alkaline aqueous solution such as an aqueous ammonia solution as a chelating agent and NaOH for pH adjustment to a coprecipitation reactor, and carrying out a coprecipitation reaction while injecting N2 to prevent oxidation of metal ions.

[0057] 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, in addition to the nickel salt, a cobalt salt, a manganese salt, an aluminum salt, etc. 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. The lithium raw material may include, for example, Li2CO3, LiOH, or a combination thereof.

[0058] Another embodiment provides a lithium secondary battery comprising a positive electrode containing the aforementioned positive electrode active material, a negative electrode, and a non-aqueous electrolyte.

[0059] 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, which may include the positive electrode active material for a lithium secondary battery according to the above embodiment. In the positive electrode active material layer, the content of the positive electrode active material may be 90% to 99% by weight of the total weight of the positive electrode active material layer.

[0060] The positive electrode active material layer may further contain a binder and / or a conductive material. In this case, the content of the binder and the conductive material may be 1% to 5% by weight, respectively, relative to the total weight of the positive electrode active material layer.

[0061] The binder plays a role in making it easier for positive electrode active material particles to adhere to each other and for the positive electrode active material to adhere to the current collector. Typical examples of binders include, but are not limited to, polyvinyl alcohol, carboxymethylcellulose, hydroxypropylcellulose, diacetylcellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylic styrene-butadiene rubber, epoxy resin, and nylon.

[0062] The conductive material is used to impart conductivity to the electrodes, and any electronically conductive material that does not cause chemical changes can be used in the battery that is constructed. Examples of conductive materials include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjenblack, and carbon fibers; metallic materials such as metal powders or metal fibers of copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; or conductive materials containing mixtures thereof.

[0063] The positive electrode current collector may be, but is not limited to, thin aluminum, thin nickel, or a combination thereof.

[0064] 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 includes a negative electrode active material.

[0065] The negative electrode active material includes a material capable of reversibly intercalating / deintercalating lithium ions, lithium metal, an alloy of lithium metal, a material capable of doping and dedoping lithium, or a transition metal oxide.

[0066] As the substance capable of reversibly intercalating / deintercalating the lithium ions, carbon substances can be used, and any of the carbon-based negative electrode active materials generally used in lithium ion secondary batteries can be used. Representative examples thereof include crystalline carbon, amorphous carbon, or both of these can be used.

[0067] 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 may be used.

[0068] As the substance capable of doping and undoping lithium, Si, SiO x (0 < x < 2, Si-Y alloy (where 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 a combination thereof and is not Si), Sn, SnO2, Sn-Y (where 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 a combination thereof and is not Sn), etc. can be mentioned.

[0069] 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.

[0070] The binder serves to make the negative electrode active material particles easily adhere to each other and also make the negative electrode active material easily adhere to the current collector.

[0071] The conductive material is used to impart conductivity to the electrode, and in the battery being configured, any electron conductive material can be used as long as it does not cause a chemical change.

[0072] As the current collector, one can be selected from the group consisting of copper foil, nickel thin, stainless steel thin, titanium thin, nickel foam, copper foam, polymer substrate coated with a conductive metal, and combinations thereof.

[0073] 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 then applying this composition to a current collector. Since such electrode manufacturing methods are widely known in the field, a detailed explanation is omitted in this specification. The solvent may be N-methylpyrrolidone or the like, but is not limited thereto.

[0074] The electrolyte comprises a non-aqueous organic solvent and a lithium salt.

[0075] The non-aqueous organic solvent acts as a medium through which ions involved in the electrochemical reaction of the battery can move.

[0076] The aforementioned lithium salt dissolves in an organic solvent and acts as a source of lithium ions within the battery, enabling the basic operation of a lithium secondary battery and playing a role in promoting the movement of lithium ions between the positive and negative electrodes.

[0077] Depending on the type of lithium secondary battery, a separator may be present between the positive and negative electrodes. Such a separator may be polyethylene, polypropylene, polyvinylidene fluoride, or a multilayer film of two or more layers thereof. Needless to say, mixed multilayer films such as a polyethylene / polypropylene two-layer separator, a polyethylene / polypropylene / polyethylene three-layer separator, or a polypropylene / polyethylene / polypropylene three-layer separator may also be used.

[0078] 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. They can also be classified by shape into cylindrical, prismatic, coin-type, pouch-type, etc., and by size into bulk type and thin-film type. The structure and manufacturing methods of these batteries are widely known in this field, so a detailed explanation will be omitted. [Examples]

[0079] The following describes examples and comparative examples of the present invention. The following examples are merely one embodiment of the present invention, and the present invention is not limited to the following examples.

[0080] [Example 1] (1) Production of cathode active material precursor Ni 0.92 Co 0.04 Mn 0.04 A precursor having the composition (OH)2 is produced by a general coprecipitation method. Specifically, NiSO4·6H2O, CoSO4·7H2O, and MnSO4·H2O are dissolved in distilled water to produce aqueous metal salt solutions. After preparing the 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 added to the coprecipitation reactor as a chelating agent, and NaOH is added to adjust the pH. The precipitate obtained from the coprecipitation process is filtered, washed with distilled water, and then dried in a 100°C oven for 24 hours to produce a cathode active material precursor with an average diameter of approximately 14.8 μm.

[0081] (2) Manufacturing of positive electrode active material The prepared precursor and LiOH·H2O are mixed in a molar ratio of 1:1.06, then placed in a tube furnace and calcined while introducing oxygen at a rate of 50 mL / min. The temperature is raised to 700°C at a rate of 10°C / min, and this temperature is maintained for 12 hours, after which the product is allowed to cool naturally to 25°C.

[0082] (3) Battery manufacturing 92% of the manufactured positive electrode active material, 4% of Super P as a conductive material, and 4% of PVDF as a binder are placed in an NMP solution, and a slurry is produced by missing the slurry. This slurry is then applied to an aluminum current collector to produce the positive electrode, and a coin half-cell is fabricated with lithium metal as the negative electrode.

[0083] [Examples 2 to 8, Comparative Examples 1 to 5, Comparative Examples 11 and 12] In the production of the positive electrode active material in Example 1, the precursor of the positive electrode active material, the positive electrode active material, and the battery are produced in the same manner as in Example 1, except that the mixing ratio of the precursor and LiOH·H2O and the heat treatment temperature are changed as shown in Table 1 below.

[0084] [Table 1]

[0085] [Evaluation Example 1: Analysis of lithium content in the cathode active material structure via neutron diffraction analysis] The positive electrode active materials produced in Examples 1 to 8 and Comparative Examples 1 to 5, 11, and 12 were subjected to neutron diffraction analysis using the Hanaro reactor at the Korea Atomic Energy Research Institute. Rietveld refinement was then performed using these results to analyze the lithium content within the positive electrode active material structure, as shown in Figure 2. Referring to Figure 2, it can be confirmed that in Examples 1 to 8, the molar content of lithium within the active material structure was in the range of 1.02 to 1.15 per mole of active material, indicating that lithium was more readily incorporated into the active material structure compared to the comparative examples.

[0086] [Evaluation Example 2: Real-time X-ray diffraction analysis during charging and discharging] Batteries manufactured in Examples 4, 8, and Comparative Example 5, where the molar ratio of lithium raw material to precursor was 1.30, were charged and discharged in the range of 2.5V to 4.25V, and real-time X-ray diffraction analysis and the resulting change in the lattice constant of the R-3m structure model were analyzed. X-ray diffraction analysis (XRD) was performed using an X-ray diffractometer (Empyrean, Malvern Panalytical) equipped with CuKα radiation (λ=1.540598Å).

[0087] Figures 3 to 5 show the real-time X-ray diffraction patterns for Example 4, Example 8, and Comparative Example 5, respectively. Referring to Figures 3 to 5, it can be seen that in Examples 4 and 8, the positional change of the X-ray diffraction peak is smaller compared to Comparative Example 5.

[0088] Figures 6 to 8 are graphs showing the changes in lattice constants obtained by real-time X-ray diffraction analysis for Example 4, Example 8, and Comparative Example 5, respectively. Referring to Figures 6 to 8, it can be seen that the changes in lattice constants are smaller in Examples 4 and 8 compared to Comparative Example 5. This indicates that the active materials produced in the examples have suppressed structural collapse and cracking due to charging and discharging, which is understood to improve the battery life characteristics.

[0089] Figures 9 to 11 show graphs illustrating the change in lattice constants obtained by real-time X-ray diffraction analysis after a second charge-discharge cycle in the 2.5V to 4.25V range for the batteries manufactured in Example 4, Example 8, and Comparative Example 5, respectively. Figures 9 to 11 also confirm that the change in lattice constants is smaller in the Examples compared to the Comparative Examples. This indicates that the change in lattice constants is not a characteristic limited to the first cycle, but rather a reversible characteristic.

[0090] Tables 2 and 3 below specifically show the changes in the c-axis grid constant. Table 2 shows the c-axis grid constants measured during the initial charge and discharge for Examples 4, 8, and Comparative Examples 5, 11, and 12. Table 3 shows the c-axis grid constants measured during the second charge and discharge for Examples 4, 8, and Comparative Examples 5, 11, and 12.

[0091] [Table 2]

[0092] [Table 3]

[0093] Referring to Tables 2 and 3, in the example, the rate of change of the c-axis lattice constant of the R-3m structure during the initial and second charge-discharge cycles in the 2.5V to 4.25V range was less than 2.3%, the maximum value of the c-axis lattice constant was less than 14.41 Å, and the change in the c-axis lattice constant was 0.35 Å or less. This indicates that the positive electrode active material of the example is structurally very stable and exhibits high lifetime characteristics without collapsing even after repeated charge-discharge cycles. Furthermore, in Comparative Examples 11 and 12, where the molar ratio of lithium to the precursor was 1.03, the rate of change of the c-axis lattice constant was relatively higher than in the example, and the degree to which the lattice constant change decreased compared to Comparative Example 1, i.e., the difference in lattice constant change due to synthesis temperature, was smaller than in Examples 5, 10, and Comparative Example 5. This shows that the change in lattice constant is a characteristic that can be controlled not only by the firing temperature but also by the amount of lithium added.

[0094] In one embodiment, by adjusting the molar ratio of lithium raw material to precursor to 1:1.06 to 1:1.3 and adjusting the firing temperature to 680°C to 780°C, the molar content of lithium present in the positive electrode active material structure was adjusted to 1.02 to 1.15. In this example, it was confirmed that the rate of change of the c-axis lattice constant during charging and discharging was less than 2.3%.

[0095] [Evaluation Example 3: PDF (Pair Distribution Function) analysis obtained by Fourier transform of neutron diffraction analysis results] To understand the local structural changes of MO bonds due to lithium-rich transition metal substitution, pair distribution function (PDF) analysis was performed by Fourier transforming the neutron diffraction analysis results of Evaluation Example 1. Figure 12 shows the PDF analysis results for the cathode active materials produced in Comparative Examples 11, 12, and 1, and Figure 13 shows the PDF analysis results for the cathode active materials produced in Examples 4, 8, and 5. Comparing Figures 12 and 13, Comparative Examples 1 and 5, synthesized at 800°C, do not show a large difference in Ni-O bond distance. This means that the lithium / transition metal ratio in the crystal structure is maintained almost constant regardless of the lithium / transition metal ratio added during synthesis. In contrast, Comparative Examples 11 and 4, synthesized at 700°C, show a phenomenon where the Ni-O bond distance decreases as lithium excess increases. This phenomenon occurs because at lower synthesis temperatures, the proportion of lithium in the structure increases and the proportion of transition metal decreases. This increases the oxidation state of Ni, which in turn increases the Ni-O bond strength and decreases the Ni-O bond distance. This means that at 700°C, the excess lithium is contained within the structure.

[0096] [Evaluation Example 4: Cation Mixture Analysis] The proportion of nickel within lithium sites, i.e., the cation mixing, was analyzed for the positive electrode active materials produced in Examples 1 to 8 and Comparative Examples 1 to 5, 11, and 12 using Rietveld analysis with neutron diffraction, and the results are shown in Figure 14. Looking at the graphs for Comparative Example 11 and Examples 1 to 4 in Figure 14, it can be seen that as the lithium content increases and more excess lithium enters the active material structure, the oxidation number of the transition metal increases, and this reduces the cation mixing. Furthermore, it can be confirmed that the cation mixing of the positive electrode active materials in Examples 1 to 8 is less than 5 atomic percent. The higher cation mixing in Examples 1 to 8 compared to Comparative Examples 1 to 5 is due to the relatively lower synthesis temperature in the Examples compared to the Comparative Examples, resulting in higher cation mixing.

[0097] [Evaluation Example 5: Analysis of Residual Impurity Content] The Rietveld analysis method using X-ray diffraction analysis was performed on the positive electrode active materials prepared in Examples 1 to 8 and Comparative Examples 1 to 5, 11, and 12 to determine the residual impurities Li2CO3 and Li 2O The content of impurities was measured, and the content of impurities in the positive electrode active material of Comparative Examples 1 to 5 is shown in Figure 15, the content of impurities in the positive electrode active material of Comparative Example 12 and Examples 5 to 8 is shown in Figure 16, and the content of impurities in the positive electrode active material of Comparative Example 11 and Examples 1 to 4 is shown in Figure 17. In Figure 15, it can be seen that in the case of the comparative examples, the excess lithium could not enter the active material structure and remained in the form of impurities such as Li2CO3 and Li2O. In contrast, in Figure 16, the impurity content in Examples 5 to 8 was greatly reduced, and in Figure 17, it can be seen that in Examples 1 to 4, there were almost no impurities and the excess lithium penetrated into the active material structure.

[0098] [Evaluation Example 6: Battery Life Characteristics Evaluation] The batteries manufactured in the examples and comparative examples were subjected to charge and discharge cycles in the 2.5V to 4.25V range for approximately 50 cycles to evaluate their lifespan. Figure 18 shows the lifespan characteristics of Comparative Example 12 and Examples 5, 6, and 8, Figure 19 shows the lifespan characteristics of Comparative Example 11 and Examples 1, 3, and 4, and Figure 20 shows the lifespan characteristics of Comparative Examples 1, 2, 3, 4, and 5. Referring to Figures 18 to 20, the comparative examples in Figure 20 show a decrease in discharge capacity with each cycle regardless of composition, resulting in poor lifespan characteristics. In contrast, the batteries of the examples in Figures 18 and 19 exhibit high capacity and long lifespan characteristics, confirming that increasing the lithium content in the positive electrode active material structure improves lifespan characteristics.

[0099] Although preferred embodiments have been described in detail above, the scope of the present invention is not limited thereto. Various modifications and improvements by those skilled in the art using the basic concepts defined in the claims also fall within the scope of the present invention.

Claims

1. A positive electrode active material containing a compound represented by the following chemical formula 1, The molar content of lithium present in the structure of the positive electrode active material, as measured by neutron diffraction analysis, is 1.02 to 1.15 per mole of the positive electrode active material. A positive electrode active material for lithium secondary batteries, wherein the rate of change of the c-axis lattice constant of the R-3m structure during charging and discharging in the range of 2.5V to 4.25V is less than 2.3%. [Chemical formula 1] Li １＋ａ (N ｂ M １ １－ｂ ) １－ａ O ２ In the above chemical formula 1, 0.02 ≤ a ≤ 0.15, 0.8 < b < 1, M １ is one or more elements selected from Co, Mn, Al, Mg, Ca, Ti, V, Cr, Zr, Nb, Mo, and W.

2. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the maximum value of the c-axis lattice constant of the R-3m structure during charging and discharging in the range of 2.5V to 4.25V is 14.410 Å or less.

3. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the c-axis lattice constant of the R-3m structure changes within the range of 13.900 Å to 14.410 Å during charging and discharging in the range of 2.5 V to 4.25 V.

4. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the nickel content in the lithium site of the positive electrode active material is less than 5 atomic percent.

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

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

7. The positive electrode active material for a lithium secondary battery according to claim 1, wherein in the chemical formula 1, 0.90 < b < 1.

8. A method for producing a positive electrode active material for a lithium secondary battery, comprising mixing a precursor containing a compound represented by the following chemical formula 11 with a lithium raw material in a molar ratio of 1:1.06 to 1:1.3, and heat-treating it in a temperature range of 680°C to 780°C, wherein the heat-treating is performed by single firing. [Chemical formula 11] Ni ｂ１１ M １１ １－ｂ１１ (OH) ２ 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, and W.

9. A positive electrode comprising the positive electrode active material described in any one of claims 1 to 7, The negative electrode and, A lithium secondary battery containing an electrolyte.

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