Method for producing positive electrode additive for lithium secondary battery

Abstract

The present invention relates to a method for producing a positive electrode additive for a lithium secondary battery. According to the present invention, a method for producing a positive electrode additive for a lithium secondary battery that exhibits a high initial irreversible capacity and has excellent air stability is provided.

Description

[Technical Field]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0139619, filed October 19, 2021, and Korean Patent Application No. 10-2022-0133499, filed October 17, 2022, and all contents disclosed in the documents of said Korean patent applications are incorporated herein by reference.

[0002] The present invention relates to a method for producing a positive electrode additive for a lithium secondary battery. [Background technology]

[0003] 2. Description of the Related Art As electronic devices become more multifunctional and consume more power, many attempts have been made to increase the capacity of lithium secondary batteries and improve their charge / discharge efficiency.

[0004] As an example, a technology has been proposed in which a positive electrode active material containing 80% or more Ni is used as the positive electrode material for a lithium secondary battery, and a metal or metal-based negative electrode active material such as SiO, Si, or SiC is used as the negative electrode together with a carbon-based negative electrode active material such as natural graphite or artificial graphite.

[0005] Metal and metal oxide-based anode active materials can achieve higher capacity than carbon-based anode active materials. However, because metal and metal oxide-based anode active materials experience significantly greater volumetric changes during charge and discharge than graphite, it is difficult to increase the metal and metal oxide content in the anode above 15%. Furthermore, when metal and metal oxide are added to the anode, irreversible reactions occur during the initial charge and discharge cycles, resulting in greater lithium loss than when carbon-based anode active materials are used. Therefore, when metal and metal oxide-based anode active materials are used, the amount of lithium lost increases as the battery capacity increases, resulting in a greater decrease in initial capacity.

[0006] Therefore, various methods have been studied to increase the capacity of lithium secondary batteries or reduce the irreversible capacity, one of which is prelithiation, which is a concept of replenishing lithium within the battery that is consumed during the formation of the SEI (solid electrolyte interphase) layer in the initial state.

[0007] Various methods have been proposed for prelithiation in batteries.

[0008] One example is a method of electrochemically lithiating the negative electrode before the battery is used. However, lithiated negative electrodes are very unstable in the atmosphere, making it difficult to scale up the process using electrochemical lithiation.

[0009] Another example is a battery using lithium metal or lithium silicide (Li) as the negative electrode. x However, since the powder is highly reactive and has poor atmospheric stability, it is difficult to establish suitable solvents and process conditions for coating the negative electrode.

[0010] One method of pre-lithiating the positive electrode is to coat the positive electrode with an amount of positive electrode material equal to the amount of lithium consumed in the negative electrode. However, since the amount of positive electrode material added increases due to the low capacity of the positive electrode material itself, the energy density and capacity per weight of the final battery decrease by the amount of the added positive electrode material.

[0011] Therefore, a material suitable for pre-lithiation of a battery at the positive electrode must have the irreversible property of releasing at least twice as much lithium as existing positive electrode materials during the first charge and not reacting with lithium during subsequent discharge. An additive that meets these conditions is called a sacrificial positive electrode material.

[0012] In the case of commercial batteries, after an electrolyte is injected into a case containing a stacked cathode, separator, and anode, the battery first undergoes a formation process to perform charge / discharge operations. During this process, an SEI layer formation reaction occurs on the anode, and gas is generated due to the decomposition of the electrolyte. During the formation process, the sacrificial cathode material releases lithium and decomposes, reacting with the electrolyte. Gases such as N2, O2, and CO2 generated during this process are collected through a gas pocket removal process.

[0013] Over-lithiated positive electrode materials, which are lithium-rich metal oxides, are often used as sacrificial positive electrode materials. Well-known over-lithiated positive electrode materials include Li6CoO4, Li5FeO4, and Li6MnO4, which have an anti-fluorite structure. Their theoretical capacities are 977 mAh / g for Li6CoO4, 867 mAh / g for Li5FeO4, and 1001 mAh / g for Li6MnO4, which are sufficient for use as sacrificial positive electrode materials. Among these, Li6CoO4 has the best electrical conductivity and favorable electrochemical properties for use as a sacrificial positive electrode material.

[0014] However, the Li5FeO4 sacrificial cathode material has drawbacks: poor air stability, rapid performance degradation when exposed to air, low electrical conductivity, and insufficient irreversible capacity. As a result, a significant amount of Li5FeO4 must be added to compensate for the large irreversible capacity in high-capacity lithium secondary batteries. This has hindered recent technological developments aimed at providing lithium secondary batteries with lower weight and improved capacity characteristics. Therefore, there is a continuing need for the development of Li5FeO4-based sacrificial cathode materials with even greater irreversible capacity. Summary of the Invention [Problem to be solved by the invention]

[0015] The present invention provides a method for producing a positive electrode additive for a lithium secondary battery that exhibits a high initial irreversible capacity and has excellent air stability. [Means for solving the problem]

[0016] According to one embodiment of the present invention, mixing and heat-treating the carbon nanotubes, the water-soluble polymer dispersant, and the iron (Fe) precursor to form an iron oxide-carbon precursor; mixing a lithium precursor and the iron oxide-carbon precursor and calcining at 500° C. or higher to form lithium-iron oxide particles; and The method for producing a positive electrode additive for a lithium secondary battery includes heat-treating the mixture of the lithium-iron oxide particles and lithium difluoro(oxalato)borate in an oxygen-containing gas atmosphere at a temperature of less than 300°C to form a lithium-iron oxide coated with a lithium difluoro(oxalato)borate-containing layer.

[0017] Hereinafter, a method for preparing the positive electrode additive for a lithium secondary battery according to an embodiment of the present invention will be described in more detail.

[0018] The terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted as meanings and concepts that correspond to the technical idea of ​​the present invention, based on the principle that the inventor can appropriately define the concept of the term in order to best explain his / her invention.

[0019] Unless otherwise defined herein, all technical and scientific terms have the same meaning as commonly understood by those of ordinary skill in the art to which this invention belongs. The terms used to describe the present invention are merely for the purpose of effectively describing specific embodiments and are not intended to limit the present invention.

[0020] As used herein, the singular forms "a," "an," and "the" include the plural forms as well, unless the context clearly indicates otherwise.

[0021] As used herein, the meaning of "comprising" embodies certain properties, regions, integers, steps, operations, elements, and / or components, and does not exclude the presence or addition of other certain properties, regions, integers, steps, operations, elements, components, and / or groups.

[0022] The present invention can be modified in various ways and can have various forms, and therefore, the following detailed description will be given by way of example of a specific embodiment, but it should be understood that this is not intended to limit the present invention to the specific disclosed embodiment, but rather to include all modifications, equivalents, and alternatives within the spirit and technical scope of the present invention.

[0023] In this specification, when the positional relationship of two parts is described, for example, "above," "on top," "below," or "to the side," one or more other parts may be located between the two parts, unless the expression "immediately" or "directly" is used.

[0024] In this specification, when a temporal relationship is described, for example, "after," "following," "next," or "before," it may also include cases where the relationship is not consecutive, unless the expression "immediately" or "directly" is used.

[0025] As used herein, the term "at least one" should be understood to include all possible combinations of one or more of the associated items.

[0026] As used herein, the term "cathode additive" refers to a material that has the irreversible property of releasing at least twice as much lithium as conventional cathode materials during initial charging of a battery and not reacting with lithium during subsequent discharge. The cathode additive can also be called a sacrificial positive electrode material. The cathode additive compensates for lithium loss, thereby restoring lost capacity and increasing battery capacity. It also prevents battery explosion by suppressing gas generation, thereby improving battery life characteristics and safety.

[0027] The present inventors have continued their research to improve the electrical conductivity and irreversible capacity of Li5FeO4-based positive electrode additives (sacrificial positive electrode materials) and to improve their air stability in a more simplified manner.

[0028] As a result of this ongoing research, we have found that a dual-coating layer cathode additive can be obtained by adding a dispersion of carbon nanotubes in the presence of a water-soluble polymer dispersant and then calcining the resulting dispersion to LiFeO-based lithium transition metal oxide particles. Furthermore, a lithium difluoro(oxalato)borate-containing layer is formed on the surface of the dual-coating layer cathode additive in an oxygen-containing gas atmosphere. A schematic cross-sectional view of one embodiment of the cathode additive can be seen in Figure 1.

[0029] The positive electrode additive has a carbon nanotube-containing layer formed on LiFeO-based lithium transition metal oxide particles, which has electrical conductivity similar to that of the particles, and can therefore have superior electrical conductivity and high irreversible capacity compared to conventional LiFeO-based positive electrode additives.

[0030] In addition, since a uniform carbon coating layer derived from the water-soluble polymer dispersant is formed on the surface of the lithium transition metal oxide particles, and carbon nanotubes are uniformly and relatively highly bound to the carbon coating layer, the positive electrode additive of the embodiment may have even higher electrical conductivity and irreversible capacity.

[0031] In addition, the positive electrode additive has a surface coated with a lithium difluoro(oxalato)borate-containing layer, and therefore exhibits excellent stability against moisture and carbon dioxide even when exposed to air.

[0032] In contrast, cathode additives that only have a carbon coating layer derived from the polymer dispersant have difficulty improving electrical conductivity and therefore have difficulty in achieving sufficient irreversible capacity.Furthermore, cathode additives in which carbon nanotubes are directly formed on additive particles also have difficulty in bonding the carbon nanotubes uniformly and at a high ratio to the additive particles, resulting in insufficient improvement in electrical conductivity, irreversible capacity, and / or capacity characteristics during charge and discharge.

[0033] The positive electrode additive for lithium secondary batteries significantly improves electrical conductivity, irreversible capacity, and charge / discharge capacity characteristics because a high proportion of carbon nanotubes are uniformly bound to the lithium transition metal oxide particles through the interaction between the carbon coating layer and the carbon nanotube-containing layer. Furthermore, the lithium difluoro(oxalato)borate-containing layer formed on the carbon nanotube-containing layer improves air stability, allowing the positive electrode additive to stably exhibit its electrical conductivity, irreversible capacity, and charge / discharge capacity characteristics.

[0034] According to one embodiment of the present invention, mixing and heat-treating the carbon nanotubes, the water-soluble polymer dispersant, and the iron (Fe) precursor to form an iron oxide-carbon precursor; mixing a lithium precursor and the iron oxide-carbon precursor and calcining at 500° C. or higher to form lithium-iron oxide particles; and The method for producing a positive electrode additive for a lithium secondary battery includes heat-treating the mixture of the lithium-iron oxide particles and lithium difluoro(oxalato)borate in an oxygen-containing gas atmosphere at a temperature of less than 300°C to form a lithium-iron oxide coated with a lithium difluoro(oxalato)borate-containing layer.

[0035] As a specific example, the step of forming the iron oxide-carbon precursor can be performed by the steps of: forming a carbon nanotube dispersion in which the carbon nanotubes are dispersed in an aqueous solvent in the presence of the water-soluble polymer dispersant; mixing the carbon nanotube dispersion and an iron (Fe) precursor in the presence of a base; reacting the carbon nanotube dispersion and the iron (Fe) precursor in the mixed solution at a temperature of 50°C to 100°C; and filtering and drying the reaction product solution, and heat-treating it at a temperature of 200°C to 300°C.

[0036] The iron oxide-carbon precursor is mixed with a lithium precursor and calcined at high temperature to form LiFeO-based lithium transition metal oxide particles (i.e., the lithium-iron oxide particles). At the same time, the water-soluble polymer dispersant is calcined on the surface of the lithium-iron oxide particles, forming a uniform carbon coating layer. Carbon nanotubes are bonded to the carbon coating layer. The lithium-iron oxide particles are then mixed with lithium difluoro(oxalato)borate (LiDFOB) and calcined in an oxygen-containing gas atmosphere at a temperature below 300°C to obtain lithium-iron oxide particles coated with a LiDFOB-containing layer.

[0037] Hereinafter, the method for preparing the positive electrode additive for a lithium secondary battery according to the embodiment will be described step by step.

[0038] The carbon nanotubes, the water-soluble polymer dispersant, and the iron (Fe) precursor are mixed and heat-treated to form an iron oxide-carbon precursor.

[0039] The water-soluble polymer dispersant may be any water-soluble polymer that can uniformly disperse carbon nanotubes in an aqueous solvent and form the carbon coating layer upon firing. Preferably, the water-soluble polymer dispersant may include one or more compounds selected from the group consisting of polyvinylpyrrolidone-based polymers, polyacrylic acid-based polymers, polyvinyl alcohol-based polymers, and hydroxyalkyl cellulose-based polymers.

[0040] The water-soluble polymer dispersant and carbon nanotubes can be dispersed and mixed in an aqueous solvent by, for example, ultrasonic spraying to form a carbon nanotube dispersion, which can then be mixed with an iron precursor or an aqueous solution thereof and a base such as ammonium hydroxide.

[0041] In order to form a carbon coating layer of appropriate thickness and content, the water-soluble polymer dispersant may be used in an amount of 0.1 to 2 parts by weight, or 0.5 to 2 parts by weight, or 0.5 to 1.5 parts by weight, based on the total content of the iron oxide-carbon precursor.

[0042] In order to form a carbon nanotube-containing layer with an appropriate content on the carbon coating layer, the carbon nanotubes may be used in an amount of 1 to 10 parts by weight, or 2 to 10 parts by weight, or 2 to 7 parts by weight, based on the total content of the iron oxide-carbon precursor.

[0043] The iron (Fe) precursor may include one or more compounds selected from the group consisting of Fe(III) nitrates, sulfates, phosphates, oxides, halides, and hydrates thereof.

[0044] As described above, after mixing the carbon nanotube dispersion and the iron precursor, the carbon nanotube dispersion and the iron precursor are stirred, and a base such as ammonium hydroxide (NH4OH) is added in an equivalent ratio to the iron precursor, followed by reaction at a temperature of 50°C to 100°C or 70°C to 90°C for 1 to 10 hours. The reaction product solution is filtered and dried, and then further heat-treated at a temperature of 200°C to 300°C or 220°C to 280°C for 2 to 15 hours or 6 to 12 hours to remove impurities. The drying step can be performed using a conventional oven, and an iron oxide-carbon precursor can be formed through this process.

[0045] Next, the lithium precursor and the iron oxide-carbon precursor are mixed and calcined at 500° C. or higher to form lithium-iron oxide particles.

[0046] The iron oxide-carbon precursor can be mixed with a lithium precursor and then calcined at a temperature above 500° C., alternatively between 500° C. and 1000° C., alternatively between 550° C. and 700° C., to form lithium-iron oxide.

[0047] The reaction between the iron oxide-carbon precursor and the lithium precursor can be an equivalent reaction. For example, when the lithium precursor is a lithium oxide such as LiO, the iron oxide-carbon precursor and the lithium precursor can be mixed in a molar ratio of 1:5, followed by high-temperature calcination.

[0048] As the lithium precursor, in addition to the lithium oxide (Li2O), any lithium precursor well known in the technical field to which the present invention pertains can be used.

[0049] Next, the mixture of the lithium-iron oxide particles and lithium difluoro(oxalato)borate (hereinafter referred to as "LiDFOB") is heat-treated in an oxygen-containing gas atmosphere at a temperature of less than 300°C to form a lithium-iron oxide coated with a LiDFOB-containing layer.

[0050] The lithium-iron oxide particles and LiDFOB can be mixed by solid-phase mixing using a conventional mixer.

[0051] In particular, the heat treatment of the mixture can be carried out in an oxygen-containing gas atmosphere at a temperature of less than 300°C.

[0052] Preferably, the oxygen-containing gas may be air.

[0053] When exposed to air, lithium-iron oxides such as LiFeO react with carbon dioxide (CO) and moisture (HO) in the air to convert to LiCO or LiOH. Therefore, it might be expected that heat-treating the lithium-iron oxide particles in air, an oxygen-containing gas, would be undesirable. However, contrary to this expectation, heat-treating a mixture of the lithium-iron oxide particles and LiDFOB in an air atmosphere at a temperature below 300°C yields a lithium-iron oxide coated with a LiDFOB-containing layer that has excellent air stability.

[0054] Preferably, the step of forming the lithium-iron oxide coated with the LiDFOB-containing layer can be performed by heat-treating the mixture in an air atmosphere at a temperature of less than 300°C, alternatively 260°C to 295°C, alternatively 260°C to 290°C, alternatively 265°C to 285°C, alternatively 270°C to 280°C.

[0055] By carrying out the heat treatment of the mixture in an oxygen-containing gas atmosphere, a positive electrode additive for a lithium secondary battery having improved air stability can be obtained compared to when the heat treatment is carried out in a typical inert gas atmosphere.

[0056] Alternatively, the heat treatment of the mixture may be performed by first heat-treating the mixture in an inert gas atmosphere and then second heat-treating the resultant mixture in an oxygen-containing gas atmosphere.

[0057] As a specific example, the step of forming the lithium-iron oxide coated with the LiDFOB-containing layer may include a first heat treatment of the mixture of the lithium-iron oxide particles and LiDFOB in an inert gas atmosphere at a temperature less than 300°C, and a second heat treatment of the resultant product of the first heat treatment in an oxygen-containing gas atmosphere at a temperature less than 300°C.

[0058] Here, the temperatures of the first heat treatment process and the second heat treatment process can be independently adjusted to a temperature below 300°C, or 260°C to 295°C, or 260°C to 290°C, or 265°C to 285°C, or 270°C to 280°C.

[0059] The LiDFOB can be used in an amount of 5.0 to 20.0 parts by weight, alternatively 5.5 to 19.0 parts by weight, alternatively 6.0 to 18.5 parts by weight, alternatively 6.5 to 18.0 parts by weight, alternatively 7.0 to 17.5 parts by weight, alternatively 7.5 to 17.0 parts by weight, alternatively 8.0 to 16.5 parts by weight, alternatively 8.0 to 16.0 parts by weight, alternatively 8.0 to 15.5 parts by weight, or alternatively 8.0 to 15.0 parts by weight, based on the total amount of the lithium-iron oxide particles.

[0060] Additives such as lithium hexafluorophosphate, lithium triflate, and lithium difluorophosphate can be further mixed with the LiDFOB. However, to fully demonstrate the effect of improving air stability by introducing the LiDFOB-containing layer, the LiDFOB-containing layer preferably contains at least 50 mol % or more, or 70 mol % or more, or 90 mol % or more of LiDFOB.

[0061] If necessary, the lithium-iron oxide coated with the LiDFOB-containing layer may be washed and dried.

[0062] As a non-limiting example, the washing step can be performed by mixing the lithium-iron oxide particles and a washing solution in a weight ratio of 1:2 to 1:10 and stirring them. The washing solution can be distilled water, ammonia water, or the like. The drying can be performed by heat treatment at a temperature of 100°C to 200°C or 100°C to 180°C for 1 to 10 hours.

[0063] Meanwhile, the positive electrode additive for lithium secondary batteries obtained by the above manufacturing method includes the lithium-iron oxide particles; a carbon coating layer formed on the lithium-iron oxide particles; a carbon nanotube-containing layer formed on the carbon coating layer; and a lithium difluoro(oxalato)borate-containing layer formed on the carbon nanotube-containing layer.

[0064] The positive electrode additive essentially contains lithium-iron oxide particles, including the compound Li5FeO4. Lithium transition metal oxide particles, such as Li5FeO4, contain a higher-than-stoichiometric ratio of lithium. Excess lithium ions migrate to the negative electrode during the initial charge / discharge process to compensate for irreversible capacity loss.

[0065] The positive electrode additive may further include conventionally known lithium transition metal oxides such as Li2NiO2 and Li6CoO4 in addition to the lithium-iron oxide Li5FeO4. However, considering the manufacturing cost and physical properties of the positive electrode additive, it is preferable that the positive electrode additive contains at least 50 mol% or more, or 70 mol% or more, or 90 mol% or more of Li5FeO4 as the lithium transition metal oxide.

[0066] The lithium-iron oxide particles may be in the form of primary particles having a volume average particle size (D50) of 0.5 μm to 45 μm, or 1 μm to 25 μm, or 5 μm to 15 μm, or secondary particles formed by aggregation of the primary particles. Within this particle size range, the lithium-iron oxide particles can be uniformly mixed with the positive electrode active material and exhibit appropriate properties within the positive electrode.

[0067] To obtain a suitable particle size distribution and volume average particle size, the lithium-iron oxide particles can be passed through a standard sieve having a mesh size corresponding to the desired particle size distribution after synthesis. The particle size distribution and volume average particle size (D50) of the lithium-iron oxide particles can be measured and calculated using a well-known laser particle size analyzer or the like.

[0068] The positive electrode additive may form a carbon coating layer on the lithium-iron oxide particles and a carbon nanotube-containing layer containing carbon nanotubes physically or chemically bonded to the carbon coating layer. The formation of the carbon coating layer and the carbon nanotube-containing layer may be confirmed by electron microscopy or XRD analysis of the positive electrode additive.

[0069] According to an embodiment of the present invention, the total content of the carbon coating layer and the carbon nanotube-containing layer may be 0.5 parts by weight to 6.0 parts by weight, alternatively 1.0 parts by weight to 6.0 parts by weight, alternatively 1.0 parts by weight to 5.9 parts by weight, alternatively 1.5 parts by weight to 5.9 parts by weight, alternatively 1.5 parts by weight to 5.8 parts by weight, based on 100 parts by weight of the total content of the positive electrode additive.

[0070] The carbon coating layer:carbon nanotube-containing layer is contained in a weight ratio of 1:4 to 1:50, alternatively 1:8 to 1:50, alternatively 1:8 to 1:30, alternatively 1:10 to 1:30, alternatively 1:10 to 1:20.

[0071] By controlling the total content and weight ratio of the carbon coating layer and the carbon nanotube-containing layer within the above ranges, the carbon coating layer does not inhibit the properties of the lithium-iron oxide particles, such as irreversible capacity, and a high proportion of carbon nanotubes are uniformly bound to the carbon coating layer, thereby further improving the electrical conductivity, irreversible capacity, and capacity characteristics during charge and discharge of the positive electrode additive.

[0072] In a specific embodiment, the carbon coating layer is contained in an amount of 0.05 parts by weight to 2.0 parts by weight, or 0.06 parts by weight to 2.0 parts by weight, or 0.06 parts by weight to 1.9 parts by weight, based on 100 parts by weight of the total amount of the positive electrode additive; and the carbon nanotube-containing layer is contained in an amount of 0.4 parts by weight to 4.0 parts by weight, or 0.8 parts by weight to 4.0 parts by weight, or 0.8 parts by weight to 3.95 parts by weight, or 1.0 parts by weight to 3.95 parts by weight, or 1.0 parts by weight to 3.90 parts by weight, based on 100 parts by weight of the total amount of the positive electrode additive.

[0073] The range of each content of the carbon coating layer and the carbon nanotube-containing layer, as well as the range of their total content, can be determined by analyzing the carbon content on the surface of the positive electrode additive using well-known elemental analysis, or can be measured and calculated based on the content of the water-soluble polymer dispersant and carbon nanotubes used as raw materials.

[0074] In the positive electrode additive, the carbon coating layer may have a thickness of 10 nm to 300 nm. The carbon nanotubes in the carbon nanotube-containing layer are physically and uniformly adsorbed or chemically bonded to the carbon coating layer. Depending on the thickness of the carbon coating layer and the bonding state of the carbon nanotubes, the positive electrode additive of one embodiment may exhibit optimized irreversible capacity and capacity characteristics during charge and discharge.

[0075] The thickness of the carbon coating layer can be calculated based on the analysis results of the BET specific surface area of ​​the positive electrode additive and the carbon content described above, or can be measured by analyzing the positive electrode additive using a transmission electron microscope (TEM) or a scanning transmission electron microscope (STEM).

[0076] Meanwhile, the positive electrode additive includes a LiDFOB-containing layer formed on the carbon nanotube-containing layer.

[0077] The surface of the positive electrode additive is coated with the LiDFOB-containing layer. Referring to FIG. 1 , the positive electrode additive includes the carbon coating layer 20 formed on the lithium-iron oxide particles 10, the carbon nanotube-containing layer 30 formed on the carbon coating layer 20, and the LiDFOB-containing layer 40 formed on the carbon nanotube-containing layer 30. The LiDFOB-containing layer 40 is physically or chemically bound to the carbon nanotube-containing layer 30 formed on the carbon coating layer 20.

[0078] According to one embodiment of the present invention, the LiDFOB-containing layer is composed solely of LiDFOB. The LiDFOB-containing layer may also contain additives, such as lithium hexafluorophosphate, lithium triflate, and lithium difluorophosphate, which are known in the field of lithium secondary batteries, in addition to LiDFOB. However, to fully demonstrate the effect of improving air stability due to the introduction of the LiDFOB-containing layer, the LiDFOB-containing layer preferably contains at least 50 mol %, or 70 mol %, or 90 mol % of LiDFOB.

[0079] The formation of the LiDFOB-containing layer can be confirmed by electron microscope analysis or XRD analysis of the positive electrode additive.

[0080] According to an embodiment of the present invention, the LiDFOB-containing layer is contained in an amount of 5.0 parts by weight to 15.0 parts by weight, alternatively 5.5 parts by weight to 15.0 parts by weight, alternatively 5.5 parts by weight to 12.0 parts by weight, alternatively 5.5 parts by weight to 10.0 parts by weight, alternatively 6.0 parts by weight to 10.0 parts by weight, or alternatively 6.0 parts by weight to 9.0 parts by weight, based on 100 parts by weight of the total amount of the positive electrode additive.

[0081] To ensure that the positive electrode additive's air stability improving effect is fully achieved, the content of the LiDFOB-containing layer is preferably 5.0 parts by weight or more based on 100 parts by weight of the total content of the positive electrode additive. However, if the content of the LiDFOB-containing layer is excessively high, the irreversible capacity and charge / discharge capacity characteristics of the positive electrode additive may be reduced. Therefore, the content of the LiDFOB-containing layer is preferably 15.0 parts by weight or less based on 100 parts by weight of the total content of the positive electrode additive.

[0082] The above-described positive electrode additive can be mixed with a separate positive electrode active material to act as a sacrificial positive electrode material that compensates for the irreversible capacity of the negative electrode during the initial charge / discharge process of a lithium secondary battery, and the positive electrode active material acts after the irreversible capacity is compensated. Moreover, the positive electrode additive can also be preferably used as an additional positive electrode active material because it has improved capacity characteristics during charge / discharge.

[0083] According to yet another embodiment of the present invention, there is provided a positive electrode for a lithium secondary battery.

[0084] The positive electrode for the lithium secondary battery may include a positive electrode active material, a binder, a conductive material, and the positive electrode additive.

[0085] The positive electrode additive has the property of irreversibly releasing lithium during charge and discharge of a lithium secondary battery, and therefore, when included in the positive electrode of a lithium secondary battery, the positive electrode additive can serve as a sacrificial positive electrode material for prelithiation.

[0086] Preferably, the positive electrode for the lithium secondary battery comprises a positive electrode material including a positive electrode active material, a conductive material, the positive electrode additive, and a binder; and a current collector supporting the positive electrode material.

[0087] To increase the capacity of a battery, the ratio of anode active material in the anode must be increased, which increases the amount of lithium consumed in the SEI layer. Therefore, the designed capacity of the battery can be determined by calculating the amount of lithium consumed in the SEI layer of the anode and then back-calculating the amount of sacrificial cathode material to be applied to the cathode side.

[0088] According to one embodiment, the positive electrode additive is contained in an amount of more than 0 wt % and not more than 15 wt % based on the total weight of the positive electrode material.

[0089] In order to compensate for the irreversible lithium consumed in the formation of the SEI layer, the content of the positive electrode additive is preferably more than 0 wt % based on the total weight of the positive electrode material.

[0090] However, if the positive electrode additive is contained in excess, the content of the positive electrode active material, which exhibits reversible charge / discharge capacity, decreases, resulting in a decrease in battery capacity, and residual lithium in the battery may plate on the negative electrode, causing a short circuit or jeopardizing safety. Therefore, the content of the positive electrode additive is preferably 15 wt % or less based on the total weight of the positive electrode material.

[0091] Specifically, the content of the positive electrode additive may be greater than 0 wt %, alternatively 0.5 wt % or more, alternatively 1 wt % or more, alternatively 2 wt % or more, or alternatively 3 wt % or more, based on the total weight of the positive electrode material; and 15 wt % or less, alternatively 12 wt % or less, or alternatively 10 wt % or less.

[0092] Preferably, the content of the positive electrode additive may be 0.5 wt % to 15 wt %, alternatively 1 wt % to 15 wt %, alternatively 1 wt % to 12 wt %, alternatively 2 wt % to 12 wt %, alternatively 2 wt % to 10 wt %, or alternatively 3 wt % to 10 wt %, relative to the total weight of the positive electrode material.

[0093] The positive electrode active material may be any material capable of reversibly intercalating and deintercalating lithium ions, and may be, for example, a composite oxide or phosphate containing lithium and a metal selected from cobalt, manganese, nickel, iron, and combinations thereof.

[0094] As a non-limiting example, the positive electrode active material may be a compound represented by any one of the following chemical formulas:

[0095] Li a A1- b R b D2(0.90≦a≦1.8, 0≦b≦0.5);Li a E 1-b R b O 2-c D c (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05);LiE 2-b R b O 4-c D c (0≦b≦0.5, 0≦c≦0.05);Li a Ni 1-b-c Co b R c D d (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05, 0 <d≦2);Li a Ni 1-b-c Co b R c O2- d Z d (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05, 0 <d<2);Li a Ni 1-b-c Co b R c O 2-dZ2(0.90≦a≦1.8、0≦b≦0.5、0≦c≦0.05、0 <d<2);Li a Ni 1-b-c Mn b R c D d (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05,0 <d≦2);Li a Ni 1-b-c Mn b R c O 2-d Z d (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05,0 <d<2);Li a Ni 1-b-c Mn b R c O 2-d Z2(0.90≦a≦1.8、0≦b≦0.5、0≦c≦0.05、0 <d<2);Li a Ni b E c G d O2(0.90≦a≦1.8;0≦b≦0.9;0≦c≦0.5;0.001≦d≦0.1);Li a Ni b Co. Co c Mn d G e O2(0.90≦a≦1.8、0≦b≦0.9、0≦c≦0.5、0≦d≦0.5、0.001≦e≦0.1);Li a NiG b O2(0.90≦a≦1.8;0.001≦b≦0.1);Li a CoG b O2(0.90≦a≦1.8;0.001≦b≦0.1);Li a MnG b O2(0.90≦a≦1.8;0.001≦b≦0.1);Li a Mn2G b O4(0.90≦a≦1.8、0.001≦b≦0.1);QO2;QS2;LiQS2;V2O5;LiV2O5;LiTO2;LiNiVO4;Li (3-f) J2(PO4)3(0≦f≦2);Li (3-f) Fe2(PO4)3(0≦f≦2);

[0096] In the above chemical formula, A is Ni, Co, Mn, or a combination thereof; R is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, a rare earth element, or a combination thereof; D is O, F, S, P, or a combination thereof; E is Co, Mn, or a combination thereof; Z is F, S, P, or a combination thereof; G is Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, or a combination thereof; Q is Ti, Mo, Mn, or a combination thereof; T is Cr, V, Fe, Sc, Y, or a combination thereof; and J is V, Cr, Mn, Co, Ni, Cu, or a combination thereof.

[0097] Of course, the cathode active material may have a coating layer on its surface, or the cathode active material may be mixed with a cathode active material having a coating layer. The coating element contained in the coating layer may be Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or a mixture thereof.

[0098] According to one embodiment, the positive electrode active material is contained in an amount of 80 wt % to 95 wt % based on the total weight of the positive electrode material.

[0099] Specifically, the content of the positive electrode active material may be 80 wt % or more, alternatively 82 wt % or more, alternatively 85 wt % or more, based on the total weight of the positive electrode material; and 95 wt % or less, alternatively 93 wt % or less, alternatively 90 wt % or less.

[0100] Preferably, the content of the positive electrode active material may be 82% to 95% by weight, alternatively 82% to 93% by weight, alternatively 85% to 93% by weight, alternatively 85% to 90% by weight, based on the total weight of the positive electrode material.

[0101] The conductive material is used to impart electrical conductivity to the electrode.

[0102] The conductive material can be any material that does not cause chemical changes in the battery and has electronic conductivity. Non-limiting examples of the conductive material include carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; graphite, such as natural graphite and artificial graphite; metal powder or metal fiber, such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives. The conductive material can be one or a mixture of two or more of the above examples.

[0103] The content of the conductive material is adjusted within a range that provides an appropriate level of conductivity without causing a decrease in battery capacity, and is preferably 1 wt % to 10 wt % or 1 wt % to 5 wt % based on the total weight of the cathode material.

[0104] The binder is used to make the positive electrode material adhere well to the current collector.

[0105] Non-limiting examples of the binder include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, etc. The binder may be one or a mixture of two or more of the above examples.

[0106] The content of the binder is adjusted within a range that provides an appropriate level of adhesiveness without causing a decrease in battery capacity, and preferably, the content of the binder may be 1 wt % to 10 wt % or 1 wt % to 5 wt % based on the total weight of the cathode material.

[0107] The current collector may be made of any material that is applicable to the positive electrode of a lithium secondary battery in the technical field to which the present invention pertains, without any particular limitation.

[0108] Non-limiting examples of the current collector include stainless steel; aluminum; nickel; titanium; sintered carbon; or aluminum or stainless steel surface treated with carbon, nickel, titanium, silver, or the like. Preferably, the current collector has a thickness of 3 μm to 500 μm. The current collector may have a surface with fine irregularities to enhance the adhesive strength of the positive electrode material. The current collector may have various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0109] The positive electrode for the lithium secondary battery is formed by laminating a positive electrode material including the positive electrode active material, the conductive material, the positive electrode additive, and a binder on the current collector.

[0110] According to yet another embodiment of the present invention, there is provided a lithium secondary battery including the positive electrode for a lithium secondary battery, a negative electrode, a separator, and an electrolyte.

[0111] The lithium secondary battery includes a cathode containing the cathode additive. This suppresses gas generation at the cathode during charging and discharging, thereby improving safety and lifespan. The lithium secondary battery also exhibits high discharge capacity, excellent output power, and capacity retention.

[0112] Therefore, the lithium secondary battery is used as an energy supply source with improved performance and safety in the fields of portable electronic devices such as mobile phones, laptops, tablet computers, mobile batteries, and digital cameras; and in the fields of transportation such as electric cars, electric motorcycles, and personal mobility devices.

[0113] The lithium secondary battery includes an electrode assembly wound between a positive electrode and a negative electrode with a separator interposed therebetween, and a case in which the electrode assembly is housed, and the positive electrode, the negative electrode, and the separator are impregnated with an electrolyte.

[0114] The lithium secondary battery may have various shapes such as a prismatic shape, a cylindrical shape, a pouch shape, and the like.

[0115] The matters relating to the positive electrode are replaced with the above-mentioned matters.

[0116] The negative electrode may include a negative electrode material including a negative electrode active material, a conductive material, and a binder; and a current collector that supports the negative electrode material.

[0117] The negative electrode active material may include a material capable of reversibly intercalating and deintercalating lithium ions, lithium metal, a lithium metal alloy, a material capable of doping and dedoping lithium, and a transition metal oxide.

[0118] The material capable of reversibly intercalating and deintercalating lithium ions may be a carbonaceous material, such as crystalline carbon, amorphous carbon, or a mixture thereof. Specifically, the carbonaceous material may be natural graphite, artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitches, mesophase pitch-based carbon fiber, carbon microbeads, petroleum or coal tar pitch-derived cokes, soft carbon, or hard carbon.

[0119] The lithium metal alloy may be an alloy of lithium with 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, Sn, Bi, Ga, and Cd.

[0120] The material capable of doping and dedoping with lithium is Si, Si-C composite, SiO x(0 < x < 2), Si-Q alloy (where Q 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 Group 15 element, a Group 16 element, a transition metal, a rare earth element, and combinations thereof; provided that Si is excluded), Sn, SnO2, Sn-R alloy (where R 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 Group 15 element, a Group 16 element, a transition metal, a rare earth element, and combinations thereof; provided that Sn is excluded), and the like. And as the substance capable of doping and undoping lithium, at least one of the above examples and SiO2 can be mixed and used. Q and R can be Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Tl, Ge, P, As, Sb, Bi, S, Se, Te, Po, and the like.

[0121] And the transition metal oxide can be vanadium oxide, lithium vanadium oxide, lithium titanate, and the like.

[0122] Preferably, the negative electrode contains one or more negative electrode active materials selected from the group consisting of a carbonaceous material and a silicon compound.

[0123] That is, according to another embodiment of the present invention, there is provided a lithium secondary battery including a positive electrode for the lithium secondary battery; a negative electrode containing one or more negative electrode active materials selected from the group consisting of a carbonaceous material and a silicon compound; a separator; and an electrolyte.

[0124] Here, the carbonaceous material is at least one substance selected from the group consisting of the natural graphite, artificial graphite, kish graphite, pyrolytic carbon, mesophase pitch, mesophase pitch-based carbon fiber, carbon microspheres, petroleum or coal-based coke, soft carbon, and hard carbon described above. And the silicon compound is a compound containing Si described above, that is, Si, Si-C composite, SiO x (0 < x < 2), and can be the Si-Q alloy, mixtures thereof, or mixtures of at least one of these and SiO2.

[0125] Also, the negative electrode may contain micro-silicon. When the negative electrode contains micro-silicon, a capacity superior to that when using a carbonaceous material as the negative electrode active material can be achieved. Specifically, when using specific micro-silicon in the silicon compound, a residual capacity of 80% or more can be maintained even after 500 or more charge and discharge cycles, and an energy density significantly superior to that of conventional lithium secondary batteries can be realized. Further, when the negative electrode contains micro-silicon, the charge and discharge life of a solid battery using a solid electrolyte can be greatly increased, and the charging rate can also be greatly improved at room temperature.

[0126] The size of the micro-silicon is not particularly limited. For example, the micro-silicon can have a diameter of 100 μm or less, or a diameter of 1 μm to 100 μm, or a diameter of 1 μm to 20 μm.

[0127] According to one embodiment, the negative electrode active material is contained at 85% to 98% by weight based on the total weight of the negative electrode material.

[0128] Specifically, the content of the negative electrode active material can be 85% by weight or more, or 87% by weight or more, or 90% by weight or more; and 98% by weight or less, or 97% by weight or less, or 95% by weight or less based on the total weight of the negative electrode material.

[0129] Preferably, the content of the negative electrode active material may be 85% to 97% by weight, alternatively 87% to 97% by weight, alternatively 87% to 95% by weight, or alternatively 90% to 95% by weight, based on the total weight of the negative electrode material.

[0130] The conductive material, the binder, and the current collector contained in the negative electrode material are substituted with the contents described above for the positive electrode for a lithium secondary battery.

[0131] The separator separates the positive electrode and the negative electrode and provides a path for lithium ions to move. Any separator that can be used as a separator for a lithium secondary battery in the technical field of the present invention can be used without particular limitation. The separator preferably has low resistance to ion movement of the electrolyte and excellent wettability with the electrolyte.

[0132] Specifically, the separator may be a porous polymer film made of a polyolefin polymer such as polyethylene, polypropylene, ethylene-butene copolymer, ethylene-hexene copolymer, or ethylene-methacrylate copolymer. The separator may be a multilayer film formed by laminating two or more layers of the porous polymer film. The separator may be a nonwoven fabric containing glass fiber, polyethylene terephthalate fiber, or the like. The separator may be coated with a ceramic component or a polymer material to ensure heat resistance or mechanical strength.

[0133] Meanwhile, the electrolyte may be any electrolyte applicable to lithium secondary batteries in the technical field to which the present invention pertains, without any particular limitation, and may be, for example, an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, etc.

[0134] Specifically, the electrolyte may include a non-aqueous organic solvent and a lithium salt.

[0135] The non-aqueous organic solvent may be any organic solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move.

[0136] Specifically, the non-aqueous organic solvent may be an ester-based solvent such as methyl acetate, ethyl acetate, γ-butyrolactone, or ε-caprolactone; an ether-based solvent such as dibutyl ether or tetrahydrofuran; a ketone-based solvent such as cyclohexanone; an aromatic hydrocarbon-based solvent such as benzene or fluorobenzene; dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), or propylene carbonate (propylene carbonate). The solvent may be a carbonate solvent such as ethylene carbonate (PC); an alcohol solvent such as ethyl alcohol and isopropyl alcohol; a nitrile such as R-CN (R is a hydrocarbon group having 2 to 20 carbon atoms and having a linear, branched, or cyclic structure, and containing a double-bonded aromatic ring or an ether bond); an amide such as dimethylformamide; a dioxolane such as 1,3-dioxolane; or sulfolane.

[0137] Among the above examples, it is preferable to use a carbonate-based solvent as the non-aqueous organic solvent.

[0138] In particular, in consideration of the charge / discharge performance of the battery and compatibility with the sacrificial cathode material, it is preferable to use a mixture of a cyclic carbonate (e.g., ethylene carbonate, propylene carbonate) having high ionic conductivity and a high dielectric constant and a linear carbonate (e.g., ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate) having a low viscosity as the nonaqueous organic solvent. In this case, mixing the cyclic carbonate and the linear carbonate in a volume ratio of 1:1 to 1:9 is advantageous for achieving the above-mentioned performance.

[0139] Furthermore, it is preferable to use, as the non-aqueous organic solvent, a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 1:2 to 1:10; or a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 1-3:1 to 9:1.

[0140] The lithium salt contained in the electrolyte dissolves in the non-aqueous organic solvent and acts as a source of lithium ions in the battery, enabling basic operation of the lithium secondary battery and promoting the movement of lithium ions between the positive electrode and the negative electrode.

[0141] Specifically, the lithium salt may be LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCFSO, LiC4F9SO, LiN(C2F5SO3), LiN(C2F5SO2), LiN(CF3SO2), LiN(SO2F), (LiFSI, lithium bis(fluorosulfonyl)imide), LiCl, LiI, LiB(C2O4), etc. Preferably, the lithium salt may be LiPF, LiFSI, or a mixture thereof.

[0142] The lithium salt is contained in the electrolyte at a concentration of 0.1 M to 2.0 M. The lithium salt contained in the above concentration range can provide the electrolyte with appropriate conductivity and viscosity, thereby exhibiting excellent electrolyte performance.

[0143] Optionally, the electrolyte contains an additive for the purpose of improving the life characteristics of the battery, suppressing the decrease in battery capacity, and improving the discharge capacity of the battery.

[0144] For example, the additive may be a haloalkylene carbonate compound such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivative, sulfur, quinoneimine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride, etc. The additive is contained in an amount of 0.1 wt % to 5 wt % based on the total weight of the electrolyte. [Effects of the Invention]

[0145] According to the present invention, a method for producing a cathode additive for a lithium secondary battery that exhibits high initial irreversible capacity and excellent air stability is provided. The cathode additive produced by the above method can compensate for irreversible capacity loss in high-capacity lithium secondary batteries and effectively suppress gas generation in the battery and the resulting fire and explosion. [Brief explanation of the drawings]

[0146] [Figure 1] 1 is a simplified schematic diagram showing a cross section of a positive electrode additive particle for a lithium secondary battery according to one embodiment of the present invention. [Figure 2] 1 is a scanning electron microscope (SEM) image of the positive electrode additive prepared in Example 1. [Figure 3] 1 is a scanning electron microscope (SEM) image of the positive electrode additive prepared in Comparative Example 3. [Figure 4] 1 shows the results of X-ray diffraction (XRD) analysis of the positive electrode additives prepared in Examples and Comparative Examples. [Figure 5] 1 shows the results of X-ray diffraction (XRD) analysis of the positive electrode additives prepared in Examples and Comparative Examples. [Figure 6] 1 shows the results of X-ray diffraction (XRD) analysis of the positive electrode additives prepared in Examples and Comparative Examples. [Figure 7] 1 shows the results of X-ray diffraction (XRD) analysis of the positive electrode additives prepared in Examples and Comparative Examples. [Figure 8] 1 shows the results of X-ray diffraction (XRD) analysis of the positive electrode additives prepared in Examples and Comparative Examples. [Figure 9] 1 shows the results of X-ray diffraction (XRD) analysis of the positive electrode additives prepared in Examples and Comparative Examples. [Figure 10] 1 shows the results of X-ray diffraction (XRD) analysis of the positive electrode additives prepared in Examples and Comparative Examples. [Figure 11] 1 shows the results of X-ray diffraction (XRD) analysis of the positive electrode additives prepared in Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0147] The functions and effects of the present invention will be described in more detail below with reference to specific embodiments of the present invention. However, these are presented as examples to facilitate understanding of the invention. The following embodiments are not intended to limit the scope of the invention in any way, and it will be apparent to those skilled in the art that various changes and modifications can be made within the scope and technical concept of the present invention.

[0148] Example 1 (1) Manufacturing of cathode additives The positive electrode additive of Example 1 was prepared using a 0.2 L reactor and a mechanical stirrer according to the following method.

[0149] An aqueous dispersion of carbon nanotubes manufactured by LG Chem was used. The aqueous dispersion was prepared by adding 5.83 wt% carbon nanotubes (CNTs) and 1.0 wt% polyvinylpyrrolidone (Acros organics, Mw 50,000 g / mol), a water-soluble polymer dispersant, to 200 ml of DI water and mixing them with an ultrasonic tip for 10 minutes.

[0150] 0.6 mol of iron(III) nitrate nonahydrate (Daejeon Chemicals and Metals, 242.328 g) was dissolved in 600 ml of DI water and slowly poured into a flask containing 28 g of the CNT aqueous dispersion (CNT content = 3.3 wt% relative to the iron oxide-carbon precursor (Fe2O3-CNT precursor) formed in the subsequent step) and stirred for 30 minutes. Next, 1.8 mol (252.36 g) of NH4OH was slowly poured into the flask and stirred for 30 minutes, followed by a reaction at 80°C for 6 hours.

[0151] After the reaction was complete, the mixture was left to stand for 30 minutes, the upper layer solution was discarded, and the mixture was filtered and dried for 12 hours in a convection oven at 120°C. The dried powder was then heat-treated in an air atmosphere at 250°C for 6 hours to remove impurities, yielding an iron oxide-carbon precursor (Fe2O3-CNT precursor).

[0152] Li2O (Ganfeng Lithium Co., Ltd.) and the Fe2O3-CNT precursor were uniformly mixed in a molar ratio of 5:1 and fired in a heat treatment furnace at 600 °C (heated over 2 hours, maintained for 6 hours) under an Ar atmosphere to obtain lithium-iron oxide.

[0153] 100 parts by weight of the lithium-iron oxide particles were mixed with 8.0 parts by weight of lithium difluoro(oxalato)borate (Sigma-Aldrich) using a mixer, and the mixture was baked in a heat treatment furnace in an air atmosphere at 270°C for 1 hour and then quenched to obtain the positive electrode additive of Example 1.

[0154] (2) Manufacture of lithium secondary batteries The lithium transition metal oxide, carbon black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were mixed in an organic solvent (N-methylpyrrolidone) in a weight ratio of 90:4:6 to prepare a cathode material slurry. The cathode material slurry was applied to one side of a 15 μm thick aluminum foil current collector, which was then rolled and dried to prepare a cathode (punch size: Φ14 mm).

[0155] A coin cell-type lithium secondary battery was fabricated by preparing the cathode, anode, separator, and electrolyte. The anode was a 300 μm thick Li metal (punched size: Φ14 mm). The electrolyte was a non-aqueous organic solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) in a volume ratio of 1:2:1, with 1.0 M LiPF6 and 2 wt% vinylene carbonate (VC) dissolved in the solvent. The separator was a PE resin separator (WL20C, 20 μm, manufactured by W-scope).

[0156] Example 2 A positive electrode additive of Example 2 and a lithium secondary battery including the same were prepared in the same manner as in Example 1, except that the content of the lithium difluoro(oxalato)borate was increased to 15.0 parts by weight based on 100 parts by weight of the lithium-iron oxide.

[0157] Example 3 A positive electrode additive of Example 3 and a lithium secondary battery including the same were manufactured in the same manner as in Example 1, except that the content of the CNT aqueous dispersion was increased to 34 g (CNT content relative to the Fe2O3-CNT precursor formed in the subsequent process = 4.0 wt%).

[0158] Example 4 Lithium-iron oxide particles were obtained in the same manner as in Example 1. 100 parts by weight of the lithium-iron oxide particles were mixed with 15.0 parts by weight of lithium difluoro(oxalato)borate (Sigma-Aldrich) using a mixer.

[0159] The mixture was baked in a heat treatment furnace in an air atmosphere at 280° C. for 1 hour, and then quenched to obtain the positive electrode additive of Example 4.

[0160] A lithium secondary battery was manufactured in the same manner as in Example 1 using the positive electrode additive.

[0161] Example 5 Lithium-iron oxide particles were obtained in the same manner as in Example 1. 100 parts by weight of the lithium-iron oxide particles were mixed with 15.0 parts by weight of lithium difluoro(oxalato)borate (Sigma-Aldrich) using a mixer.

[0162] The mixture was first baked in a heat treatment furnace under an argon gas atmosphere at 270°C for 1 hour, and the resultant was second baked under an air atmosphere at 270°C for 1 hour, followed by quenching to obtain a cathode additive of Example 5.

[0163] A lithium secondary battery was manufactured in the same manner as in Example 1 using the positive electrode additive.

[0164] Comparative Example 1 1.494 g of Li2O (Ganfeng Lithium Co., Ltd.) and 1.597 g of Fe2O3 (Sigma-Aldrich) were solid-phase mixed (molar ratio Li2O:Fe2O3 = 5:1). The mixture was pressed into pellets and calcined in a heat treatment furnace at 270°C in an air atmosphere for 1 hour to produce the cathode additive of Comparative Example 1.

[0165] A lithium secondary battery of Comparative Example 1 was fabricated in the same manner as in Example 1, except that the positive electrode additive was used.

[0166] Comparative Example 2 1.494 g of Li2O (Ganfeng Lithium Co., Ltd.) and 1.597 g of Fe2O3 (Sigma-Aldrich) were solid-phase mixed (molar ratio Li2O:Fe2O3 = 5:1). 0.4 g of polyvinylpyrrolidone (Acros Organics, Mw 50,000 g / mol) was added to the mixture (4 g of polyvinylpyrrolidone was added based on 0.1 mol of the resulting positive electrode additive (Li5FeO4)). The mixture was pressed into pellets and calcined in a heat treatment furnace in an air atmosphere at 270°C for 1 hour to produce the positive electrode additive of Comparative Example 2.

[0167] A lithium secondary battery of Comparative Example 2 was fabricated in the same manner as in Example 1, except that the positive electrode additive was used.

[0168] Comparative Example 3 1.494 g of Li2O (Ganfeng Lithium Co., Ltd.) and 1.597 g of Fe2O3 (Sigma-Aldrich) were solid-phase mixed (molar ratio Li2O:Fe2O3 = 5:1). 10 wt% of carbon nanotubes (CNTs) were added to the mixture and mixed. The mixture was formed into pellets using a press and fired in a heat treatment furnace in an air atmosphere at 270°C for 1 hour to prepare the positive electrode additive of Comparative Example 3. A lithium secondary battery of Comparative Example 3 was fabricated in the same manner as in Example 1, except for using the positive electrode additive.

[0169] Comparative Example 4 Lithium-iron oxide particles were obtained in the same manner as in Example 1. 100 parts by weight of the lithium-iron oxide particles were mixed with 15.0 parts by weight of lithium difluoro(oxalato)borate (Sigma-Aldrich) using a mixer.

[0170] The mixture was baked in a heat treatment furnace in an argon gas atmosphere at 270° C. for 1 hour, and then quenched to obtain a positive electrode additive of Comparative Example 4.

[0171] A lithium secondary battery was manufactured in the same manner as in Example 1 using the positive electrode additive.

[0172] Comparative Example 5 Lithium-iron oxide particles were obtained in the same manner as in Example 1. 100 parts by weight of the lithium-iron oxide particles were mixed with 15.0 parts by weight of lithium difluoro(oxalato)borate (Sigma-Aldrich) using a mixer.

[0173] The mixture was baked in a heat treatment furnace in an air atmosphere at 320° C. for 1 hour, and then quenched to obtain a positive electrode additive of Comparative Example 5.

[0174] A lithium secondary battery was manufactured in the same manner as in Example 1 using the positive electrode additive.

[0175] Comparative Example 6 Lithium-iron oxide particles were obtained in the same manner as in Example 1. 100 parts by weight of the lithium-iron oxide particles were mixed with 6 parts by weight of oxalic acid, 4 parts by weight of boric acid, and 11 parts by weight of lithium bis(fluorosulfonyl)imide (Li-FSI, Nippon Shokubai Co., Ltd.) using a mixer.

[0176] The mixture was baked in a heat treatment furnace in an air atmosphere at 270° C. for 1 hour, and then quenched to obtain a positive electrode additive of Comparative Example 6.

[0177] A lithium secondary battery was manufactured in the same manner as in Example 1 using the positive electrode additive.

[0178] Comparative Example 7 Lithium-iron oxide particles were obtained in the same manner as in Example 1. 100 parts by weight of the lithium-iron oxide particles were mixed with 6 parts by weight of oxalic acid, 4 parts by weight of boric acid, and 6 parts by weight of lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI, J&H Chemical Co.) using a mixer.

[0179] The mixture was baked in a heat treatment furnace in an air atmosphere at 270° C. for 1 hour, and then quenched to obtain a positive electrode additive of Comparative Example 7.

[0180] A lithium secondary battery was manufactured in the same manner as in Example 1 using the positive electrode additive.

[0181] Experimental Example 1 Scanning electron microscope (SEM) images of the positive electrode additives prepared in Example 1 and Comparative Example 3 are shown in FIG. 2 (Example 1) and FIG. 3 (Comparative Example 3).

[0182] The results of X-ray diffraction analysis (D8 Endeavor, Bruker) for the positive electrode additives prepared in the examples and comparative examples are shown in FIG. 4 (Example 1), FIG. 5 (Example 2), FIG. 6 (Example 3), FIG. 7 (Example 5), FIG. 8 (Comparative Example 1), FIG. 9 (Comparative Example 2), FIG. 10 (Comparative Example 3), and FIG. 11 (Comparative Example 4).

[0183] The results of the scanning transmission microscope and XRD analysis confirmed that the positive electrode additive of the example formed a lithium transition metal oxide of Li5FeO4, and that a double coating layer of a polyvinylpyrrolidone (PVP)-derived carbon coating layer and a carbon nanotube-containing layer was formed on the lithium transition metal oxide particles to a thickness of 10 nm to 300 nm. Furthermore, it was confirmed that a lithium difluoro(oxalato)borate-containing layer was formed on the surface of the lithium transition metal oxide.

[0184] 5 (Example 2) and 11 (Comparative Example 4), Example 2, which was fired in an air atmosphere, showed almost no peaks in the 2θ value range of 26 to 28 degrees, while Comparative Example 4, which was fired in an argon gas atmosphere, showed many relatively strong peaks in this 2θ value range. These results confirm that when heat-treated in an air atmosphere, a better LiDFOB coating effect can be achieved with almost no impurities.

[0185] Experimental Example 2 (1) Charge / discharge capacity The lithium secondary batteries prepared in the examples and comparative examples were subjected to a charge-discharge experiment by charging them at a constant current of 60 mA / g and a constant voltage of 30 mA / g at 45° C. to 4.25 V, and then discharging them at a constant current of 10 mA / g to 2.5 V. The charge capacity and discharge capacity were calculated from the charge-discharge experiment.

[0186] (2) Charge capacity retention rate after aging The lithium secondary batteries prepared in the examples and comparative examples were stored in a chamber under an air atmosphere maintained at a temperature of 30°C and a relative humidity of 33% (33RH%) for 6 hours. The charge-discharge experiments were then performed on the lithium secondary batteries under the same conditions. The ratio of the charge capacity after storage to the charge capacity before storage in the chamber (capacity retention, %) was calculated.

[0187] [Table 1]

[0188] Referring to Table 1 above, it was confirmed that the lithium secondary batteries of the Examples exhibited a charge capacity of 550 mAh / g or more, a high charge capacity retention rate of 85% or more after aging, and maintained a color similar to that of the electrode film before aging, thereby having excellent air stability.

[0189] In contrast, the lithium secondary battery of Comparative Example 1 exhibited a relatively high charge capacity, but the charge capacity retention rate after aging was significantly low. The lithium secondary battery of Comparative Example 2 exhibited poor charge capacity, and the electrode film was distorted during the aging experiment, making it impossible to measure the charge capacity retention rate. The lithium secondary battery of Comparative Example 3 exhibited good charge capacity, but the electrode film was distorted to such an extent that performance measurement was impossible during the aging experiment. The lithium secondary battery of Comparative Example 4 exhibited good charge capacity, but it was confirmed that the charge capacity retention rate after aging was relatively low.

[0190] The lithium secondary battery of Comparative Example 5, which contained a positive electrode additive that had been heat-treated at a relatively high temperature, exhibited a significant decrease in charge capacity. This was confirmed to be due to the loss of carbon components (carbon coating layer, carbon nanotube-containing layer) caused by the heat treatment at a high temperature. However, the charge capacity retention rate after aging in the lithium secondary battery of Comparative Example 5 increased slightly. This is presumably because the loss of the carbon components reduces the specific surface area of ​​the positive electrode additive, resulting in a relative improvement in the coating efficiency of lithium difluoro(oxalato)borate.

[0191] In the lithium secondary batteries of Comparative Examples 6 and 7, a combination of compounds having a similar corresponding structure was added instead of lithium difluoro(oxalato)borate, but it was confirmed that the charge capacity was relatively low and the charge capacity retention rate after aging was also significantly low.

[0192] Although the present invention has been described above with reference to limited embodiments and drawings, the present invention is not limited thereto, and it is of course possible for a person skilled in the art to which the present invention pertains to make various modifications and variations within the technical spirit of the present invention and the equivalent scope of the claims set forth below. [Explanation of symbols]

[0193] 10 Lithium-iron oxide particles 20 carbon coating layers 30 Carbon nanotube-containing layer 40 Lithium difluoro(oxalato)borate-containing layer

Claims

1. mixing and heat-treating the carbon nanotubes, the water-soluble polymer dispersant, and the iron precursor to form an iron oxide-carbon precursor; mixing a lithium precursor and the iron oxide-carbon precursor and calcining at 500° C. or higher to form lithium-iron oxide particles; and The method for preparing a positive electrode additive for a lithium secondary battery includes heat-treating the mixture of the lithium-iron oxide particles and lithium difluoro(oxalato)borate in an oxygen-containing gas atmosphere at a temperature of less than 300°C to form a lithium-iron oxide coated with a lithium difluoro(oxalato)borate-containing layer.

2. 2. The method for producing a positive electrode additive for a lithium secondary battery according to claim 1, wherein the lithium difluoro(oxalato)borate is mixed in an amount of 5.0 to 20.0 parts by weight based on the lithium-iron oxide particles.

3. 2. The method of claim 1, wherein the step of forming the lithium-iron oxide coated with the lithium difluoro(oxalato)borate-containing layer is performed by heat-treating the mixture in an air atmosphere at a temperature of 260° C. to 295° C.

4. forming a lithium-iron oxide coated layer containing lithium difluoro(oxalato)borate; a step of subjecting the mixture of lithium-iron oxide particles and lithium difluoro(oxalato)borate to a primary heat treatment under an inert gas atmosphere at a temperature of less than 300°C; and 4. The method for preparing a positive electrode additive for a lithium secondary battery according to claim 1, further comprising the step of subjecting the resultant of the first heat treatment to a second heat treatment in an oxygen-containing gas atmosphere at a temperature of less than 300°C.

5. 2. The method for producing a positive electrode additive for a lithium secondary battery according to claim 1, wherein the water-soluble polymer dispersant comprises one or more compounds selected from the group consisting of polyvinylpyrrolidone-based polymers, polyacrylic acid-based polymers, polyvinyl alcohol-based polymers, and hydroxyalkyl cellulose-based polymers.

6. 2. The method of claim 1, wherein the water-soluble polymer dispersant is used in an amount of 0.1 to 2 parts by weight based on the total amount of the iron oxide-carbon precursor.

7. 2. The method for producing a positive electrode additive for a lithium secondary battery according to claim 1, wherein the carbon nanotubes are used in an amount of 1 to 10 parts by weight based on the total amount of the iron oxide-carbon precursor.

8. 2. The method for producing a positive electrode additive for a lithium secondary battery according to claim 1, wherein the iron precursor comprises one or more compounds selected from the group consisting of Fe(III) nitrates, sulfates, phosphates, oxides, halides, and hydrates thereof.

9. The step of forming the iron oxide-carbon precursor comprises: forming a carbon nanotube dispersion in which the carbon nanotubes are dispersed in an aqueous solvent in the presence of the water-soluble polymer dispersant; mixing the carbon nanotube dispersion and an iron precursor in the presence of a base; reacting the carbon nanotube dispersion and the iron precursor in the mixture at a temperature of 50°C to 100°C; and 10. The method for preparing a positive electrode additive for a lithium secondary battery according to claim 1, comprising the steps of filtering and drying the reaction product solution, and heat-treating the solution at a temperature of 200 to 300°C.

10. The positive electrode additive for a lithium secondary battery is the lithium-iron oxide particles, a carbon coating layer formed on the lithium-iron oxide particles; a carbon nanotube-containing layer formed on the carbon coating layer; and The method for producing a positive electrode additive for a lithium secondary battery according to claim 1 , further comprising forming a lithium difluoro(oxalato)borate-containing layer on the carbon nanotube-containing layer.

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