Positive electrode additive for lithium secondary battery, positive electrode active material comprising same, positive electrode, and lithium secondary battery

Abstract

The present invention relates to a cathode additive for a lithium secondary battery and a cathode active material, a cathode and a lithium secondary battery comprising the same, in particular, the cathode additive represented by Formula 1 is formed on the surface of a carbon material contained in a cathode active material and is not dissolved in an electrolyte, which plays a role of electrically separating the cathode and the anode, thus improving the battery performance by suppressing side reactions in the battery.

Description

Technical Field

[0001] This application claims priority based on Korean Patent Application No. 10-2021-0001836, filed on January 7, 2021, and Korean Patent Application No. 10-2021-0004278, filed on January 12, 2021, the entire contents of which are incorporated herein by reference.

[0002] This invention relates to a positive electrode additive for lithium secondary batteries, as well as a positive electrode active material containing the additive, a positive electrode, and a lithium secondary battery. Background Technology

[0003] Until recently, there has been considerable interest in developing high-energy-density batteries using lithium as the anode. For example, lithium metal has attracted significant attention as an anode active material for electrochemical batteries due to its low weight and high capacity compared to other electrochemical systems with lithium-intercalated carbon anodes and nickel or cadmium electrodes, where the energy density is reduced by the increased weight and volume of the anode containing non-electroactive materials. Lithium metal anodes, or anodes primarily composed of lithium metal, offer the opportunity to form lighter batteries with higher energy densities compared to batteries such as lithium-ion batteries, nickel-metal hydride batteries, or nickel-cadmium batteries. This characteristic is highly desirable for batteries used in lightweight portable electronic devices such as mobile phones and laptops.

[0004] This type of lithium battery cathode active material is known, and the cathode active material contains sulfur-containing cathode active material with sulfur-sulfur bonds and achieves high energy capacity and rechargeability through the electrochemical breaking (reduction) and reforming (oxidation) of sulfur-sulfur bonds.

[0005] As described above, the theoretical energy density of a lithium-sulfur secondary battery using lithium and alkali metals as negative electrode active materials and sulfur as positive electrode active material is 2,800 Wh / kg, and the theoretical sulfur capacity is 1,675 mAh / g, which is significantly higher than other battery systems. Furthermore, because sulfur is abundant, inexpensive, and environmentally friendly, lithium-sulfur secondary batteries have attracted attention as an energy source for portable electronic devices.

[0006] However, sulfur, used as the positive electrode active material in lithium-sulfur secondary batteries, is a non-conductor, making it difficult for electrons generated by the electrochemical reaction to migrate. Furthermore, polysulfides (Li₂S₈ to Li₂S₄) generated during the charging and discharging processes of lithium-sulfur secondary batteries dissolve, and lithium sulfide (Li₂S₂ / Li₂S) and sulfur have poor conductivity and slow kinetics for the electrochemical reaction, leading to a decline in battery life and rate performance.

[0007] The results have been reported that when benzo[ghi]peryleneimide (BPI) is introduced as a cathode additive to improve such problems in lithium-sulfur secondary batteries, BPI acts as a redox mediator and can enhance the kinetics of electrochemical reactions and improve battery performance and lifespan (Laura CHGerber et al.; "Three-Dimensional Growth of Li2S in Lithium-Sulfur Batteries Promoted by a Redox Mediator"; Nano Lett. 2016, 16, 1, 549-554).

[0008] However, BPI tends to dissolve in ether or carbonate solvents used in the electrolyte of lithium-sulfur secondary batteries. When BPI dissolves in the electrolyte, electrons are transferred between the positive and negative electrodes, which need to be electrically isolated. This can cause internal short circuits and lead to a decrease in battery performance.

[0009] Therefore, there is a need to develop an additive that does not dissolve in the electrolyte of lithium secondary batteries, including lithium-sulfur secondary batteries, and can act as a catalyst for the reaction in the positive electrode.

[0010] Existing technical documents

[0011] [Non-patent literature]

[0012] (Non-patent literature 1) Nano Letters 2016, 16, 1, 549-554; Laura CH Gerber et al.; "Three-Dimensional Growth of Li2S in Lithium-Sulfur Batteries Promoted by a Redox Mediator" Summary of the Invention

[0013] [Technical Issues]

[0014] The purpose of this invention is to provide a positive electrode additive for lithium secondary batteries, which can act as a catalyst for redox mediator in the positive electrode without dissolving in the electrolyte of lithium secondary batteries.

[0015] Another object of the present invention is to provide a positive electrode active material comprising the aforementioned positive electrode additive for lithium secondary batteries, a positive electrode, and a lithium secondary battery.

[0016] [Technical Solution]

[0017] To achieve the above objectives, one embodiment of the present invention provides a positive electrode additive for lithium secondary batteries represented by the following formula 1:

[0018] <Formula 1>

[0019]

[0020] Where R represents a carbon chain.

[0021] Another embodiment of the present invention provides a method for preparing a positive electrode additive for lithium secondary batteries, the method comprising the step of reacting benzo[a]perylene anhydride (BPA) with a linear hydrocarbon molecule containing an amino group at at least one end.

[0022] Another embodiment of the present invention provides a positive electrode active material for a lithium secondary battery comprising a carbon material, wherein the positive electrode active material comprises the positive electrode additive on the surface of the carbon material.

[0023] Another embodiment of the present invention provides a positive electrode for a lithium secondary battery, the positive electrode for a lithium secondary battery comprising the positive electrode additive.

[0024] Another embodiment of the present invention provides a lithium secondary battery, the lithium secondary battery comprising a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte.

[0025] [Beneficial Effects]

[0026] The positive electrode additive for lithium secondary batteries according to the present invention does not dissolve in the electrolyte, thus preventing the degradation of battery performance caused by internal short circuits that occur when the positive electrode additive dissolves in the electrolyte.

[0027] Furthermore, the positive electrode additive for lithium secondary batteries exhibits electrochemical activity while being adsorbed onto the carbon material contained in the positive electrode of the lithium secondary battery, and acts as a catalyst for the reaction that takes place in the positive electrode, thus enhancing the electrochemical activity of the battery.

[0028] Furthermore, the positive electrode additive for lithium secondary batteries exhibits electrochemical activity while being adsorbed onto the carbon material contained in the positive electrode of the lithium secondary battery, thus enhancing the electrochemical activity of the battery. Attached Figure Description

[0029] Figure 1 This is a schematic diagram showing the raw materials used to synthesize the cathode additives in the examples and comparative examples.

[0030] Figure 2The graphs shown represent the FT-IR (Fourier transform infrared spectroscopy) measurements of the mixed solutions containing the respective positive electrode additives of Example 1 and Comparative Example 1.

[0031] Figure 3 A photograph is shown illustrating the color change over time of a mixed solution containing the positive electrode additive and carbon material (MWCNT) of Example 1.

[0032] Figure 4 This is a graph showing the electrochemical activity of a coin cell based on the presence or absence of the positive electrode additive of Example 1.

[0033] Figure 5 The graph shows the life performance of the respective coin batteries of Example 2 and Comparative Examples 1 and 3, depending on the presence or absence of adsorption of positive electrode additives.

[0034] Figure 6 The graph shows the life performance of the respective coin batteries of Examples 2, 4 and Comparative Example 3, depending on the presence or absence of adsorption of positive electrode additives. Detailed Implementation

[0035] The invention will be described in more detail below to aid in understanding it.

[0036] The terms or words used in this specification and claims should not be construed as limited to their ordinary or dictionary meanings, but rather should be interpreted as meanings and concepts corresponding to the technical ideas of the invention, based on the principle that the inventors can appropriately define the concepts of the terms to describe the invention in the best possible way.

[0037] Positive electrode additives for lithium secondary batteries

[0038] This invention relates to a positive electrode additive for lithium secondary batteries, and to a positive electrode additive for lithium secondary batteries that is adsorbed onto a carbon material used in a conventional positive electrode for lithium secondary batteries and thereby has the physical property of being insoluble in an electrolyte, and is electrochemically active while being adsorbed onto the surface of the carbon material.

[0039] In this invention, the positive electrode additive for lithium secondary batteries can be represented by the following formula 1:

[0040] <Formula 1>

[0041]

[0042] Where R represents a carbon chain.

[0043] The additive of Formula 1 has a structure in which a carbon chain is introduced into benzo[ghi]peryleneimide (BPI). The form of the carbon chain is not particularly limited, and can be, for example, a linear, cyclic, or branched carbon chain.

[0044] Furthermore, when the carbon chain is linear, the adsorption process of carbon materials can be performed more easily compared to when the carbon chain is cyclic or branched, and the process efficiency can be improved. For example, the linear carbon chain can be an aliphatic carbon chain.

[0045] BPI is an imide compound synthesized through the dehydration condensation of molecules containing benzo[a]perylene anhydride (BPA) and an amine, and is used as a positive electrode additive for lithium secondary batteries. However, because BPI is soluble in electrolyte, electrons transfer between the positive and negative electrodes, which need to be electrically isolated, while dissolving in the electrolyte. This leads to a problem of degraded battery performance due to internal short circuits.

[0046] However, when carbon chains are introduced into the BPI as shown in Formula 1 above, they do not dissolve in the electrolyte because carbon chains do not dissolve well in common electrolyte solvents. Here, the electrolyte refers to an electrolyte for lithium secondary batteries, which may, for example, be an electrolyte containing carbonate solvents and / or ether solvents.

[0047] When the number of carbon atoms in the carbon chain is small, the solubility of the electrolyte becomes uncontrolled, making it difficult to prevent carbon atoms from dissolving in the electrolyte. Conversely, when the number of carbon atoms is large, the proportion of benzo[a]perylene anhydride (BPA) that can participate in the electrochemical reaction decreases, which may reduce the effectiveness of the additive per unit mass and may also unnecessarily increase the battery weight. Therefore, R can be an alkyl group with 8 to 12 carbon atoms.

[0048] Furthermore, when the additive of Formula 1 contains ether chains or the like to replace carbon chains, the affinity with polar electrolyte solvents is enhanced by atoms with high electronegativity, such as oxygen. As a result, the additive is easily dissolved in the electrolyte, which leads to the transfer of electrons between the positive and negative electrodes, and the battery performance may decrease due to internal short circuits.

[0049] Method for preparing positive electrode additives for lithium secondary batteries

[0050] The present invention also relates to a method for preparing a positive electrode additive for lithium secondary batteries, the method comprising the steps of: reacting (i) benzo[a]perylene or a derivative thereof with (ii) a hydrocarbon molecule containing an amino group at at least one end.

[0051] In this invention, the benzo[a]perylene derivative can be benzo[a]perylene anhydride (BPA), amino-containing benzo[a]perylene, or carboxyl-containing benzo[a]perylene. Considering its reactivity with hydrocarbon molecules, the benzo[a]perylene derivative is preferably benzo[a]perylene anhydride (BPA).

[0052] Furthermore, in this invention, the hydrocarbon molecule containing an amino group at at least one end can be an alkylamine having 8 to 12 carbon atoms. For example, an alkylamine having 8 to 12 carbon atoms can be octylamine (C8H12H2O). 19 N), 1-aminodecane (C10) or dodecylamine (C12).

[0053] Furthermore, the reaction can be a dehydration condensation reaction, which includes the following steps: dissolving the reaction material in an organic solvent; refluxing the product at a temperature of 100°C to 200°C; and cooling the product. Here, the organic solvent is not particularly limited, as long as it is an organic solvent commonly used in the positive electrode reaction of lithium secondary batteries, and examples may include DMF (dimethylformamide).

[0054] The reaction temperature can be above 100℃, above 120℃, above 140℃ but below 160℃, below 170℃, or below 200℃. When the reaction temperature is below 100℃, the reaction rate is low and the target compound may not be obtained. When the reaction temperature exceeds 200℃ or is significantly higher than the boiling point of the solvent, bubbles will be generated due to vaporization, and the experimental equipment may be damaged.

[0055] Furthermore, considering the extent to which the additive of Formula 1 is fully synthesized, reflux can be carried out for 5 to 30 hours. Specifically, reflux can be carried out for more than 5 hours or more than 10 hours, or less than 20 hours or less than 30 hours. When the reflux time is less than 10 hours, the reaction is incomplete, and the target product may not be obtained. When the reflux time exceeds 30 hours, over-reaction occurs, and the process efficiency may decrease.

[0056] Furthermore, cooling can be to room temperature, which can be above 20°C, above 23°C and below 27°C, or below 30°C, for example, 25°C. When the cooling temperature is below 20°C, the cooling time increases, resulting in an unnecessary extension of the process time, while when the cooling temperature exceeds 30°C, the cooling is insufficient, which may reduce the yield in the precipitation step.

[0057] In addition, after cooling, the process may include adding methanol and stirring the product to precipitate the reaction product in the solution. The stirring time can range from 30 minutes to 3 hours. For example, the stirring time can be more than 30 minutes or more than 1 hour, and can be less than 2 hours or less than 3 hours. When the stirring time is less than 30 minutes, sufficient precipitation time cannot be ensured, thus reducing the yield, while when the stirring time exceeds 3 hours, the process efficiency may decrease.

[0058] In addition, after the stirring step, a vacuum filtration and washing step can be included to improve the purity.

[0059] Positive electrode active materials for lithium secondary batteries

[0060] In this invention, the positive electrode active material comprises a carbon material, and the positive electrode active material contains a positive electrode additive represented by Formula 1 on the surface of the carbon material:

[0061] <Formula 1>

[0062]

[0063] Where R represents a carbon chain.

[0064] The positive electrode additive represented by Formula 1 is the same as that described above.

[0065] Furthermore, the carbon material is a porous carbon material, and the cathode additive can be adsorbed and bound to any one or more surfaces on the outer and inner surfaces of the porous carbon material.

[0066] Furthermore, in the positive electrode active material for lithium secondary batteries of the present invention, the content of additives can be 1 to 15% by weight relative to the total weight of carbon materials. When the content of additives is less than 1% by weight, the effect obtained by including additives is not obvious, while when the content of additives exceeds 15% by weight, the content of additives exceeds the adsorption limit of carbon materials, and some additives dissolve in the electrolyte, thereby causing self-discharge, or the energy density of the battery may decrease due to the increase in weight.

[0067] In this invention, the positive electrode active material layer may include the above-mentioned positive electrode active material, as well as conductive material and adhesive.

[0068] Conductive materials are used to provide conductivity to the electrodes, and in the resulting battery, conductive materials can be used without particular restrictions, as long as they are electronically conductive and do not cause chemical changes. Specific examples of conductive materials may include: graphite, such as natural or artificial graphite; carbon materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermally cracked carbon black, or carbon fibers; powders or fibers of metals such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide or potassium titanate; conductive metal oxides, such as titanium oxide; conductive polymers, such as polyphenylene derivatives, etc., wherein one or more mixtures may be used. The content of conductive material relative to the total weight of the positive electrode active material layer can typically be from 1% to 30% by weight.

[0069] The adhesive enhances the adhesion between the positive electrode active material particles and the adhesion strength between the positive electrode active material and the positive electrode current collector. Specific examples of adhesives may include: polyvinylidene fluoride (PVDF), PVDF-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers of the above materials, wherein one or a mixture of two or more can be used. The adhesive content relative to the total weight of the positive electrode active material layer can be from 1% to 30% by weight.

[0070] In the positive electrode active material for lithium secondary batteries of the present invention, the positive electrode active material may further contain sulfur. Specifically, in the positive electrode active material for lithium secondary batteries of the present invention, a sulfur-carbon composite material obtained by compounding the above-mentioned carbon material with adsorbed additives with sulfur powder can be prepared. There are no particular limitations on the method for preparing the sulfur-carbon composite material, and methods commonly used in the art for preparing sulfur-carbon composite materials can be used.

[0071] For example, the melt diffusion method can be used to prepare sulfur-carbon composites. The melt diffusion method involves heating sulfur to melt it, thereby allowing the sulfur to diffuse into carbon particles. Here, heat treatment can include various direct or indirect heating methods.

[0072] The method for preparing the sulfur-carbon composite material according to the present invention may include: (S1) mixing sulfur and carbon; and heat-treating the sulfur and carbon mixture formed in step (S1). Specifically, the temperature during heat treatment is 100 to 200°C, preferably 110 to 190°C, more preferably 120 to 180°C, and the heat treatment may be performed using a melt diffusion method. When the temperature is below the above range, the sulfur-carbon composite material itself may not be able to be prepared because the process of sulfur dissolving and penetrating into the carbon cannot occur. When the temperature exceeds the above range, the loss rate increases due to sulfur vaporization, and the sulfur-carbon composite material undergoes denaturation. Therefore, when used as a positive electrode material for lithium secondary batteries, the effect of improving battery performance may not be significant.

[0073] Furthermore, when sulfur and carbon materials are included in the positive electrode active material for lithium secondary batteries of the present invention, the weight ratio of sulfur to carbon materials and additives can be from 1:1 to 1:0.1, preferably from 1:0.5 to 1:0.2. When the proportion of sulfur is higher than the above range, the resistance of the battery may increase because carbon has insufficient conductivity, while when the proportion of sulfur is lower than the above range, the weight ratio of sulfur is too low, which will excessively reduce the energy density of the battery.

[0074] Positive electrode for lithium secondary batteries

[0075] The present invention also relates to a positive electrode for a lithium secondary battery comprising the aforementioned positive electrode active material.

[0076] Preferably, the additive of Formula 1 contained in the positive electrode is not soluble in carbonate solvents and / or ether solvents commonly used as electrolytes in lithium secondary batteries, and is therefore suitable for use in electrodes of lithium-sulfur secondary batteries containing sulfur-carbon composite materials as positive electrode active materials.

[0077] The positive electrode active material for lithium secondary batteries of the present invention can be obtained by adsorbing the additive of Formula 1 onto the surface of the carbon material contained in the positive electrode. Because the bonding strength generated by adsorption is high, the additive of Formula 1 remains insoluble in the electrolyte even when the battery is in operation, thus preventing phenomena such as battery short circuits caused by the additive of Formula 1 dissolving in the electrolyte. Furthermore, because the additive of Formula 1 can adhere to the surface of the carbon material without chemical bonding, chemical / electrochemical interference that may occur when other desired chemical materials are added for chemical bonding can be prevented. Moreover, because adsorption occurs rapidly, the reaction to the surface can be completed within minutes.

[0078] In this invention, the weight ratio of the additive in Formula 1 to the carbon material can be from 0.01:1 to 0.15:1. Specifically, the weight ratio can be 0.01:1 or more, 0.02:1 or more, 0.03:1 or more, or 0.04:1 or more, and 0.15:1 or less, 0.09:1 or less, 0.08:1 or less, or 0.06:1 or less. When the weight ratio is less than 0.01:1, because the content of the additive in Formula 1 is relatively small, there may be no significant change. However, when the weight ratio exceeds 0.15:1, the content of the additive exceeds the adsorption limit of the carbon material, thereby causing some of the additive to dissolve in the electrolyte, resulting in self-discharge, or the energy density of the battery may decrease due to the increase in weight.

[0079] Lithium secondary batteries

[0080] The present invention also relates to a lithium secondary battery comprising a positive electrode, a negative electrode, a separator between the positive and negative electrodes, and an electrolyte, wherein the positive electrode comprises a positive electrode active material comprising an additive of Formula 1. The additive of Formula 1 may be included in the positive electrode active material layer described later.

[0081] The electrolyte of the present invention may contain organic solvents and lithium salts.

[0082] There are no particular restrictions on the use of organic solvents, as long as they can serve as a medium for the migration of ions participating in the electrochemical reaction of the battery, and they can preferably be used in one or more ways selected from ether solvents and carbonate solvents.

[0083] Specifically, as organic solvents, the following can be used: ester solvents, such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents, such as dibutyl ether or tetrahydrofuran; ketone solvents, such as cyclohexanone; aromatic solvents, such as benzene and fluorobenzene; carbonate solvents, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents, such as ethanol and isopropanol; nitriles, such as R-CN (R is a linear, branched, or cyclic hydrocarbon group from C2 to C20, and may contain double bonds, aromatic rings, or ether bonds); amides, such as dimethylformamide; dioxolane, such as 1,3-dioxolane; sulfolane, etc. Among these solvents, carbonate solvents are preferred, and even more preferred are mixtures of cyclic carbonates (e.g., ethylene carbonate, propylene carbonate, etc.) with high ionic conductivity and high dielectric constant that enhance the charging and discharging performance of the battery, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, etc.). In this case, the electrolyte exhibits excellent performance when the cyclic carbonate and the linear carbonate are mixed in a volume ratio of about 1:1 to about 1:9.

[0084] Lithium salts can be used without particular restrictions, as long as they are compounds capable of providing lithium ions used in lithium secondary batteries. Specifically, the following substances can be used as lithium salts: LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, etc. It is preferable to use lithium salts in a concentration range of 0.1M to 2.0M. When the concentration of the lithium salt is included in the above range, the electrolyte has suitable conductivity and viscosity, thus exhibiting excellent electrolyte performance, and lithium ions can migrate effectively.

[0085] To improve battery life performance, suppress battery capacity degradation, and increase battery discharge capacity, in addition to the electrolyte components mentioned above, the electrolyte may also contain one or more additives, such as: haloalkyl carbonate compounds (e.g., difluoroethylene carbonate), pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, (condensed) glycol dimethyl ethers, hexamethylphosphoryltriamine, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted... The additives may include azole ketones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxyethanol, or aluminum trichloride. The content of the additives may be from 0.1% to 5% by weight relative to the total weight of the electrolyte.

[0086] In this invention, the positive electrode includes a positive current collector and a positive active material layer, wherein the positive active material layer is formed on the positive current collector and includes the aforementioned positive active material.

[0087] In the positive electrode, there are no particular restrictions on the positive current collector, as long as it is conductive and does not cause chemical changes to the battery. Materials used include, for example, stainless steel, aluminum, nickel, titanium, calcined carbon; or aluminum or stainless steel with a surface treated with carbon, nickel, titanium, silver, etc. Furthermore, the positive current collector can typically have a thickness from 3 μm to 500 μm, and fine irregularities can be formed on its surface to improve the adhesion strength of the positive electrode active material. For example, the positive current collector can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0088] Besides using the aforementioned positive electrode active material, a conventional positive electrode preparation method can be used to prepare the positive electrode. Specifically, a composition comprising the aforementioned positive electrode active material and optionally a binder and a conductive material for forming a positive electrode active material layer is coated onto a positive electrode current collector, then dried and rolled to prepare the positive electrode. Here, the types and contents of the positive electrode active material, binder, and conductive material are as described above.

[0089] As a solvent, solvents commonly used in the art can be used, and examples include dimethyl sulfoxide (DMSO), isopropanol, N-methylpyrrolidone (NMP), acetone, water, etc., wherein one or a mixture of two or more can be used alone. Considering the coating thickness and manufacturing yield of the slurry, the amount of solvent used is sufficient as long as it dissolves or disperses the positive electrode active material, conductive material, and binder, and achieves a viscosity sufficient to achieve excellent thickness uniformity when subsequently coated to prepare the positive electrode.

[0090] As another method, the positive electrode can also be prepared by casting the composition for forming the positive electrode active material layer onto a separate carrier and pressing the film layer obtained by peeling it off from the carrier onto the positive electrode current collector.

[0091] In this invention, the negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

[0092] The negative electrode active material layer optionally includes a negative electrode active material, a binder, and a conductive material.

[0093] As a negative electrode active material, compounds capable of reversibly inserting or deintercalating lithium can be used. Specific examples of such negative electrode active materials may include: carbon materials, such as artificial graphite, natural graphite, graphitized carbon fibers, or amorphous carbon; (semi-)metallic materials capable of forming alloys with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; and (semi-)metal oxides that can be doped or undoped with lithium, such as SiO₂. β (0<β<2), SnO2, vanadium oxide or lithium vanadium oxide; or composite materials containing (semi-)metallic materials and carbon materials, such as Si-C composite materials or Sn-C composite materials, wherein any one or more mixtures can be used. Additionally, lithium metal thin films can be used as the negative electrode active material. Furthermore, as carbon materials, both low-crystallinity carbon and high-crystallinity carbon can be used. Representative examples of low-crystallinity carbon can include soft carbon and hard carbon, and representative examples of high-crystallinity carbon can include high-temperature calcined carbon, such as irregular, plate-like, flake-like, spherical or fibrous natural or artificial graphite, condensed graphite, pyrolytic carbon, mesophase pitch-based carbon fibers, mesophase carbon microspheres, mesophase pitch, and coke derived from petroleum or coal tar pitch.

[0094] The binder, conductive material, and negative electrode current collector can be selected with reference to the composition described above for the positive electrode, but are not limited thereto. Furthermore, the method for forming the negative electrode active material layer on the negative electrode current collector can include coating methods known as those for the positive electrode, and is not particularly limited thereto.

[0095] In this invention, the separator separates the negative and positive electrodes and provides a migration channel for lithium ions. Separators commonly used in lithium secondary batteries can be used without particular limitation, but separators with low resistance to electrolyte ion migration and excellent electrolyte retention are particularly preferred. Specifically, porous polymer membranes can be used, such as those prepared from polyolefin polymers (e.g., ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers); or laminated structures of two or more layers from the above porous polymer membranes. Furthermore, common porous nonwoven fabrics can be used, such as nonwoven fabrics made from high-melting-point glass fibers, polyethylene terephthalate fibers, etc. Additionally, coated separators containing ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength, and single-layer or multi-layer structures can be selectively used.

[0096] Preferred implementation scheme

[0097] Preferred embodiments are provided below to illustrate the invention; however, the following embodiments are for illustrative purposes only, and it will be apparent to those skilled in the art that various modifications and variations can be made within the scope and technical concept of the invention, and such modifications and variations also fall within the scope of the appended claims.

[0098] Figure 1 This is a schematic diagram illustrating the raw materials used to synthesize the positive electrode additive in the embodiments and comparative examples, and in the following embodiments and comparative examples, the raw materials used are... Figure 1 The raw materials shown are used to synthesize additives.

[0099] Preparation Example 1: Preparation of Benzo[a]perylene anhydride (BPA)

[0100] In a three-necked flask, a glass topper, a thermocouple, and a reflux condenser are installed and placed in a heating mantle.

[0101] 123.0 g of maleic anhydride was introduced into the mixture and dissolved at 75 °C. After adding 8.04 g of perylene, the mixture was heated to 240 °C.

[0102] 16.5g of chloroquinone was introduced into the mixture, which was then refluxed for 10 minutes, cooled to 140°C, and 160ml of xylene heated to 60°C was added to obtain a mixture.

[0103] The mixture was cooled to 90°C and then filtered. The filtered mixture was introduced into an ethyl acetate / chloroform (2:1 (volume / volume)) solution and purified by repeating the washing process of heating to 65°C and filtration twice.

[0104] The purified mixture was then vacuum dried at 70°C to prepare BPA.

[0105] Example 1

[0106] (1) Preparation of positive electrode additives

[0107] 8 g of BPA prepared in Preparation Example 1 and 45 mmol of octylamine (C8) were introduced and refluxed at 160 °C overnight, and then cooled to room temperature to obtain the synthesized compound.

[0108] 300 ml of methanol was introduced into the compound, and the mixture was stirred at room temperature for 1 hour.

[0109] The stirred mixture was then vacuum filtered, washed several times with methanol, and dried to prepare a positive electrode additive (C8 A-BPI) for lithium secondary batteries.

[0110] (2) Preparation of positive electrode active material

[0111] The additive prepared in Preparation Example 1 and multi-walled carbon nanotubes (MWCNTs, CNano) as the carbon material were mixed in tetrahydrofuran (THF) solvent. The mixture was then dried at 80°C for 1 day to prepare a positive electrode active material in the form of MWCNT powder adsorbed with the additive from Preparation Example 1. Here, the additive and carbon material were mixed such that, based on the finally manufactured MWCNT powder, the additive content was 4 parts by weight relative to 100 parts by weight of MWCNT powder.

[0112] (3) Manufacturing of the positive electrode and coin cell

[0113] The above-prepared positive electrode active material and carboxymethyl cellulose (CMC) binder were mixed in a weight ratio of 96:4 to prepare an aqueous slurry. The positive electrode active material was MWCNT powder adsorbed with the additives of Preparation Example 1. The aqueous slurry was coated onto aluminum foil and dried, and then punched into Φ14 holes to serve as a reference electrode. A lithium electrode was used as the counter electrode, thereby manufacturing a coin cell. In manufacturing the coin cell, a polyethylene membrane (16 μm, Celgard) was used as the separator, and an electrolyte of 1M LiTFSI in a mixed solvent of DOL and DME (1:1 (volume / volume)) was used as the electrolyte.

[0114] Example 2

[0115] (1) Preparation of positive electrode additives

[0116] The positive electrode additive (C10 A-BPI) for lithium secondary batteries was prepared in the same manner as in Example 1, except that 1-aminodecane (C10) was used instead of octylamine.

[0117] (2) Preparation of positive electrode active material

[0118] Subsequently, sulfur and MWCNT powder adsorbed with positive electrode additive (C10 A-BPI) at a ratio of 4 parts by weight relative to 100 parts by weight, as in Example 1 above, were mixed at a ratio of 1:0.33. The resulting mixture was then heat-treated at 150°C for 1 hour to prepare sulfur-carbon composite powder as a positive electrode active material.

[0119] (3) Manufacturing of the positive electrode and coin cell

[0120] A sulfur-carbon composite powder and carboxymethyl cellulose (CMC) binder were mixed in a weight ratio of 96:4 to prepare an aqueous slurry. The aqueous slurry was coated onto aluminum foil and dried, then punched into Φ14 holes for use as a reference electrode, and a lithium electrode was used as the counter electrode, thus manufacturing a coin cell. In manufacturing the coin cell, a polyethylene membrane (16 μm, Celgard) was used as the separator, and an electrolyte of 1 M LiTFSI in a mixed solvent of DOL and DME (1:1 (volume / volume)) was used as the electrolyte.

[0121] Example 3

[0122] Except for using dodecylamine (C12) instead of octylamine, a positive electrode additive (C12 A-BPI), a positive electrode active material, and a coin cell-type lithium-sulfur secondary battery were prepared in the same manner as in Example 1.

[0123] Example 4

[0124] Except for using MWCNT powder with 8 parts by weight of positive electrode additive (C10A-BPI) adsorbed relative to 100 parts by weight of MWCNT powder as the positive electrode active material, the positive electrode active material and the coin cell type lithium-sulfur secondary battery were prepared in the same manner as in Example 2.

[0125] Comparative Example 1

[0126] Except for using tetraethylene glycol monoamine instead of octylamine (C8), the positive electrode additive, positive electrode active material, positive electrode and coin cell for lithium secondary batteries were prepared in the same manner as in Example 1.

[0127] Comparative Example 2

[0128] The coin battery was manufactured in the same manner as in Example 1, except that ordinary MWCNT powder without adsorbed additives was used as the positive electrode active material.

[0129] Comparative Example 3

[0130] The coin battery was manufactured in the same manner as in Example 2, except that ordinary MWCNT powder without adsorbed additives was used as the positive electrode active material.

[0131] Experiment Example 1: Solubility Experiment of Electrolyte Solvent for Lithium Secondary Batteries

[0132] Solubility experiments were conducted to determine whether the additives prepared in Example 1 and Comparative Example 1 could dissolve in the electrolyte solvent (Example 1: C8 linear carbon chain, Comparative Example 1: C8 linear ether chain).

[0133] Dimethoxyethane (DME), commonly used in lithium-sulfur secondary batteries, was used as the solvent in the solubility experiment.

[0134] 0.1 g of each of the positive electrode additives from Example 1 and Comparative Example 1 were introduced into 10 g of dimethoxyethane solvent, and after mixing the mixture for 1 day, FT-IR (Fourier Transform Infrared Spectroscopy) was performed to detect the presence of solute in the mixed solution. A Nicolet iS5 (Thermo Fisher Scientific Solutions) was used for FT-IR measurement.

[0135] Figure 2 The graphs shown represent the FT-IR measurement results of the mixed solutions containing the respective positive electrode additives of Example 1 and Comparative Example 1.

[0136] like Figure 2 As shown, it was confirmed that the C=O peak, which is characteristic of the positive electrode additive material, did not appear in the mixture containing the positive electrode additive of Example 1, while the C=O peak appeared in the mixture containing the positive electrode additive of Comparative Example 1.

[0137] These results show that the additive of Example 1 is not soluble in DME solvent, while the additive of Comparative Example 1 is soluble in DME solvent.

[0138] Experiment Example 2: Carbon Absorption Experiment

[0139] A carbon absorption experiment was conducted to determine whether the positive electrode additive of the lithium secondary battery prepared in Example 1 showed absorption of carbon materials.

[0140] The positive electrode additive and carbon material of the lithium secondary battery prepared in Example 1 were mixed in tetrahydrofuran (THF) solvent, and the mixed solution was visually observed immediately after mixing and one day after mixing. Multi-walled carbon nanotubes (MWCNT, CNano) were used as the carbon material. Furthermore, based on the total weight of the mixed solution, 1 wt% and 25 wt% of the additive and carbon material were mixed, respectively (additive to carbon material weight ratio of 0.04:1).

[0141] Figure 3 A photograph is shown illustrating the color change over time of a mixed solution containing the positive electrode additive and carbon material (MWCNT) of Example 1.

[0142] like Figure 3 As shown, it was confirmed that after mixing the positive electrode additive of Example 1 and MWCNT in THF solvent, the mixed solution immediately showed a yellow color (before adsorption), while the yellow color disappeared (after adsorption) after 1 day had passed since mixing.

[0143] These results show that in a mixed solution containing the additive of Example 1 and MWCNT as a carbon material, the additive of Example 1 is adsorbed onto the carbon material and does not dissolve in the solvent.

[0144] Experiment Example 3: Experiments related to the electrochemical activity of batteries

[0145] Electrochemical activity experiments were conducted on coin batteries containing the respective positive electrode active materials prepared in Example 1 and Comparative Example 2.

[0146] The positive electrode active material of Example 1 is MWCNT powder adsorbed with a positive electrode additive (C8A-BPI) prepared using octylamine (C8) as a raw material, and the positive electrode active material of Comparative Example 1 is ordinary MWCNT powder without adsorbed positive electrode additive.

[0147] The respective coin cells of Example 1 and Comparative Example 2 were rammed back and forth three times at a rate of 10 mV / s in the range of 1.8 V to 2.9 V, and the change in current with voltage was measured during the third round trip (VMP3, Biologic).

[0148] Figure 4 This is a graph showing the electrochemical activity of a coin cell based on the presence or absence of the positive electrode additive of Example 1.

[0149] like Figure 4 As shown, it was confirmed that the coin cell containing the positive electrode additive of Example 1 with MWCNT adsorbed has electrochemical activity in the general driving range of 1.8V to 2.5V of lithium-sulfur secondary batteries.

[0150] These results show that the cathode additive of Example 1 will enhance the electrochemical activity of lithium secondary batteries, including lithium-sulfur secondary batteries.

[0151] Experiment Example 4: Confirmation of Lifetime Performance

[0152] The coin batteries of Example 2 and Comparative Examples 1 and 3 were charged and discharged three times at a constant current of 0.1C within a range of 1.8V to 2.5V, and their capacity was measured under continuous charging and discharging conditions at a constant current of 0.2C / 0.5C (charge / discharge) within the same voltage range. The results are shown below. Figure 5 (Using a potentiostat from PNE Corporation)

[0153] like Figure 5 As shown, it was confirmed that although there was no problem with the lifespan reduction in Example 2, the lifespan of Comparative Example 1 decreased sharply because the catalyst dissolved in the electrolyte.

[0154] Experimental Example 5: Confirmation of the change in voltage with discharge capacity at each discharge rate

[0155] For the respective coin batteries of Examples 2, 4, and Comparative Example 3, the voltage change with discharge capacity at each discharge rate was measured, and the lifespan performance was compared. The results are shown below. Figure 6 middle.

[0156] like Figure 6 As shown, it can be seen that Examples 2 and 4 have higher discharge voltages compared to Comparative Example 3.

[0157] The invention has been described above with reference to limited embodiments and accompanying drawings. However, the invention is not limited thereto, and those skilled in the art can make various modifications and variations within the scope of the invention and equivalent to the appended claims.

Claims

1. A positive electrode additive for lithium secondary batteries represented by Formula 1 below: <Formula 1> in, R is a linear carbon chain with 8 to 12 carbon atoms. The positive electrode additive does not dissolve in the electrolyte.

2. A method for preparing the positive electrode additive for lithium secondary batteries according to claim 1, the method comprising the following steps: (i) benzo[a]perylene or a benzo[a]perylene derivative reacts with (ii) a hydrocarbon molecule containing an amino group at at least one end.

3. The method for preparing a positive electrode additive for lithium secondary batteries according to claim 2, wherein, The benzo[a]perylene derivative is benzo[a]perylene anhydride, an amino-containing benzo[a]perylene, or a carboxyl-containing benzo[a]perylene.

4. The method for preparing a positive electrode additive for lithium secondary batteries according to claim 2, wherein, The hydrocarbon molecule containing an amino group at at least one end is an alkylamine having 8 to 12 carbon atoms.

5. The method for preparing a positive electrode additive for lithium secondary batteries according to claim 2, wherein, The reaction is a dehydration condensation reaction involving reflux at a temperature of 100°C to 200°C followed by cooling.

6. A positive electrode active material for lithium secondary batteries, said positive electrode active material comprising: Carbon materials in, The positive electrode active material comprises the positive electrode additive as described in claim 1 on the surface of the carbon material.

7. The lithium secondary battery positive electrode active material according to claim 6, wherein, The carbon material is a porous carbon material, and The cathode additive is adsorbed and bound to one or more surfaces, both the outer and inner surfaces, of the porous carbon material.

8. The positive electrode active material for lithium secondary batteries according to claim 6, wherein, Based on the total weight of the carbon material, the content of the cathode additive is from 1% to 15% by weight.

9. The positive electrode active material for lithium secondary batteries according to claim 6, wherein the positive electrode active material further comprises sulfur.

10. The positive electrode active material for lithium secondary batteries according to claim 9, wherein, The ratio of sulfur to the total weight of the carbon material and the additive is from 1:1 to 1:0.

1.

11. A positive electrode for a lithium secondary battery, comprising the positive electrode active material as described in claim 6.

12. A lithium secondary battery, comprising: positive electrode; negative electrode; A separator, the separator being located between the positive electrode and the negative electrode; and Electrolyte in, The positive electrode is the positive electrode as described in claim 11.

13. The lithium secondary battery according to claim 12, wherein, The electrolyte contains one or more solvents selected from ether solvents and carbonate solvents.

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