Active material of positive electrode, positive electrode and secondary lithium battery including the same
Patent Information
- Application Number
- ES2022736855T
- Authority / Receiving Office
- ES · ES
- Patent Type
- Patents
- Current Assignee / Owner
- Priority Date
- 2021-01-12
- Filing Date
- 2022-01-06
- Publication Date
- 2026-09-14
- Estimated Expiration
- 2042-01-06
Abstract
Description
Active material of positive electrode, positive electrode and secondary lithium battery including the same Technical field This disclosure relates to a positive electrode active material, a positive electrode, and a secondary lithium battery that includes the same. Background of the technique Until recently, there has been considerable interest in developing high-energy-density batteries that use lithium as a negative electrode. For example, compared to other electrochemical systems that have a carbon negative electrode inserted with lithium and a nickel or cadmium electrode—which reduces battery energy density by increasing the weight and volume of the negative electrode with the presence of non-electroactive materials—lithium metal has low weight and high capacity properties and has therefore attracted much attention as an active negative electrode material in electrochemical batteries. A lithium metal negative electrode, or a negative electrode consisting primarily of lithium metal, offers the opportunity to create batteries that are lighter and have a higher energy density compared to batteries such as lithium-ion, nickel-metal hydride, or nickel-cadmium batteries.Such characteristics are highly preferred with batteries for portable electronic devices such as mobile phones and laptops where a premium is paid for low weight. Such types of positive electrode active materials for a lithium battery are known, and positive electrode active materials include a positive electrode active material containing sulfur that includes a sulfur-sulfur bond and achieve high energy capacity and rechargeability from electrochemical cleavage (reduction) and reformation (oxidation) of the sulfur-sulfur bond. A lithium-sulfur secondary battery, which uses lithium and an alkali metal as the negative electrode active material and sulfur as the positive electrode active material, as described above, has a theoretical energy density of 2,800 Wh / kg and a theoretical sulfur capacity of 1,675 mAh / g, which are significantly higher than those of other battery systems. Furthermore, given sulfur's advantages of being inexpensive due to its abundant resources and its environmentally friendly nature, lithium-sulfur secondary batteries have garnered attention as a power source for portable electronic devices. However, sulfur, used as the active positive electrode material in a lithium-sulfur secondary battery, is a non-conductor, making it difficult for electrons generated by an electrochemical reaction to migrate. Furthermore, the polysulfide (Li₂S₂ to Li₂S₄) generated during the charging and discharging of the lithium-sulfur secondary battery elutes, and lithium sulfide (Li₂S₂ / Li₂S) and sulfur have poor electrical conductivity and slow kinetics for electrochemical reactions, leading to declining battery life and discharge rate properties. A research result has been reported whereby, when benzo[ghi]peryleneimide (BPI) is introduced as a positive electrode additive to improve such problems in a lithium-sulfur secondary battery, the BPI acts as a redox mediator and is able to enhance the kinetics of an electrochemical reaction and boost the performance and lifetime properties of a battery (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). However, BPI tends to dissolve in an ether-based solvent or a carbonate-based solvent used in an electrolyte solution of a secondary lithium-sulfur battery, and when BPI dissolves in an electrolyte solution, electrons are transferred between a positive electrode and a negative electrode that need to be electrically separated, causing a battery performance decline problem by inducing an internal short circuit. Consequently, the development of an additive has been required that does not dissolve in an electrolyte solution of a lithium secondary battery including a lithium-sulfur secondary battery, and that is capable of playing the role of a catalyst for a reaction that is advanced at a positive electrode. Previous technique document Non-patent document (Non-patent document 1) Nano Lett.2016, 16, 1, 549-554; Laura CH Gerber et al.; "Three-Dimensional Growth of Li2S in Lithium-Sulfur Batteries Promoted by a Redox Mediator" Chemistr y - A European Journal, vol. 20, no. 29, 9081-9093, discloses the synthesis of N-(n-butyl)benzo[ghi]perylene-1, 2dicarboxyamide. Organic Letters, vol. 12, no. 11, 2656–2659, discloses the synthesis of N-(n-dodecyl)benzo[ghi]perylene-1,2-dicarboxiamide. US patent 2017 / 222226 A1 discloses an energy storage device, wherein the cathode comprises a metal sulfide and a redox mediator having a redox potential suitable for reducing or oxidizing the metal sulfide, wherein the metal sulfide is N-(n-aryl)benzo[ghi]perylene-1,2-dicarboxiamide. [Technical problem] An object of the present disclosure is to provide a positive electrode active material, a positive electrode, and a lithium secondary battery that includes a positive electrode additive for a lithium secondary battery capable of acting as a catalyst as a redox mediator in a positive electrode without dissolving in an electrolyte solution for a lithium secondary battery. [Technical solution] To achieve the above objectives, an embodiment of the present disclosure provides a positive electrode active material for a lithium secondary battery, comprising: a carbon material; and a positive electrode additive on a carbon material surface, wherein the positive electrode additive is represented by the following Formula 1: , where, R is a carbon chain. Another embodiment of the present disclosure provides a positive electrode for a secondary lithium battery that includes the positive electrode additive. Another embodiment of the present disclosure provides a secondary lithium battery comprising the positive electrode, a negative electrode, a separator interposed between them, and an electrolyte solution. Additional achievements are disclosed in the dependent claims. Advantageous effects A positive electrode additive for a lithium secondary battery used in accordance with this disclosure does not dissolve in an electrolyte solution and, therefore, a battery performance decline phenomenon caused by an internal short circuit that occurs when a positive electrode additive dissolves in an electrolyte solution can be avoided. Furthermore, the positive electrode additive for a lithium secondary battery exhibits electrochemical activity while adsorbed onto a carbon material included in a positive electrode of a lithium secondary battery, and plays the role of a catalyst for a reaction that is advanced at the positive electrode and, consequently, the electrochemical activity of the battery can be enhanced. Furthermore, the positive electrode additive for a lithium secondary battery exhibits electrochemical activity while adsorbed onto a carbon material included in a positive electrode of a lithium secondary battery and, consequently, the electrochemical activity of the battery can be enhanced. Description of the drawings Figure 1 is a schematic diagram showing raw materials used to synthesize a positive electrode additive in examples and comparative examples. Figure 2 shows graphs, each of which shows an FT-IR (Fourier transform infrared spectroscopy) measurement result for a mixture solution that includes a positive electrode additive from each of Example 1 and Comparative Example 1. Figure 3 shows photographs that show a color change over time for a mixture solution that includes a positive electrode additive from Example 1 and a carbon material (MWCNT). Figure 4 is a graph showing the electrochemical activity of a button cell depending on the presence or absence of adsorption of a positive electrode additive from Example 1. Figure 5 is a graph showing the lifetime properties of a button cell from each of Example 2 and Comparative Examples 1 and 3 depending on the presence or absence of adsorption of a positive electrode additive. Figure 6 is a graph showing the lifetime properties of a button cell from each of Example 2, Example 4, and Comparative Example 3 depending on the presence or absence of adsorption of the positive electrode additive. Best way Hereafter in this document, the present disclosure will be described in more detail to help in understanding the present disclosure. The terms or words used in this specification and in the claims are not to be interpreted in a manner limited to common or dictionary meanings, but rather as meanings and concepts corresponding to the technical ideas of this disclosure, based on the principle that the inventors can adequately define the concepts of terms to best describe the invention. Positive electrode additive for a lithium secondary battery A positive electrode additive for a lithium secondary battery is disclosed, and a positive electrode additive for a lithium secondary battery adsorbed on a carbon material used in a general positive electrode for a lithium secondary battery and having, therefore, the physical property of not dissolving in an electrolyte solution and having electrochemical activity while adsorbed on a surface of the carbon material. In this disclosure, the positive electrode additive for a lithium secondary battery is represented by the following Formula 1: where, R is a carbon chain. The Formula 1 additive has a structure in which a carbon chain is inserted into benzo[ghi]perilenimide (BPI). The carbon chain is not particularly limited in shape and can be, for example, a linear, cyclic, or branched carbon chain. Furthermore, when the carbon chain is linear, an adsorption process can be carried out more easily for a carbon material compared to when the carbon chain is cyclic or branched, and the efficiency of the process can be enhanced. For example, the linear carbon chain can be an aliphatic carbon chain. BPI is an imide compound synthesized by the dehydration condensation of molecules including benzoperylene anhydride (BPA) and an amine, and is used as a positive electrode additive for a secondary lithium battery. However, because BPI dissolves in an electrolyte solution, electrons are transferred between a positive and a negative electrode that must be electrically separated while dissolving in the electrolyte solution. This causes a battery performance decline by inducing an internal short circuit. However, when a carbon chain is introduced into the BPI as in the Formula 1 above, it does not dissolve in an electrolyte solution because a carbon chain does not dissolve well in a common electrolyte solution solvent. In this document, "electrolyte solution" means an electrolyte solution for a lithium secondary battery and, for example, may mean an electrolyte solution that includes a carbonate-based solvent and / or an ether-based solvent. When the number of carbon atoms in the carbon chain is small, the solubility of an electrolyte solution is not controlled, making it difficult to provide a function to prevent the carbon atoms from dissolving. Conversely, when the number of carbon atoms is large, the proportion of benzoperylene anhydride (BPA) capable of participating in an electrochemical reaction decreases, which can reduce the additive's effect per mass and unnecessarily increase the battery's weight. Therefore, R can be an alkyl group with 8 to 12 carbon atoms. Furthermore, when the Formula 1 additive includes an ether or similar chain instead of a carbon chain, the affinity with a polar electrolyte solution solvent is enhanced by an atom such as oxygen that has a high electronegativity, and therefore the additive dissolves easily in an electrolyte solution leading to the transfer of electrons between a positive and a negative electrode, and battery performance may decline by causing an internal short circuit. Method for preparing a positive electrode additive for a lithium secondary battery A method for preparing a positive electrode additive for a lithium secondary battery is also disclosed, and the method may include reacting (i) benzoperylene or a derivative thereof, and (ii) a carbon molecule including an amine group at at least one end. The benzoperylene derivative can be benzoperylene anhydride (BPA), benzoperylene containing an amine group, or benzoperylene containing a carboxyl group, and considering the reactivity with a carbon molecule, the benzoperylene derivative can preferably be benzoperylene anhydride (BPA). Furthermore, a carbon molecule that includes an amine group at at least one end can be an alkylamine, which has 8 to 12 carbon atoms. For example, an alkylamine with 8 to 12 carbon atoms can be octylamine (C8H19N), 1-aminodecane (C10), or dodecylamine (C12). Furthermore, the reaction may be a dehydration condensation reaction involving steps of dissolving the reaction materials in an organic solvent, refluxing the resulting mixture at 100–200 °C, and cooling it. In this document, the organic solvent is not specifically limited, provided it is one that can be commonly used in a positive electrode reaction of a secondary lithium battery. Examples of such solvents include DMF (dimethylformamide). The reaction temperature can be 100 °C or higher, 120 °C or higher, or 140 °C or higher, and 160 °C or lower, 170 °C or lower, or 200 °C or lower. When the reaction temperature is lower than 100 °C, the reaction rate is low and the target compound may not be obtained. When the reaction temperature exceeds 200 °C or is significantly higher than the boiling point of the solvent, vaporization bubbles are generated and the experimental apparatus may be damaged. Furthermore, reflux can be carried out for 5 to 30 hours, assuming the Formula 1 additive is synthesized to a sufficient degree. Specifically, reflux can be carried out for 5 hours or more, 10 hours or more, 20 hours or less, or 30 hours or less. When the reflux time is less than 10 hours, the reaction is not complete, and therefore the target product may not be obtained. When the reflux time exceeds 30 hours, an overreaction occurs, which can decrease process efficiency. Furthermore, the cooling can be ambient temperature cooling, and in this document, ambient temperature can be 20°C or higher, or 23°C or higher, and 27°C or lower, or 30°C or lower, and for example, it can be 25°C. When the cooling temperature is lower than 20°C, the cooling time increases, resulting in an unnecessarily long process time, and when the cooling temperature exceeds 30°C, the cooling is insufficient and, therefore, the yield in the precipitation stage may be reduced. In addition, after cooling, a step of adding methanol and stirring the mixture to precipitate the reaction product from the solution can be included. The stirring time can range from 30 minutes to 3 hours. For example, the stirring time can be 30 minutes or more, or 1 hour or more, or 2 hours or less, or 3 hours or less. When the stirring time is less than 30 minutes, sufficient time for precipitation is not ensured, reducing the yield, and when the stirring time exceeds 3 hours, it can decrease the efficiency of the process. In addition, after the stirring stage, a vacuum filtration and washing stage of the mixture can also be included, and the purity can be enhanced. Active material of the positive electrode for a lithium secondary battery In this disclosure, the active positive electrode material includes a carbon material and a positive electrode additive represented by the following Formula 1 on a surface of the carbon material: where, R is a carbon chain. The positive electrode additive represented by Formula 1 is the same as described above. Furthermore, the carbon material is a porous carbon material, and the positive electrode additive can bond by adsorption to any one or more of an external and an internal surface of the porous carbon material. Furthermore, in the active positive electrode material for a secondary lithium battery of this disclosure, the additive may be included at a concentration of 1 to 15% by weight relative to the total weight of the carbon material and the additive. When the additive content is less than 1% by weight, the effect of including the additive becomes negligible, and when it exceeds 15% by weight, the additive content surpasses an adsorption limit of the carbon material, and some of the additives elute into an electrolyte solution, causing self-discharge, or the energy density of a battery may be reduced by an increase in weight. In this disclosure, a positive electrode active material layer may include, along with the positive electrode active material described above, a conductor and a binder. The conductor is used to provide conductivity to an electrode and, in the resulting battery, can be used without any particular limit as long as it conducts electrons without causing chemical changes. Specific examples include graphite, such as natural or synthetic graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, or carbon fiber; metal powders such as copper, nickel, aluminum, or silver, or metal fibers; conductive filaments such as zinc oxide or potassium titanate; oxides of conductive metals such as titanium oxide; and conductive polymers such as polyphenylene derivatives or similar materials. A single type or a mixture of two or more types may be used. The conductor can commonly be included at up to 30% by weight of the total weight of the positive electrode active material layer. The binder plays a role in enhancing the adhesion between particles of the positive electrode active material and the adhesive strength between the positive electrode active material and a positive electrode current collector. Specific examples of binders may include poly(vinylidene fluoride) (PVDF), a vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), poly(vinyl alcohol), polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, an ethylene-propylene-diene polymer (EPDM), a sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, various copolymers thereof, or similar materials; and among these, a single type or a mixture of two or more types may be used. The binder can be included from 1 to 30% by weight with respect to the total weight of the positive electrode active material layer. In the positive electrode active material for a lithium secondary battery of the present disclosure, the positive electrode active material may also include sulfur. Specifically, in the positive electrode active material for a lithium secondary battery of the present disclosure, a sulfur-carbon compound may be prepared by combining the carbon material adsorbed with the additive described above with sulfur powder. A method for preparing the sulfur-carbon compound is not particularly restricted, and methods commonly used in the art for preparing a sulfur-carbon compound may be employed. For example, the sulfur-carbon compound can be prepared using a fusion diffusion method. The fusion diffusion method is a preparation technique that involves penetrating carbon particles with sulfur by melting it through heating. In this document, the heat treatment may include various direct or indirect heating methods. The sulfur-carbon composite material according to this disclosure may include (S1) mixing sulfur and carbon; and heat-treating the sulfur-carbon mixture formed in step (S1). Specifically, the temperature during heat treatment is 100 to 200 °C, preferably 110 to 190 °C, and more preferably 120 to 180 °C, and the heat treatment may use the melt diffusion method. When the temperature is lower than the above range, the sulfur-carbon composite itself may not be formed because the dissolution and permeation process of sulfur in carbon does not proceed. When the temperature exceeds the above range, the rate of loss increases due to sulfur vaporization, and the sulfur-carbon composite is denatured. Therefore, any battery performance improvement effect may be negligible when used as a positive electrode material in a secondary lithium battery. Furthermore, when sulfur and a carbon material are included in the positive electrode active material for a lithium secondary battery of this disclosure, the weight ratio of sulfur to the total weight of the carbon material and the additive may be from 1:1 to 1:0.1, and preferably from 1:0.5 to 1:0.2. When the sulfur ratio is higher than the above range, the resistance of a cell may increase because the conductivity from the carbon is insufficient, and when the sulfur ratio is lower than the above range, the weight ratio of sulfur is excessively low, excessively reducing the energy density of a battery. Positive electrode for a secondary lithium battery This disclosure also relates to a positive electrode for a secondary lithium battery that includes the positive electrode active material. Preferably, the Formula 1 additive included in the positive electrode does not dissolve in a carbonate-based solvent and / or an ether-based solvent commonly used as an electrolyte solution of a lithium secondary battery and may therefore be suitable as a positive electrode of a lithium-sulfur secondary battery that includes a sulfur-carbon compound as a positive electrode active material. The active positive electrode material for a lithium secondary battery of this disclosure can be obtained by adsorbing the Formula 1 additive onto a carbon material surface included in the positive electrode. Due to the strong bonding force from adsorption, the Formula 1 additive may not dissolve in an electrolyte solution even when a battery is powered on, and consequently, a phenomenon such as a battery short circuit caused by the Formula 1 additive dissolving in an electrolyte solution can be avoided. Furthermore, because the Formula 1 additive can bond to a carbon material surface without forming chemical bonds, the chemical / electrochemical intervention that can occur when other chemical materials are added to form chemical bonds can be avoided.Furthermore, because adsorption takes place rapidly, the reaction of bonding to the surface can be completed within a few minutes. In this disclosure, the above Formula 1 additive and the carbon material, including the additive, may have a weight ratio of 0.01:1 to 0.15:1, and specifically, the weight ratio may be 0.01:1 or higher, 0.02:1 or higher, 0.03:1 or higher, or 0.04:1 or higher, and 0.15:1 or lower, 0.09:1 or lower, 0.08:1 or lower, or 0.06:1 or lower. When the weight ratio is less than 0.01:1, no significant changes may occur because the Formula 1 additive content is small, and when it exceeds 0.15:1, the additive content exceeds an adsorption limit of the carbon material and therefore some of the additives are eluted into an electrolyte solution, causing self-discharge, or the energy density of a battery may be reduced by an increase in weight. Secondary lithium battery This disclosure also relates to a secondary lithium battery comprising a positive electrode, a negative electrode, a separator interposed between them, and an electrolyte solution, and the positive electrode comprising the positive electrode active material comprising the Formula 1 additive. The Formula 1 additive may be incorporated into a layer of positive electrode active material and / or a layer of negative electrode active material as described below. The electrolyte solution in this disclosure may include an organic solvent and a lithium salt. The organic solvent may be used without particular limitation as long as it is capable of performing the function of a medium through which ions involved in an electrochemical reaction of a battery migrate, and may preferably include one or more types selected from ether-based solvents and carbonate-based solvents. Specifically, as the organic solvent, ester-based solvents such as methyl acetate, ethyl acetate, β-butyrolactone and β-caprolactone can be used; ether-based solvents such as dibutyl ether or tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methylethyl carbonate (MEC), ethylmethyl carbonate (EMC), ethylene carbonate (EC) and propylene carbonate (PC); alcohol-based solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN, wherein R is a C2 to C20 hydrocarbon group of linear, branched or cyclic structure, and may include a double bond, an aromatic ring or an ether linkage; amides, such as dimethylformamide; dioxolanes such as 1,3-dioxolane; sulfolanes or the like.Among these, carbonate-based solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate, propylene carbonate, or similar) with high ionic conductivity and a high dielectric constant capable of enhancing the charging and discharging performance of a battery, and a linear carbonate-based compound with low viscosity (e.g., ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, or similar) is preferred. In this case, the performance of the electrolyte solution is superior when the cyclic carbonate and the linear carbonate are mixed in a volume ratio of approximately 1:1 to approximately 1:9. Lithium salts can be used without any particular limitation, provided they are compounds capable of supplying lithium ions for use in a secondary lithium battery. Specifically, lithium salts such as LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, or similar compounds can be used. The concentration of the lithium salt is preferably between 0.1 M and 2.0 M. When the lithium salt concentration falls within this range, the electrolyte has appropriate conductivity and viscosity, resulting in excellent electrolyte performance, and the lithium ions can migrate efficiently. In order to enhance the service life properties of a battery, inhibit a decrease in battery capacity, enhance the discharge capacity of a battery, and the like, the electrolyte may include, in addition to the electrolyte components described above, one or more types of additives such as, for example, a haloalkylene carbonate-based compound such as difluoroethylene carbonate, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, n-glime, hexaphosphoric acid triamide, a nitrobenzene derivative, sulfur, a quinoneimine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this document, the additive may be included from 0.1 to 5% by weight with respect to a total weight of the electrolyte. In this disclosure, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and including the positive electrode active material described above. At the positive electrode, the positive electrode current collector is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, or similar materials can be used. Furthermore, the positive electrode current collector can typically be 3 to 500 µm thick, and micro-irregularities can be formed on its surface to increase its bonding strength with the positive electrode active material. The positive electrode current collector can be used in various forms, such as films, sheets, foils, nets, porous bodies, foams, and non-woven textiles. The positive electrode can be prepared using a common positive electrode preparation method, except that the positive electrode active material described above is used. Specifically, a composition for forming a positive electrode active material layer, including the positive electrode active material described above, and optionally a binder and a conductor, is applied as a coating onto the positive electrode current collector, then dried and laminated to prepare the positive electrode. In this document, each type and content of the positive electrode active material, binder, and conductor are as described above. As a solvent, solvents commonly used in the technique may be employed, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, water, or similar solvents. A single type or a mixture of two or more types may be used. The quantity of solvent used is sufficient provided that, considering the coating thickness of the paste and the preparation yield, the active material of the positive electrode, the conductor, and the binder are dissolved or dispersed, and that the resulting viscosity is sufficient to achieve excellent thickness uniformity when applied as a coating for subsequent preparation of the positive electrode. As another method, the positive electrode can also be prepared by pouring the composition to form a layer of positive electrode active material onto a separate support, and applying a film obtained by peeling it off this support onto the positive electrode current collector. In this disclosure, the negative electrode includes a negative electrode current collector and a layer of negative electrode active material disposed over the negative electrode current collector. The negative electrode active material layer optionally includes a binder and a conductor along with the negative electrode active material. As the active material of the negative electrode, compounds capable of reversibly intercalating or deintercalating lithium may be used. Specific examples include carbon materials such as artificial graphite, natural graphite, graphitized carbon fiber, or amorphous carbon; metal compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; metal oxides capable of doping and dedoping lithium such as SiO₂ (0 < β < 2), SnO₂, vanadium oxides, or lithium-vanadium oxides; or compounds that include metal compounds and carbon materials such as Si-C compounds or Sn-C compounds, or similar compounds, and among these, any one or a mixture of two or more may be used. Additionally, a thin film of lithium metal may also be used as the active material of the negative electrode.Furthermore, carbon materials, including low-crystallinity carbon, high-crystallinity carbon, and similar materials, can all be used. Representative examples of low-crystallinity carbon include soft carbon and hard carbon, while representative examples of high-crystallinity carbon include high-temperature baked carbon, such as artificial graphite or amorphous natural graphite, in plate, flaky, spherical, or fibrous forms, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microspheres, mesophase pitches, and cokes derived from coal tar or petroleum pitch. The binder, conductor, and current collector of the negative electrode can be selected with reference to, but are not limited to, the compositions of the positive electrode described above. Furthermore, the method of forming the active material layer of the negative electrode current collector can include coating methods known as those used for the positive electrode, and is not specifically limited to them. In this disclosure, the separator separates the negative and positive electrodes and provides a migration path for lithium ions. Separators commonly used in lithium-ion secondary batteries may be used without limitation, and those with excellent electrolyte solution retention capacity and low resistance to electrolyte ion migration are particularly preferred. Specifically, porous polymer films may be used, for example, porous polymer films prepared with a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof.In addition, common porous nonwoven textile materials can also be used, for example, nonwoven textiles made of high-melting-point fiberglass, poly(ethylene terephthalate) fiber, and similar materials. Furthermore, a coated separator incorporating a ceramic component or polymer material can be used to ensure heat resistance or mechanical strength, and a single-layer or multi-layer structure can be selectively employed. Hereafter in this document, preferred examples are provided to clarify this disclosure; however, the following examples are for illustrative purposes only. Figure 1 is a schematic diagram showing raw materials used to synthesize a positive electrode additive in examples and comparative examples, and in the following examples and comparative examples, raw materials as shown in Figure 1 were used to synthesize the additive. Example preparation 1: Preparation of benzoperylene anhydride (BPA) In a three-necked flask, each neck was fitted with a glass lid, a thermocouple, and a reflux condenser, and placed on a heating mantle. 123.0 g of maleic anhydride was introduced and dissolved at 75 °C, and after adding 8.04 g of perylene, the result was heated to 240 °C. 16.5 g of chloranyl was introduced, the result was refluxed for 10 minutes, then cooled to 140 °C, and 160 ml of xylene heated to 60 °C was added to obtain a mixture. The mixture was cooled to 90 °C and then filtered. The filtered mixture was introduced into an ethyl acetate / chloroform solution (2:1 (v / v)) and purified by repeating the washing, heating to 65 °C, and filtration processes twice. After that, the purified mixture was vacuum-dried at 70 °C to prepare BPA. Example 1 (1) Preparation of positive electrode additive 8 g of BPA prepared in Preparation Example 1 and 45 mmol of octylamine (C8) were introduced, refluxed overnight at 160 °C and then cooled to room temperature to obtain a synthesized compound. 300 ml of methanol were introduced into the compound and the mixture was stirred for 1 hour at room temperature. After that, the stirred mixture was vacuum filtered, then washed several times with methanol and dried to prepare a positive electrode additive for a secondary lithium battery (C8 A-BPI). (2) Preparation of active material for positive electrode The additive prepared in Preparation Example 1 and multi-walled carbon nanotubes (MWCNTs, CNano Company) as a carbon material were mixed in a tetrahydrofuran (THF) solvent. The mixture was then dried for 1 day at 80 °C to prepare a positive electrode active material in the form of MWCNT powder onto which the additive from Preparation Example 1 was adsorbed. In this document, the additive and carbon material were mixed such that the additive was present at a ratio of 4 parts by weight to 100 parts by weight of MWCNT powder, based on the final MWCNT powder produced. (3) Positive Electrode and Button Cell Fabrication The MWCNT powder adsorbed with the additive from Preparation Example 1, the previously prepared positive electrode active material, and a carboxymethylcellulose (CMC) binder were mixed in a 96:4 weight ratio to prepare an aqueous paste. The aqueous paste was applied as a coating to aluminum foil, dried, and perforated at 14 π for use as a reference electrode. A button-type cell was fabricated using a lithium electrode as a counter electrode. When fabricating the button-type cell, a polyethylene separator (16 µm, Celgard) was used, and a 1 M LiTFSI electrolyte solution in a 1:1 (v / v) solvent mixture of DOL and DME was used as the electrolyte solution. Example 2 (1) Preparation of positive electrode additive A positive electrode additive (C10 A-BPI) of a lithium secondary battery was prepared in the same way as in Example 1, except that 1-aminodecane (C10) was used instead of octylamine. (2) Preparation of active material for positive electrode After that, sulfur and MWCNT powder in which the positive electrode additive (C10 A-BPI) was adsorbed in 4 parts by weight to 100 parts by weight of MWCNT as in Example 1 above were mixed in parts by weight of 1:0.33, and the result was heat treated for 1 hour at 150 °C to prepare a sulfur-carbon composite powder that is a positive electrode active material. (3) Manufacturing of positive electrode and button type cell The sulfur-carbon composite powder and a carboxymethylcellulose (CMC) binder were mixed in a 96:4 weight ratio to prepare an aqueous paste. The aqueous paste was applied as a coating onto aluminum foil, dried, and perforated at 14 π to be used as a reference electrode. A button-type cell was fabricated using a lithium electrode as a counter electrode. A polyethylene separator (16 µm, Celgard) was used for the button-type cell, and a 1 M LiTFSI electrolyte solution in a 1:1 (v / v) solvent mixture of DOL and DME was used as the electrolyte. Example 3 A positive electrode additive (C12 A-BPI) of a lithium secondary battery, a positive electrode active material, and a button cell-type lithium-sulfur secondary battery were prepared in the same manner as in Example 1, except that dodecylamine (C12) was used instead of octylamine. Example 4 A positive electrode active material and a button cell-type lithium-sulfur secondary battery were prepared in the same manner as in Example 2, except that, as the positive electrode active material, MWCNT powder was used in which the positive electrode additive (C10 A-BPI) of Example 2 was adsorbed at 8 parts by weight to 100 parts by weight of the MWCNT powder. Comparative Example 1 A positive electrode additive of a lithium secondary battery, a positive electrode active material, a positive electrode, and a button-type cell were prepared in the same manner as in Example 1, except that tetraethylene glycol monoamine was used instead of octylamine (C8). Comparative Example 2 A button-type cell was fabricated in the same way as in Example 1, except that general MWCNT powder was used in which no additive was adsorbed as the positive electrode active material. Comparative Example 3 A button-type cell was fabricated in the same way as in Example 2, except that general MWCNT powder was used in which no additive was adsorbed as the positive electrode active material. Experimental Example 1: Solvent Solubility Experiment of Electrolyte Solution for a Lithium Secondary Battery A solubility experiment was conducted to determine whether the additive prepared in each of Example 1 and Comparative Example 1 dissolved in an electrolyte solution solvent (Example 1: C8 linear carbon chain, Comparative Example 1: C8 linear ether chain). Dimethoxyethane (DME), commonly used in a lithium-sulfur secondary battery, was used as the solvent for the solubility experiment. 0.1 g of the positive electrode additive from each of Example 1 and Comparative Example 1 were introduced into 10 g of a dimethoxyethane solvent, and after mixing the mixture for 1 day, an FT-IR (Fourier transform infrared spectroscopy) measurement was performed to check for the presence of a solute in the mixture solution. In this document, the FT-IR measurement was performed using a Nicolet iS5 instrument (Thermo Fisher Scientific Solutions LLC). Figure 2 shows graphs, each of which shows the FT-IR measurement result for the mixture solution including the positive electrode additive from each of Example 1 and Comparative Example 1. As shown in Figure 2, a C=O peak that is a feature of the positive electrode additive material was identified as not appearing in the mixture solution including the positive electrode additive of Example 1, and a C=O peak appeared in the mixture solution including the positive electrode additive of Comparative Example 1. From these results, it can be seen that the additive in Example 1 did not dissolve in the DME solvent, and the additive in Comparative Example 1 did dissolve in the DME solvent. Experimental Example 2: Carbon Absorption Capacity Experiment A carbon absorption capacity experiment was conducted to determine whether the positive electrode additive of a lithium secondary battery prepared in Example 1 exhibits absorption capacity for a carbon material. The positive electrode additive of a lithium secondary battery prepared in Example 1 and a carbon material were mixed in a tetrahydrofuran (THF) solvent, and the mixture solution was visually observed immediately after mixing and one day after mixing. In this document, multi-walled carbon nanotubes (MWCNTs, CNano Company) were used as the carbon material. Furthermore, the additive and the carbon material were mixed at 1 wt% and 25 wt%, respectively, based on the total weight of the mixture solution (the additive and carbon material had a weight ratio of 0.04:1). Figure 3 shows photographs that show a color change over time for the mixture solution that includes the positive electrode additive from Example 1 and the carbon material (MWCNT). As shown in Figure 3, the mixing solution was identified as exhibiting a yellow color immediately after mixing the positive electrode additive from Example 1 and the MWCNT in the THF solvent (before adsorption), while the yellow color disappeared when 1 day had passed after mixing (after adsorption). From these results, it was seen that, in the mixture solution that includes the additive from Example 1 and the MWCNT which is a carbon material, the additive from Example 1 was adsorbed onto the carbon material and did not dissolve in the solvent. Experimental Example 3: Experiment on the relationship with the electrochemical activity of the battery For the button-type cell that includes the positive electrode active material prepared in each of Example 1 and Comparative Example 2, an experiment on electrochemical activity was carried out. The positive electrode active material of Example 1 is MWCNT powder on which the positive electrode additive (C8 A-BPI) prepared using octylamine (C8) as raw material was adsorbed, and the positive electrode active material of Comparative Example 1 is general MWCNT powder on which no positive electrode additive was adsorbed. The button-type cell of each of Example 1 and Comparative Example 2 went back and forth three times in the 1.8 to 2.9 V section at a rate of 10 mV / s, and a voltage-change-dependent current was measured on the third round trip (VMP3, BioLogic). Figure 4 is a graph showing the electrochemical activity of the button cell depending on the presence or absence of adsorption of the positive electrode additive from Example 1. As shown in Figure 4, the button-type cell that included MWCNT on which the positive electrode additive of Example 1 was adsorbed was identified to have electrochemical activity in a range of 1.8 to 2.5 V, a general drive range of a lithium-sulfur secondary battery. From these results, it can be seen that the positive electrode additive of Example 1 will enhance the electrochemical activity of a lithium secondary battery that includes a lithium-sulfur secondary battery. Experimental Example 4: Identification of shelf-life properties The button cell in each of Example 2 and Comparative Examples 1 and 3 was charged and discharged three times within a range of 1.8 to 2.5 V with a constant current of 0.1 C, and the battery capacity was measured under continuous charge and discharge conditions within the same voltage range with a constant current of 0.2 C / 0.5 C (charge / discharge). The results are shown in Figure 5 (a potentiostat from PNE was used). As shown in Figure 5, it was identified that, while Example 2 had no life decline problem, the life declined rapidly in Comparative Example 1 as the catalyst eluted in the electrolyte solution. Experimental Example 4: Identifying the discharge capacity-dependent change in voltage for each discharge rate For the button cell in each of Example 2, Example 4, and Comparative Example 3, a capacity-dependent change in voltage was measured for each discharge rate, and the lifetime properties were compared. The results are shown in Figure 6. As shown in Figure 6, it could be seen that Example 2 and Example 4 had an increased discharge voltage compared to Comparative Example 3.
Claims
1. A positive electrode active material for a lithium secondary battery, comprising: a carbon material; and a positive electrode additive on a surface of the carbon material, wherein the positive electrode additive is represented by the following Formula 1: , wherein R is a carbon chain.
2. The positive electrode active material for a lithium secondary battery of claim 1, wherein the carbon material is a porous carbon material, and the positive electrode additive is adsorbed onto any one or more of an external and an internal surface of the porous carbon material.
3. The positive electrode active material for a lithium secondary battery of claim 1, wherein the positive electrode additive is included in an amount of 1 to 15% by weight based on the total weight of the carbon material and the positive electrode additive. 4.The positive electrode active material for a lithium secondary battery of claim 1, further comprising sulfur.
5. The positive electrode active material for a lithium secondary battery of claim 4, wherein the weight ratio of sulfur to the total weight of the carbon material and the positive electrode additive is from 1:1 to 1:
0.
6. The positive electrode active material for a lithium secondary battery of claim 1, wherein R is a linear carbon chain.
7. The positive electrode active material for a lithium secondary battery of claim 6, wherein R is an alkyl group having 8 to 12 carbon atoms.
8. A positive electrode for a lithium secondary battery, comprising the positive electrode active material of claim 1. 9.A lithium secondary battery comprising: the positive electrode of claim 8; a negative electrode; a separator interposed therebetween; and an electrolyte solution.
10. The lithium secondary battery of claim 9, wherein the electrolyte solution comprises one or more types of solvents selected from ether-based solvents and carbonate-based solvents.