Negative electrode structure including heteroatom-doped carbon nanotube, manufacturing method therefor, and lithium battery using same

Abstract

The present invention relates to a negative electrode structure including heteroatom-doped carbon nanotubes, a manufacturing method therefor, and a lithium battery using same. The technical gist of the present invention is to include: a current collector; heteroatom-doped carbon nanotubes grown vertically on a surface of the current collector, wherein defects are formed in the carbon nanotubes as heteroatoms are doped into the carbon of the nanotubes, thereby generating active sites on the surface; and lithium metal injected into the space between the heteroatom-doped carbon nanotubes.

Description

A cathode structure comprising heteroatom-doped carbon nanotubes, a method for manufacturing the same, and a lithium battery using the same

[0001] The present invention relates to a cathode structure comprising heteroatom-doped carbon nanotubes, a method for manufacturing the same, and a lithium battery using the same.

[0002] Due to advancements in high-performance battery technology required for electric vehicles, energy storage systems (ESS), and portable electronic devices, interest in next-generation batteries capable of simultaneously ensuring energy density and safety is growing. While currently widely used lithium-ion batteries possess high energy density and long lifespan characteristics, there are limitations to improving energy density due to the theoretical limitations of active materials in the cathode and anode. Consequently, lithium battery technologies that surpass the performance of existing lithium-ion batteries are attracting attention, and in particular, lithium-sulfur batteries, lithium metal batteries, or lithium all-solid-state batteries are being researched as next-generation battery technologies.

[0003] Among these, lithium-sulfur batteries are attracting attention due to their excellent theoretical energy density. Lithium-sulfur batteries use sulfur as the positive electrode active material, and their theoretical capacity reaches 1,675 mAh / g. Lithium metal is used as the negative electrode, and the theoretical capacity of lithium metal is very high at 3,830 mAh / g. With this combination, lithium-sulfur batteries can theoretically provide an energy density of about 2,600 Wh / kg, allowing for performance that significantly surpasses the energy density of currently commercialized lithium-ion batteries (about 200 to 300 Wh / kg).

[0004] However, lithium-sulfur batteries are still facing difficulties in commercialization. The biggest problem is the formation of dendrites on the lithium metal anode. Lithium dendrites are a phenomenon in which they grow in the shape of tree branches on the surface of the lithium metal anode during the charging and discharging process. If these dendrites continue to grow, they can penetrate the separator and cause a short circuit between the anode and cathode, which not only reduces the battery's lifespan but can also cause serious safety issues.

[0005] Furthermore, lithium-sulfur batteries have a problem in which polysulfides, formed by the reaction of sulfur (S) and lithium (Li) during the charging and discharging process, migrate within the electrolyte, leading to the loss of cathode active material and a decrease in reaction efficiency. This phenomenon is called the polysulfide shuttle effect and is one of the main causes of shortened battery life.

[0006] Recently, various technologies have been researched to solve these problems. Research is underway to apply 3D structured cathodes or carbon nanotube-based cathodes to enhance the safety of lithium metal cathodes.

[0007] In this regard, 'Carbon nanotube carpet on and grown from copper (US 2022-0359859 A1)' presented a cathode technology using carbon nanotubes grown in a vertical arrangement on a copper substrate. This technology aimed to suppress dendrite growth and improve the lifespan and safety of the battery by allowing lithium ions to be inserted and extracted between the vertically arranged carbon nanotubes during the charging and discharging process.

[0008] However, these carbon nanotube-based cathodes have limitations in that they lack affinity for lithium, so lithium is not deposited uniformly, and consequently, they cannot completely prevent dendrite formation.

[0009] As such, there is an urgent need for a new cathode technology for lithium-sulfur batteries that can dramatically improve battery life, increase processability, and increase energy density. This technology must be able to fundamentally suppress dendrite formation and allow lithium to be uniformly deposited based on high affinity with lithium metal.

[0010] Accordingly, the inventors, taking into account these technical requirements, researched a new cathode structure capable of improving the stability and performance of lithium metal cathodes and maximizing the lifespan of lithium batteries such as lithium-sulfur batteries. As a result, they confirmed that a technology capable of effectively suppressing dendrite growth and having excellent affinity with lithium could be utilized, and developed this to complete the present invention.

[0011] To solve the above problems, the present invention provides a negative electrode structure comprising heteroatom-doped carbon nanotubes to improve electrochemical performance by enhancing lithium affinity, a method for manufacturing the same, and a lithium battery using the same as a technical problem.

[0012] To solve the above technical problem, the present invention provides a cathode structure comprising a heteroatom-doped carbon nanotube, wherein the cathode structure comprises: a current collector; a lithium metal layer formed on the current collector; and a carbon nanotube vertically grown on the surface of the current collector within the lithium metal layer, wherein the carbon nanotube is a heteroatom-doped carbon nanotube in which a defect is formed on the carbon nanotube as a heteroatom is doped into the carbon, thereby creating an active site on the surface.

[0013] In the present invention, the heteroatom is characterized by being at least one selected from the group consisting of sulfur (S), boron (B), nitrogen (N), phosphorus (P), and oxygen (O).

[0014] In the present invention, the carbon nanotube is characterized by being vertically grown on the surface of the current collector for a length of 3 to 50 μm.

[0015] In the present invention, the heteroatom-doped carbon nanotube is characterized by satisfying the following relationship 1.

[0016] [Relationship 1]

[0017] 1.1 ≤ I D / I G

[0018] However, I D / I G 1,360 ± 50 cm in the wavenumber region of the Raman spectrum -1 Maximum peak intensity (I) measured at D ) and, 1,580 ± 50 cm -1 Maximum peak intensity (I) measured at G It is a value calculated as the ratio of ).

[0019] Meanwhile, to solve the above technical problem, the present invention provides a method for manufacturing a cathode structure comprising heteroatom-doped carbon nanotubes, characterized by comprising: a step of vertically growing carbon nanotubes on the surface of a current collector by supplying a carbon source gas and a heteroatom-containing doping gas onto the current collector; and a step of forming a lithium metal layer by injecting lithium metal onto the current collector on which the carbon nanotubes have grown, wherein the heteroatom-doped carbon nanotubes are vertically grown from the surface of the current collector and, as heteroatoms are doped into the carbon, defects are formed in the carbon nanotubes, thereby creating active sites on the surface.

[0020] In the present invention, the heteroatom is characterized by being at least one selected from the group consisting of sulfur (S), boron (B), nitrogen (N), phosphorus (P), and oxygen (O).

[0021] In the present invention, prior to the step of manufacturing the heteroatom-doped carbon nanotube, the invention further comprises the step of sequentially depositing and forming a buffer layer and a catalyst layer on the surface of the current collector.

[0022] On the other hand, in order to solve the above technical problem, the present invention provides a lithium battery comprising the above-mentioned cathode structure.

[0023] According to the present invention, which is a means for solving the above problem, the affinity with lithium is greatly improved by doping heteroatoms into vertically grown carbon nanotubes. In particular, the active sites created by heteroatom doping promote the smooth electrodeposition and desorption of lithium ions, which has the advantage of increasing the electrochemical reaction rate and efficiency of the cathode.

[0024] In addition, by doping heteroatoms into the vertical growth structure of carbon nanotubes, the growth of lithium dendrites can be suppressed, and the uniformly distributed active sites induce the even electrodeposition and desorption of lithium metal, thereby minimizing the problem of electrode volume expansion and preventing safety issues such as short circuits.

[0025] In addition, heteroatom-doped carbon nanotubes can maintain structural stability even during the repeated insertion and extraction of lithium ions, so they exhibit minimal electrode performance degradation even under long-term cycling conditions and have the advantage of maintaining a high cycle life and stable capacity.

[0026] In addition, since the cathode structure of the present invention is manufactured through a deposition process such as PECVD, it is easy to mass-produce and optimize the process, and the process time is shortened and cost-effective is high because heteroatom doping and vertical growth of carbon nanotubes are performed simultaneously.

[0027] In addition, the cathode structure of the present invention can be utilized not only as a cathode for lithium-sulfur batteries but also as a cathode for lithium metal batteries or lithium all-solid-state batteries using lithium metal as the cathode, thereby having the effect of contributing to the commercialization and advancement of battery technology.

[0028] Figure 1 is a conceptual diagram showing the process of electrodeposition and desorption of lithium metal on a general copper current collector.

[0029] Figure 2 is a conceptual diagram showing the process of electrodeposition and desorption of lithium metal in heteroatom-doped carbon nanotubes of the present invention.

[0030] Figure 3 is a conceptual diagram showing the mechanism of lithium ion electrodeposition in heteroatom-doped carbon nanotubes of the present invention.

[0031] Figures 4a, 4b, 4c, 4d, and 4e are SEM photographs and EDX analysis images of Example 2.

[0032] Figures 5a and 5b are SEM images of Comparative Example 2.

[0033] Figures 6a and 6b are SEM images of Comparative Example 4.

[0034] Figure 7 is a graph showing the Raman spectrum of a negative electrode for a lithium-sulfur battery.

[0035] Figure 8 is a graph showing the XPS N1s spectrum of a negative electrode for a lithium-sulfur battery.

[0036] Figure 9 shows deching cell data of a negative electrode for a lithium-sulfur battery.

[0037] Figure 10 is full cell data of a negative electrode for a lithium-sulfur battery.

[0038] The present invention is susceptible to various modifications and may have various embodiments, and specific embodiments are illustrated in the drawings and described in detail. However, this is not intended to limit the invention to specific embodiments, and it should be understood that the invention includes all modifications, equivalents, and substitutions that fall within the spirit and scope of the invention. Similar reference numerals have been used for similar components in the description of each drawing.

[0039] The terms used in this invention are used merely to describe specific embodiments and are not intended to limit the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this invention, terms such as "comprising" or "having" are intended to specify the presence of the features, numbers, steps, reactions, components, or combinations thereof described in the specification, and should be understood as not precluding the existence or addition of one or more other features, numbers, steps, reactions, components, or combinations thereof.

[0040] Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Terms such as those defined in commonly used dictionaries should be interpreted as having a meaning consistent with their meaning in the context of the relevant technology, and should not be interpreted in an ideal or overly formal sense unless explicitly defined in this application.

[0041] Before describing the present invention, the process of plating and stripping lithium metal on a general copper current collector (Cu foil) is explained. Figure 1 is a conceptual illustration of this process, showing the mechanism of plating and stripping lithium metal on the surface of the copper current collector step by step.

[0042] As shown in Figure 1, in the initial stage, no lithium metal exists on the copper current collector. Subsequently, as lithium ions are reduced, lithium metal (Li metal) is thinly and uniformly plated on the surface of the copper current collector. When stripping proceeds after the lithium is plated, the lithium metal is ionized and returns to the electrolyte.

[0043] As these electrodeposition and desorption processes are repeated, the deposition of lithium metal on the surface of the copper current collector begins to proceed unevenly and eventually grows into the form of lithium dendrites. These dendrites can penetrate the separator and cause a short circuit with the anode, which can lead to battery performance degradation, shortened lifespan, and even safety issues. Therefore, it is essential to design a cathode structure that induces uniform electrodeposition of lithium metal and suppresses dendrite growth.

[0044] Accordingly, the present invention provides a negative electrode structure for a lithium battery manufactured by directly growing carbon nanotubes doped with heteroatoms. The present invention provides a negative electrode structure capable of improving the performance and lifespan of a lithium battery by increasing the affinity between carbon nanotubes and lithium, by directly growing carbon nanotubes through a deposition technique, such as plasma-enhanced chemical vapor deposition (PECVD), while simultaneously doping them with heteroatoms, thereby breaking the carbon bonds of the carbon nanotubes and forming defects, and creating active sites on the surface of carbon nanotubes grown vertically on the surface of a collecting body.

[0045] That is, the cathode structure comprises a current collector; a lithium metal layer formed on the current collector; and a carbon nanotube vertically grown on the surface of the current collector within the lithium metal layer, wherein the carbon nanotube is a heteroatom-doped carbon nanotube in which a defect is formed on the carbon nanotube as a heteroatom is doped with carbon, thereby creating an active site on the surface.

[0046] The above current collector is a substrate for use as a negative current collector in a lithium battery, such as a lithium-sulfur battery, and acts as a mechanical support to relieve stress caused by volume changes during the electrodeposition and desorption process of lithium metal, and helps to suppress dendrite growth by inducing uniform lithium deposition.

[0047] The above current collector may be selected from one or more types from the group consisting of foil, film, sheet, mesh, net, foam, porous body, and nonwoven fabric, and the shape of the current collector is not limited, and various types of current collectors other than the above types may be used.

[0048] The metal of the current collector used in the present invention may be composed of one or more components selected from copper (Cu), nickel (Ni), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), zinc (Zn), molybdenum (Mo), tungsten (W), silver (Ag), gold (Au), ruthenium (Ru), platinum (Pt), iridium (Ir), aluminum (Al), tin (Sn), bismuth (Bi), antimony (Sb), and stainless steel. Among these, copper is generally used, but nickel or stainless steel may also be mainly used.

[0049] A buffer layer is formed by depositing on at least one surface of the current collector. As the first layer deposited on the surface of the current collector, the buffer layer lowers the interfacial resistance between the current collector and the lithium metal and alleviates corrosion and mechanical stress. The buffer layer can be made of aluminum with a thickness of 20 nm, and the deposition process is carried out through processes such as e-beam deposition using an e-beam evaporator or sputtering.

[0050] The catalyst layer is deposited on the buffer layer to enhance reactivity with lithium and help suppress dendrite growth while promoting the uniform precipitation of lithium. Fe with a thickness of 1 nm can be used for the catalyst layer, and, similar to the buffer layer, the deposition process is carried out through processes such as e-beam deposition or sputtering.

[0051] For reference, e-beam deposition is a method of forming a thin film on the surface of a current collector after evaporating a target metal (e.g., Al, Fe) using an electron beam, and sputtering is a method of depositing atoms on a current collector by ejecting them from a target using plasma ions.

[0052] Heteroatom-doped carbon nanotubes are carbon nanotubes that grow vertically on the surface of a current collector through a deposition process, and as heteroatoms are doped into the carbon of the carbon nanotubes, defects are formed in the carbon nanotubes, creating active sites on the surface.

[0053] The influence of the vertically grown length of heteroatom-doped carbon nanotubes in the negative electrode structure of a lithium battery is explained as follows. In the first case, if the vertically grown carbon nanotube length is too short (less than 10 μm), the contact area with the electrolyte is limited, and the lithium storage capacity is reduced. Additionally, if the carbon nanotube length is too short, there is a possibility that the current collector will be directly exposed, leading to uneven lithium electrodeposition and desorption. In the second case, the surface area increases with carbon nanotube lengths in the range of 10 to 30 μm, which increases the contact area with the electrolyte and can increase lithium storage capacity. Furthermore, since the electrical contact between the current collector and the carbon nanotubes increases, high electrical precision can be maintained, resulting in excellent electrochemical and mechanical stability. In the third case, if the carbon nanotube length exceeds 50 μm, the density increases, hindering interaction with the electrolyte, and the electron conduction path becomes too long, leading to increased resistance. In particular, since lithium ions do not diffuse into the interior of carbon nanotubes, the lithium storage capacity is lower than theoretical, resulting in low electrochemical and mechanical stability and ultimately degrading cycle performance.

[0054] For the reasons mentioned above, in the present invention, it is preferable that the length of the carbon nanotube grown vertically from the surface of the current collector be in the range of 3 to 50 μm. If the length of the carbon nanotube is grown vertically to less than 3 μm, sufficient surface area and space for the electrodeposition and desorption of lithium ions are not secured, making it difficult to achieve uniform deposition of lithium metal. This increases the likelihood of lithium dendrite formation and may lead to a decrease in battery performance and a shortened lifespan. If the length of the vertically grown carbon nanotube exceeds 50 μm, the structural stability of the carbon nanotube may decrease, and excessively long carbon nanotubes may cause problems such as reduced electrolyte penetration during the electrodeposition and desorption process, thereby lowering charge / discharge efficiency and impairing the durability and stability of the battery.

[0055] For this reason, maintaining the vertically grown length of carbon nanotubes in the range of 3 to 50 µm is important for smooth electrodeposition and desorption of lithium ions, dendrite suppression, and optimizing the performance and lifespan of the battery.

[0056] At this time, the heteroatom doped into the vertically grown carbon nanotube may be at least one selected from the group consisting of sulfur (S), boron (B), nitrogen (N), phosphorus (P), and oxygen (O).

[0057] Figure 2 conceptually illustrates the process of lithium metal electrodeposition and desorption on these heteroatom-doped carbon nanotubes. By referring to the enlarged portion of the carbon nanotube grown perpendicularly on the surface of the current collector (Cu foil), a structure in which heteroatoms are doped into the carbon of the carbon nanotube is confirmed.

[0058] For example, nitrogen doped with heteroatoms may be doped including pyridinic N, pyrrolic N, graphitic N, or a combination thereof.

[0059] In this nitrogen-doped structure, carbon bonds of carbon nanotubes are broken to create active sites on the surface of the carbon nanotubes, which induces uniform electrodeposition of lithium metal and inhibits dendrite growth. When lithium metal is uniformly electrodeposited in the spaces between vertically grown carbon nanotubes, the lithium metal is stably desorbed during the charging and discharging process, and helps prevent the formation of lithium dendrites on the current collector. This enables a significant improvement in the lifespan and safety of the battery.

[0060] The mechanism by which active sites are generated is illustrated in detail in FIG. 3. Specifically, FIG. 3 is a conceptual diagram showing the mechanism of lithium ion electrodeposition in the heteroatom-doped carbon nanotubes of the present invention. When some of the carbon atoms arranged in a hexagonal structure of the carbon nanotube are doped with nitrogen, carbon bonds are broken or rearranged, thereby generating active sites. The active sites provide a site where lithium ions can be stably adsorbed, and at these active sites, lithium ions are reduced and electrodeposited onto a lithium metal layer.

[0061] As lithium ions diffuse inward along the active sites of vertically grown carbon nanotubes, the lithium ions are uniformly distributed and electrodeposition is successful, thereby ensuring an even distribution of lithium metal and suppressing the growth of lithium dendrites caused by non-uniform electrodeposition.

[0062] In heteroatom-doped carbon nanotubes, the amount of defects caused by heteroatom doping significantly affects electrochemical performance; if the defects are too few, active sites are not sufficiently generated, which may hinder the smooth electrodeposition and desorption of lithium ions. Conversely, if the defects are excessive, the structural stability of the carbon nanotubes is reduced and electrical conductivity decreases, which can degrade battery performance. Therefore, the amount of defects must be maintained within an appropriate range, and Raman spectroscopy was used to quantitatively evaluate this. In the Raman spectrum, the D band (I D ) represents defects in carbon nanotubes, and the G band (I G ) exhibits a distinctive crystalline carbon structure.

[0063] I D / I G The ratio is an indicator representing the degree of defects in carbon nanotubes, and the I of heteroatom-doped carbon nanotubes D / I G It is desirable that it satisfies relation 1.

[0064] [Relationship 1]

[0065] 1.1 ≤ I D / I G

[0066] Here I D / I G 1,360 ± 50 cm in the wavenumber region of the Raman spectrum -1 Maximum peak intensity (I) measured at D ) and, 1,580 ± 50 cm -1 Maximum peak intensity (I) measured at G It is a value calculated as the ratio of ).

[0067] When the above relationship 1 is satisfied, the amount of carbon nanotube defects is optimized, enabling uniform electrodeposition and desorption of lithium ions, and not only can the energy density and lifespan of the battery be improved, but the optimal amount of defects also plays an important role in suppressing lithium dendrite growth and increasing the stability of the battery.

[0068] Meanwhile, a method for manufacturing a cathode structure including heteroatom-doped carbon nanotubes as described above comprises the steps of: sequentially depositing and forming a buffer layer and a catalyst layer on the surface of a current collector (S10); supplying a carbon source gas and a heteroatom-containing doping gas onto the current collector to vertically grow carbon nanotubes on the surface of the current collector (S20); and forming a lithium metal layer by injecting lithium metal onto the current collector on which the carbon nanotubes have grown (S30).

[0069] According to the manufacturing method described above, first, a buffer layer and a catalyst layer are sequentially deposited and formed on the surface of the current collector. Since the technique for depositing and forming the buffer layer and the catalyst layer has been previously explained, a description thereof will be omitted here.

[0070] Next, a step is performed to vertically grow carbon nanotubes on the surface of a current collector by supplying a carbon source gas and a heteroatom-containing doping gas onto the current collector (S20). The above step is a step of manufacturing heteroatom-doped carbon nanotubes in which active sites are created on the surface by doping the carbon of the carbon nanotubes with heteroatoms, by placing the current collector in a reactor, introducing a carrier gas, carbon gas, and a heteroatom-containing doping gas under a vacuum atmosphere, and then performing a deposition process at 700 to 1,000 °C to vertically grow carbon nanotubes on the surface of the current collector.

[0071] The above deposition process can be performed using at least one of Chemical Vapor Deposition (CVD) and Plasma Enhanced Chemical Vapor Deposition (PECVD). Through the above deposition process, carbon nanotubes are vertically grown on the surface of the current collector.

[0072] The above-mentioned carrier gas is an inert or reactive gas used to control the atmosphere of the reactor in the deposition process of the present invention, and one or more of argon (Ar) and hydrogen (H2) are used. This carrier gas maintains a constant gas flow during the deposition process and enables the reaction materials of the carbon gas and the doping gas to be transported to the surface of the current collector.

[0073] The above carbon gas is a source gas that provides carbon atoms necessary for the growth of carbon nanotubes, and one or more of acetylene (C2H2), methane (CH4), and ethylene (C2H4) can be used. During the deposition process, this carbon gas forms carbon radicals that react with metal particles (e.g., Fe) on the catalyst layer on the surface of the current collector to grow into carbon nanotubes.

[0074] To dope the carbon nanotubes with heteroatoms, a doping gas containing heteroatoms is additionally injected into the reactor in addition to the carbon gas for carbon nanotube growth. As the doping gas, at least one of nitrogen gas (N2) and ammonia gas (NH3) may be used, and if nitrogen gas or ammonia gas is used, nitrogen among the heteroatoms is doped into the vertically grown carbon nanotubes.

[0075] The above-mentioned doping gas is injected together during the vertical growth process of carbon nanotubes, but in some cases, it is also possible to inject it continuously after the vertical growth of the carbon nanotubes. However, injecting the doping gas into the reactor simultaneously with the carrier gas and carbon gas is effective for nitrogen doping.

[0076] Here, the deposition process via PECVD can be performed by applying high-frequency power (RF Power) of 200 W or more and less than 400 W to the reactor. When high-frequency power of less than 200 W is applied, the growth rate of carbon nanotubes decreases, and heteroatom doping is insufficient, which may result in weak defect formation. This leads to a lack of active sites, which degrades the electrodeposition and desorption performance of lithium ions, and consequently, the electrochemical properties of the electrode may not meet expectations. When high-frequency power exceeding 400 W is applied, excessive energy is supplied, which may cause the structure of carbon nanotubes to grow irregularly or be damaged. Furthermore, excessive defect formation may weaken the mechanical strength of the carbon nanotubes, and nitrogen doping among heteroatoms is undesirable because it may increase unfavorable bonds such as Graphitic N instead of Pyridinic N, thereby reducing interaction with lithium ions.

[0077] When introducing nitrogen gas for nitrogen doping among the above heteroatoms, it is desirable to control the flow rate in the range of 160 sccm or more and less than 320 sccm. If the nitrogen gas flow rate is less than 160 sccm, the nitrogen supply is insufficient, resulting in a lack of nitrogen doped into the carbon nanotubes. Consequently, it is difficult to completely form active sites, which leads to a decrease in the electrodeposition and desorption performance of lithium ions. Additionally, if nitrogen gas is introduced at a flow rate of less than 160 sccm, the defects in the carbon nanotubes become insufficient, which may result in a negligible improvement in battery performance. If the nitrogen gas is controlled at a flow rate exceeding 320 sccm, excessive nitrogen is introduced, which may lead to an excessive increase in structural defects of the carbon nanotubes. This increases the likelihood of forming unfavorable nitrogen bonds, such as Graphitic N, instead of Pyridinic N, and may reduce the mechanical strength of the carbon nanotubes. In addition, the formation of excessive defects in carbon nanotubes can cause the lithium ion migration path to become irregular, which may reduce the electrochemical stability of the electrode and shorten the battery life.

[0078] Next, a cathode structure is manufactured including a step of forming a lithium metal layer by injecting lithium metal onto a current collector on which carbon nanotubes have been grown, wherein the lithium metal layer is formed by injecting lithium metal into the lithium metal layer, and the carbon nanotubes are vertically grown from the surface of the current collector and have defects formed in the carbon nanotubes as heteroatoms are doped into the carbon, thereby creating active sites on the surface (S30).

[0079] In other words, lithium metal penetrates uniformly between the vertically aligned structures of carbon nanotubes to form a lithium metal layer, thereby suppressing volume changes that occur during the electrodeposition and desorption processes of lithium. Active sites formed on heteroatom-doped carbon nanotubes enhance interactions with lithium ions, promoting high-speed diffusion and electrochemical reactions of lithium.

[0080] In addition, during the process of injecting lithium foil into the spaces between heteroatom-doped carbon nanotubes, the high conductivity and mechanical strength of the carbon nanotubes ensure a uniform distribution of lithium and improve the structural stability of the electrode, thereby suppressing lithium dendrite formation and maintaining stable performance even during long charge-discharge cycles. Through this process, the anode structure combining heteroatom-doped carbon nanotubes and lithium metal can solve the problems of non-uniform growth and volume expansion of lithium metal, and a high-performance anode capable of providing high capacity and long lifespan in lithium-sulfur batteries, lithium metal batteries, or all-solid-state batteries is completed.

[0081]

[0082] The embodiments of the present invention will be described in more detail below. However, the following embodiments are provided merely to aid in understanding the present invention and do not limit the scope of the present invention.

[0083]

[0084] <Example 1>

[0085] Al and Fe were deposited to a thickness of 20 nm and 1 nm on a copper substrate using an e-beam evaporator process. The Al used in this process served as a buffer layer, while Fe served as a catalyst. The substrate, after the above process was completed, was placed in a PECVD apparatus, and the vacuum was set to 10^(-2 to 3) torr. Subsequently, 20 sccm of argon, 60 sccm of hydrogen, 80 sccm of acetylene, and 240 sccm of nitrogen gas were introduced, heated to 900 ℃, and grown for 1 hour with an RF power of 300 W. In this process, argon and hydrogen acted as carrier gases, while acetylene and nitrogen acted as source gases. A lithium foil (20 μm) was placed on the grown nitrogen-doped vertically grown carbon nanotube, and the anode was prepared by heating it on a hot plate at 250 ℃.

[0086] <Example 2>

[0087] A cathode was manufactured in the same manner as in Example 1, but the flow rate of nitrogen gas in the PECVD process was changed to 280 sccm and the RF power to 350 W.

[0088] <Comparative Example 1>

[0089] A lithium foil with a thickness of 20 μm was prepared as the cathode.

[0090] <Comparative Example 2>

[0091] A cathode was manufactured in the same manner as in Example 1, but without introducing nitrogen gas, the gas flow rates were set to 200 sccm of argon, 100 sccm of hydrogen, and 40 sccm of acetylene, and the RF power was set to 100 W.

[0092] <Comparative Example 3>

[0093] A cathode was manufactured in the same manner as in Example 1, but with the nitrogen gas flow rate changed to 160 sccm and the RF power to 100 W.

[0094] <Comparative Example 4>

[0095] A cathode was manufactured in the same manner as in Example 1, but with the nitrogen gas flow rate changed to 160 sccm and the RF power to 200 W.

[0096] <Comparative Example 5>

[0097] A cathode was manufactured in the same manner as in Example 1, but with the nitrogen gas flow rate changed to 320 sccm and the RF power to 300 W.

[0098] <Comparative Example 6>

[0099] A cathode was manufactured in the same manner as in Example 1, but with the nitrogen gas flow rate changed to 240 sccm and the RF power to 400 W.

[0100] The deposition conditions for PECVD RF Power and N2 flow rate in Examples 1 and 2 and Comparative Examples 1 to 6 are summarized and shown in Table 1 below.

[0101] Classification PECVD RF Power (W) N2 Flow Rate (sccm) Example 1 N-VACNT (300 W / N2 240 sccm) 300 240 Example 2 N-VACNT (350 W / N2 280 sccm) 350 280 Comparative Example 1 Li foil -- Comparative Example 2 VACNT 100 -- Comparative Example 3 N-VACNT (100 W / N2 160 sccm) 100 160 Comparative Example 4 N-VACNT (200 W / N2 160 sccm) 200 160 Comparative Example 5 N-VACNT (300 W / N2 320 sccm) 300 320 Comparative Example 6 N-VACNT (400 W / N2 240 sccm) 400 240

[0102] <Test Example 1> Analysis of Growth Length and Structure of Nitrogen-Doped Carbon Nanotubes

[0103] In this test example, SEM images were observed and EDX analysis was performed to compare the shapes of the cathode structures prepared in Example 1, Example 2, and Comparative Examples 2 to 6.

[0104] The carbon nanotubes prepared in Example 1 were grown uniformly vertically on the surface of a copper current collector, and their length was confirmed to be approximately 40 μm. This vertical arrangement indicates that the carbon nanotubes have a high density and good alignment, which is advantageous for the uniform distribution of lithium metal during the electrodeposition and desorption processes of lithium ions. In particular, the appropriate length and vertical arrangement structure of the vertically grown carbon nanotubes contribute to maximizing the contact area with the electrolyte and improving the electrochemical performance and mechanical stability of the electrode.

[0105] Regarding Example 2, the SEM images in Figs. 4a and 4b confirm that the length of the vertically grown carbon nanotubes is within the range of 15 to 20 μm compared to the case of Example 1, and that lithium is uniformly electrodeposited. This demonstrates that the carbon nanotubes grew well vertically. In the EDX layer image in Fig. 4c, the distribution of carbon, aluminum, iron, copper, and oxygen is even, indicating that the carbon nanotubes grew well on the buffer layer and catalyst layer. Examining the nitrogen distribution in Fig. 4d confirms that nitrogen is uniformly doped into the carbon nanotubes. Examining the carbon distribution in Fig. 4e, it can be seen that the carbon nanotubes have a high density and are evenly distributed.

[0106] Regarding Comparative Example 2, in the cathode structure manufactured without using nitrogen gas, carbon nanotubes grew irregularly, and some carbon nanotubes did not grow vertically but showed a bent shape. In particular, the carbon nanotubes grew to a length exceeding 80 μm, and it was confirmed that this caused uneven electrodeposition of lithium and reduced the stability of the cathode.

[0107] In the case of Comparative Example 3, it was confirmed that carbon nanotubes grew to a length of several tens of micrometers. That is, in Comparative Example 3, although carbon nanotubes grew well, nitrogen doping did not occur well due to the low RF Power of 100 W and the low flow rate of 160 sccm, so lithium infusion did not occur well.

[0108] In Comparative Example 4, the nitrogen gas flow rate was set to 160 sccm, the same as in Comparative Example 3, but the RF Power was increased to 200 W. In the cathode structure manufactured, as the carbon nanotubes grew to a length of nearly 200 μm, the vertical alignment of some carbon nanotubes was incomplete and showed a bent shape.

[0109] In Comparative Example 5, when the RF Power was set to 300 W, the same as in Example 1, and the nitrogen gas flow rate was set to 320 sccm, the proportion of acetylene gas in the total flow rate was lowered, so the carbon nanotubes grew to a length of only 4 μm, and most of the carbon nanotubes did not grow well.

[0110] In Comparative Example 6, the nitrogen gas flow rate was set to 240 sccm, the same as in Example 1, but the RF Power was increased to 400 W. Although the carbon nanotubes grew to a length of about 15 μm, the power was too high, so the carbon nanotubes grew sparsely and the density of the carbon nanotube array was low.

[0111] <Test Example 2> Analysis of Nitrogen Doping and Defect Structure via Raman Spectrum

[0112] In this test example, Raman spectrum analysis was performed to evaluate the defect structure and nitrogen doping degree of carbon nanotubes used in the anode for a lithium-sulfur battery. In this regard, FIG. 7 is a graph showing the Raman spectra of anode structures prepared according to Example 1, Example 2, Comparative Example 2, Comparative Example 4, Comparative Example 5, and Comparative Example 6. Note that FIG. 7a represents Comparative Example 2, b represents Comparative Example 4, c represents Example 1, d represents Comparative Example 5, e represents Comparative Example 6, and f represents Example 2.

[0113] D band in the Raman spectrum (approx. 1,360 cm⁻¹) -1 ) refers to defects in carbon nanotubes, the G band (approx. 1,580 cm⁻¹) -1 ) indicates the crystallinity of carbon. I D / I G The higher the ratio, the more defects exist in the carbon nanotubes, and this is closely related to the degree of nitrogen doping. A D / A G The ratio is also used as an indicator to evaluate the amount of defects.

[0114] Ratio of peak intensities in the Raman spectrum I D / I G Regarding the value, I of Example 1 D / I G is 1.172, Example 2 is 1.195, Comparative Example 2 is 1.018, and Comparative Example 3 is I because nitrogen doping was not properly achieved due to low power and low nitrogen gas flow rate. D / I G The value could not be calculated, and in Comparative Example 4, the defect caused by nitrogen doping did not increase significantly with 1.106, while in Comparative Example 5 it was 1.172 and in Comparative Example 6 it was 1.242.

[0115] A D / A G As for the values, Example 1 is 1.194, Example 2 is 1.215, Comparative Example 2 is 0.965, Comparative Example 4 is 0.967, Comparative Example 5 is 1.199, and Comparative Example 6 is 1.216.

[0116] As a result of the analysis, I in Examples 1, 2, 5, and 6 compared to Comparative Examples 2 and 4. D / I G and A D / A G The value increased. This means that more nitrogen doping was achieved than under the conditions of Comparative Example 2 and Comparative Example 4. In particular, it was confirmed that nitrogen doping proceeded effectively when the RF power was 200 W or more and the nitrogen gas flow rate was 160 sccm or more.

[0117] Therefore, to effectively manufacture nitrogen-doped carbon nanotubes, an RF power of 200 W or more and a sufficient nitrogen flow rate of 160 sccm or more are required, and it was confirmed through this test example that the negative electrode performance of a lithium-sulfur battery can be improved through these optimal conditions.

[0118] <Test Example 3> Analysis of Nitrogen Doping Status via XPS N1s Spectrum

[0119] In this test example, XPS N1s spectrum analysis was performed to evaluate the nitrogen doping state and bonding type of the negative electrode structures for lithium-sulfur batteries prepared in Example 1, Example 2, Comparative Example 4, Comparative Example 5, and Comparative Example 6.

[0120] In this regard, FIG. 8 is a graph showing the N1s spectrum of carbon nanotubes used in a negative electrode structure for a lithium-sulfur battery. However, FIG. 8 a represents Comparative Example 2, b represents Comparative Example 4, c represents Example 1, d represents Comparative Example 5, e represents Comparative Example 6, and f represents Example 2.

[0121] In the graph of Fig. 8, peaks of Pyridinic N, Pyrrolic N, and Graphitic N bonds appear, with the peak position of Pyrrolic N being approximately 400–401 eV and the peak position of Graphitic N being approximately 402–403 eV. Additionally, since the Pyridinic N bond among the bonds between carbon nanotubes and nitrogen has a significant effect on the electronic conductivity and lithium affinity of carbon nanotubes, a Pyridinic N peak appearing at approximately 398–399 eV is observed in the XPS N1s results.

[0122] As a result of the XPS analysis in Fig. 8, the Pyridinic N peak was clearly identified in Example 1, and in the case of Example 2, the Pyridinic N peak increased, confirming that nitrogen doping was effectively achieved. In other words, it was confirmed that Pyridinic N bonds were effectively formed in Comparative Example 4, Example 1, and Example 2. In particular, the highest intensity Pyridinic N peak was observed in Example 2. This means that nitrogen doping was most appropriately achieved under conditions of RF power of 350 W and N2 flow rate of 280 sccm.

[0123] On the other hand, in Comparative Examples 5 and 6, although the number of defects due to nitrogen doping increased, Pyridinic N bonds were not sufficiently formed. This was determined to be because the nitrogen gas flow rate was too high (≥ 320 sccm) or the RF power was too high (≥ 400 W), causing other types of defects to be generated instead of Pyridinic N bonds.

[0124] Therefore, through this test example, it was confirmed that the optimal nitrogen doping conditions for the negative electrode of a lithium-sulfur battery are an RF power (W) of 200 or more and less than 400, and a nitrogen gas flow rate (sccm) of 160 or more and less than 320.

[0125] <Test Example 4> Analysis of Lithium Electrodeposition* and Desorption Performance through Symmetric Cell Evaluation

[0126] In this test example, a symmetrical cell test was performed to evaluate the performance of the anode for a lithium-sulfur battery. For this purpose, the anodes prepared in the examples and comparative examples were punched to a diameter of 16 mm, and then the anodes were placed on both sides with a separator (diameter 19 mm, Wscope 16 μm) in between. A symmetrical cell was prepared using a solution of 1 mol LiTFSI and DOL / DME mixed in a 1:1 ratio as the electrolyte. Subsequently, 1 mAh / cm² 2 1-2-5-10 mA / cm² in terms of capacity 2 After performing lithium electrodeposition and desorption for 5 cycles each at a current density of , again at 1 mA / cm² 2 Evaluation was performed at the current density of .

[0127] FIG. 9 shows a symmetric cell data graph of a negative electrode for a lithium-sulfur battery prepared through the examples and comparative examples. FIG. 9(a) is Comparative Example 1, with 5 mA / cm² 2A short circuit occurred after about 29 hours at the current density. In Comparative Example 2 of Fig. 9(b), a short circuit occurred after about 48 hours. In the case of Example 2 according to Fig. 9(d), the cycle was maintained stably for more than 250 hours, showing excellent performance as a lithium-sulfur battery anode. On the other hand, in Comparative Example 4 of Fig. 9(c), a high overvoltage was observed, and it was confirmed that the insertion and extraction of lithium were unstable.

[0128] In addition, compared to Comparative Example 1 using vertically grown carbon nanotubes, Example 2 showed significantly lower overvoltage even at high current densities. This means that lithium insertion and extraction are carried out smoothly even under high power conditions, and lithium dendrite formation is suppressed.

[0129] These results confirmed that the cathode structure incorporating nitrogen-doped vertically grown carbon nanotubes significantly improves the performance of lithium-sulfur batteries and maintains stable performance, particularly under high power and long-duration cycling conditions.

[0130] <Test Example 5> Analysis of Electrochemical Performance of Lithium-Sulfur Battery through Full-Cell Evaluation

[0131] In this test example, a full cell test was performed to evaluate the performance of a negative electrode for a lithium-sulfur battery. To this end, the negative electrodes prepared for the example and comparative example were each punched to a diameter of 14 mm, and the positive electrodes were punched to a diameter of 12 mm. Then, the positive and negative electrodes were placed with a separator (diameter 19 mm, Wscope 16 μm) in between. As the electrolyte, a DOL / DME mixed solution containing 1 mol of LiTFSI and LiNO3 was used to prepare the full cell.

[0132] The cathode was prepared in a ratio of S@MWCNT : SPB : CMC / SBR = 8 : 1 : 1. A sulfur-carbon composite cathode was fabricated by vapor-depositing sulfur onto carbon via heat treatment after mixing sulfur and multi-walled carbon nanotubes (MWCNT) in a weight ratio of 2 : 1. Subsequently, a slurry was prepared by mixing CMC / SBR as a binder and SPB as a conductive agent, and the cathode was formed by casting; the loading amount of this cathode was approximately 0.8 mg / cm². 2 was.

[0133] A YP50F coated separator was used as the separator, and the cutoff voltage of the full cell was set to 1.8 to 2.8 V. The full cell test was performed under conditions of 0.1-0.5-1-2-3-5 C-rate, with 5 cycles for each C-rate, followed by repeating the 0.1 C-rate.

[0134] In this regard, FIG. 10 shows a full cell data graph of a negative electrode for a lithium-sulfur battery. FIG. 10(a) shows the full cell performance of Comparative Example 1, which showed a constant capacity under low C-rate conditions, but exhibited unstable cycle performance with a rapid decrease in capacity at a high C-rate (5 C). Comparative Example 2 in FIG. 10(b) showed slightly improved performance compared to Comparative Example 1, but the capacity decreased significantly and cycle stability declined at high current densities of 3 C or higher.

[0135] Example 2 of Fig. 10(c) maintained high capacity and stable cycle performance even at a high C-rate (5 C). This is because the nitrogen-doped carbon nanotube structure helped with the uniform electrodeposition and desorption of lithium, thereby improving the electrochemical performance of the electrode.

[0136] These results confirmed that a cathode structure incorporating heteroatom-doped carbon nanotubes provides high capacity and excellent cycle stability in lithium-sulfur batteries even under high power conditions. This was found to be because heteroatom-doped vertically grown carbon nanotubes suppress lithium dendrite formation and ensure smooth movement of lithium ions.

[0137] The results of the above test examples were summarized and presented in Table 2 below.

[0138] CNT growth length (㎛) I D / I G (Raman)A D / A G (Raman)Pyridinic N(XPS)Li infusion Example 1 40 1.17 21.194 ○○ Example 2 15 ~ 20 1.19 51.215 ◎○ Comparative Example 1 ----- Comparative Example 2 40 ~ 90 1.01 80.965 ×× Comparative Example 3 tens--- × Comparative Example 4 ~ 20 0 1.10 60.967 ○× Comparative Example 5 41.17 21.199 △× Comparative Example 6 15 1.24 21.216 △×

[0139] In summary, the present invention is characterized by providing a cathode structure in which heteroatom-doped carbon nanotubes are grown vertically directly on the surface of a current collector, and active sites are generated as carbon bonds are broken by doping the carbons of the carbon nanotubes with heteroatoms. According to these characteristics, the electrodeposition and desorption of lithium can be stably achieved through the uniform vertical alignment of carbon nanotubes and the formation of active sites by heteroatom doping.

[0140] In particular, by suppressing lithium dendrite formation even under high current density and long-term cycling conditions, and enabling efficient insertion and extraction of lithium with low overvoltage, the output performance and lifespan of the lithium-sulfur battery can be significantly improved.

[0141] By directly growing heteroatom-doped carbon nanotubes to manufacture a cathode structure, it can be utilized not only as a cathode for lithium-sulfur batteries but also as a cathode for lithium metal batteries, lithium all-solid-state batteries, etc., thus having the advantage of significantly improving battery life.

[0142] The foregoing description is merely an illustrative explanation of the technical concept of the present invention, and those skilled in the art to which the present invention pertains will be able to make various modifications and variations within the scope of the essential characteristics of the present invention. Accordingly, the embodiments disclosed in the present invention are intended to explain, not to limit, the technical concept of the present invention, and the scope of the technical concept of the present invention is not limited by such embodiments. The scope of protection of the present invention shall be interpreted by the claims, and all technical concepts within an equivalent scope shall be interpreted as being included within the scope of rights of the present invention.

Claims

1. Entire house; A lithium metal layer formed on the above current collector; Inside the above lithium metal layer, It includes carbon nanotubes grown vertically on the surface of the above-mentioned current collector, and A cathode structure comprising a heteroatom-doped carbon nanotube, characterized in that the above carbon nanotube is a heteroatom-doped carbon nanotube in which defects are formed in the carbon nanotube as heteroatoms are doped into the carbon, thereby creating active sites on the surface.

2. In Paragraph 1, The above heteroatom is, A cathode structure comprising a heteroatom-doped carbon nanotube, characterized by being at least one selected from the group consisting of sulfur (S), boron (B), nitrogen (N), phosphorus (P), and oxygen (O).

3. In Paragraph 1, The above carbon nanotubes are, A cathode structure comprising heteroatom-doped carbon nanotubes, characterized by being vertically grown to a length of 3 to 50 μm on the surface of the above-mentioned current collector.

4. In Paragraph 1, The above heteroatom-doped carbon nanotubes are, A cathode structure comprising a heteroatom-doped carbon nanotube characterized by satisfying the following relationship 1: [Relationship 1] 1.1 ≤ I D / I G (However, I D / I G 1,360 ± 50 cm in the wavenumber region of the Raman spectrum -1 Maximum peak intensity (I) measured at D ) and, 1,580 ± 50 cm -1 Maximum peak intensity (I) measured at G It is a value calculated as the ratio of ).

5. A step of vertically growing carbon nanotubes on the surface of a current collector by supplying a carbon source gas and a heteroatom-containing doping gas onto the current collector; and The step of forming a lithium metal layer by injecting lithium metal onto a current collector on which the carbon nanotubes are grown; A method for manufacturing a cathode structure comprising a heteroatom-doped carbon nanotube, characterized by including a heteroatom-doped carbon nanotube that is vertically grown from the surface of the current collector inside a lithium metal layer, and in which a defect is formed in the carbon nanotube as heteroatoms are doped into the carbon, thereby creating an active site on the surface.

6. In Paragraph 5, The above heteroatom is, A method for manufacturing a cathode structure comprising a heteroatom-doped carbon nanotube, characterized by being at least one selected from the group consisting of sulfur (S), boron (B), nitrogen (N), phosphorus (P), and oxygen (O).

7. In Paragraph 5, Prior to the step of manufacturing the above heteroatom-doped carbon nanotubes, A method for manufacturing a cathode structure comprising heteroatom-doped carbon nanotubes, further comprising the step of sequentially depositing and forming a buffer layer and a catalyst layer on the surface of the above-mentioned current collector.

8. A lithium battery comprising a negative electrode structure according to any one of claims 1 to 4.

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