阅读说明：本技术 在内部形成空洞的锂-碳复合体及其制造方法 (Lithium-carbon composite having cavity formed therein and method for producing same ) 是由 姜埈 金光浩 于 2017-12-12 设计创作，主要内容包括：本发明涉及一种在内部形成空洞的锂-碳复合体及其制造方法,其要旨在于,包括：向具有芳族环的有机溶剂添加锂前驱体并进行混合的步骤；向上述有机溶剂内配置一对金属丝的步骤；通过溶液中的等离子体放电而形成在碳元素体中掺杂锂的锂-碳复合体的步骤；以及,对上述锂-碳复合体内的氢进行去除并为了在上述锂-碳复合体的内部形成空洞而对上述锂-碳复合体进行退火处理的步骤。借此,能够利用溶液中的等离子体放电简单地合成出锂-碳复合体,而且能够通过对所合成的锂-碳复合体进行退火处理而在内部形成空洞,从而提升适用上述锂-碳复合体的锂可充电电池的锂充放电性能。(The present invention relates to a lithium-carbon composite having a cavity formed therein and a method for manufacturing the same, and the lithium-carbon composite includes: a step of adding a lithium precursor to an organic solvent having an aromatic ring and mixing; disposing a pair of wires in the organic solvent; a step of forming a lithium-carbon composite body in which lithium is doped in a carbon element body by plasma discharge in a solution; and a step of removing hydrogen in the lithium-carbon composite and annealing the lithium-carbon composite to form a cavity in the lithium-carbon composite. Thus, a lithium-carbon composite can be easily synthesized by plasma discharge in a solution, and a cavity can be formed inside the synthesized lithium-carbon composite by annealing the lithium-carbon composite, thereby improving the lithium charge and discharge performance of a lithium rechargeable battery to which the lithium-carbon composite is applied.)

1. A method for producing a lithium-carbon composite having a cavity formed therein, comprising:

a method for producing a lithium-carbon composite having a cavity formed therein, comprising:

a step of adding a lithium precursor to an organic solvent having an aromatic ring and mixing;

disposing a pair of wires in the organic solvent;

a step of forming a lithium-carbon composite body in which lithium is doped in a carbon element body by plasma discharge in a solution; and the number of the first and second groups,

and a step of annealing the lithium-carbon composite in order to remove hydrogen in the lithium-carbon composite and form a cavity in the lithium-carbon composite.

2. The method for producing a lithium-carbon composite body having a cavity formed therein according to claim 1, wherein:

the organic solvent is an organic solvent which is present in a solution at normal temperature and is composed of hydrocarbon containing no other element but only carbon and hydrogen.

3. The method for producing a lithium-carbon composite body having a cavity formed therein according to claim 2, wherein:

the organic solvent is selected from the group consisting of xylene, benzene, toluene, and combinations thereof.

4. The method for producing a lithium-carbon composite body having a cavity formed therein according to claim 1, wherein:

the lithium precursor is cyclopentadiene lithium.

5. The method for producing a lithium-carbon composite body having a cavity formed therein according to claim 1, wherein:

the step of forming a lithium-carbon composite as described above, comprising:

and a step of forming a lithium-carbon composite in which lithium is doped in the carbon element body by a process of decomposing the lithium precursor and the carbon precursor by plasma discharge and then polymerizing the decomposed lithium precursor and carbon precursor by applying a bipolar pulse dc power to the wire.

6. The method for producing a lithium-carbon composite body having a cavity formed therein according to claim 1, wherein:

in the above-described step of annealing the lithium-carbon composite,

the diffusion of lithium makes the internal structure of the carbon element disordered, and the lithium is aggregated by diffusion, thereby forming a cavity in a region where no lithium is present in the carbon element.

7. The method for producing a lithium-carbon composite body having a cavity formed therein according to claim 6, wherein:

and additionally forming a cavity in the carbon element body through a lithium removing process of the aggregated lithium.

8. A lithium-carbon composite having a cavity formed therein, characterized in that:

in the lithium-carbon composite in which the hollow is formed inside,

comprises doping lithium in a carbon element body by plasma discharge in a solution and forming a hollow inside by annealing treatment, and is formed at L iC_xWherein x is 4 to 2-y, and y is 0.001 to 1.999.

Technical Field

The present invention relates to a lithium-carbon composite having a cavity formed therein and a method for manufacturing the same, and more particularly, to a lithium-carbon composite having a cavity formed therein and a method for manufacturing the same, which can easily synthesize a lithium-carbon composite by plasma discharge in a solution, form a cavity (voids) in the interior by annealing the synthesized lithium-carbon composite, and form an additional cavity by a delithiation process to maximize a lithium storage capacity.

Background

Recently, as various electronic devices are miniaturized and lightened, demands for ultra-large power storage systems are rapidly increased, and thus, worldwide attention to new energy sources is continuously increased. Among them, a great deal of research activity has been focused on the field of lithium rechargeable batteries that are environmentally friendly, have high energy density, can realize rapid charge and discharge, and have excellent cycle stability. In particular, since there are various types of carbon-based, metal-based, and oxide-based materials used as cathode active materials of lithium rechargeable batteries, and they play a central role in increasing high-output and high-density energy power, a great deal of research and commercialization thereof have been carried out.

However, although silicon exhibits high theoretical capacity characteristics, it is difficult to commercialize it because of a large structural change occurring during alloying of silicon and lithium and an expansion of 400% or more in volume compared to the original size, silicon is easily cracked when volume expansion occurs as described above, and mechanical stress (stress) caused by the volume change causes a reduction in mechanical stability (mechanical stability), and thus, a problem of a reduction in interfacial capacity occurs due to a reduction in electrical contact between silicon-lithium alloy-forming electrodes, and thus, a problem of a reversible charge/discharge (electrolyte) occurs, and a problem of a reduction in interfacial capacity occurs due to a reduction in surface charge/discharge (electrolyte) of a solid.

Attempts have therefore also been made to minimize the problem of volume expansion as described above by various means, among which the most representative is a method of suppressing volume expansion using a void space (void space) inside a carbon element body by encapsulating silicon in an egg-shell (yolk-shell) structure or a core-shell (core-shell) structure inside the carbon element body. As another method, a method of suppressing volume expansion by lithium oxide generated while alloying by using a metal oxide material capable of alloying with lithium is currently being attempted. However, the former method has a problem of low manufacturing efficiency due to high cost, multi-stage processing, and long process time, and the latter method has a problem of an increase in irreversible capacity due to the generation of lithium oxide and a need to overcome low conductivity of the metal oxide itself.

Disclosure of Invention

Problems to be solved by the invention

Accordingly, an object of the present invention is to provide a lithium-carbon composite in which a void is formed inside by fundamentally solving problems of electrode degradation and electrical conductivity due to volume expansion by excluding the use of materials such as silicon and metal oxides from a source and maximizing the storage capacity of lithium by using only a carbon material, and a method for manufacturing the same.

It is another object of the present invention to provide an internally-formed lithium-carbon composite which can be easily synthesized by plasma discharge in a solution, and in which a large number of voids (cavities) are formed by annealing and delithiation (delithiation) of the synthesized lithium-carbon composite, thereby improving the lithium charge/discharge performance, and a method for manufacturing the same.

Means for solving the problems

In order to achieve the above object, a method for producing a lithium-carbon composite having a cavity formed therein according to the present invention is characterized by comprising: a step of adding a lithium precursor to an organic solvent having an aromatic ring and mixing; disposing a pair of wires in the organic solvent; a step of forming a lithium-carbon composite body in which lithium is doped in a carbon element body by plasma discharge in a solution; and a step of removing hydrogen in the lithium-carbon composite and annealing the lithium-carbon composite to form a cavity in the lithium-carbon composite.

The organic solvent is an organic solvent which is present in the solution at room temperature and is composed of Hydrocarbon (HC) containing only carbon (C) and hydrogen (H) without other elements, and is preferably selected from the group consisting of xylene (xylene), benzene (toluene), toluene (toluene), and combinations thereof.

The lithium precursor is preferably cyclopentadienyl lithium (lithium cyclopentadienyl), and the pair of wires is preferably a metal material having a melting point of more than 2000 ℃.

Further, the step of forming a lithium-carbon composite described above includes: and a step of forming a lithium-carbon composite in which lithium is doped in the carbon element body by applying a bipolar pulsed direct current power source to the wire, decomposing (decomposing) the lithium precursor and the carbon precursor by plasma discharge (plasma discharge), and then polymerizing (polymerizing).

The diffusion of lithium makes the internal structure of the carbon element disordered, and the lithium can be aggregated by diffusion, so that a void is preferably formed in a region where no lithium is present in the carbon element, and a void is preferably additionally formed in the carbon element through a process of delithiating the aggregated lithium.

In the step of annealing the lithium-carbon composite, the hydrogen bonded to the carbon element body is removed to bond carbon to the original hydrogen bonding site, thereby realizing carbon-carbon bonding, and the heat treatment is preferably performed at 400 ℃.

In order to achieve the above object, the lithium-carbon composite having a cavity formed therein according to the present invention is characterized by including a lithium-carbon composite in which a carbon element is doped with lithium by plasma discharge in a solution and a cavity is formed inside by annealing treatment, and further comprising L iC_xThe composition ratio of (A) to (B). (wherein x is 4 to 2-y and y is 0.001 to 1.999).

Effects of the invention

With the configuration of the present invention as described above, it is possible to obtain a lithium-carbon composite body and a manufacturing method thereof, which fundamentally solves the problems of deterioration of an electrode due to volume expansion, electrical conductivity, and the like by excluding the use of materials such as silicon and metal oxide from a source and can maximize the storage capacity of lithium by using only a carbon material.

Further, it is possible to obtain a lithium-carbon composite in which a large number of voids (cavities) can be formed inside by simply synthesizing by plasma discharge in a solution and annealing and delithiating (delithiation) the synthesized lithium-carbon composite, and thus lithium charge and discharge performance can be improved.

Drawings

Fig. 1 is a sequence diagram of a method for producing a lithium-carbon composite in which a void is formed, to which an embodiment of the present invention is applied.

Fig. 2 is an explanatory view of a method for producing a lithium-carbon composite.

Fig. 3 is a photograph illustrating plasma discharge in a solution.

Fig. 4 is a morphology (morphology) photograph of the lithium-carbon complex.

FIG. 5 is a schematic view showing a lithium-carbon composite in the form of carbon black.

FIG. 6 is a mapping image of time-of-flight secondary ion mass spectrometry (ToF-SIMS) of a lithium-carbon complex.

Fig. 7 is a graph showing Atomic percentage (%)) confirmation by etching of the lithium-carbon composite.

FIG. 8 is an X-ray photoelectron spectroscopy nuclear spectrum (XPS coreLEVELLEVEL spectrum) obtained by etching of a lithium-carbon complex.

Fig. 9 is an X-ray photoelectron spectroscopy (XPS) spectrum for confirming the composition of the lithium-carbon composite based on the annealing temperature and time.

FIG. 10 is an L i Nuclear Magnetic Resonance (NMR) spectrum of the lithium-carbon complex.

FIG. 11 is a C-H cross-polarized magic angle spin nuclear magnetic resonance (CPMAS NMR) spectrum of a lithium-carbon complex.

FIG. 12 is a schematic diagram of a lithium-carbon composite illustrating a delithiation process.

Fig. 13 is a state diagram of a lithium-carbon composite based on annealing temperature.

Fig. 14 is a graph illustrating discharge capacity based on the number of charge and discharge cycles of the lithium-carbon composite.

Fig. 15 is a Cyclic Voltammetry (CV) chart for charging and discharging a lithium-carbon composite.

Fig. 16 is a graph illustrating the discharge capacity of a lithium-carbon composite based on the type of organic solvent used in plasma discharge in a solution.

Detailed Description

Next, a lithium-carbon composite having a cavity formed therein and a method for manufacturing the same to which embodiments of the present invention are applied will be described in detail with reference to the accompanying drawings.

Lithium-carbon composites, packages, suitable for use in the inventionIncludes a lithium-carbon composite body in which lithium is doped in a carbon element body by plasma discharge in a solution and voids (voids) are formed inside by annealing treatment, and is formed at L iC_xThe composition ratio of (A) to (B). (wherein x is 4 to 2-y and y is 0.001 to 1.999.)

As a method for producing the lithium-carbon composite described above, as shown in fig. 1, first, in step S1, a lithium precursor is added to an organic solvent having an aromatic ring and mixed.

In order to obtain a lithium-carbon composite having a cavity formed therein, a lithium precursor is added to an organic solvent having an aromatic ring and mixed. In this case, an organic solvent containing carbon, which can be used as a carbon (C) source and can form carbon radicals (carbon chemical), is used as the organic solvent. Among them, an organic solvent having an aromatic ring (aromatic ring) is used in the present invention.

The organic solvent is largely classified into an organic solvent having a linear structure and an organic solvent having an aromatic ring, and when the organic solvent having a linear structure is used, a carbon composite formed by the subsequent steps has a plate-like shape and the yield is very low. When the carbon composite body having a plate-like shape as described above is formed, it is not suitable for doping lithium in the inside thereof, and therefore an organic solvent having an aromatic ring of a nonlinear structure is used in the present invention. When the organic solvent having an aromatic ring as described above is selected and used, since carbon is aggregated in a spherical form, lithium can be doped into the inside, and the object of the present invention, that is, the organic solvent can be formed in a shape in which a cavity can be formed inside.

Among them, as a suitable solvent of the organic solvent having an aromatic ring, an organic solvent which exists in a solution at normal temperature and is composed of Hydrocarbon (HC) containing only carbon (C) and hydrogen (H) without containing other elements is preferably used. That is, the solvent can be selected from the group consisting of xylene (xylene), benzene (toluene), toluene (toluene), and combinations thereof, wherein the solvent with the highest yield is xylene. This is because when other elements such as oxygen (O) are contained in the organic solvent in addition to carbon and hydrogen, lithium is likely to be oxidized.

Lithium cyclopentadienylithium (lithium cyclopentadienylithium) is preferably used as the lithium precursor to be added to the organic solvent, because lithium cyclopentadienylcan be effectively dissolved in the organic solvent and is a sample harmless to the human body. That is, since lithium precursors other than cyclopentadienide lithium are cross-dissolved in an organic solvent, lithium cannot be doped into carbon, and the lithium precursors are samples harmful to the human body, and thus are not suitable for use in tests.

In step S2, a pair of wires is disposed in an organic solvent.

In order to be able to form a lithium-carbon composite by plasma discharge, as shown in fig. 2, a chamber, a pair of electrodes located in the chamber, and a power supply section for applying power to the electrodes are provided. In this case, the chamber is used to store an organic solvent and a lithium precursor therein and provide a space in which plasma discharge occurs. A pair of electrodes facing each other are disposed in the chamber, and one wire (metalwire) is disposed at each end of each electrode, so that the pair of wires are disposed facing each other in a line along the longitudinal direction. The metal wire is immersed in an organic solvent stored in a chamber, and a lithium-carbon composite to which the present invention is applied is produced by plasma discharge.

The wire is a material that can be arranged to form a plasma discharge in an organic solvent by power transmitted from the electrode. The material of the wire as described above should consist of a metal having a melting point exceeding 2000 c. When the melting point of the wire is 2000 ℃ or lower, the wire is melted by plasma discharge and mixed between the lithium precursor and carbon in the form of metal particles, and thus a desired lithium-carbon composite cannot be obtained. That is, in order to obtain a lithium-carbon composite containing no metal particles, it is preferable to arrange a metal wire made of a metal having a melting point exceeding 2000 ℃ and perform plasma discharge. Among them, the wire having a melting point of more than 2000 ℃ is preferably selected from the group consisting of hafnium (Hf), tantalum (Ta), niobium (Nb), molybdenum (Mo), tungsten (W), technetium (Tc), rhenium (Re), ruthenium (Ru), osmium (Os), iridium (Ir), and combinations thereof.

When the pair of wires are arranged in a line along the longitudinal direction, the wires are preferably arranged with an interval of 1 to 2 mm. When the interval between the wires is less than 1mm, there is a problem in that the lithium-carbon complex generated between the wires is intercalated between the wires because the interval is excessively small, thereby hindering the subsequent generation of the lithium-carbon complex and thus causing the end of the plasma discharge. In addition, when the interval between the wires exceeds 2mm, since the organic solvent is a nonpolar (nonpolar) solvent having no dielectric constant, an excessively large interval between the wires may cause a problem in that a plasma discharge cannot be formed. Therefore, the interval between the pair of wires is preferably 1 to 2mm, which is most suitable for forming plasma discharge.

In step S3, a lithium-doped lithium-carbon composite is formed by plasma discharge in the solution.

When a bipolar pulsed direct current power supply is applied to a pair of wires that are installed at both ends of an electrode and immersed in an organic solvent in step S2, as shown in fig. 2 and 3, a plasma discharge is instantaneously formed and lithium is doped into the above-described carbon element body by a process of decomposing (decomposing) a lithium precursor and a carbon precursor in carbon derived from the organic solvent by means of the plasma discharge and then polymerizing (polymerizing). That is, by forming discharge in an organic solvent through a wire, it is possible to form lithium radicals (lithium radius) using a lithium precursor and carbon radicals using carbon contained in the organic solvent, and then immediately aggregate each other to cause polymerization (polymerization). When polymerization occurs, carbon atoms are aggregated with each other to form a carbon element body, and at this time, lithium radicals are inserted between the carbon atoms while the carbon is aggregated, thereby doping lithium atoms in a form in which carbon is arranged around the lithium atoms. That is, since the lithium-carbon composite having the above-described structure can form a composite form in which lithium atoms having a very small diameter are formed and carbon surrounds the lithium atoms, the volume change of lithium ions can be reduced even when the lithium ions are charged and discharged in the lithium atoms in the lithium rechargeable battery, thereby preventing problems in terms of mechanical stress (stress) and mechanical stability (mechanical stability) due to the volume change.

The pulse width of the plasma discharge is preferably 3.0. mu.s or less. When the pulse width exceeds 3.0. mu.s, a problem of transition to arc plasma rather than occurrence of plasma discharge will be caused. When the arc plasma (arc plasma) state is maintained for a certain time or more, the temperature of the organic solvent may be increased due to the generation of a large amount of current, and when the temperature of the organic solvent is increased, the production of the lithium-carbon complex may be interrupted due to the boiling evaporation of the organic solvent. In addition, the sputtering rate of the electrodes in the arc plasma state is increased, which causes an increase in the interval between the electrodes and thus causes a problem in that the plasma cannot be continuously formed. Therefore, the pulse width of the applied power source is preferably 3.0 μ s or less.

The frequency of the applied pulsed dc power supply is preferably 15kHz or more, because a plasma shutdown problem may be caused when the frequency is less than 15 kHz. The voltage of the plasma discharge is preferably in the range of 1500 to 2000V, and when the voltage is less than 1500V, the plasma may be turned off during the plasma discharge due to insufficient voltage. In addition, the plasma may transition to an arc plasma when the voltage exceeds 2000V. When the transition is made to arc plasma, the polymerization of carbon is not smoothly completed due to the change in the properties of carbon, and thus a desired lithium-carbon composite cannot be obtained.

The reason why the lithium-carbon composite is formed by plasma discharge in a solution rather than by gas-phase plasma discharge is that lithium oxide, which is an irreversible substance, is highly likely to be formed when oxygen atoms are introduced during the synthesis of the lithium-carbon composite, and thus cannot help to improve the performance of the substance. Therefore, it is necessary to perform plasma discharge in an organic solvent composed of hydrocarbon in the absence of oxygen.

The lithium-carbon complex obtained by the method as described above can be obtained from an organic solvent by using one or all of fractionation, washing, filtration, and precipitation, and the obtained lithium-carbon complex can completely remove the remaining organic solvent by drying. That is, a lithium-doped lithium-carbon composite in a pure powder form can be obtained.

In step S4, hydrogen is removed by annealing the lithium-carbon composite and a cavity is formed inside.

The lithium-carbon composite produced by step S3 is in a state where lithium is doped into the inside of the polymerized carbon element, and the lithium-carbon composite in an amorphous state can be converted into nanocrystalline graphite (nanocrystalline graphite) in a crystalline state by removing hydrogen by annealing (annealing) the lithium-carbon composite as described above, in the course of the annealing process described above, the nanocrystalline graphite will be agglomerated between the flakes (flakes), and by further performing the annealing process, the agglomerated lithium will form voids between the flakes and be intensively inserted between the single-layer graphite layers inside the nanocrystalline graphite, whereby the lithium-carbon composite will become L iC in which voids are formed and 1 or more lithium atoms are doped into every 2 lithium atoms_xStructure, i.e. will convert to a high density (dense) crystalline state and form L iC_xAnd (5) structure. (wherein x is 4 to 2-y and y is 0.001 to 1.999.)

When lithium atoms aggregated in such a structure are desorbed from the carbon element bodies in a delithiation (delithiation) process, new voids are additionally formed in the carbon element bodies.

Specifically, by applying the lithium-carbon composite to a lithium rechargeable battery, lithium in the lithium-carbon composite can be charged and discharged through a charging and discharging process of lithium. In this case, lithium originally present in the lithium-carbon composite, which is not to be aggregated, is desorbed during lithium discharge, and voids are formed at the positions where the aggregated lithium is originally present. The formed cavity can provide a space in the subsequent lithium charging process, so that the charging and discharging performance of lithium is further improved. Since the voids as described above are continuously generated in the lithium-carbon composite, the charge and discharge performance and cycle of the lithium rechargeable battery are significantly superior to those of the existing lithium rechargeable batteries.

Further, hydrogen (H) bonded to the carbon element body is also removed by annealing treatment because carbon-carbon bonds are not formed smoothly in the carbon element body when hydrogen is present, and carbon and hydrogen are bonded. That is, the carbon element bodies cannot be formed in a sufficient size to form voids, and the carbon element bodies can only exist independently in a small size. In the case described above, the voids can be formed by aggregating lithium in the interior of the carbon element body and then releasing it in the subsequent process.

The annealing temperature in this case is preferably 400 ℃ or higher, and when the annealing is performed at a temperature lower than 400 ℃, not only is there a problem that lithium atoms are not smoothly doped into the inside of the carbon element body, but also the annealing may occur in a state of a lithium precursor, and L iC necessary for the present invention cannot be formed_xA lithium-carbon composite. Further, there is a problem that carbon-carbon bonding cannot be achieved and some hydrogen-bonded portion remains because hydrogen inside the lithium-carbon composite cannot be completely removed. Therefore, the temperature of the annealing treatment should be 400 ℃ or higher.

The composition of the lithium-carbon composite manufactured through the steps S1 to S4 as described above was L iC_x(wherein x is 4 to 2-y and y is 0.001 to 1.999) and 4 to 6 wt% of lithium is contained with respect to 100 wt% of the entire lithium-carbon composite₃、LiC₆A lithium-carbon composite with a higher equal carbon ratio, but it is difficult to form L iC as described in the present invention_2-xHowever, not only L iC having high output density, high energy density and excellent cycle stability can be produced by the present invention_2-xA lithium-carbon composite, and can produce L iC by very simple engineering₂、LiC₃And L iC₄。

Next, embodiments to which the present invention is applied will be described in more detail.

< example 1 >: production of lithium-carbon composite bodies

As an electrode for plasma discharge, a tungsten wire (purity 99.9%, nilacocorp., Japan) having a diameter of 1mm was used. In order to concentrate the energy, the electrodes were covered with a ceramic tube with a protrusion length of 1 mm. Next, the electrode was placed in a beaker in which lithium cyclopentadienyl (lithium cyclopentadienyl) was dissolved in xylene (xylene), and plasma discharge was performed using a bipolar pulsed DC power supply (Kurita, Japan). At this time, the voltage was controlled to 2.0kV, the pulse frequency was controlled to 100kHz and the pulse width was controlled to 0.7 μ s, and the test was performed under the conditions of room temperature and atmospheric pressure.

In order to increase the electrical conductivity of the synthesized lithium-carbon (L i-C) composite and remove hydrogen present in the lithium-carbon composite, the carbon material was charged into a tube furnace (tube furnace) and then annealed at 500 ℃ for 3 hours in a nitrogen atmosphere, and in this case, the annealing was performed at 10 ℃ for 10 minutes^-1The tube furnace was then cooled.

< example 2> determination of L i-C characteristics

For the carbon material produced in the examples described above, its L i-C morphology was observed by high angle annular dark field image scanning transmission electron microscopy (HADDF-STEM) using a TA L OS F200X apparatus, furthermore, the lithium distribution of the carbon species was investigated using ION-TOF equipment (Munster, Germany) by performing time-of-flight secondary ION mass spectrometry (TOF-SIMS), the chemical composition of the surface was characterized using K-a L PHA + X-ray photoelectron spectroscopy (XPS) system (thermodissher Scientific, u.k.). the X-ray photoelectron spectroscopy was performed by scanning monochromatic Al K α lines (1486.6 eV; spot size (spot), 400 μm.) the angle between the sample surface and the analyzer was 90 °, the X-ray beam acquired the elemental information of C1s, O1s, L i1s during all scans, using a spot size of 400 × 400 μm.

Depth profiling (depth profiling) refers to an operation of experimentally obtaining a plotted value of the presence amount or concentration of atoms with respect to the surface depth, which is performed using an argon (Ar) gas ion source of an apparatus operating at 3kV and 1 μ a, and which has a gradient of 45 ° with respect to the surface normal in an area of 1.5 × 3 mm.

To investigate the state of lithium in L i-C, a 7L i Nuclear Magnetic Resonance (NMR) measurement of the solid state was performed at 400MHz using a AVANCE III HD energy spectrometer (Bruker, Germany) L i-C surface area was calculated using Brunauer-Emmett-Teller (BET) method (Autosorb-iQ, quantachroms instrument, USA), and the sample was degassed at 200 ℃ for 2 hours before performing the Brunauer-Emmett-Teller (BET) measurement.

< example 3> preparation of L i-C electrode and electrochemical characterization

L i-C was performed using a 2032 coin-type half-cells (Wellcos Corp.) as the working electrode, by mixing the active material, conductive carbon black (TIMCA L Graphite)&Carbon Super P) and a polyacrylic acid binder (average Mw of 3000000) were mixed in distilled water at a weight ratio of 7:1:2 to form a slurry. The mixing was performed for 30 minutes at a revolution speed of 400 to 2000rpm and a rotation speed of 160 to 800rpm using an AR-100 conditioning mixer (THINKY Corp.), and the homogenization was performed for 10 minutes at a revolution speed of 2200 rpm. Next, the above slurry was uniformly applied to a copper foil (Cu foil) with a doctor blade. Next, the copper foil was pressed using a roll press after removing the solvent by drying the copper foil overnight at 50 ℃. At this time, the mass load of the electrode material on the current collector was about 0.8mg/cm²。

In an argon-filled glove box as a button cell with lithium metal foil and as a counter electrode, 1M L iPF in a 1:1(v/v) mixture of ethylene carbonate and dimethyl carbonate₆Electrochemical testing was performed using BCS-805Biologic cell test System (Biologic, France) at 0.005V (vs. L i/L i)⁺) By performing cyclic voltammetry using the same workstation, 0.1-0.0V (vs. L i/L i) at a scan rate of 0.2mV/s⁺) The reduction in the voltage range and the oxidation peak were investigated.

Fig. 4 is a morphology (morphology) photograph of the lithium-carbon complex. The lithium-carbon composite body to which the present invention is applied is synthesized by plasma discharge in a solution, and the average amount of the lithium-carbon composite body is about 6 to 7g per hour. The lithium-carbon composite described above was investigated using a Transmission Electron Microscope (TEM). It was confirmed that the obtained lithium-carbon composite was in the form of carbon black (carbon black) as shown in fig. 5. Fig. 6 is a mapping image of time-of-flight secondary ion mass spectrometry (ToF-SIMS) of a lithium-carbon complex capable of demonstrating that lithium in carbon is uniformly distributed.

Fig. 7 is a result of performing high-resolution X-ray photoelectron spectroscopy (XPS) on a surface in order to investigate the composition of doped lithium. In order to confirm the composition of lithium in the lithium-carbon composite, the Atomic percentage (%)) of the upper portion of the lithium-carbon composite was measured while cutting the upper portion. As shown in fig. 7, it was found that the results of confirming the atomic percentages based on the etching time were 4 to 6 wt% on average, whereby it was possible to judge that lithium atoms were mostly present on the carbon spheres. Further, it was confirmed that the amount of lithium on the surface was about 0.75 wt%.

X-ray photoelectron spectroscopy (XPS) depth profile analysis was performed to confirm the presence of lithium inside the carbon and investigate the carbon composition distribution at different depths FIG. 8 illustrates an X-ray photoelectron spectroscopy nuclear level spectrum (XPscore level spectrum) illustrating the results of etching carbon by performing sputtering from 100 to 1900s in units of 300s, L iC to which the present invention is applied_2-xThe main peak is observed in 55.2-55.6eV, whereas L iC is observed₂、LiC₃And L iC₆Etc. other compositions only small peaks were observed.

FIG. 9 is an X-ray photoelectron spectroscopy (XPS) spectrum for confirming the composition of a lithium-carbon composite based on the annealing temperature and time, in which Peak-1 (Peak-1) corresponds to a lithium atom or a lithium precursor (55.2eV), and Peak-2 (Peak-2) corresponds to L iC_2-x(55.2-55.6 eV). It can be found that the lower the temperature at which the heat treatment is performed, the lower the proportion of its Peak-1 (Peak-1) and that at higher temperatures, i.e. 500 ℃, the Peak-1 (Peak-1) is significantly higher than the Peak-2 (Peak-2). That is, it can be found that the intensity of Peak-1 (Peak-1) gradually increases with increasing temperatureA new Peak, Peak-2 (Peak-2), is reduced and appears, and the intensity of the appeared Peak-2 (Peak-2) is gradually increased.

Further, it can be found from an X-ray photoelectron spectroscopy (XPS) spectrum confirming the composition of the lithium-carbon composite body based on the annealing treatment time that the Peak-1 (Peak-1) is gradually decreased and the Peak-2 (Peak-2) is gradually increased as the annealing treatment time is increased from 1 minute to 3 hours. This is because lithium precursor or lithium atoms in a doped state undergo diffusion movement with an increase in temperature and then excessively condense at a specific position of the carbon matrix.

In addition, more information on the atomic state of the lithium-carbon composite was obtained by comparing the result of a solid state magic angle spinning (NMR) Nuclear Magnetic Resonance (NMR) spectrum of the lithium-carbon composite with the result of X-ray photoelectron spectroscopy (XPS) analysis. Illustrated in fig. 10 is⁷L i Nuclear Magnetic Resonance (NMR) results, no metal band (metal band) appeared in a of FIG. 10, and only one band was clearly observed at 0 ppm.

B of fig. 10 is a Nuclear Magnetic Resonance (NMR) image enlarged for a narrower range, and shows a chemical shift change by indicating the shift (shift) observed with a dotted line. It was found that the peak position did not change sharply until 300 c when the heat treatment was performed, but the chemical shifts of the samples annealed at 400 c and 500c for 1.5 hours showed a tendency to shift to lower frequencies (upward electric field). In addition, almost no Peak shift occurred in the samples annealed at 300 ℃ for 1.5 hours and 3 hours, while two components such as Peak-1 (Peak-1) and Peak-2 (Peak-2) occurred in the samples annealed at 200 ℃ for 1 minute and 300 ℃ for 1 minute.

The narrower Peak-2 (Peak-2) at the lower side of 1ppm of the half width appearing on the sample annealed at 300 ℃ is a very fast shift, originating from diamagnetic lithium material (e.g., detached from precursor during heat treatment or synthesis), while Peak-1 (Peak-1) originates from slow motion (e.g., present in lithium adsorbed to solid molecules or immobilized in molecular structure) and diamagnetic lithium species.

It can be confirmed from FIG. 10 that Peak-1 (Peak-1) and Peak-2 (Peak-2) of the central Peak are deconvoluted (deconvo) in the Lorentzian (L oretzian) line shape and Gaussian (Gaussian) line shape, respectively, and the calculation results of the relative ratios of the peaks are shown in Table 1. the chemical shifts, half widths and relative ratios of Peak-1 (Peak-1) and Peak-2 (Peak-2) on the 7L i Nuclear Magnetic Resonance (NMR) spectrum of the lithium-carbon composite annealed at different temperatures are shown in Table 1.

[ TABLE 1 ]

The relative proportion of Peak-2 (Peak-2) was reduced at 200 ℃ for 1 minute, but the difference was not significant. But the intensity of Peak-2 (Peak-2) (10%) was significantly lower at 300 c when heat treated than at 200 c. In the sample subjected to the annealing treatment at 400 ℃ for 1 minute, the Peak-2 (Peak-2) in the lithium spectrum was hardly disappeared, and only the shape of the Peak-1 (Peak-1) was exhibited in a wide range. Further, the samples subjected to the heat treatment at 500 ℃ for 1 minute, 1.5 hours, and 3 hours showed a tendency to move from-0.3 ppm to-0.4 ppm, but the difference was very small.

Illustrated in fig. 11 is¹³C-¹The H cross-polarization magic angle spin nuclear magnetic resonance (CPMAS NMR) spectrum was measured to confirm that the peak shift of lithium is related to the change in the carbon crystal structure.¹³C-¹The H cross polarization magic angle spin nuclear magnetic resonance (CPMAS NMR) spectra were obtained from samples that were not annealed (as-pretreated), samples that were annealed at 200 ℃ for 1 minute, and samples that were annealed at 300 ℃ for 1 minute, and samples that were treated at 400 ℃ or higher were not observed. The spectrum of the sample without annealing treatment and the sample annealed at 200 ℃ for 1 minute can be obtainedIt was confirmed that sp was present in the range of 10 to 0ppm³Carbon peak value. This indicates that a large number of alkyl groups (alkyl groups) are present in the lithium-carbon complex, thereby forming a structure in a random (random) and amorphous (amorphous) state.

But sp in the sample annealed at 300 ℃ for 1 minute³The carbon peak almost disappeared and was only sp²The carbon peak remained, indicating that the alkyl group derived from the precursor remained in the prepared sample in carbon and lithium or the decomposition product thereof. In contrast, sp can be found at 400 ℃ or higher²The carbon peak also almost disappears, passing sp²The disappearance of the carbon peak can conclude that hydrogen has disappeared and that carbon is attached in the corresponding position.

The presence of alkyl groups as described above indicates that the graphite flake is still short and random and takes on an irregular shape, suggesting that it is in an extremely amorphous graphite morphology. In the spectrum of the sample annealed at 300 c for 1 minute, the amorphous form was low compared to the sample annealed at 200 c for 1 minute because the alkyl group which may cause the graphite plate to be separated disappeared. In addition, sp was neutralized in a sample annealed at 200 ℃ for 1 minute²The peak corresponding to carbon is relatively smaller than that of the sample that was not annealed, suggesting sp bonded to hydrogen²The carbon linking sites remain and indicate that the graphite plates are not remotely linked. A larger peak could not be obtained in the sample treated at a temperature above 400 c, indicating that almost no hydrogen was present and that the graphite plate was present in a relatively expanded state could be confirmed.

From the results described above, it can be estimated that the samples annealed at 400 ℃ were⁷L i change in chemical shift value and passage of Nuclear Magnetic Resonance (NMR)¹³C-¹The graphite forms deduced by H cross polarization magic angle spin nuclear magnetic resonance (CPMAS NMR) analysis have a correlation relationship.

As shown in fig. 12, the internal structure of the lithium-carbon composite to which the present invention is applied is in an amorphous state and exists in a state where a large number of voids are formed inside. Therefore, lithium doped into the carbon can exist in a very free state in the void. Considering that the Peak-1 (Peak-1) ratio after the annealing treatment at 300 ℃ or lower is high, it is presumed that a large amount of lithium exists in the pores of the lithium-carbon composite in an adsorbed state. But graphitization will occur with an increase in the annealing temperature and thus carbon will grow into a nano-crystalline graphite structure.

As a result, a large number of nano-voids will disappear and lithium will accumulate between the graphite plates, causing all of the lithium to be absorbed into the narrow voids, as evidenced by the complete disappearance of Peak-2 (Peak-2) at 400 deg.C₂Therefore, dense lithium as described above is the same as predicted by X-ray photoelectron spectroscopy (XPS), and can be at L iC_2-xExist in the form of (1).

As shown in FIG. 13, lithium in a lithium-carbon composite (as-prepared) exists in a simple polymerized state within carbon in a state where annealing is not performed, however, the arrangement state of a carbon element body and lithium is changed when annealing is performed, and the carbon element body is converted from amorphous carbon (amorphous carbon) into a nano-crystalline graphite (nanocrystalline graphite) shape, when annealing is performed at 300 ℃, the carbon element body will be in a state of being connected to each other to some extent and lithium will exist therein, when annealing is performed at 400 ℃, the connection of the carbon element body will be longer than 300 ℃ and lithium will exist in a state of being aggregated to each other, and when annealing is performed at 500 ℃, the aggregation of lithium will be more severe than 400 ℃ and thus converted into a shape of being formed by the aggregation of lithium voids, when annealing temperature is increased, the aggregation of the carbon element body and lithium will be increased and thereby L iC is formed_2-xL iC as described above_2-xLithium-carbon compositeThe combination will form a crystalline structure.

Fig. 14 is a graph illustrating the Discharge Capacity (mAh/g)) based on the number of charge and Discharge cycles (cyclic fibers), when compared with the theoretical Capacity (L i-C500 C.E) of the lithium-carbon composite annealed at 500 ℃, the actual Discharge Capacity (L i-C500 Discharge Capacity) of the lithium-carbon composite is about 800mAh/g at the initial stage, and gradually increases to 1000mAh/g close to the theoretical Capacity as the number of cycles increases, which indicates that lithium originally condensed in the lithium-carbon composite is desorbed and voids are formed in the corresponding region when the lithium is charged and discharged, as the number of charge and Discharge cycles increases, lithium can be easily charged at the corresponding position at the next charging, thereby increasing the charge and Discharge Capacity.

This can be confirmed by the graph illustrated in FIG. 15, FIG. 15 being L i-C500 at 0.2mVs over a voltage range of 0.005-3V^-1A Cyclic Voltammetry (CV) graph of charging and discharging at the scan rate of (a). This confirmed that stable charge/discharge characteristics were exhibited even with the lithium-carbon composite to which the present invention was applied, and that the 2 nd and 3 rd current values were larger than the 1 st current value.

Fig. 16 is a graph illustrating the discharge capacity of the number of cycles of a lithium-carbon composite synthesized based on the type of organic solvent used in plasma discharge in a solution. In this case, benzene (benzone), toluene (toluene), and xylene (xylene) were used as the organic solvent, and the discharge capacity characteristics were substantially the same in the three cases, and it was confirmed that the highest discharge capacity was obtained for xylene among the three organic solvents. Thus, the most suitable organic solvent during plasma discharge in solution is xylene.

The reason why the lithium-carbon composite produced by the present invention has a capacity 3 times as large as that of conventional graphite is as follows. 1) Formation of voids by movement and concentration of doped lithium, and 2) desorption of doped lithium during charge and discharge and formation of voids in regions where lithium originally exists. The lithium-carbon composite to which the present invention is applied as described above maintains a high capacity state until 250 cycles, and gradually decreases to 90% or less of the initial capacity in about 300 cycles thereafter.

Therefore, by plasma discharge in a solution according to the present invention, a lithium-carbon composite body in which a cavity is formed inside can be obtained by manufacturing a lithium-carbon composite body by a simple method and by annealing it. Since the lithium-carbon composite as described above forms a void inside, the charge and discharge performance of lithium is more excellent and the life and performance of a lithium rechargeable battery to which the lithium-carbon composite is applied will increase.