KR20220071426A - Anode active material coated with nitrogen-doped carbon for sodium ion secondary battery and method of preparing the same - Google Patents

Abstract

Disclosed is a negative electrode active material for a sodium secondary battery including: tin-selenide; and a nitrogen-doped carbon-based compound bonded to at least a part of the tin-selenide surface. The negative electrode active material for a sodium secondary battery has excellent lifespan characteristics and charge/discharge capacity.

Description

Nitrogen-doped carbon-coated negative active material for sodium secondary battery and manufacturing method thereof

It relates to a nitrogen-doped carbon-coated negative active material for a sodium secondary battery and a method for manufacturing the same.

As the field of application of lithium-ion batteries such as portable electronic devices and electric vehicles is expanding, the demand for high-power, large-capacity, long-life lithium-ion batteries is increasing. However, the reserves of lithium are not sufficient, the price competitiveness to satisfy the increasing demand for lithium ion batteries is low, and the biocompatibility is low for applications such as implantable biomedical devices and electronic skin functions that can check health similarly. There is a limit to being

On the other hand, sodium ion batteries are attracting attention as alternative batteries for lithium ion batteries because they have abundant sodium reserves, are nearly 10 times cheaper than lithium ion batteries, and have excellent biocompatibility and electrochemical properties similar to those of lithium. However, sodium ion has an ionic radius of 55% greater than that of lithium ion, and for this reason, the energy density is lowered, thereby exhibiting unfavorable kinetic properties in inserting and extracting sodium ions. It is difficult to insert large sodium ions into the graphite layer of the carbon material, and accordingly, the graphite material is generally not applicable as an anode material in a sodium ion battery.

In order to solve this, metal chalcogenides are attracting attention as negative electrode materials for sodium ion batteries, and among them, tin chalcogenides can provide a high capacity according to conversion and alloying reactions in the cycling process, but in the prior art Since volume expansion and particle agglomeration due to the insertion/release of sodium ions, which are problems of metal compounds, still occur, it acts as a factor degrading the electrochemical performance of the sodium ion battery.

In order to solve this problem, the development of an anode active material for a sodium secondary battery that is suitable for a sodium ion battery and has improved battery life characteristics and charge/discharge capacity is required.

In order to solve the problems of the prior art, an object of the present invention is to provide an anode active material for a sodium secondary battery having excellent lifespan characteristics and charge/discharge capacity, and a method for manufacturing the same.

According to one aspect tin-selenide; and a nitrogen-doped carbon-based compound bonded to at least a portion of the tin-selenide surface.

In an embodiment, the tin-selenide may include SnSe and SnSe ₂ .

In one embodiment, the negative active material may be in the form of nanoflakes.

In an embodiment, the average thickness of the anode active material may be 10 to 30 nm.

In one embodiment, the average pore diameter of the negative active material may be 10 ~ 20㎚.

According to another aspect, there is provided an anode for a sodium secondary battery in which a slurry including the anode active material for a sodium secondary battery, a binder, and a conductive material is applied to a current collector.

In one embodiment, the binder is polyvinylidene fluoride, polyvinyl fluoride, polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene , ethylene-propylene (EPDM) rubber, styrene-butyrene rubber, and may be at least one selected from the group consisting of fluororubber.

According to another aspect, (a) mixing the tin precursor and the selenium precursor in a solvent; (b) adding the mixture of (a) to a sealed reactor and reacting at 150 to 200° C. for 10 to 15 hours to prepare tin-selenide; and (c) mixing and dry grinding with the tin-selenide and carbon precursor, followed by calcining in a nitrogen atmosphere;

In one embodiment, in step (c), a nitrogen-doped carbon coating layer may be formed on at least a portion of the tin-selenide surface.

In one embodiment, the step (c) may be performed at 400 ~ 600 ℃, 1-3 hours conditions.

In an embodiment, the calcination of step (c) may be performed under conditions of increasing 5° C. per minute.

According to one aspect, tin-selenide; And by applying the negative active material coated with a nitrogen-doped carbon-based compound bonded to at least a portion of the tin-selenide surface to the secondary battery, volume expansion occurring during the charging and discharging process can be suppressed, and thus the lifespan of the battery Characteristics and electrical characteristics can be improved at the same time.

The effect of one aspect of the present specification is not limited to the above-described effect, but it should be understood to include all effects that can be inferred from the configuration described in the detailed description or claims of the present specification.

1 is a schematic view showing a method of manufacturing a negative active material for a sodium secondary battery according to an embodiment of the present specification.
2 is an XRD and Raman spectrum analysis result of a negative active material for a sodium secondary battery according to an embodiment of the present specification.
3 is an XPS analysis result of a negative active material for a sodium secondary battery according to an embodiment of the present specification.
4 is a HR FE-SEM and EDX analysis results of the anode active material for a sodium secondary battery according to an embodiment of the present specification.
5 is a FE-TEM and HR FE-TEM analysis result of the negative active material for a sodium secondary battery according to an embodiment of the present specification.
6 is a nitrogen adsorption/desorption isotherm graph and BJH pore size analysis results of a negative active material for a sodium secondary battery according to an embodiment of the present specification.
7 is a graph showing a differential capacity (dQ/dV) of a secondary battery including a negative electrode for a sodium secondary battery and a performance evaluation graph according to an embodiment and a comparative example of the present specification.
8 is a performance evaluation graph of a secondary battery including an anode for a sodium secondary battery according to an embodiment and a comparative example of the present specification.

Hereinafter, one aspect of the present specification will be described with reference to the accompanying drawings. However, the description of the present specification may be implemented in several different forms, and thus is not limited to the embodiments described herein. And in order to clearly explain one aspect of the present specification in the drawings, parts irrelevant to the description are omitted, and similar reference numerals are attached to similar parts throughout the specification.

Throughout the specification, when a part is "connected" with another part, this includes not only the case where it is "directly connected" but also the case where it is "indirectly connected" with another member interposed therebetween. . In addition, when a part "includes" a certain component, this means that other components may be further provided without excluding other components unless otherwise stated.

When a range of numerical values is recited herein, the values have the precision of the significant figures provided in accordance with the standard rules in chemistry for significant figures, unless the specific range is otherwise stated. For example, 10 includes the range of 5.0 to 14.9 and the number 10.0 includes the range of 9.50 to 10.49.

According to one aspect tin-selenide; and a nitrogen-doped carbon-based compound bonded to at least a portion of the tin-selenide surface.

In a sodium secondary battery, metal chalcogenides have been proposed as an anode material for a sodium secondary battery to improve the disadvantageous dynamic properties according to the conventional sodium ion radius, and among them, tin chalcogenides are converted and Although it is possible to provide a high capacity according to the alloying reaction, there is a problem in that volume expansion and agglomeration of particles still occur due to the insertion/release of sodium ions.

To solve this, the tin-selenide may include a nitrogen-doped carbon-based compound bonded to at least a portion of the surface, and specifically, the nitrogen-doped carbon-based compound may be coated. The nitrogen-doped carbon matrix compound (N-doped carbon matrix) minimizes volume expansion during insertion/release of sodium ions to increase electrical conductivity, thereby improving lifespan characteristics when finally applied to a secondary battery. In addition, nitrogen doped into the carbon-based compound may improve electrical conductivity and surface wettability, and may provide external defects capable of increasing a diffusion site for diffusion of sodium ions.

The negative active material may be tin-selenide dispersed on a continuous nitrogen-doped conductive carbon matrix. When the matrix is doped with nitrogen, volume expansion of tin-selenide by sodium ions may be further suppressed.

According to one non-limiting example, the negative active material may include a carbon-based compound doped with nitrogen by coating at least a portion of the tin-selenide surface with dopamine hydrochloride and calcining in a nitrogen atmosphere, but It is not limited. If nitrogen is doped without the use of hydrazine, which is a toxic substance, it is more environmentally friendly and nitrogen can be doped more stably.

The tin-selenide may include SnSe and SnSe ₂ . Since the tin-selenide contains both SnSe and SnSe ₂ , an alloying/dealloying reaction may be actively performed, and thus sodium ions may be easily captured and released.

The negative active material may be in the form of nanoflakes, and due to the layered crystal structure according to the nanoflake structure, a transport channel and an active area for sodium ion diffusion increase, so that the electrochemical performance of the negative active material may be improved. .

Meanwhile, the average thickness of the negative active material may be 10 to 30 nm, for example, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, 16 nm, 17 nm, 18 nm, 19 nm , 20 nm, 21 nm, 22 nm, 23 nm, 24 nm, 25 nm, 26 nm, 27 nm, 28 nm, 29 nm or 30 nm, but is not limited thereto. If the average thickness of the anode active material is less than 10 nm, cracks in the layered crystal structure may occur, and if it is more than 30 nm, the active area may decrease and electrochemical performance may be deteriorated.

The average pore diameter of the negative active material may be 10 to 20 nm, for example, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, 16 nm, 17 nm, 18 nm, 19 nm or It may be 20 nm, but is not limited thereto. If the average pore diameter of the anode active material is less than 10 nm, diffusion of sodium ions may be reduced to deteriorate electrochemical performance, and if it exceeds 20 nm, the diameter may be larger than necessary, thereby reducing electrochemical performance.

In addition, in order to achieve the above object, according to another aspect, a slurry comprising the anode active material for a sodium secondary battery, a binder and a conductive material is applied to a current collector, a negative electrode for a sodium secondary battery is provided.

The binder is polyvinylidene fluoride, polyvinyl fluoride, polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene (EPDM) ) may be at least one selected from the group consisting of rubber, styrene-butyrene rubber, and fluororubber.

The type of the conductive material is not particularly limited as long as it has conductivity without causing a chemical change in the battery. For example, the conductive material is graphite such as natural graphite or artificial graphite; carbon black, such as carbon black, denka black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; conductive fibers such as carbon fibers and metal fibers; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskeys such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; It may be a conductive material such as a polyphenylene derivative.

The weight ratio of the anode active material for the sodium secondary battery, the binder, and the conductive material may be 50 to 95: 1 to 25: 1 to 25, respectively. If the weight ratio of the negative active material is less than 50, the lifespan characteristics and conductivity of the battery may be deteriorated, and if it exceeds 95, the content of the binder and the conductive material may be relatively decreased, thereby reducing adhesion.

The type of the current collector is not particularly limited as long as it has conductivity without causing a chemical change in the battery. For example, the current collector may be copper, stainless steel, aluminum, nickel, titanium, calcined carbon, a copper or stainless steel surface treated with carbon, nickel, titanium, silver, etc., or an aluminum-cadmium alloy. . In addition, by forming fine irregularities on the surface, the bonding strength of the negative electrode active material can be strengthened, and it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwovens. The current collector may have a thickness of 3 μm to 500 μm.

The secondary battery may have a structure in which an electrolyte containing sodium salt is impregnated in an electrode assembly having a structure in which a separator is interposed between a positive electrode and a negative electrode.

The separator is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength may be used. The pore diameter of the separator is generally 0.01 μm to 10 μm, and the thickness may be 5 μm to 300 μm. For example, the separator may include an olefin-based polymer such as chemical resistance and hydrophobic polypropylene; It may be a sheet or non-woven fabric made of glass fiber or polyethylene. In addition, when a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may serve as a separator.

The sodium salt-containing electrolyte is composed of a sodium salt and a solvent, and may optionally further include an additive. The solvent of the electrolyte may be aqueous or non-aqueous, but is not limited thereto. The electrolyte may be an organic, inorganic liquid or solid electrolyte, but is not limited thereto.

The additive is fluoroethylene carbonate (FEC), tertiary-difluoroethylene carbonate (t-DFEC), vinyl carbonate (VC), ethylene sulfite (ethylene sulfite, ES) ) and the like, but is not limited thereto.

An example of the solvent is a non-aqueous organic solvent, and the non-aqueous organic solvent is N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butylo Lactone, 1,2-dimethoxyethane, tetrahydroxyfranc, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile , nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivative, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydro It may be an aprotic organic solvent such as a furan derivative, ether, methyl pyropionate, or ethyl propionate, but is not limited thereto.

An example of the electrolyte is an organic solid electrolyte, and the organic solid electrolyte is a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, a poly agitation lysine, polyester sulfide, polyvinyl alcohol. , polyvinylidene fluoride, may be a polymerization agent including an ionic dissociation group, but is not limited thereto.

An example of the electrolyte is an inorganic solid electrolyte, and the inorganic solid electrolyte is Na ₂ S-SiS ₂ , Na ₂ S-GeS ₂ , NaTi ₂ (PO ₄ ) ₃ , NaFe ₂ (PO ₄ ) ₃ , Na ₂ ( SO ₄ ) ₃ , Fe ₂ (SO ₄ ) ₂ (PO ₄ ), Fe ₂ (MoO ₄ ) ₃ It may be, but is not limited thereto.

Examples of the sodium salt include NaClO ₄ , NaPF ₆ , NaAsF ₆ , NaSbF ₆ , NaBF ₄ , NaCF ₃ SO ₃ , NaN(SO ₂ CF ₃ ) ₂ , lower aliphatic sodium carboxylate salt, NaAlCl ₄ and the like. , a mixture of two or more thereof may be used. Among these, it is preferable to use one containing at least one selected from the group consisting of fluorine-containing NaPF ₆ , NaAsF ₆ , NaSbF ₆ , NaBF ₄ , NaCF ₃ SO ₃ and NaN(SO ₂ CF ₃ ).

1 is a schematic view showing a method of manufacturing a negative active material for a sodium secondary battery according to an embodiment of the present specification. Referring to FIG. 1 , a method for preparing a negative active material for a sodium secondary battery includes (a) mixing a tin precursor and a selenium precursor in a solvent; (b) adding the mixture of (a) to a sealed reactor and reacting at 150 to 200° C. for 10 to 15 hours to prepare tin-selenide; and (c) mixing and dry grinding with the tin-selenide and carbon precursor, followed by calcining in a nitrogen atmosphere.

In step (a), the tin precursor and the selenium precursor are each added to a solvent and stirred to prepare each solution, for example, a solution A containing a tin precursor and a solution B containing a selenium precursor, and then A uniform mixture can be prepared by mixing solutions A and B.

The mixture prepared in step (a) is put into the sealed reactor in step (b), and reacted at 150 to 200° C. for 10 to 15 hours to prepare tin-selenide. After the reaction in the reactor is completed, the product may be cooled to room temperature, and the product may further include a washing step of washing in a washing solution. By drying under the conditions, black tin-selenide powder can be prepared.

The conventional method of synthesizing selenide uses a selenium precursor in which selenium powder is dissolved in a hydrazine solution, but hydrazine hydrate has a problem in that it adversely affects health due to its strong toxicity. In step (a), each solution is prepared by adding a tin precursor and a selenium precursor to a solvent, and the solution is mixed, and in step (b), it is reacted at 150 to 200° C. and 10 to 15 hours. By preparing tin-selenide, it is possible to prepare tin-selenide without the use of hydrazine hydrate. In addition, if tin-selenide is prepared using unstable hydrazine hydrate, the rate characteristics may be deteriorated when applied as an active material, but the synthesis method not only produces tin-selenide in an environmentally friendly manner, but also produces tin-selenide. Structural stability can also be improved.

In step (c), the tin-selenide powder and carbon precursor, for example, dopamine hydrochloride, are put in a mortar, and physical grinding is performed without a separate solvent for 0.5 to 2 hours, making it simpler than before. Mixtures can be prepared. After pulverization, the anode active material may be prepared by calcining in a nitrogen atmosphere, and a nitrogen-doped carbon-based compound coating layer may be formed on at least a portion of the tin-selenide surface of the anode active material. Meanwhile, the calcination of step (c) may be performed at 400 to 600° C. for 1 to 3 hours, and the calcination temperature may be performed under conditions of increasing 5° C. per minute.

Hereinafter, embodiments of the present specification will be described in more detail. However, the following experimental results describe only representative experimental results among the above examples, and the scope and content of the present specification may not be construed as reduced or limited by the examples. Each effect of the various embodiments of the present specification not explicitly presented below will be specifically described in the corresponding section.

Tin selenide/NC may be manufactured according to an embodiment of the present invention below, but the scope of the present invention is not limited thereto. /NC means a state in which N-doped carbon is coated on the tin selenid.

sample

Tin (IV) chloride pentahydrate (Tin (IV) chloride pentahydrate, SnCl ₄ 5H ₂ O), ethyl alcohol, and ethylene glycol were purchased from Daejeong Chemical, and dopamine hydrochloride and oleylamine were purchased from Sigma Aldrich. , selenium dioxide (SeO ₂ ) was purchased from Kanto Chemicals. All chemicals met analytical grade and were used without further purification.

Example

Solution A was prepared by adding 25 mM SnCl ₄ ·5H ₂ O to 30 ml of ethyl alcohol with continuous stirring. 50 mM SeO ₂ was added to 45 ml of ethylene glycol with continuous stirring, and then 5 ml of oleylamine was added to prepare a transparent yellow solution B. Solution A was added dropwise to solution B and stirred for 15 minutes to prepare a homogeneous solution. The prepared mixed solution was transferred to a Teflon-lined autoclave and heated at 180° C. for 12 hours. After the reaction was completed, the autoclave was cooled to room temperature to collect a black product.

The collected black product was washed with toluene:ethyl alcohol 1:2 mixed solution until the supernatant became colorless. After washing, the product was dried at 80° C. for 12 hours to obtain black tin selenide powder. 0.3 g of Tin selenide powder and 0.074 of dopamine hydrochloride were physically mixed and ground using a mortar and pestle. The pulverized powder was calcined at 500° C. for 2 hours at a rate of 5° C./min in a nitrogen atmosphere to prepare nitrogen-doped carbon-coated tin selenide/NC powder.

comparative example

Tin selenide powder without coating was prepared.

Experimental Example 1: XRD and Raman spectrum analysis

2 is an XRD and Raman spectrum analysis result of a negative active material for a sodium secondary battery according to an embodiment of the present specification. For XRD analysis, crystal structure was analyzed using X-ray diffraction (XRD, M18XHF-SRA, Mac Science) of Cu-Kα radiation (1.5406 Å), and Raman spectrum analysis was performed using HR-Raman spectrometers (inVia Raman microscopes). and analyzed.

Referring to FIG. 2( a ), an XRD pattern of tin selenide, tin selenide/NC, and nitrogen-doped carbon (N-doped carbon) is shown. The nitrogen-doped carbon did not show a peak, and it can be confirmed that this is due to the nitrogen-doped carbon being amorphous. In addition, Tin selenide is JCPDS no. 48-1224 (SnSe) as well as JCPDS no. A peak of 23-0602 (SnSe ₂ ) was observed. According to the standard diffraction peak, it can be confirmed that the prepared sample contains a mixed phase of orthorhombic SnSe and hexagonal SnSe ₂ . Tin selenide/NC showed a diffraction peak similar to that of Tin selenide, and the peak of nitrogen-doped carbon did not appear due to the formation of amorphous carbon. The sharp diffraction peaks of Tin selenide and Tin selenide/NC indicate well-formed crystals.

In order to confirm the presence of carbon, Raman analysis of tin selenide/NC was performed, which is shown in FIG. 2(b). Referring to FIG. 2(b), the characteristics of E _g and A _1g modes of Tin selenide/NC were 116 cm ^-1 and 185 cm ^-1 , respectively. In contrast, the peaks located at 1350 cm ^-1 and 1596 cm ^-1 confirm that the D and G bands were formed due to the vibration in the plane of the sp ² bonded carbon atom and the vibration mode of the sp ³ bonded carbon atom in the amorphous carbon. . Accordingly, the presence of carbon in Tin selenide/NC can be confirmed by Raman analysis.

Experimental Example 2: XPS analysis

3 is an XPS analysis result of the anode active material for a sodium secondary battery according to an embodiment of the present specification, and specifically, X-ray photoelectron spectroscopy (XPS, MultiLab2000, Thermo VG) using monochromatic Al X-rays (Al-Kα line: 1486.6 eV) Scientific System) was used to analyze the composition and chemical balance of the element surface.

In FIG. 3(a), it can be seen a scan spectrum indicating the presence of Sn, Se, C, and N elements in Tin selenide/NC. Figure 3(b) shows the core level spectrum of Sn 3d, and two peaks are observed at 486.8 eV and 495.3 eV, and it can be confirmed that this is due to the photoelectron emission of Sn 3d _5/2 and Sn 3d _3/2 , respectively. have. According to the previously reported literature on SnSe and SnSe ₂ , it can be confirmed that the partial occurrence of the oxidation state of Sn ²⁺ and Sn ⁴⁺ in Tin selenide/NC coincides.

In Fig. 3(c), the peaks located at 54.1 eV and 55.2 eV indicate the photoelectron emission of Se 3d, which means the Se ^2- oxidation state of Se. 3(d) and (e) show the C 1s and N 1s peaks of the nitrogen-doped carbon matrix present in tin selenide/NC. The C 1s spectrum shows peaks at 284.5 eV, 285 eV, and 286 eV, and it can be confirmed that CeC, CeN, and CeO are used, respectively. In addition, it can be confirmed that the N 1s core level spectrum includes peaks of pyridinic-N, pyrrolic-N, and graphitic-N. The presence of pyridinic-N and graphitic-N in the tin selenide/NC reduces the resistance interface when the tin selenide/NC is applied to a sodium secondary battery, and reduces sodium ion diffusion It is expected to improve the physical properties of tin selenide/NC by improving it.

Experimental Example 3: SEM and EDX analysis

4 is a HR FE-SEM and EDX analysis result of an anode active material for a sodium secondary battery according to an embodiment of the present specification, and a high-resolution field emission scanning electron microscope (HR) equipped with an energy dispersive X-ray spectrometer (EDX, Oxford Instruments). FE-SEM, MERLIN, Carl Zeiss) were used for analysis.

4(a) to 4(d), it can be confirmed that the tin selenide/NC was formed into nanoflakes with non-uniform agglomerated sizes, and the average thickness of the nanoflake particles was about 20 nm. can be checked

In a sodium secondary battery, the layered crystal structure according to the nanoflake structure may be advantageous for electrochemical performance. Specifically, the nano-flake structure significantly increases the electrode contact area in order to provide more transport channels and active regions for sodium ion diffusion, thereby improving the speed, thereby implementing improved cycling.

4(g) to (j) show the elemental composition of Tin selenide/NC.

4(g) to (j), it can be seen that the elements of tin selenide/NC are uniformly distributed throughout. In particular, the uniform distribution of the nitrogen-doped carbon matrix in tin selenide/NC can not only improve the conductivity but also reduce the extreme volume change and agglomeration that may occur during the alloying/dealloying reaction.

Experimental Example 4: TEM analysis

5 is a FE-TEM and HR FE-TEM analysis result of the anode active material for a sodium secondary battery according to an embodiment of the present specification, and was analyzed using a field emission transmission electron microscope (FE-TEM, JEOL JEM-2100F).

5(a) to (e) are FE-TEM images of tin selenide/NC, confirming that tin selenide/NC forms aggregated nanoflakes. The crystal plane of the nanoflakes can be analyzed by irradiating a Fourier transform (FT) pattern and a FE-TEM to obtain a selected-area electron diffraction (SAED) pattern.

5(e) is a plan view of the nanoflakes corresponding to the (101) plane of the hexagonal SnSe ₂ , and it can be confirmed that the parallel lattice pattern has an interval between the planes of about 0.29 nm. In particular, the limited-field electron diffraction pattern at the diffraction point clearly shown in Fig. (e) indicates that Tin selenide/NC has polycrystalline properties. In addition, it can be confirmed that diffraction points along the (411) and (311) crystal planes represent orthorhombic SnSe, and the (101) crystal plane represents hexagonal SnSe ₂ .

Accordingly, it can be confirmed that SnSe and SnSe ₂ phases were properly formed in Tin selenide/NC.

Experimental Example 5: Analysis of pore size

6 is a nitrogen adsorption/desorption isotherm graph and BJH pore size analysis result of a negative active material for a sodium secondary battery according to an embodiment of the present specification, a surface analyzer (BELSORP-max, MP), a Brunauer-Emmett-Teller (BET) and The average pore diameter, distribution and specific surface area were analyzed using the Barrett-Joyner-Halenda (BJH) method.

Fig. 6(a) shows the adsorption/desorption isotherm of tin selenide/NC, and the isotherm shows a type IV hysteresis curve, and thus it can be confirmed that tin selenide/NC contains typical mesoporous pores. It can be seen that the average pore diameter and specific surface area are respectively 14.9 nm and 21.6 m 2 g ^-1 . The mesoporous structure can facilitate sodium ion kinetics as well as enhance electrolyte diffusion into the electrode while cycling is performed.

Thermogravimetric analysis (TGA) was performed at 30-900°C conditions at a ramping rate of 10°C per minute in the atmosphere to analyze the amount of nitrogen-doped carbon present in tin selenide/NC. The weight loss rates of tin selenide/NC and tin selenide were 34.8% and 50.1%, respectively, and the amount of nitrogen-doped carbon in tin selenide/NC was 23.4%. The electrical conductivity of tin selenide/NC and tin selenide was measured with a 4-point probe It was measured by the method (4-point probe method, MCP-T610, Mitsubishi). The electrical conductivity of tin selenide at room temperature was 4.6 x 10 ^-6 Scm ^-1 , and the electrical conductivity of tin selenide/NC was 2.9 x 10 ^-4 Scm ^-1 . Accordingly, it can be confirmed that the electrical conductivity of Tin selenide/NC was improved as the nitrogen-doped carbon was coated.

production example

Tin selenide/NC (negative electrode active material), Denka black (conductive material), and carboxymethyl cellulose (CMC, binder) prepared according to the example were dissolved in purified water in a weight ratio of 80: 10: 10, respectively, to form a slurry After manufacturing, it is applied to copper foil using the doctor blade method, dried naturally, dried in a drying oven at 80° C. for 5 hours to remove residual moisture, rolled with a roll press, and punched into a circular disk, The electrode was prepared by maintaining it at 80°C under a vacuum atmosphere for 3 hours, and a coin-type CR2032 type cell was polished as a reference electrode using the prepared electrode disk, and due to a separator made of glass fiber (Whatman, GF/D), A negative electrode for a blocked sodium secondary battery was prepared.

An electrode assembly was prepared using the prepared negative electrode, pure sodium metal foil as the positive electrode, and a separator made of glass fiber (Whatman, GF/D). After putting the electrode assembly in a pouch and connecting a lead wire, 5 wt% of fluoroethylene carbonate (FEC) is added to an ethylene carbonate (EC) and propylene carbonate (PC) solution in a volume ratio of 1:1 in which 1M NaClO ₄ salt is dissolved. Sodium secondary battery was assembled by injecting the solution contained as an electrolyte and sealing it in an argon glove box in which humidity and oxygen content were maintained at 5 ppm or less. All electrochemical measurements were performed at 25°C unless otherwise specified.

Comparative Preparation Example

Tin selenide (cathode active material), Denka black (conductive material), and carboxymethyl cellulose (CMC, binder) prepared according to Comparative Example were dissolved in purified water in a weight ratio of 80: 10: 10 to prepare a slurry. After that, it is applied on copper foil using a doctor blade method, dried naturally, dried in a drying oven at 80° C. for 5 hours to remove residual moisture, and then rolled with a roll press, punched on a circular disk, and vacuum atmosphere The electrode was prepared by maintaining it at 80°C for 3 hours under A negative electrode for a sodium secondary battery was prepared.

An electrode assembly was prepared using the prepared negative electrode, pure sodium metal foil as the positive electrode, and a separator made of glass fiber (Whatman, GF/D). After putting the electrode assembly in a pouch and connecting a lead wire, 5 wt% of fluoroethylene carbonate (FEC) is added to an ethylene carbonate (EC) and propylene carbonate (PC) solution in a volume ratio of 1:1 in which 1M NaClO ₄ salt is dissolved. Sodium secondary battery was assembled by injecting the solution contained as an electrolyte and sealing it in an argon glove box in which humidity and oxygen content were maintained at 5 ppm or less. All electrochemical measurements were performed at 25°C unless otherwise specified.

Experimental Example 6: Performance evaluation of secondary battery 1

7 is a graph and performance evaluation graph of differential capacity (dQ/dV) of a discharge curve of a secondary battery including a negative electrode for a sodium secondary battery according to an embodiment and a comparative example of the present specification, and the evaluation of the charge/discharge capacity performance is an electrochemical Analysis was performed at 0.01 to 3.0 V using a cycler (Battery Tester 05001, HTC).

7(a) and (b) are graphs of differential capacity (dQ/dV) of discharge curves of sodium secondary batteries including negative electrodes of Preparation Examples and Comparative Examples, respectively, and were performed in the range of 0.01 to 3.0V. The initial reduction process has reduction peaks at 1.0V and 0.78V, which are analyzed to be caused by conversion and alloying reactions and formation of a solid electrolyte interface (SEI) layer. However, the reduction peak shown by the tin selenide/NC electrode appears sharper than the reduction peak of the tin selenide electrode, which is due to the increase in the formation of the solid electrolyte interface as nitrogen-doped carbon is combined in the tin selenide/NC electrode. is considered to be followed.

In addition, as the number of cycles increases, the sharp peak decreases, which confirms that the formation of the solid electrolyte interface is stable and limited in the first cycle. After the first cycle, two distinct peaks appear in the reduction region, which is analyzed to indicate excellent reversibility as the solid electrolyte interfacial layer is stably formed on the tin selenide/NC electrode. In addition, the peak observed at 1V is indicated by decomposition of tin selenide into Sn and Na ₂ Se, which can be confirmed by the peaks observed at ~0.78V and ~0.2V in the Sn and Na alloy reaction.

On the other hand, according to the oxidation region, it can be confirmed that oxidation peaks appear at 0.2 V and 0.66 V, which is analyzed to be due to the formation of metal Sn due to the dealloying reaction of Na _3.75 Sn. In addition, the peaks appearing at ~1.4V and ~1.5V are analyzed to be due to the reversible conversion reaction and the formation of tin selenide.

The overall electrochemical reaction mechanism is shown in the following formulas (1) to (4).

(1) SnSe + 2Na ⁺ + 2e ^- ↔ Sn + Na ₂ Se

(2) Sn + 3.75Na ⁺ + 3.75e ^- ↔ Na _3.75 Sn

(3) SnSe ₂ + 4Na ⁺ + 4e ^- ↔ Sn + 2Na ₂ Se

(4) Sn + 3.75Na ⁺ + 3.75e ^- ↔ Na _3.75 Sn

On the other hand, it can be confirmed that all redox peaks after the first cycle almost overlap, indicating the reversible electrochemical behavior of the tin selenide/NC electrode. In addition, it can be seen that the differential capacity graph of the tin selenide electrode shows a similar peak with slight fluctuations and unstableness compared to the tin selenide/NC electrode, and the intensity of the peak decreases with the number of cycles, which is lower than that of the tin selenide electrode. Cycling performance.

7(c) and (d) show the charging/discharging characteristics for 1, 2, 10, and 50 cycles at a voltage range of 0.01 to 3.0 V and a current density of 200 mAg ^-1 of the tin selenide electrode and the tin selenide/NC electrode. . The tin selenide electrode and the tin selenide/NC electrode show similar electrochemical reactions, and it is analyzed that the plateau in the charge/discharge characteristic corresponds to the peak observed in the differential capacity graph.

On the other hand, it can be seen that the initial discharge capacity of the tin selenide electrode was 436 mAhg ^-1 , and the initial discharge capacity of the tin selenide/NC electrode was 428 mAhg ^-1 . As such, it can be seen that the initial discharge capacity of the tin selenide/NC electrode is slightly lower than that of the tin selenide electrode. It is analyzed that the presence of the nitrogen-doped carbon matrix does not affect the capacity improvement, but improves the problems caused by volume expansion/contraction during the alloying/dealloying reaction and helps to improve the conductivity. Accordingly, it can be confirmed that the charging and discharging characteristics of the tin selenide/NC electrode in 1 to 50 cycles are similar due to the improved sodium storage property of the reversibility. On the other hand, it can be seen that the charge and discharge characteristics of the tin selenide electrode show a distinct change, which is analyzed to be due to the deterioration of the structural stability and reversibility of the electrode during the electrochemical reaction.

7(e) and (f) show the cycling performance of the tin selenide electrode and the tin selenide/NC electrode at a current density of 200 mAg ^-1 . The initial discharge capacity of the tin selenide electrode was 463 mAhg ^-1 , and as the number of cycles increased, the charge and discharge capacity was 99 mAhg ^-1 in the 100th cycle. On the other hand, the initial discharge capacity of the tin selenide/NC electrode was 460 mAhg ^-1 , and as the number of cycles increased, the current density of 200 mAg ^-1 converged to a charge and discharge capacity of 348 mAhg ^-1 until the 100th cycle was completed. , it can be confirmed that this is a value 3.5 times higher than that of the tin selenide electrode.

Experimental Example 7: Performance evaluation of secondary battery 2

8 is a performance evaluation graph of a secondary battery including an anode for a sodium secondary battery according to an embodiment and a comparative example of the present specification.

8(a) shows the cycling performance at a current density of 400 mAg ^-1 of the tin selenide/NC electrode, and shows a discharge capacity of 260 mAhg ^-1 in the 150th cycle. Accordingly, it can be confirmed that the tin selenide/NC electrode has improved electrochemical performance as the number of cycles and current density, which indicate the cycleability of the electrode, increased, and the percentage of carbon doped with nitrogen was increased to obtain improved electrical conductivity. By doing so, it can be confirmed that the electrochemical performance capable of maintaining the capacity can be improved. In addition, while the first cycle showed a coulombic efficiency of 95%, it can be confirmed that the coulombic efficiency increased to 98% after the first cycle due to the stable formation of the solid electrolyte interfacial layer after that. Accordingly, it can be predicted that the nano flake structure and the nitrogen-doped carbon matrix present in tin selenide/NC will improve the electrochemical properties when applied as an anode for a sodium secondary battery.

FIG. 8(b) is for evaluating the rate characteristics of the tin selenide electrode and the tin selenide/NC electrode, and was performed in a current density range of 50 to 1600 mAg ⁻¹ . In detail, the Tin selenide / NC electrode applied 50mAg ^-1 , 100 mAg ^-1 , 200 mAg ^-1 , 400 mAg ^-1 , 800 mAg ^-1 and 1600 mAg ^-1 currents, and the results were respectively 503 mAhg ^-1 , 405 mAhg ^-1 , 377 mAhg ^-1 , 334 mAhg ^-1 , 285 mAhg ^-1 , and 234 mAhg ^-1 , indicating excellent rate characteristics. In addition, it can be confirmed that the reversibility is excellent by showing a reversible capacity of 507 mAhg ^-1 without loss of capacity when a current of 50 mAg ^-1 is re-applied. This is analyzed to be due to the excellent reversibility and structural stability of the tin selenide/NC electrode even after cycling at a high current density of 1600 mAg ^-1 . In contrast, the tin selenide electrode exhibited a discharge capacity of 39 mAhg ^-1 when a current of 1600 mAg ^-1 was applied, and a reversible discharge capacity of 362 mAhg ^-1 when a current of 50 mAg ^-1 was re-applied, indicating low reversibility. have.

Fig. 8(c) shows a graph of charging and discharging characteristics according to current density, and it can be seen that the tin selenide/NC electrode exhibits high charging and discharging capacity even at high current density. On the other hand, it can be seen that the tin selenide electrode exhibited a charge/discharge capacity lower than 80% at a current density of 1600 mAg ^-1 . This difference in electrochemical performance is due to the nitrogen-doped carbon matrix, and it is analyzed that the electrochemical performance of the tin selenide electrode is lowered due to low conductivity and volume expansion and contraction during cycling.

8 (d) and (e) are the results of EIS (Electrochemical Impedance System) measurement and analysis to evaluate the electrochemical performance of the tin selenide electrode and the tin selenide/NC electrode, respectively, with an amplitude of 5 mV and a frequency of 100 kHz to 0.01 Hz The range was analyzed using an electrochemical analyzer (Iviumstat, Ivium Technologies). EIS plots of fresh cells and aged cells for 1, 2, and 3 days under open circuit potential (OCP) for tin selenide electrodes and tin selenide/NC electrodes are shown. It can be seen that all EIS profiles contain one semicircle in the high frequency region according to the charge transfer resistance (R _ct ) and exhibit a linear behavior in the low frequency region known as the Warburg impedance. In particular, in Fig. 8(e), it can be seen that the charge transfer resistance (R _ct ) values of the tin selenide / NC electrode represent 1015 Ω, 964 Ω, 686 Ω and 230 Ω in the fresh cell, 1, 2, and 3 days of aging, respectively. have. In contrast, in FIG. 8(d), it can be seen that the charge transfer resistance (R _ct ) value of the tin selenide electrode is 1977Ω, 1612Ω, 987Ω, and 676Ω in the fresh cell, 1, 2, and 3 days aging cells, respectively. .

In addition, the radius of the semicircle for the tin selenide electrode and the tin selenide/NC electrode decreased with time from the fresh cell to the cells aged 1, 2, and 3 days, so that the wettability increased, resulting in an aging-related charge transfer resistance (R _ct ) value decreased. The tin selenide/NC electrode can easily transfer the charge provided by the improved conductivity according to the nitrogen-doped carbon matrix, so it can be confirmed that the charge transfer resistance (R _ct ) value is lower than that of the tin selenide electrode, which It is analyzed that the improved electrode integrity is due to the improved electronic contact between the active materials as the nitrogen-doped carbon matrix is formed into a coated composite.

In general, boundary diffusion in EIS spectra corresponds to a low frequency region, and such boundary diffusion is mostly determined by the size and shape of particles. In addition, as shown in FIG. 8(f), the Warburg impedance is measured based on the fresh cells of the tin selenide/NC electrode and the cells aged for 1, 2, and 3 days. Referring to FIG. 8(f), the tin selenide/NC electrode exhibited a lower Warburg impedance value as the aging period increased, which is analyzed to be due to the improved diffusivity of sodium ions during the charge/discharge cycle. , it can be confirmed that the nitrogen-doped carbon matrix plays an important role in improving the electrochemical performance of the sodium secondary battery.

In order to confirm the structural stability of the electrode, the electrode was analyzed after 20 cycles. The tin selenide electrode experienced volume expansion/contraction after 20 cycles, resulting in numerous cracks. Tin selenide/NC electrodes did not crack during 20 cycles, unlike tin selenide electrodes. Tin selenide can reduce the lifespan characteristics of the electrode due to volume expansion/contraction and low conductivity. can

The description of the present specification described above is for illustration, and those of ordinary skill in the art to which one aspect of the present specification belongs can easily transform it into other specific forms without changing the technical idea or essential features described in this specification. you will be able to understand Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive. For example, each component described as a single type may be implemented in a dispersed form, and likewise components described as distributed may be implemented in a combined form.

The scope of the present specification is indicated by the following claims, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be construed as being included in the scope of the present specification.

Claims (11)

tin-selenide; and
A negative active material for a sodium secondary battery comprising a; nitrogen-doped carbon-based compound bonded to at least a portion of the tin-selenide surface. According to claim 1,
The tin-selenide is an anode active material for a sodium secondary battery comprising SnSe and SnSe ₂ . According to claim 1,
The negative active material is an anode active material for a sodium secondary battery in the form of nanoflakes. According to claim 1,
The negative active material for a sodium secondary battery having an average thickness of 10 to 30 nm of the negative active material. According to claim 1,
The negative active material for a sodium secondary battery having an average pore diameter of 10 to 20 nm of the negative active material. A negative electrode for a sodium secondary battery in which a slurry comprising the negative active material for a sodium secondary battery according to any one of claims 1 to 5, a binder, and a conductive material is applied to a current collector. 7. The method of claim 6,
The binder is polyvinylidene fluoride, polyvinyl fluoride, polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene (EPDM) ) At least one negative electrode for a sodium secondary battery selected from the group consisting of rubber, styrene-butyrene rubber, and fluororubber. (a) mixing the tin precursor and the selenium precursor in a solvent;
(b) adding the mixture of (a) to a sealed reactor and reacting at 150 to 200° C. for 10 to 15 hours to prepare tin-selenide; and
(c) mixing with the tin-selenide and carbon precursor, dry grinding, and calcining in a nitrogen atmosphere; 9. The method of claim 8,
In the step (c), a nitrogen-doped carbon coating layer is formed on at least a portion of the tin-selenide surface. 9. The method of claim 8,
The step (c) is a method of manufacturing a negative active material for a sodium secondary battery performed at 400 ~ 600 ℃, 1-3 hours conditions. 9. The method of claim 8,
The method for producing a negative active material for a sodium secondary battery, wherein the calcination of step (c) is performed under the condition that the temperature is increased by 5° C. per minute.

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