KR20210099501A - Transition metal substituted Sodium vanadium fluorophosphate/Sodium Vanadium Phosphate composite for a Sodium Ion Storage Material - Google Patents

Abstract

The present invention relates to a sodium ion storage material of a doped compound, and an electrode material for a sodium ion battery including the sodium ion storage material, an electrode for the sodium ion battery, and the sodium ion battery, capable of improving a discharge capacity while representing an excellent charging/discharging cycle. In detail, the present invention relates to the sodium ion storage material including a compound comprising a (Na_3V_(2-x)M_x(PO_4)_2F_3)/(Na_3V_(2-y)M_y(PO_4)_3) complex body. Accordingly, when the sodium ion storage material is used, the vanadium content is reduced while the high discharge capacity is maintained. When the sodium ion storage material is applied to the sodium ion battery, which is a secondary battery, and a sodium-magnesium hybrid ion battery, the excellent charging/discharging cycle characteristic may be represented.

Description

Transition metal substituted Sodium vanadium fluorophosphate/Sodium Vanadium Phosphate composite for a Sodium Ion Storage Material}

The present invention relates to a sodium ion storage material of a doped compound and an electrode material for a sodium ion battery comprising the same, an electrode for a sodium ion battery and a sodium ion battery.

A secondary battery based on sodium, which has a low production cost and is widely distributed on earth, can be said to be the most suitable energy storage source for the next-generation energy storage system. In addition, sodium is an element belonging to the same group as lithium in the periodic table, and in terms of its electrochemical behavior and production method, it has the advantage that the production base used in the existing lithium secondary battery can be used as it is.

Recently, research on sodium based secondary batteries using sodium instead of lithium is being re-evaluated. Since sodium has abundant resource reserves, if a secondary battery using sodium instead of lithium can be manufactured, the secondary battery can be manufactured at low cost. In addition, nickel and cobalt may not be used in a cathode material for a sodium ion battery, unlike nickel and cobalt at high cost in a cathode material for a lithium ion battery.

However, compared to the conventional lithium secondary battery, the sodium secondary battery shows low characteristics in terms of output and energy storage density due to the difference in physical characteristics. In the case of an energy storage system, it is a device that stores power generated by solar cells or wind power generation at a time when the demand is small and supplies energy during a time when the demand for energy surges. It is important to build a large-capacity storage system at a cost.

Therefore, for the commercialization of such a sodium ion battery, research on a cathode active material, which is an important material, is being actively conducted. In particular, polyanion-based Na ₃ V ₂ (PO ₄ ) ₂ F ₃ (NVPF), Na ₃ V ₂ O ₂ (PO ₄ )F (NVOPF), And, Na ₃ V ₂ (PO ₄ ) ₃ (NVP) has been intensively studied. However, these three materials contain a large amount of vanadium, and the high price of vanadium is a big obstacle to commercialization. The price of vanadium is similar to that of nickel. Therefore, it is essential to reduce the vanadium content for commercialization.

Korean Patent Publication No. 10-2015-0045818 (published on April 29, 2015)

SUMMARY OF THE INVENTION An object of the present invention is to provide a sodium ion storage material capable of exhibiting excellent charge/discharge cycle characteristics while reducing vanadium content and improving discharge capacity.

Another object of the present invention is to provide an electrode material for a sodium ion battery capable of exhibiting excellent charge/discharge cycle characteristics while improving discharge capacity.

Another object of the present invention is to provide a sodium ion battery and a magnesium-sodium mixed ion battery capable of exhibiting excellent charge/discharge cycle characteristics while improving discharge capacity.

Other objects and advantages of the present invention will become more apparent from the following detailed description of the invention, claims and drawings.

According to an embodiment of the present invention, Na ₃ V _2-x M _x (PO ₄ ) ₂ F ₃ /Na ₃ V _2-x M _x (PO ₄ ) ₃ (M = Fe,Zn,Cr,Mn,Cu , Zn, 0 < x ≤ 2) relates to a sodium ion storage material containing.

In the present invention, the electrode material may further include a binder.

in the present invention The binder is polyvinylidene fluoride, polyvinyl alcohol, polyacrylic acid, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butyrene rubber, and may include at least one selected from the group consisting of fluororubber.

In the present invention, the electrode material may further include a conductive material.

According to another embodiment of the present invention, it relates to an electrode for a sodium ion battery comprising the electrode material according to the present invention.

According to another embodiment of the present invention, it relates to a sodium ion battery comprising the electrode and the electrolyte according to the present invention.

By using a sodium ion storage material and an electrode material for sodium ion batteries that lower the content of vanadium, which is an expensive raw material by doping a heterogeneous element of the present invention, high discharge capacity can be maintained, and furthermore, when applied to a sodium ion battery, which is a secondary battery, excellent charge Discharge cycle characteristics may be exhibited.

1 (a) shows an XRD graph of a material having an Fe content of 8.9 at% among the electrode materials prepared in Example 1 of the present invention.
Figure 1 (b) shows a low magnification transmission electron micrograph of the electrode material according to the present invention.
Figure 1 (c) shows a high magnification transmission electron micrograph of the electrode material according to the present invention.
Figure 1 (d) shows the ratio of NVPF, NVP, and Na ₅ V _1-x Fe _x (PO ₄ ) ₂ F ₂ in the electrode material according to the present invention.
Figure 1 (e) shows the elemental mapping of NVPF / NVP containing 8.9% Fe according to the present invention.
Figure 2 (a) shows the electrode material charging and discharging profile (profile) according to the Fe content of the compound prepared in Example 1 of the present invention at 0.5 C.
2 (b) shows the current density of the electrode material according to the Fe content of the compound prepared in Example 1 of the present invention (0.5 C, 1 C, 3 C, 5 C, 10 C, 1 C = 128 mAh/g) The change in charge capacity and discharge capacity according to the graph is shown.
Figure 2 (c) is a graph showing the change in charge capacity and discharge capacity according to the number of cycles at 10 C of the electrode material (0, 4.9%, 8.9%) according to the Fe content of the prepared compound.
Figure 2 (d) is a graph showing changes in the average discharge voltage, discharge capacity, and energy density of the electrode material according to the Fe content of the compound prepared in Example 1 of the present invention.
3 is a graph showing changes in charge capacity and discharge capacity according to the number of cycles of the electrode material doped with Fe, Ti, Cu, Zn, Cr and Mn of the compound prepared at 0.5 C.
Figure 4 (a) shows the cyclic voltammetry profile of the electrode material without Fe added at voltage scan rates of 0.5 mV/s, 1 mV/s, 3 mV/s, and 5 mV/s in the 2 V ~ 4.45 V section. it has been shown
4 (b) shows the cyclic voltammetry profile of the electrode material in which Fe is added as much as 0.2 at voltage scan rates of 0.5 mV/s, 1 mV/s, 3 mV/s, and 5 mV/s in the 2 V to 4.45 V section. it has been shown
Figure 4 (c) shows the cyclic voltammetry profile of the electrode material in which Fe is added as much as 8.9% at voltage scan rates of 0.5 mV/s, 1 mV/s, 3 mV/s, and 5 mV/s in the 2V to 4.45 V section. it has been shown
FIG. 4(d) shows the change in the diffusion coefficient of sodium ions in each voltage plateau based on FIGS. 4(a), (b), and (c).
Figure 5 (a) shows the 1st, 3rd, 5th, and 10th charge and discharge profiles at 0.128 mA/g of a CuS-NVPF/NVP (Fe 8.9 %) sodium ion battery.
Figure 5 (b) shows the charge and discharge profiles at 0.128 mA / g, 0.384 mA / g, 0.640 mA / g, 1.28 A / g of the CuS-NVPF / NVP (Fe 8.9 %) sodium ion battery.
Figure 5 (c) shows the cycle stability at 0.128 mA / g, 0.384 mA / g, 0.640 mA / g, 1.28 A / g of the CuS-NVPF / NVP (Fe 8.9 %) sodium ion battery.
Figure 5 (d) shows the superiority of the CuS-NVPF / NVP (Fe 8.9 %) sodium ion battery compared to the previously reported sodium ion battery.
Figure 5 (e) is a graph showing the long-life cycle stability of CuS-NVPF / NVP (Fe 8.9 %).

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiment of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. Further, the embodiments of the present invention are provided in order to more completely explain the present invention to those of ordinary skill in the art.

The inventors of the present invention, when using a doped hetero element as a sodium ion storage material or an electrode material for a sodium ion battery, can exhibit excellent charge/discharge cycle characteristics while reducing the vanadium content and maintaining high discharge capacity unlike before. found

According to an embodiment of the present invention, to a sodium ion storage material, Na ₃ V _2-x M _x (PO ₄ ) ₂ F ₃ /Na ₃ V _2-y M _y (PO ₄ ) ₂ F ₃ (0 <x, y≤2) may include a compound composed of a complex. Na ₃ V _2-x M _x (PO ₄ ) ₂ F ₃ /Na ₃ V _2-y M _y (PO ₄ ) ₂ F ₃ (0<x,y≤2) In the complex, M is a metal, and the complex is The metal is doped. Preferably, M may be a transition metal, more preferably M may be Fe. The transition metal in the composite may be doped in an atomic percent of 0% to 15% compared to the initial amount of vanadium in the composite. In the example of the present invention, up to ~15% was implemented, but it will be obvious that more than 15% can be doped according to experimental conditions.

The conductive carbon may be coated in an amount of more than 2 wt% and 5 wt% or less, preferably 2.5 wt% to 5 wt%, based on the weight of the electrode active material. When the amount of conductive carbon is too large, the amount of the nanoplate is relatively decreased to decrease overall battery characteristics, and when too small, it is not preferable because the effect of improving electrical conductivity cannot be exhibited.

In the present invention, the electrode active material may be coated with a conductive material to further improve electrical conductivity. In the present invention, the conductive material may be at least one selected from the group consisting of conductive carbon, noble metals, and metals. In particular, in the case of coating with conductive carbon, it is preferable because the conductivity can be effectively increased without significantly increasing the manufacturing cost and weight.

In the present invention, the conductive carbon may be at least one selected from the group consisting of carbon black, carbon nanotubes, and graphene, but is not limited thereto.

In the present invention, the conductive carbon may be coated in an amount of more than 2 wt% and 5 wt% or less, preferably 2.5 wt% to 5 wt%, based on the weight of the electrode active material. When the amount of conductive carbon is too large, the amount of the nanoplate is relatively decreased to decrease overall battery characteristics, and when too small, it is not preferable because the effect of improving electrical conductivity cannot be exhibited.

In the present invention, the conductive carbon may be applied to the surface of the electrode active material particles, for example, the surface of the electrode active material particles may be coated to a thickness of 0.1 nm to 20 nm.

In the present invention, in the case of the primary particles coated with the conductive carbon in an amount of 0.5 to 1.5% by weight based on the total weight of the electrode active material, the thickness of the carbon coating layer may be about 0.1 nm to 2.0 nm.

In the present invention, the electrode material may further include a binder and optionally a conductive material in addition to the electrode active material.

In the present invention, the binder is for well adhering the electrode active material particles to each other and improving the binding of the positive electrode active material to the current collector, and the specific type thereof is not particularly limited, but for example, polyvinylidene fluoride, polyvinyl alcohol, Polyacrylic acid, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer ( EPDM), sulfonated EPDM, styrene butyrene rubber, fluororubber, and various copolymers.

In the present invention, the binder may be included in an amount of 1 to 20 parts by weight based on 100 parts by weight of the electrode active material, but is not limited thereto.

The solvent of the binder in the present invention is not particularly limited, but, for example, N-methylpyrrolidone (NMP), acetone, or water may be used.

In the present invention, when the solvent is included in an amount of 1 to 10 parts by weight based on 100 parts by weight of the positive electrode active material, the operation for forming the negative electrode material is easy.

In addition, the electrode material of the present invention may optionally further include a conductive material in order to further improve electrical conductivity. As the conductive material, any one generally used for sodium ion batteries may be used, and examples thereof include carbon-based materials such as carbon black, acetylene black, Ketjen black, and carbon fiber; metal-based substances such as metal powders such as copper, nickel, aluminum, and silver, or metal fibers; A conductive material including a conductive polymer such as a polyphenylene derivative or a mixture thereof can be used. The content of the conductive material may be appropriately adjusted and used.

In addition, in the present invention, it is preferable that the particle size of the conductive material is 2 nm to 1 μm. When the particle size of the conductive material is less than 2 nm, there is a problem in that it is difficult to form a uniform slurry in the electrode manufacturing process, and when it exceeds 1 μm, there is a problem in that the electrical conductivity of the electrode cannot be improved.

In addition, in the present invention, the electrode active material and the conductive material may be included in a weight ratio of 9:1 to 99:1, and more preferably, may be included in a weight ratio of 9:1, but is not limited thereto.

In the present invention, the terms “sodiate” and “sodiate” may refer to a process of adding sodium to an electrode material.

As used herein, the terms "desodiate" and "desodiate" may refer to a process of removing sodium from an electrode material.

In the present invention, the terms “charge” and “charge” may refer to a process of providing electrochemical energy to a battery.

In the present invention, the terms “discharge” and “discharge” may refer to a process of removing electrochemical energy from a battery, for example, when the battery is used to perform a desired operation.

In the present invention, the term "anode" may refer to an electrode (sometimes referred to as a cathode) where electrochemical reduction and sodiumization occurs during the discharge process.

In the present invention, the term “cathode” may refer to an electrode (sometimes referred to as an anode) in which electrochemical oxidation and desodiumization occur during the discharge process.

According to another embodiment of the present invention, it relates to a method for manufacturing an electrode active material according to the present invention, (a) sodium fluoride (Sodium fluoride), ammonium metavanadate (Ammonium metavanadate), ammonium phosphate monobasic (Ammonium phosphate) monobasic) and citric acid are mixed and heated to form a precursor mixture, (b) drying the precursor mixture; and (c) heating the dried precursor mixture.

Each of the above steps in the present invention will be described in detail as follows.

(a) step: after mixing sodium fluoride, ammonium metavanadate, ammonium phosphate monobasic and citric acid, heating to form a precursor mixture can

Here, to synthesize NVPF/NVP doped by 0%, 4.9%, 8.9%, 11.5%, 14.8% of Fe, sodium fluoride may be 3 mmol, ammonium metavanadate 2 mmol, 1.8 mmol, 1.65 mmol, 1.6 mmol, 1.5 mmol, Iron nitrate x mmol (0 mmol, 0.20 mmol, 0.35 mmol, 0.40 mmol, 0.5 mmol), ammonium phosphate 2 mmol, citric acid It may be 1.6 mmol, but is not limited thereto. On the other hand, for the synthesis of NVPF/NVP doped by Ti, Cu, Zn, Cr, and Mn by 2.8%, 3.6%, 1.9%, 1.9%, and 2.6%, respectively, TiO and metal nitrate were used instead of iron nitrate. Each of 0.15 mmol, 0.15 mmol, 0.1 mmol, 0.1 mmol, 0.1 mmol, and 0.1 mmol may be used, but is not limited thereto.

Step (b): The precursor mixture may be dried.

The drying may be performed at 80° C., and then may be additionally performed at 120° C., but is not limited thereto, and the drying may be performed in a vacuum oven, but is not limited thereto.

Step (c): heating the dried precursor mixture may be performed.

The drying may be performed at 300° C., and thereafter may be additionally performed at 650° C., but is not limited thereto, and the heating may be performed in an argon atmosphere, but is not limited thereto.

According to another embodiment of the present invention, it relates to a sodium ion battery comprising the electrode and the electrolyte according to the present invention. When the electrode according to the present invention is a positive electrode, the negative electrode is manufactured by directly coating the negative electrode material containing the negative electrode active material on the current collector, or by casting on a separate support and laminating the negative electrode material film peeled from the support on the current collector. It can be obtained with a negative electrode plate. In the present invention, the negative electrode is not limited to the enumerated form and may be in a form other than the above form.

When the electrode according to the present invention is a positive electrode, a compound capable of reversible intercalation and deintercalation of sodium (a sodium intercalation compound) may be used as the negative active material. A more specific negative electrode active material is not particularly limited, but a sodium-transition metal composite oxide is preferable. As the sodium-transition metal composite oxide, for example, NaMn ₂ O ₄ , NaNiO ₂ , NaCoO ₂ , NaFeO ₂ , NaNi _0.5 Mn _0.5 O ₂ , NaCrO _{2 ,} Na _0.9 Mg _0.05 Ni _0.5 Mn _0.5 O ₂ and Na _0.9 Ca _0.05 It may be at least one selected from the group consisting of _{Ni 0.5} Mn _0.5 O _{2 .} In some cases, two or more kinds of positive electrode active materials may be used in combination.

In the present invention, the negative electrode material may further include a binder, a solvent, and optionally a conductive material in addition to the negative electrode active material layer.

In the present invention, the binder is for well adhering the electrode active material particles of the nanoplate to each other and improving the bonding of the electrode active material to the current collector, and the specific type thereof is not particularly limited, but, for example, polyvinyl alcohol, polyacrylic acid , carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymer including ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoro Loethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, and the like may be used, but is not limited thereto.

In the present invention, the binder may be included in an amount of 1 to 20 parts by weight based on 100 parts by weight of the negative active material, but is not limited thereto.

In the present invention, the type of solvent of the binder is not particularly limited, but for example, N-methylpyrrolidone (NMP), acetone, or water may be used.

In the present invention, the solvent is included in an amount of 1 to 10 parts by weight based on 100 parts by weight of the negative electrode active material, so that the operation for forming the negative electrode material is easy.

In addition, the negative electrode material of the present invention may optionally further include a conductive material in order to further improve electrical conductivity. As the conductive material, any one generally used for sodium ion batteries may be used, and examples thereof include carbon-based materials such as carbon black, acetylene black, Ketjen black, and carbon fiber; metal-based substances such as metal powders such as copper, nickel, aluminum, and silver, or metal fibers; A conductive material including a conductive polymer such as a polyphenylene derivative or a mixture thereof can be used. The content of the conductive material may be appropriately adjusted and used.

In addition, in the present invention, it is preferable that the particle size of the conductive material is 2 nm to 1 μm. When the particle size of the conductive material is less than 2 nm, there is a problem in that it is difficult to form a uniform slurry in the electrode manufacturing process, and when it exceeds 1 μm, there is a problem in that the electrical conductivity of the electrode cannot be improved.

In addition, in the present invention, the negative active material and the conductive material may be included in a weight ratio of 1:9 to 99:1, and more preferably, may be included in a weight ratio of 1:1 to 9:1, but is not limited thereto.

In the present invention, the current collector may generally have a thickness of 3 μm to 500 μm. In the present invention, the current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and may be, for example, aluminum, nickel, titanium, calcined carbon, copper or stainless steel, optionally As a result, their surfaces may be surface-treated with carbon, nickel, titanium, silver, or the like, or may be an aluminum-cadmium alloy. In addition, the bonding strength of the positive electrode active material may be strengthened by forming fine irregularities on the surface, and may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam body, a nonwoven body, and the like.

In the present invention, the electrolyte includes a liquid electrolyte, a solid electrolyte, or a combination thereof, the liquid electrolyte includes a sodium salt, an organic solvent, or a combination thereof, and the solid electrolyte includes a polymer compound. , an inorganic solid electrolyte, or a combination thereof.

In the present invention, when the electrolyte is a liquid electrolyte, it includes an electrolyte salt and a solvent.

In the present invention, the electrolyte salt is specifically sodium-containing hydroxide (eg, sodium hydroxide (NaOH), etc.), borate (eg, sodium metaborate (NaBO ₂ ), borax (Na ₂ B ₄ O ₇ ), boric acid (H ₃ BO ₃ ), etc.), phosphates (eg, trisodium phosphate (Na ₃ PO ₄ ), sodium pyrophosphate (Na ₂ HPO ₄ ), etc.), chloric acid (eg, NaClO _{4 ,} etc.), NaAlCl ₄ , NaAsF ₆ , NaBF ₄ , NaPF ₆ , NaSbF ₆ , NaCF ₃ SO ₃ or NaN(SO ₂ CF ₃ ) ₂ , and the like, and any one or a mixture of two or more thereof may be used.

In the present invention, the electrolyte may include 2 to 5% by weight of the electrolyte salt based on the total weight of the electrolyte.

In addition, in the present invention, the solvent may be used without any particular limitation as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the solvent is an aqueous solvent such as water or alcohol; or a non-aqueous solvent such as an ester solvent, an ether solvent, a ketone solvent, an aromatic hydrocarbon solvent, an alkoxyalkane solvent, or a carbonate solvent. Among these, it can be used individually by 1 type or in mixture of 2 or more types.

In particular, a preferred solvent in the present invention may be an ester solvent, and specific examples include methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, Methyl propionate, ethyl propionate, γ-butyrolactone, decanolide, γ-valerolactone, mevalono and lactone (mevalonolactone), γ-caprolactone (γ-caprolactone), δ-valerolactone (δ-valerolactone), or ε-caprolactone (ε-caprolactone).

More preferably, among the ether-based solvents, diethylene glycol dimethyl ether (diglyme), dibutyl ether, tetraglyme, 2-methyltetrahydrofuran (2-methyltetrahydrofuran), Or tetrahydrofuran (tetrahydrofuran), etc. are mentioned.

Specific examples of the ketone-based solvent in the present invention include cyclohexanone and the like. Specific examples of the aromatic hydrocarbon-based organic solvent include benzene, fluorobenzene, chlorobenzene, iodobenzene, toluene, fluorotoluene, or xylene. (xylene) and the like. Examples of the alkoxyalkane solvent include dimethoxy ethane or diethoxy ethane.

Specific examples of the carbonate solvent in the present invention include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (methylpropylcarbonate, MPC), ethylpropyl carbonate (ethylpropylcarbonate) , EPC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), butylenes carbonate (BC), or fluoroethylene carbonate (FEC).

In the present invention, when the electrolyte is a solid electrolyte, specifically, an organic polymer electrolyte such as a polyethylene oxide-based polymer compound, a polymer compound containing at least one of a polyorganosiloxane chain, or a polyoxyalkylene chain may be used. there is. In addition, a so-called gel-type thing in which a non-aqueous electrolyte solution is maintained in a high molecular compound can also be used. In addition, it is also possible to use an inorganic solid electrolyte such as _{Na 2 S-SiS 2, Na} 2 S-GeS 2 , such as sulfide electrolyte, NaZr ₂ (PO ₄₎ NASICON-type electrolyte such as _3. When these solid electrolytes are used, safety may be further improved.

Moreover, when using a solid electrolyte in the sodium secondary battery of this invention, a solid electrolyte may serve as a separator, and in that case, a separator may not be required.

In addition, the electrolyte contains additives (hereinafter referred to as 'other additives') that can be generally used in the electrolyte for the purpose of improving the lifespan characteristics of the battery, suppressing the reduction of the battery capacity, and improving the discharge capacity of the battery, in addition to the electrolyte components. may include more.

In the present invention, it is preferable to further add 0.1 to 5% by weight of fluoroethylene carbonate to the electrolyte.

The sodium ion battery of the present invention uses a solid electrolyte as an electrolyte as described above, and may not require a separate separator when the solid electrolyte serves as a separator, but otherwise may further include a separator. .

In the present invention, the separator separates the negative electrode and the positive electrode, and provides a passage for sodium ions to move, and any separator commonly used in batteries using sodium ions may be used. That is, one having low resistance to ion movement of the electrolyte and excellent moisture content of the electrolyte may be used. For example, it is selected from glass fiber, polyester, Teflon, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), or a combination thereof, and may be in the form of a nonwoven fabric or a woven fabric. Preferably, a polyolefin-based polymer separator such as polyethylene or polypropylene is mainly used for the sodium secondary battery, and a coated separator containing a ceramic component or a polymer material may be used to secure heat resistance or mechanical strength, and optionally single-layer or multi-layer structure can be used.

In addition, the sodium secondary battery of the present invention may further include fluoroethylene carbonate (FEC), vinyl carbonate (VC), or a combination thereof as an additive.

Hereinafter, the present invention will be described in more detail through examples. These examples are only for illustrating the present invention in more detail, and it will be apparent to those of ordinary skill in the art that the scope of the present invention is not limited by these examples according to the gist of the present invention. .

Example

[Example 1] Preparation of compound

A general sol-gel (sol-gel) method was used to synthesize the NVPF/NVP complex. On the other hand, the synthesis method according to this embodiment is for synthesizing the NVPF single phase, but according to the Fe content, NVP and Na ₅ V _1-x Fe _x (PO ₄ ) ₂ F _{2 are} also included as minor phases, so the prepared sample is NVPF It is called /NVP. Sodium fluoride, Ammonium metavanadate, Ammonium phosphate monobasic and Citric acid were purchased from Sigma-Aldrich. To synthesize NVPF/NVP doped with Fe by 0%, 4.9%, 8.9%, 11.5%, 14.8%, sodium fluoride is 3 mmol, ammonium metavanadate 2 mmol, 1.8 mmol, 1.65 mmol, 1.6 mmol, 1.5 Mix mmol, Iron nitrate x mmol (0 mmol, 0.20 mmol, 0.35 mmol, 0.40 mmol, 0.5 mmol), 2 mmol of ammonium phosphate and 1.6 mmol of citric acid and heat until a blue liquid is obtained. A mixture was formed. For the synthesis of NVPF/NVP doped with Ti, Cu, Zn, Cr, and Mn by 2.8%, 3.6%, 1.9%, 1.9%, and 2.6%, respectively, TiO and metal nitrate were 0.15 instead of iron nitrate, respectively. mmol, 0.15 mmol, 0.1 mmol, 0.1 mmol, 0.1 mmol, 0.1 mmol were used. Thereafter, the precursor mixture was placed in a vacuum oven and dried at 80° C. until gelled, and then completely dried at 120° C. Thereafter, the resulting precursor mixture was ground and compressed to make pellets, and then heated at 300° C. for 4 hours in an argon atmosphere, followed by additional heating at 650° C. for 8 hours. The content of the heterogeneous transition metal of the synthesized material was confirmed through an inductively coupled plasma.

A transmission electron microscope (TEM, JEOL2100F) and an X-ray diffraction analyzer (XRD, RIGAKU, D/MAX-2500) were used to confirm the three-dimensional structure and crystal structure of the synthesized compound.

[Example 2] XRD graph, X-ray diffraction (XRD) and shape analysis of the compound

XRD analysis was performed on the synthesized electrode materials. As a result, it was confirmed that all electrode materials contained both Na ₃ V ₂ (PO ₄ ) ₂ F ₃ (NVPF) and Na ₃ V ₂ (PO ₄ ) ₃ (NVP) forms, depending on the Fe content. It was also found to contain a small amount of Na ₅ V _1-x Fe _x (PO ₄ ) ₂ F ₂ phase. Therefore, the electrode material is called NVPF/NVP (Fe x %, x = vanadium replacement rate). In the case of NVPF/NVP (Fe 8.9 %), which showed the best performance, the particle size was less than 1 μm, and most were found to be distributed between 10 and 500 nm. The fact that the synthesized electrode material contains both NVPF and NVP types was also confirmed through the high-resolution transmission electron microscope image of FIG. 1c . The image in Figure 1c is an image obtained from the NVPF/NVP (Fe 8.9%) specimen. The ratio of NVPF, NVP, Na ₅ V _1-x Fe _x (PO ₄ ) ₂ F _{2 in the} electrode material is 81.16:18.84:0 when Fe is not included, and 81.67:18.33: when Fe is included 4.9%: When 0 and 8.9% of Fe were included, it was confirmed to be 73.63:19.45:6.92 (FIG. 1d). Figure 1e is an image showing the element distribution through EDS mapping in the NVPF / NVP (Fe 8.9%) specimen, it can be confirmed that Fe is well doped.

[Example 3] Charge and discharge profile of sodium ion battery according to Fe content of compound

Charge and discharge profiles of NVPF/NVPs (Fe 0 %, 4.9 %, 8.9 %) according to the Fe content were obtained at a current density of 0.5 C (Fig. 2a). As a result, the average voltage maintained a value higher than 3.7 V until Fe was increased to 4.9% (FIG. 2d). When Fe was increased more than 4.9%, the average voltage started to drop below 3.7 V. When Fe was included in 8.9%, the average voltage decreased to 3.67 V, but the discharge capacity reached the highest value of ~120 mAh/g. achieved. In addition, considering the price of vanadium and Fe, when 8.9% of Fe is included, the vanadium content can be reduced by 8.9%, which means that the price of raw materials can be reduced by about 8.9%. The prices of V ₂ O ₅ and iron ore are 14175 usd/ton and 96.7 usd/ton, respectively (as of January 24, 2020)

[Example 4] Sodium storage performance of sodium ion battery according to Fe content of compound

When Fe was not included, the charge capacity of NVPF/NVP was 119.1 mAh/g and the discharge capacity was 107.48 mAh/g. When Fe was doped by 4.9%, 8.9%, 11.5%, and 14.9%, the charge capacity was 120.97 mAh/g, 129.17 mAh/g, 124.39 mAh/g, and 124.14 mAh/g, respectively, and the discharge capacity was 112.61 mAh/g, respectively. , 119.96 mAh/g, 117.34 mAh/g, and 113.46 mAh/g. (Fig. 2a) When the Fe content was 8.9%, the charging and discharging capacities were the highest at 129.2 mAh/g and 119.96 mAh/g, respectively, and the performance was higher while replacing vanadium in terms of storage capacity (Fig. 2a). When the Fe content exceeded 8.9%, it was confirmed that the charge and discharge capacity and the discharge voltage were lowered. When the Fe content was 11.5% and 14.9%, the average discharge voltage was 3.63 and 3.60 V, respectively. However, in both cases, the performance was still higher in terms of energy density than when Fe was not doped. The energy densities were 392.11, 415.9, 433.73, 421.13, and 403.43 Wh/kg when no Fe was doped and when the Fe doping amount was 4.9%, 8.9%, 11.5%, and 14.9%, respectively.

Fe-doped NVPF/NVP showed higher performance even at high current density. As a result of measuring the performance according to the current density of NVPF/NVPs (Fe 0 %, 4.9 %, 8.9 %), NVPF/NVP without Fe showed a discharge capacity of 87.9 mAh/g at 10 C, whereas the NVPF /NVPs (Fe 4.9 %, 8.9 %) showed discharge capacities of 101 mAh/g, 108 mAh/g, and 108 mAh/g, respectively.

[Example 5] Sodium storage performance of electrode material according to doping with transition metal other than Fe

As can be seen from FIG. 3 , in the case of transition metals other than Fe, even if the doping amount exceeds 3.6% depending on the transition metal and the content is increased, the performance is lower than before doping, so it is not useful.

[Example 6] Sodium ion diffusivity analysis of NVPF/NVP doped with Fe.

Sodium ion diffusivity and electrical conductivity are factors that affect sodium ion storage performance. Since the performance of the NVPF/NVP composite significantly increases with Fe doping, it is expected that the above two factors will also change significantly.

To measure this, the sodium ion conversion coefficient was obtained by cyclic voltammetry. If the current peak increases linearly with the voltage scan rate, the sodium ion diffusion coefficient can be obtained using the Randles-Sevcik equation.

는 전류피크이며, n은 참여하는 전자갯수 (여기서 n = 2), A는 실효면적, D는 확산계수, C는 소듐 농도, v는 전압스캔속도 임) 해당식과 도 4a,b,c를 이용하여 구한 소듐확산계수를 Fe의 농도에 따라 그래프로 나타내었다 (도 4d). 그 결과, Fe를 도핑할수록, 소듐 확산 계수가 눈에 띄게 높아지는 것을 확인할 수 있었다.(here,

is the current peak, n is the number of participating electrons (here n = 2), A is the effective area, D is the diffusion coefficient, C is the sodium concentration, v is the voltage scan rate) Sodium diffusion coefficient obtained by doing so was graphed according to the concentration of Fe (FIG. 4d). As a result, it was confirmed that the sodium diffusion coefficient was remarkably increased as Fe was doped.

Although the above results have been described in detail with respect to the present invention, the scope of the present invention is not limited thereto, and various modifications and variations are possible within the scope without departing from the technical spirit of the present invention described in the claims. It will be apparent to those of ordinary skill in the art. In particular, in this embodiment, the maximum performance was shown when the Fe content was 8.9%, but this may be sufficiently varied depending on the synthesis method, the sintering temperature and atmospheric conditions or the pellet compression pressure. In the same vein, the ratio of NVPF, NVP, Na ₅ V _1-x Fe _x (PO ₄ ) ₂ F ₂ can also be sufficiently different.

[Example 7] NVPF/NVP doped with 8.9% Fe as a cathode material for a sodium ion battery full cell

A sodium ion battery was constructed using bulk copper sulfide and NVPF/NVP (Fe 8.9 %) as the negative electrode and the positive electrode, respectively. In order to optimize the performance of the sodium ion battery, the bulk copper sulfide was used after repeatedly charging and discharging sodium before using it as an electrode. The mass ratio of positive electrode: negative electrode material was ~3.43:1.

The completed sodium ion battery was charged to 4.25 V in the first cycle, and then operated in a voltage range of 0 ~ 4.05 V. As a result, it showed an average voltage of 2.09 V at 1 C (128 mAh/g, based on the positive electrode), and achieved an energy density of 162.2 Wh/kg (based on the mass of the negative electrode + positive electrode) ( FIG. 5A ). In addition, it was confirmed that energy densities of 148.59 Wh/kg, 138.27 Wh/kg, and 114.6 Wh/kg can be obtained as the current density is increased to 3 C, 5 C, and 10 C, respectively, and stable storage at each current density capacity (Fig. 5b,c). The completed sodium ion battery shows high performance even compared to the sodium ion battery reported so far ( FIG. 5d ). In addition, the completed sodium ion battery has excellent long-life cycle performance, and maintained a storage capacity of ~95% even after 450 charge/discharge cycles, and maintained a storage capacity of ~88% even after 1250 charge/discharge cycles (Fig. 5c,e) .

Claims (14)

Na ₃ V _2-x M _x (PO ₄ ) ₂ F ₃ single phase or Na ₃ V _2-x M _x (PO ₄ ) ₂ F ₃ /Na ₃ V _2-y M _y (PO ₄ ) ₃ complex (0 <x, y≤2) and M is a transition metal, sodium ion storage material. According to claim 1,
The transition metal is any one selected from the group consisting of Fe, Ti, Cu, Cr, Mn and Zn, sodium ion storage material. According to claim 1,
The transition metal is Fe, sodium ion storage material. An electrode material for a sodium ion battery comprising the sodium ion storage material of claim 3 as an electrode active material. 5. The method of claim 4,
The electrode active material is coated with one or more selected from the group consisting of conductive carbon, noble metals and metals, the electrode material for sodium ion batteries. 5. The method of claim 4,
The electrode material further comprises a binder, an electrode material for a sodium ion battery. 7. The method of claim 6,
The binder is polyvinylidene fluoride, polyvinyl alcohol, polyacrylic acid, alginic acid, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene , Polyethylene, polypropylene, ethylene-propylene-diene ter polymer (EPDM), sulfonated EPDM, comprising at least one selected from the group consisting of styrene butyrene rubber and fluororubber, electrode material for sodium ion batteries. 8. The method of claim 7,
The electrode material further comprises a conductive material, the electrode material for a sodium ion battery. The electrode for sodium ion batteries containing the electrode material of any one of Claims 4-8. A sodium ion battery or sodium-magnesium hybrid ion battery comprising the electrode and electrolyte of claim 9 . (a) mixing sodium fluoride, ammonium metavanadate, ammonium phosphate monobasic, and citric acid, followed by heating to form a precursor mixture;
(b) drying the precursor mixture; and
(c) a method for producing a sodium ion storage material comprising the step of heating the dried precursor mixture. 12. The method of claim 11,
Drying the precursor mixture is carried out at 80 ℃, then the method for producing a sodium ion storage material that is additionally carried out at 120 ℃. 12. The method of claim 11,
The heating step after compressing the dried precursor mixture into pellets is carried out at 300 ℃, and then the method for producing a sodium ion storage material that is additionally carried out at 650 ℃. 12. The method of claim 11,
Heating the dried precursor mixture is a method of producing a sodium ion storage material that is performed in an argon atmosphere.

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