KR101759323B1 - Method for manufacturing cathod material of sodium rechargeable battery and sodium rechargeable battery including the same - Google Patents

Abstract

A method for preparing a cathode active material for a sodium secondary battery according to an embodiment of the present invention includes mixing and drying a sodium precursor and a phosphate precursor; And sintering the mixed and dried precursor mixture in three steps to form a compound represented by Na ₄ Mn _x Fe _{3 -x} (PO ₄ ) ₂ (P ₂ O ₇ ) The sintering temperature of each of them increases. X is 0 = x < 3.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method for producing a positive electrode active material for a sodium secondary battery, and a sodium secondary battery comprising the same. BACKGROUND ART &lt; RTI ID = 0.0 &gt;

The present invention relates to a method for producing a cathode active material for a sodium secondary battery and a sodium secondary battery including the same.

While the development of new and renewable energy and the reduction of carbon dioxide are essential, it is essential to develop a secondary battery capable of storing electric power in order to efficiently use the produced energy.

In recent years, the development of a technology capable of being applied to a large-capacity and power storage device has been actively carried out. Among them, a sodium secondary battery capable of storing more power at a low cost has been developed.

First, the problem of safety is most important in order to manufacture a large capacity battery. In the case of a conventional non-aqueous sodium secondary battery, a highly inflammable and toxic organic solution is used as a non-aqueous electrolyte, which may cause a large explosion in the event of a fire. And, the organic liquid electrolyte has a limitation in accumulating the active material due to the limit of the ionic conductivity.

In addition, expensive equipment is necessary to constitute an environment for manufacturing a nonaqueous sodium secondary battery, and the manufacturing cost is high because the cost of the sodium salt, organic solution, and separation membrane used is high. Considering that cost is the most important requirement for large capacity batteries, these problems must be solved.

On the other hand, the aqueous electrolyte can solve the problem of safety and high manufacturing cost caused by using the non-aqueous electrolyte.

First, since the aqueous electrolyte is water-free, it is safe because it is non-flammable and non-toxic and has no risk of fire or explosion, and its ionic conductivity is 100 times higher than that of the organic electrolyte. Therefore, many active materials can be accumulated, / There is an advantage that discharge can be done.

In addition, the water-soluble sodium salt and water are abundant and inexpensive, and the separator used is also inexpensive. Moreover, since the reactivity with moisture is not considered, the process is simplified and the cost is low.

SUMMARY OF THE INVENTION The present invention provides a method for producing a cathode active material having high voltage, high capacity, high efficiency, and excellent price competitiveness, and a secondary battery including the same.

A method for preparing a cathode active material for a sodium secondary battery according to an embodiment of the present invention includes mixing and drying a sodium precursor and a phosphate precursor; And sintering the mixed and dried precursor mixture in three steps to form a compound represented by Na ₄ Mn _x Fe _3-x (PO ₄ ) ₂ (P ₂ O ₇ ) The sintering temperature of each of them increases. X is 0 < x &lt; 3.

The step of sintering in the above three steps may include a first sintering at a first temperature of 250 to 350 ° C and an argon atmosphere, and a second sintering at a second temperature of 450 to 550 ° C and an argon atmosphere .

Mixing the second sintered mixture with a carbon source, and thirdly sintering the mixture mixed with the carbon source at a third temperature of 550 to 650 ° C and an argon atmosphere.

The carbon source may be at least one selected from polyvinyl alcohol, polyolefin, polyacrylonitrile, cellulose, starch, granule, acrylonitrile, divinylbenzene, vinyl acetate, glucose, cellulose acetate, pyromellitic acid, acetone and ethanol have.

The phosphate precursor may be (NH ₄ ) ₂ HPO ₄ , (NH ₄ ) H ₂ PO ₄ , or a combination thereof.

The cathode active material for the sodium secondary battery may be Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ).

A sodium secondary battery according to an embodiment of the present invention includes a cathode including a cathode active material; cathode; Aqueous electrolyte; And a separator, wherein the cathode active material includes a compound represented by Na ₄ Mn _x Fe _{3 -x} (PO ₄ ) ₂ (P ₂ O ₇ ).

The aqueous electrolyte electrolyte may further comprise a sodium salt.

Wherein the sodium salt is selected from the group consisting of NaClO ₄ , NaPF ₆ , NaAsF ₆ , NaSbF ₆ , NaBF ₄ , NaCF ₃ SO ₃ , NaN (SO ₂ CF ₃ ) ₂ , NaCl, NaC ₂ H ₃ O ₂ , Na ₂ SO ₄ , NaNO ₃ , NaBH _4, NaOH may include lower aliphatic carboxylic acid sodium salt, NaAlCl ₄ or a combination thereof.

The separator may comprise glass fiber, polyester, Teflon, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), or a combination thereof.

The cathode active material may be Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ).

According to the method for manufacturing a cathode active material for a sodium secondary battery according to an embodiment of the present invention, a cathode active material for a sodium secondary battery can be manufactured which is applicable to an aqueous electrolyte and is stable and large-capacity. In addition, the sodium secondary battery according to an embodiment of the present invention can secure a large capacity and high stability by using an aqueous electrolyte.

1 is an X-ray diffraction (XRD) graph of a cathode active material for a sodium secondary battery according to an embodiment of the present invention.
2 is an X-ray diffraction (XRD) graph of a cathode active material for a sodium secondary battery according to an embodiment of the present invention.
3 is a graph illustrating a voltage and current of a sodium secondary battery according to an embodiment of the present invention.
4 is a graph of charge / discharge of a sodium secondary battery according to an embodiment of the present invention.
FIG. 5 is an X-ray diffraction (XRD) graph of a cathode active material for a sodium secondary battery according to an embodiment of the present invention, and FIG. 6 is a XANES spectrum graph.
7 is a graph showing a life cycle of a cathode active material for a sodium secondary battery according to an embodiment of the present invention.
FIG. 8 is a graph of charge / discharge of a cathode active material for a sodium secondary battery according to an embodiment of the present invention, and FIG. 9 is a graph illustrating a life cycle of a cathode active material for a sodium secondary battery according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited thereto, and the present invention is only defined by the scope of the following claims.

The cathode active material for a sodium secondary battery according to an embodiment of the present invention can be applied to an aqueous electrolyte, and a stable and large capacity sodium secondary battery can be manufactured.

Hereinafter, a cathode active material according to one embodiment of the present invention and a method of manufacturing the same will be described in detail.

The positive electrode active material according to one embodiment of the present invention is _{Na 4 Mn x Fe 3-x} (PO 4) 2 (P 2 O 7) (0 ≤ x <3) may be, Na ₄ Fe ₃ By way of example (PO ₄ ) ₂ (P ₂ O ₇ ).

The positive electrode active material according to one embodiment of the present invention is a phosphate compound containing iron (Fe) and can be used for a high voltage secondary battery because of its high energy density, and can have a high capacity.

Hereinafter, a method for manufacturing a positive electrode active material according to an embodiment of the present invention will be described.

First, the sodium precursor Na ₄ P ₂ O ₇ , FeC ₂ O ₄ 2H ₂ O, phosphate precursors NH ₄ H ₂ PO ₄ (or (NH ₄ ) ₂ HPO ₄ ) and pyromellitic acid (carbon source ) Is adjusted to the chemical equivalent and mixed. The mixture may be mixed with the first mixture through ball milling. In this case, for example, dry ball milling can be performed using a planetary ball-milling equipment, but wet ball milling is also possible.

The ball milling time is preferably from 1 hour to 12 hours. If the time is less than 1 hour, it is not enough to dissolve, crush or mix the charged precursors. If the time exceeds 12 hours, the process time is prolonged, Because. However, in some cases, the process time may be 12 hours or more.

In addition, if the prepared precursor can produce a homogeneous mixture without performing ball milling, mixing can also be performed by a simple stirring process.

Next, when a first mixture in which the precursors are uniformly mixed is obtained, it is subjected to first sintering at a first temperature of 250 to 350 DEG C, preferably in a 300 degree argon (Ar) atmosphere. Through such a sintering process, a material having higher crystallinity can be provided.

Thereafter, the first sintered first mixture is pelletized and secondarily sintered at a second temperature of 450 to 550 ° C, preferably in an argon (Ar) atmosphere of at least 500 ° C for at least 12 hours. According to this process, Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) compound is obtained. At this time, both the first sintering and the second sintering are performed while flowing argon gas. The flow of the argon gas forms a reducing atmosphere to prevent oxidation of iron (Fe).

Finally, the Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) compound is mixed with pyromellitic acid (carbon source) at 95 to 5 wt% (Or aged) in an atmosphere of argon (Ar) at 600 ° C. for 2 hours to prepare a cathode active material according to an embodiment of the present invention.

The carbon source may be at least one selected from polyvinyl alcohol, polyolefin, polyacrylonitrile, cellulose, starch, granule, acrylonitrile, divinylbenzene, vinyl acetate, glucose, cellulose acetate, pyromellitic acid, acetone and ethanol .

Although the present invention has been described with reference to a process for sintering in an argon atmosphere, the present invention is not limited thereto, and any one selected from helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) Of course.

Such a manufacturing method is capable of mass production of the cathode active material through a simple process and a relatively low synthesis temperature. The cathode active material for a sodium secondary battery thus produced has high price competitiveness and stability.

Hereinafter, a sodium secondary battery according to an embodiment of the present invention will be described. A sodium secondary battery according to an embodiment of the present invention includes a cathode, an anode, an electrolyte, and a separator including the cathode active material described above.

The anode, the cathode, and the separator may be wound or folded to be accommodated in the battery case. An organic electrolytic solution can be injected into the battery case and sealed with a cap assembly. The battery case may have a cylindrical shape, a rectangular shape, a thin film shape, or the like, but is not limited thereto and may be a large-sized thin film type.

A separator is disposed between the anode and the cathode to form a battery structure. After the cell structure is laminated in the bi-cell structure, the cell structure is impregnated with the organic electrolyte solution, and the resultant product is received in the pouch and sealed, thereby completing the sodium secondary battery.

Hereinafter, each configuration will be described in detail.

The anode includes a current collector and a cathode active material layer formed on the current collector. The cathode active material layer includes the cathode active material according to an embodiment of the present invention.

Further, a coating layer may be provided on the surface of the positive electrode active material, or a mixture of the compound and the coating layer may be used. The coating layer may comprise, as a coating element compound, an oxide, a hydroxide of a coating element, an oxyhydroxide of a coating element, an oxycarbonate of a coating element, or a hydroxycarbonate of a coating element. The compound constituting these coating layers may be amorphous or crystalline. The coating layer may contain Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr or a mixture thereof. The coating layer forming step may be carried out by any of coating methods such as spray coating, dipping, and the like without adversely affecting the physical properties of the cathode active material by using these elements in the above compound. It is a content that can be well understood by people engaged in the field, so detailed explanation will be omitted.

The cathode active material layer may further include a binder and a conductive material.

The binder serves to adhere the positive electrode active material particles to each other and to adhere the positive electrode active material to the current collector. Typical examples thereof include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl Polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-acrylonitrile, styrene-butadiene rubber, Butadiene rubber, acrylated styrene-butadiene rubber, polyacrylic acid, epoxy resin, nylon, and the like.

The conductive material is used for imparting conductivity to the electrode. Any conductive material can be used without causing any chemical change in the battery. Examples of the conductive material include natural graphite, artificial graphite, carbon black, acetylene black, Metal powders such as black, carbon fiber, copper, nickel, aluminum, and silver, metal fibers, and the like, and conductive materials such as polyphenylene derivatives may be used alone or in combination.

As the current collector, Al may be used, but the present invention is not limited thereto.

The negative electrode includes a current collector and a negative electrode active material layer disposed on the current collector.

The negative electrode active material layer may include, but is not necessarily limited to, sodium metal, a sodium metal-based alloy, a sodium intercalating compound, or a carbon-based material and may be used as an anode active material in the art, Or can absorb / release sodium. The sodium metal-based alloy may include, but is not limited to, an alloy of aluminum, tin, indium, calcium, titanium, vanadium, etc. with sodium.

The negative electrode active material layer may be formed of metal in a thickness of 3 to 500 탆, and may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

When a negative electrode active material other than a sodium metal or a sodium alloy is used, a carbon-based material having a graphene structure or the like can be used. A mixed cathode of a material such as graphite and graphitic carbon, a mixed cathode of a carbon-based material and a metal or an alloy, and a composite cathode may be used. Examples of the carbon-based material include natural graphite, artificial graphite, mesophase carbon, expanded graphite, carbon fiber, vapor growth carbon fiber, pitch carbonaceous material, needle coke, petroleum coke, A carbonaceous material such as polyacrylonitrile-based carbon fiber or carbon black, or an amorphous carbon material synthesized by thermal decomposition of a cyclic hydrocarbon or a cyclic oxygen-containing organic compound of a 5-membered ring or a 6-membered ring.

The negative electrode active material layer may further include a binder, and may further include a conductive material.

The binder serves to adhere the anode active material particles to each other and to adhere the anode active material to the current collector. Typical examples thereof include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, polyvinyl chloride, Such as polyvinyl chloride, polyvinyl fluoride, polymers comprising ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, Styrene-butadiene rubber, epoxy resin, nylon, and the like may be used, but the present invention is not limited thereto.

The conductive material is used for imparting conductivity to the electrode. Any material can be used as long as it does not cause any chemical change in the battery. Examples of the conductive material include natural graphite, artificial graphite, carbon black, acetylene black, , Carbon-based materials such as carbon fibers; Metal powders such as copper, nickel, aluminum, and silver, or metal-based materials such as metal fibers; Conductive polymers such as polyphenylene derivatives; Or a mixture thereof may be used.

The current collector may be a copper foil, an aluminum foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foil, a copper foil, a polymer substrate coated with a conductive metal, or a combination thereof.

The positive electrode and the negative electrode are prepared by mixing an active material, a conductive material and a binder in a solvent to prepare an active material composition, and applying the composition to a current collector. The method of manufacturing the electrode is well known in the art, and therefore, a detailed description thereof will be omitted herein. As the solvent, N-methylpyrrolidone or the like can be used, but it is not limited thereto.

The electrolyte may be a liquid electrolyte, and the liquid electrolyte may include a sodium salt and an aqueous solution.

Examples of the sodium salt that can be used in the sodium secondary battery of the present invention include NaClO ₄ , NaPF ₆ , NaAsF ₆ , NaSbF ₆ , NaBF ₄ , NaCF ₃ SO ₃ , NaN (SO ₂ CF ₃ ) ₂ , NaCl, NaC ₂ H ₃ O ₂ , Na ₂ SO ₄ , NaNO ₃ , NaBH ₄ , NaOH lower aliphatic carboxylic acid sodium salt and NaAlCl ₄ , and mixtures of two or more of these may also be used.

The separation membrane separates the cathode and the anode, and provides a passage for sodium ions. Any separator can be used as long as it is commonly used in a sodium secondary battery. That is, it is possible to use an electrolyte having a low resistance to ion movement and an excellent ability to impregnate an electrolyte. For example, selected from glass fibers, polyester, Teflon, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), or a combination thereof, and may be nonwoven fabric or woven fabric. For example, a polyolefin-based polymer separator such as polyethylene, polypropylene, or the like is mainly used for the sodium secondary battery, and a coated separator containing a ceramic component or a polymer material may be used for heat resistance or mechanical strength. Alternatively, Structure.

Hereinafter, physical properties and effects of the cathode active material according to one embodiment of the present invention will be described.

Experimental Example  One

Synthesis of materials (carbon-coated Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ))

To meet the above chemical equivalents, precursors for solid phase reactions include Na ₄ P ₂ O ₇ 4.65999 g of powder, 8.65737 g of FeC ₂ O _{4 2} H ₂ O and 3.7657 g of NH ₄ H ₂ PO ₄ were prepared. The precursors thus prepared were mixed with 5 weight percent of pyromellitic acid based on the total weight of the precursors, followed by dry ball milling.

Next, the mixed powders were first sintered in a 300 ° C argon atmosphere for 6 hours through the ball milling. The thus dried powder was made into pellets, and the pellets were secondarily sintered for 12 hours in an argon atmosphere at 500 degrees.

Then, the secondary sintered powder and pyromellitic acid were mixed at 95 to 5 weight percent and sintered in an argon atmosphere at 600 ° C for 2 hours to prepare a cathode active material.

Characteristic evaluation of materials

Hereinafter, evaluation of the characteristics of the cathode active material prepared according to Experimental Example 1 will be described.

FIG. 1 is a graph of XRD analysis of powder sintered in an argon atmosphere at 500.degree. C. in Experimental Example 1, and FIG. 2 is a graph showing XRD analysis of final powder sintered in an argon atmosphere at 600.degree. The XRD analysis preferably uses a Rietveld refinement.

 1 and 2, the cathode active material prepared according to the above-described production method has almost no difference between the actually produced powder and the theoretical calculated value, It can be seen that this high material is synthesized.

Next, FIG. 3 is a graph showing the results of measurement of the activity of the anode prepared by attaching carbon-coated Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) prepared in Experimental Example 1 to an SUS mesh network and 1 mol of NaSO ₄ And a cyclic voltammetry experiment was conducted using a saturated calomel electrode (SCE) as a reference electrode containing a dissolved electrolyte. Experimental conditions were -0.7 to 0.7 V vs. 1 mV / s sweep rate. SCE.

Referring to FIG. 3, the maximum redox reaction of Na / Na ⁺ generally observed in Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) is confirmed and reversible electrochemical reaction occurs to a considerable extent.

Referring to FIG. 4, the charging / discharging curve shows that the capacity of about 95 mAh / g is within the voltage range of -0.7 to 0.7 V vs. SCE, which is very similar to the electrochemical charge and discharge curve determined by non-aqueous electrolyte.

Next, referring to FIG. 5, it can be confirmed that the charge / discharge reaction in the cathode active material / water-based electrolyte including Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) is based on intercalation. FIG. 6 shows the change in intensity with energy change through the XANES spectrum. It can be seen that the oxidation number of Na reversibly changes between 0 and 1 according to charge and discharge.

As shown in FIG. 7, Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) serving as a cathode active material has a current density of 10 C (~1.29 A / g), a voltage of -0.19 V to 0.55 V (> 85%) over a period of 1000 cycles at a constant temperature range.

The following Figs. 8 and 9, to a charge / discharge curve of a battery consisting of a positive electrode and the negative electrode containing the _{Na 4 Fe 3 (PO 4)} 2 (P 2 O 7). According to this, the capacity of ~ 50 mAh / g can be obtained, and it is confirmed that the lifetime can be maintained up to 10000 cycles when charge / discharge is performed at 10C rate.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims. As will be understood by those skilled in the art. It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive.

Claims (11)

Mixing and drying a sodium precursor, and a phosphate precursor and pyromellitic acid; And
Sintering the mixed and dried precursor mixture in three steps,
To form a compound represented by Na ₄ Mn _x Fe _3-x (PO ₄ ) ₂ (P ₂ O ₇ )
The respective sintering temperatures over the three steps are high,
Wherein the sintering step comprises:
First sintering at a first temperature of from 250 to 350 DEG C and in an argon atmosphere,
Second sintering at a second temperature of 450-550 < 0 &gt; C and an argon atmosphere,
Mixing the second sintered mixture with pyromellitic acid, and
And a third sintering in an argon atmosphere at a third temperature of 550 to 650 ° C .; and a method of producing the cathode active material for an aqueous sodium-
X is 0 &lt; x &lt; 3.
delete delete delete The method of claim 1,
Wherein the phosphate precursor is (NH ₄ ) ₂ HPO ₄ , (NH ₄ ) H ₂ PO ₄ , or a combination thereof. In claim 1,
Wherein the cathode active material for the sodium secondary battery is Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ). delete delete delete delete delete

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