WO2015122705A1 - Positive electrode active material for sodium secondary battery, and method for preparing same - Google Patents

Abstract

The present invention relates to a positive electrode active material for a sodium secondary battery, and a method for preparing the same. The positive electrode active material for the sodium secondary battery according to the present invention is structurally more stable by replacing a part of the transition metal with Li, and accordingly, the thermal stability and life characteristics of the sodium battery comprising the positive electrode active material are greatly improved.

Description

Cathode active material for sodium secondary battery and manufacturing method thereof

The present invention relates to a cathode active material for sodium secondary batteries and a method of manufacturing the same.

At present, as a secondary battery having a high energy density, a lithium ion secondary battery using a nonaqueous electrolyte solution in which an electrolyte salt is dissolved in a nonaqueous solvent and allowing lithium ions to move between a positive electrode and a negative electrode so that charge and discharge is performed is widely used. Since lithium secondary batteries can be used for large secondary batteries, such as large power supplies for automobiles such as electric vehicles and hybrid vehicles, and distributed power storage power supplies, the demand for them is increasing. However, since lithium secondary batteries use a lot of rare metals such as cobalt, nickel, and lithium, there is a concern about supply of such rare metals due to the demand for large secondary batteries.

On the other hand, a sodium secondary battery is examined as a nonaqueous electrolyte secondary battery which can solve the worry of supply of a battery material. The sodium secondary battery comprises a positive electrode containing a positive electrode active material capable of doping and undoping sodium ions, a negative electrode containing a negative electrode active material capable of doping and undoping sodium ions, and a nonaqueous electrolyte containing sodium ions. It is composed. Since sodium secondary batteries use abundant and inexpensive sodium as a material, it is expected to be able to supply large secondary batteries in large quantities by making them practical.

In the sodium secondary battery, like lithium ions of a lithium secondary battery, charge and discharge of the battery occurs by sodium ions reciprocating between the negative electrode and the positive electrode through an electrolyte.

Japanese Unexamined Patent Application Publication No. 2007-287661 has a positive electrode made of a composite metal oxide obtained by firing a raw material having a composition ratio of Na, Mn and Co (Na: Mn: Co) of 0.7: 0.5: 0.5 and a negative electrode made of sodium metal. Secondary batteries are described in detail. Further, Japanese Patent Laid-Open No. 2005-317511 discloses α-NaFeO ₂ having a hexagonal crystal (layered rock salt) crystal structure as a sum metal oxide, specifically, by mixing Na ₂ O ₂ and Fe ₃ O ₄ . This composite metal oxide was obtained by baking at 600-700 degreeC in air.

However, the conventional sodium secondary battery has a low lifespan characteristic, that is, a discharge capacity retention rate when repeated charging and discharging is low, and thermal stability is low, and there is a need to improve it.

An object of the present invention is to provide a cathode active material for a new composition of sodium secondary battery, a sodium secondary battery positive electrode comprising the same and a sodium secondary battery comprising the same in order to solve the problems of the prior art as described above.

Another object of the present invention is to provide a method for producing a cathode active material for sodium secondary battery according to the present invention.

The present invention provides a cathode active material for sodium secondary batteries represented by the following formula (1) to solve the above problems.

[Formula 1]

Na _x Li _a [Ni _y Fe _z Mn _1-yzb M _b ] _1-a O ₂

(In Formula 1, M is an element selected from the group consisting of Co, Cr, Zr, Nb, Cu, V, Ti, Zn, Al, Ga, Mg, B and combinations thereof,

0.8≤x≤1.2, 0.01≤a≤0.1, 0.05≤y≤0.9, 0.05≤z≤0.9, 0≤b≤0.9, 0.05≤1-y-z-b≤0.9

The cathode active material for a sodium secondary battery according to the present invention is structurally more stable by preventing a migration phenomenon in which Fe ³⁺ is converted to Fe ⁴⁺ and transferred to Na ⁺ site by substituting a part of the transition metal with Li, thereby ^improving the life characteristics and thermal properties. Improve stability.

The cathode active material for sodium secondary battery according to the present invention is characterized by having a spherical particle size of 5 to 15 μm and a monodisperse particle size. When the particle size of the positive electrode active material is less than 5㎛, the specific surface area of the positive electrode active material increases, but the input of a binding solution or electrolyte and stable contact may be inhibited. Therefore, a large amount of binder is required for binding of the positive electrode active material, thereby deteriorating battery characteristics. Can be. On the other hand, when the particle size of the positive electrode active material is more than 15㎛, battery characteristics may be reduced by reducing the specific surface area of the positive electrode active material.

The cathode active material for sodium secondary battery according to the present invention is characterized in that 2θ in XRD shows three peaks in a range of 30 ° to 40 °.

The cathode active material for a sodium secondary battery according to the present invention is characterized in that the peak (104) having a main peak in the range of 40 ° to 45 ° in 2θ in XRD.

The cathode active material for sodium secondary battery according to the present invention is characterized by having a peak of 6Li ⁺ , 7Li ⁺ obtained by cation analysis by time-of-flight secondary ion mass spectrometry.

The time-of-flight secondary ion mass spectrometry (TOF-SIMS) equipment is equipped with TOF, which is a mass spectrometer, in a SIMS device. Specifically, SIMS equipment is a device that can obtain chemical components and surface structures by analyzing ions (cations or anions) emitted when the primary ions collide with the surface of the analyte. The TOF mass spectrometer, on the other hand, is a device with excellent mass resolution capable of simultaneously measuring ions of all masses with a high ion passage rate, and TOF-SIMS equipment forms secondary ions of analytically useful molecules to directly display molecular information. It is highly sensitive to elements as well as molecules, and has high spatial resolution by finely focused ion beams.

The cathode active material for a sodium secondary battery according to the present invention is characterized by having a peak of Ni ³⁺ at 855 to 860 eV in oxidation number analysis by X-ray photoelectron spectroscopy (XPS).

The present invention also provides

Mixing a cathode active material precursor, a sodium compound, and a lithium compound for a sodium secondary battery; And

It provides a method for producing a cathode active material for sodium secondary battery of the present invention comprising the step of heat-treating the mixture.

In the method for producing a cathode active material for sodium secondary battery of the present invention, the cathode active material precursor for sodium secondary battery is characterized in that represented by any one of the following formula (2).

Ni _y Fe _z Mn _1-yzb M _b (OH) ₂

Ni _y Fe _z Mn _1-yzb M _b C ₂ O ₄

[Ni _y Fe _z Mn _1-yzb M _b ] ₃ O ₄

(In Formulas 2 to 4, M is an element selected from the group consisting of Co, Cr, Zr, Nb, Cu, V, Ti, Zn, Al, Ga, Mg, B, and combinations thereof,

0.05≤y≤0.9, 0.05≤z≤0.9, 0≤b≤0.9, 0.05≤1-y-z-b≤0.9)

In the method for producing a cathode active material for sodium secondary battery of the present invention, the sodium compound is characterized in that selected from sodium carbonate, sodium nitrate, sodium acetate, sodium hydroxide, sodium hydroxide hydrate, sodium oxide or a combination thereof.

In the method for producing a cathode active material for sodium secondary battery of the present invention, the sodium compound per 1 mol of the cathode active material precursor for sodium secondary battery is characterized in that it is mixed in a ratio of 0.8 to 1.5 mol.

In the method for producing a cathode active material for sodium secondary battery of the present invention, the lithium compound is characterized in that any one selected from the group consisting of lithium nitrate, lithium acetate, lithium carbonate, lithium hydroxide, and combinations thereof. .

In the method for producing a cathode active material for sodium secondary battery of the present invention, the heat treatment step is characterized in that the heat treatment at 600 ℃ to 1000 ℃. If the heat treatment temperature is less than 600 ℃ unreacted metal particles may remain lower than the melting point of the metal contained in the positive electrode active material, if more than 1000 ℃ non-uniformization of the elements constituting the positive electrode active material proceeds to the positive electrode active material The lifespan of the sodium secondary battery may be reduced.

The present invention also provides a sodium secondary battery positive electrode comprising a cathode active material for sodium secondary battery of the present invention and a sodium secondary battery comprising the same.

The cathode active material for a sodium secondary battery according to the present invention is structurally more stable by substituting a part of the transition metal with Li, thereby greatly improving the thermal stability and life characteristics of the sodium battery including the cathode active material.

Figure 1 shows the XRD measurement results of the positive electrode active material prepared in one embodiment and comparative example of the present invention.

Figure 2 shows the results of Tof-SIMS measurement of the positive electrode active material prepared in one embodiment and comparative example of the present invention.

Figure 3 shows the XPS measurement results of the positive electrode active material prepared in one embodiment and comparative example of the present invention.

Figure 4 shows the results of measuring the initial charge and discharge characteristics of a battery including a cathode active material prepared in one embodiment and comparative example of the present invention.

5 to 8 show the results of measuring charge and discharge characteristics of a battery including a cathode active material prepared in Examples and Comparative Examples of the present invention.

9 and 10 show the results of DSC measurement of a battery including the cathode active material prepared in Examples and Comparative Examples of the present invention.

11 shows the results of XRD measurement after charge and discharge of the positive electrode active materials prepared in Examples and Comparative Examples of the present invention.

12 shows the measurement results of the rate characteristic of a battery including a cathode active material prepared in one embodiment and comparative example of the present invention.

FIG. 13 shows measurement results of life characteristics of a battery including a cathode active material prepared in Examples and Comparative Examples.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the present invention is not limited by the following examples.

Example Preparation of Positive Electrode Active Material

The reactor was charged with 4 L of distilled water, stirred at 1000 rpm while adding ammonia to adjust the pH inside the reactor to 7 and the internal temperature was maintained at 50 ° C. 4 M NaOH solution was added as a second pH adjusting agent to adjust the internal pH of the reactor to 10.2 and maintained for 30 minutes. NiSO ₄ ㆍ 6H ₂ O, FeSO ₄ ㆍ 7H ₂ O, MnSO ₄ ㆍ 5H ₂ O were mixed in an equivalent ratio with an aqueous solution of a transition metal compound, and added into the reactor together with NH ₄ OH as a complexing agent to give Ni _0.25 Fe _0.25 Mn _0.5 ( A precursor represented by OH) ₂ was prepared. Sodium carbonate and lithium carbonate were mixed with the precursor and stirred, followed by heat treatment to prepare a cathode active material represented by Na _1.0 Li _0.05 [Ni _0.25 Fe _0.25 Mn _0.5 ] _0.95 O ₂ .

Comparative Example

A cathode active material represented by Na _1.0 [Ni _0.25 Fe _0.25 Mn _0.5 ] O ₂ was prepared in the same manner as in Example except that only sodium carbonate was mixed with a precursor represented by Ni _0.25 Fe _0.25 Mn _0.5 (OH) ₂ . .

Experimental Example XRD Measurement

XRD of the cathode active materials prepared in Examples and Comparative Examples was measured and the results are shown in FIG. 1. As shown in FIG. 1, in the XRD of the positive electrode active material prepared in the embodiment of the present invention, it can be seen that 2θ shows three peaks in a range of 30 ° to 40 °.

Experimental Example TOF-SIMS Measurement

The cation analysis result obtained by using a time-of-flight secondary ion mass spectrometer (TOF-SIMS) for the cathode active materials prepared in Examples and Comparative Examples is shown in FIG. 2. Indicated.

As shown in FIG. 2, the positive electrode active material prepared in Examples of the present invention exhibits 6Li ⁺ and 7Li ⁺ peaks, and the positive electrode active material of the Comparative Example does not show peaks.

Experimental Example X-ray photoelectron spectroscopy (XPS) measurement

3 shows the results of measuring the oxidation number change of the transition metal using XPS for the cathode active materials prepared in Examples and Comparative Examples.

As shown in FIG. 3, in the case of the cathode active material prepared in the embodiment of the present invention, Ni ³⁺ may appear at 855 to 860 eV, indicating that Ni ²⁺ is partially changed to Ni ³⁺ .

Preparation Example Sodium Battery Production

Composite metal oxide cathode active material prepared in the above examples or comparative examples, acetylene black (manufactured by Denki Chemical Co., Ltd.) as a conductive material, and PVDF (manufactured by KK Corporation, polyvinylidene difluoride polyflon) as a binder Poly Vinylidene DiFluoride Polyflon)) was weighed so as to have a composition of positive electrode active material: conductive material: binder = 85:10: 5 (weight ratio).

Thereafter, first, the composite metal oxide cathode active material and acetylene black are sufficiently mixed with agate mortar, and N-methyl-2-pyrrolidone (NMP: manufactured by Tokyo Kasei Kogyo Co., Ltd.) is appropriately added to the mixture. Furthermore, PVDF was added and mixed to make it uniform and slurry. The obtained slurry was apply | coated to the thickness of 100 micrometers using an applicator on the aluminum foil of 40 micrometers thickness which is an electrical power collector, it was put into the dryer, and it dried sufficiently, removing NMP, and obtained the positive electrode sheet. The positive electrode sheet 1 thus prepared was punched out with an electrode puncher to a diameter of 1.5 cm, and then sufficiently compressed by a hand press to prepare a positive electrode.

1 M NaClO to which a positive electrode prepared with the aluminum foil faced downward was placed in the recess of the lower part of the coin cell (manufactured by Hosen Kabushiki Co., Ltd.), followed by addition of 5 vol% of fluoroethylene carbonate (FEC) as a nonaqueous electrolyte. _A sodium secondary battery was produced by combining a polypropylene porous membrane (thickness 20 μm) with a ₄ / propylene carbonate and a separator and a sodium metal as a negative electrode.

Experimental Example Measurement of Initial Charge and Discharge Characteristics

Initial charge and discharge characteristics were measured for each of the batteries including the cathode active materials prepared in Examples and Comparative Examples, which were manufactured in the preparation examples, and the results are shown in FIG. 4.

Experimental Example Measurement of Charge and Discharge Characteristics

The charge and discharge characteristics were measured at 0.2 C and 0.5 C conditions for each of the batteries including the cathode active materials prepared in Examples and Comparative Examples, which were prepared in the preparation examples, and the results are shown in FIGS. 5 to 8.

Experimental Example Thermal Stability Measurement

Thermal safety by DSC measurement was measured for each of the batteries including the cathode active materials prepared in Examples and Comparative Examples, prepared in the above preparation examples, and the results are shown in FIGS. 9 and 10.

As shown in FIGS. 9 and 10, in the case of the cathode active material including Li according to an embodiment of the present invention, the ignition temperature is 297.6 ° C. and the temperature of the ignition point is 20 ° C. or more higher than that of the comparative example. In addition, it can be seen that the thermal stability is greatly improved in the case of the cathode active material including Li by 30% or more by the embodiment of the present invention.

Experimental Example XRD Measurement after Charge and Discharge

XRD was measured after charge and discharge experiments for each of the batteries including the cathode active materials prepared in Examples and Comparative Examples, and the results are shown in FIG. 11.

As shown in FIG. 11, in the positive electrode active material including Li according to an embodiment of the present invention, the O 3 structure is maintained even after charging and discharging. In the comparative example, the O 3 crystal structure was not maintained due to migration of Fe ions and changed to P 3. You can check it.

Experimental Example

For each of the batteries including the cathode active materials prepared in Examples and Comparative Examples, the rate characteristics were measured at 0.1 C charging conditions and 0.1 C to 5 C discharge conditions at room temperature, and the results are shown in FIG. 12.

As shown in FIG. 12, in the case of the cathode active material including Li according to the embodiment of the present invention, it can be seen that the rate characteristic is greatly improved compared to the cathode material not doped with conventional Li.

Experimental Example Measurement of Life Characteristics

For each of the batteries including the cathode active material prepared in Examples and Comparative Examples, the life characteristics of 200 cycles at 0.5 C at room temperature were measured and the results are shown in FIG. 13.

As shown in FIG. 13, the positive electrode active material including Li exhibits a capacity retention rate of 76% for 200 cycles according to an embodiment of the present invention, and shows that the life characteristics of the positive electrode active material not doped with Li are greatly improved. Can be.

The cathode active material for sodium secondary battery according to the present invention is structurally more stable by substituting a part of the transition metal with Li, and accordingly, thermal stability and lifespan characteristics of the sodium battery including the cathode active material may be greatly improved.

Claims (14)

1. Sodium secondary battery positive electrode active material represented by the formula (1).

   [Formula 1]

   Na _x Li _a [Ni _y Fe _z Mn _1-yzb M _b ] _1-a O ₂

   (In Formula 1, M is an element selected from the group consisting of Co, Cr, Zr, Nb, Cu, V, Ti, Zn, Al, Ga, Mg, B and combinations thereof,

   0.8≤x≤1.2, 0.01≤a≤0.1, 0.05≤y≤0.9, 0.05≤z≤0.9, 0≤b≤0.9, 0.05≤1-y-z-b≤0.9
2. The method of claim 1,

   The cathode active material for sodium secondary battery has a spherical particle size of 5 to 15 μm, and the particle size is monodisperse positive electrode active material for sodium secondary battery.
3. The method of claim 1,

   The cathode active material for sodium secondary battery is a cathode active material for sodium secondary battery, characterized in that 2θ in XRD shows three peaks in the range of 30 ° to 40 °.
4. The method of claim 3, wherein

   The cathode active material for sodium secondary battery is a cathode active material for sodium secondary battery, characterized in that the peak (104) peak in the range 2θ in the range 40 ° to 45 ° in XRD.
5. The method of claim 1,

   The cathode active material for sodium secondary battery is a cathode active material for sodium secondary battery, characterized in that it has a peak of 6Li ⁺ , 7Li ⁺ obtained by cation analysis by time-of-flight secondary ion mass spectrometry.
6. The method of claim 1,

   The cathode active material for sodium secondary battery is a cathode active material for sodium secondary battery having a peak of Ni ³⁺ at 855 to 860 eV in the oxidation number analysis by X-ray photoelectron spectroscopy (XPS).
7. Mixing a cathode active material precursor, a sodium compound, and a lithium compound for a sodium secondary battery; And

   Heat treatment of the mixture; Method of producing a cathode active material for sodium secondary battery of claim 1 comprising a.
8. The method of claim 7, wherein

   The cathode active material precursor for the sodium secondary battery is a method of producing a cathode active material for sodium secondary battery that is represented by any one of the formulas (2) to 4 below.

   Ni _y Fe _z Mn _1-yzb M _b (OH) ₂

   Ni _y Fe _z Mn _1-yzb M _b C ₂ O ₄

   [Ni _y Fe _z Mn _1-yzb M _b ] ₃ O ₄

   (In Formulas 2 to 4, M is an element selected from the group consisting of Co, Cr, Zr, Nb, Cu, V, Ti, Zn, Al, Ga, Mg, B, and combinations thereof,

   0.05≤y≤0.9, 0.05≤z≤0.9, 0≤b≤0.9, 0.05≤1-y-z-b≤0.9)
9. The method of claim 7, wherein

   The sodium compound is sodium carbonate, sodium nitrate, sodium acetate, sodium hydroxide, sodium hydroxide hydrate, sodium oxide or a method for producing a cathode active material for a sodium secondary battery, characterized in that combinations thereof.
10. The method of claim 7, wherein

    The sodium compound per one mole of the positive electrode active material precursor for the sodium secondary battery is a method for producing a positive electrode active material for sodium secondary battery, characterized in that the mixture of 0.8 to 1.5 moles.
11. The method of claim 7, wherein

    The lithium compound is a lithium nitrate, lithium acetate, lithium carbonate, lithium hydroxide, and a method for producing a cathode active material for sodium secondary battery, characterized in that any one selected from the group consisting of a combination thereof.
12. The method of claim 7, wherein

    In the heat treatment step, the cathode active material for sodium secondary battery, characterized in that the heat treatment at 600 ℃ to 1000 ℃.
13. A sodium secondary battery positive electrode comprising the cathode active material for sodium secondary battery of claim 1.
14. A sodium secondary battery comprising the cathode for sodium secondary battery of claim 13.

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