KR101948549B1 - Cathode Active Material for Secondary Battery - Google Patents

Abstract

The present invention relates to a secondary battery, the positive electrode active material, specifically, a secondary battery positive electrode active material according to the invention are sodium transition satisfying ^{_{Na 3.12-x2 A c x1 M}} 1 a y1 M 2 b y2 (P 2 O 7) 2 It has the advantage of structural stability due to the strong PO bond of the olivine-structured sodium transition metal phosphate, which contains metal pyrophosphate, and smooth insertion and desorption of Na ion having a large ion radius is performed, There is an advantage that the charging / discharging speed can be remarkably improved.

Description

Technical Field [0001] The present invention relates to a cathode active material for a secondary battery,

The present invention relates to a cathode active material for a secondary battery, and more particularly, to a cathode active material for a secondary battery capable of intercalating / deintercalating ions in a structure very rapidly.

Phosphorous materials based on olivine structure such as LiFePO _{4 in} lithium ion batteries are one of commercially available cathode materials and are highly stable due to strong PO bonding.

Therefore, efforts have been made to use such materials having a similar structure as a cathode material in a sodium secondary battery. Recently, a technical approach has been made to utilize an olivine phosphate material such as NaMPO ₄ (M = Fe, Co, Ni, Mn) in sodium secondary batteries as disclosed in U.S. Publication No. 20110008233. However, since the sodium ion has a much larger ion size than the lithium ion size, the Na ion is not smoothly inserted / desorbed due to the size of the ion, which is problematic in terms of dynamic characteristics (kinetic aspects) The battery performance is not shown. Therefore, it is necessary to develop new materials with much better dynamic characteristics than conventional NaFePO ₄ materials.

U.S. Publication No. 20110008233

An object of the present invention is to provide a cathode active material for a secondary battery having excellent structural stability and reversible ion insertion and desorption, and a secondary battery having the same.

The present invention provides a cathode active material for a secondary battery, wherein the cathode active material for a secondary battery according to the present invention contains a sodium transition metal pyrophosphate satisfying the following formula (1).

 (Formula 1)

Na _{3 .12-x 2} A ^c _{x 1} M ₁ ^a _y1 M ₂ ^b _y2 (P ₂ O ₇ ) ₂

In formula (1), A ^c represents one or more selected elements (A) selected from alkali metals and alkaline earth metals having a valence of c of 1 or 2, x 1 is a real number satisfying 0? X 1? 0.5, x 2 is c and x 1 of a product (x2 = x1 * c), and, M ₁ ^a is a transition metal and 12 to 14 group having a valence of a mean one or more than one selected element (M _1), and M ₂ ^b is a valence of b mean one or more than one selected element (M ₂₎ in the transition metal and 12 to Group 14 with and, a and b, and the same as or different from each other, and a and b are independently 2 to an integer from 4 to each other, y1 is 0 <y1? 2.44, and y2 is a real number satisfying 0? y2? 2.44.

In the cathode active material for a secondary battery according to an embodiment of the present invention, the sodium transition metal pyrophosphate may be in a triclinic phase.

In the cathode active material for a secondary battery according to an embodiment of the present invention, the sodium transition metal pyrophosphate may be a P-1 space group.

In the cathode active material for a secondary battery according to an embodiment of the present invention, the sodium transition metal pyrophosphate may satisfy (y1 * a) + (y2 * b) = 4.88 in the formula (1).

In the cathode active material for a secondary battery according to an embodiment of the present invention, M1 and M2 of the sodium transition metal pyrophosphate are independently selected from Co, Ni, Fe, Mn, V, Cu, Ti, Al, Cr, May be one or more selected elements.

In the cathode active material for a secondary battery according to an embodiment of the present invention, A of the sodium transition metal pyrophosphate may be one or more selected from Li, Mg, and Ca.

In the cathode active material for a secondary battery according to an embodiment of the present invention, the sodium transition metal pyrophosphate may contain at least iron (Fe), manganese (Mn), or iron and manganese.

The cathode active material for a secondary battery according to an embodiment of the present invention may have a capacity of 80 mAh / g or more under charge / discharge conditions of 1.7 V / 4.0 V and 0.05 C.

The cathode active material for a secondary battery according to an embodiment of the present invention may be for a sodium secondary battery.

The cathode active material for a secondary battery according to an embodiment of the present invention may further contain carbon.

The present invention includes a sodium secondary battery including the above-mentioned cathode active material.

Since the cathode active material for a secondary battery according to the present invention contains the sodium transition metal pyrophosphate according to Formula 1, it has an advantage of structural stability due to strong PO bonding of an olivine structure sodium transition metal phosphate, Na ion can be smoothly inserted and desorbed, so that the reversibility at the time of charging / discharging and the charging / discharging rate can be remarkably improved.

1 is a scanning electron microscope (SEM) image of sodium iron pyrophosphate prepared,
FIG. 2 is a graph showing the results of X-ray diffraction analysis of sodium iron pyrophosphate, iron-manganese (Fe _1.22 Mn _1.22 ) pyrophosphate, and manganese pyrophosphate produced,
FIG. 3 is a diagram showing the crystal structure of sodium iron pyrophosphate prepared through Rietveld refinement using the GSAS program,
4 is a schematic diagram showing the crystal structure of sodium iron pyrophosphate prepared,
FIG. 5 is a graph showing charge / discharge characteristics of the sodium iron pyrophosphate phosphate secondary battery manufactured,
6 is a graph showing the reversible capacity according to the number of charge / discharge cycles of the sodium iron pyrophosphate phosphate secondary battery manufactured,
FIG. 7 is a graph showing the change (rate characteristic) of the reversible capacity with increasing current density of the sodium iron pyrophosphate phosphate secondary cell produced,
FIG. 8 is a diagram showing the crystal structure of sodium iron pyrophosphate in a state of being filled with sodium iron pyrophosphate prepared through Rietveld refinement using the GSAS program.

Hereinafter, the cathode active material according to the present invention will be described in detail. Hereinafter, the technical and scientific terms used herein will be understood by those skilled in the art without departing from the scope of the present invention. Descriptions of known functions and configurations that may be unnecessarily blurred are omitted.

The cathode active material for a secondary battery according to the present invention contains a sodium transition metal pyrophosphate satisfying the following formula (1).

 (Formula 1)

^{_{Na 3.12-x2 A c x1 M}} 1 a y1 M 2 b y2 (P 2 O 7) 2

In formula (1), A ^c represents one or more selected elements (A) selected from alkali metals and alkaline earth metals having a valence of c of 1 or 2, x 1 is a real number satisfying 0? X 1? 0.5, x 2 is c and x 1 of a product (x2 = x1 * c), and, M ₁ ^a is a transition metal and 12 to 14 group having a valence of a mean one or more than one selected element (M _1), and M ₂ ^b is a valence of b mean one or more than one selected element (M ₂₎ in the transition metal and 12 to Group 14 with and, a and b, and the same as or different from each other, and a and b are independently 2 to an integer from 4 to each other, y1 is 0 <y1? 2.44, and y2 is a real number satisfying 0? y2? 2.44.

If below, in the following described with the formula, A, is not given the atom (a, b, c), such as M ₁ or M _2, symbols (A, M ₁ or M ₂₎ other than the atom is a compound according to formula (I) means an element to be contained in and, a ^c, M ₁ ^a, mark (a ^c, M ₁ ^a, M ₂ ^b), including, atom if given the atom, such as M ₂ ^b is a compound structure according to formula 1 The ionic state of the corresponding valence of each element, i.e., + c, + a, + b, can be referred to.

In the cathode active material for a secondary battery according to an embodiment of the present invention, M ₁ and M ₂ in Formula 1 may be the same element having different valences, or different elements having different valences. Further, M ₁ may be one or two or more elements having a valence of a, and M ₂ may be one or two or more elements having a valence of b.

In the positive electrode active material for a secondary battery according to an embodiment of the present invention, A may be selected from one or more of alkali metals and alkaline earth metals other than sodium (Na).

In the cathode active material for a secondary battery according to an embodiment of the present invention, M ₁ or M ₂ may contain at least a transition metal.

In the cathode active material for a secondary battery according to an embodiment of the present invention, the sodium transition metal pyrophosphate represented by Chemical Formula 1 may satisfy (y1 * a) + (y2 * b) = 4.88.

In the cathode active material for a secondary battery according to an embodiment of the present invention, the transition metal is at least one selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Tc, Ru, Ag, Hf, Ta, W, Re, Os, Ir, Pt and Au, and Group 12 to 14 may include Zn, Al, Ga, In, Tl, Ge, Sn and Pb .

In the cathode active material for a secondary battery according to an embodiment of the present invention, the alkali metal may include Li, K, Rb and Cs groups, and the alkaline earth metal may include Be, Mg, Ca, Sr and Ba .

In the cathode active material for a secondary battery according to an embodiment of the present invention, the sodium transition metal pyrophosphate represented by Chemical Formula 1 may be a discharge state composition of the cathode active material. At this time, the discharging state is a state in which the sodium secondary battery provided with the anode containing the cathode active material is discharged from 1.7 (discharge) to 4.0 V (charge) vs. When the charge / discharge is performed at 0.05 C in the Na / Na + voltage range, it means that the discharge is completed to 1.7 V and may include the discharge state of the first charge / discharge cycle (charge-discharge sequence). In detail, the sodium secondary battery in which the charge / discharge cycle is performed is obtained by mixing the cathode active material according to the present invention: conductive material: binder in a weight ratio of 7: 1.5: 1.5 and adding N-methylpyrrolidone (NMP) A positive electrode active material slurry was coated on an aluminum foil and dried in a vacuum oven at 120 캜 for 10 hours to prepare a positive electrode and a sodium metal as an opposite electrode, and 0.8 M NaClO _{4 / (} ethylenecarbonate + diethylenecarbonate, 1: ₁ vol.ratio) And may include a used battery.

Attempts have been made to use a sodium transition metal phosphate such as NaFePO _{4 in a} cathode active material for a sodium secondary battery. However, since the sodium transition metal phosphate has an olivine structure, when an ionic radius such as Na ion is very large, It has a structural limitation that reversible insertion and desorption of ions are not smooth. Accordingly, when the sodium transition metal phosphate of the olivine structure is used as the cathode active material of the secondary battery, not only the reversibility of the reaction but also the charge / discharge rate is very low.

Since the cathode active material for a sodium secondary battery according to the present invention contains a sodium transition metal pyrophosphate represented by Chemical Formula 1, it has a strong structural stability even in the insertion and desorption of ions due to a strong PO bond, and also has an ionic radius The reversibility and charging / discharging speed at the time of charging / discharging can be remarkably improved.

In the cathode active material for a secondary battery according to an embodiment of the present invention, the sodium transition metal pyrophosphate may be a triclinic phase and may have a crystal structure of P-1 space group. By this structure, the sodium transition metal pyrophosphate can have a tunnel structure in which sodium ions (Na ⁺ ) move smoothly in the [100] and [010] directions.

In the cathode active material for a secondary battery according to an embodiment of the present invention, A contained in the sodium transition metal pyrophosphate may be one or more selected from Li, Mg, and Ca. Li, Mg and Ca have ionic radii similar to that of Na and can be substituted at the site of sodium (sodium ion) in the crystal structure of the sodium transition metal pyrophosphate to form a substitutional solid solution have.

In the cathode active material for a secondary battery according to an embodiment of the present invention, M ₁ and M ₂ contained in the sodium transition metal pyrophosphate according to Formula 1 are independently selected from Co, Ni, Fe, Mn, V, Cu, Ti , Al, Cr, Mo, and Nb. Preferably, the transition metals M1 and M2 contained in the sodium transition metal pyrophosphate according to formula (I) may be different from each other.

In the cathode active material for a secondary battery according to an embodiment of the present invention, M ₁ and M ₂ contained in the sodium transition metal pyrophosphate according to formula (1) are independently from each other at least one or more than two of Co, Ni, Fe and Mn And M ₁ and M ₂ may be different from each other.

Specifically, the cathode active material for a secondary battery according to an embodiment of the present invention may contain a sodium transition metal pyrophosphate satisfying the following formula 1-1.

(1-1)

Na _3.12 M ₃ ^II _{2.44- y 5} M ₄ ^b _{y 4} (P ₂ O ₇ ) ₂

In formula (1-1), M ₃ ^II is one or more elements (M ₃ ) selected from Co, Ni, Fe, Mn, V, Cu, Ti, Al, Cr, Mo and Nb having a valence of 2 and M ₄ ^b is at least one element (M ₄ ) selected from Co, Ni, Fe, Mn, V, Cu, Ti, Al, Cr, Mo and Nb having a valence of b; b is an integer of 3 or 4 y5 = y4 * b, and y5 is a real number satisfying 0? y5?

As the Co, Ni, Fe, Mn, V, Cu, Ti, Al, Cr, Mo and Nb have similar ionic radii, the position of the transition metal (e.g., Fe) in the crystal structure of the sodium transition metal pyrophosphate may be substituted in the site, and a substitutional solid solution may be formed.

Specifically, the cathode active material for a secondary battery according to an embodiment of the present invention may contain a sodium transition metal pyrophosphate satisfying the following formula 1-2.

(1-2)

Na _3.12 M ₅ ^II _2.44 (P ₂ O ₇ ) ₂

In formula 1-2, M ₅ ^II is one or more selected elements (M ₅ ) from Co, Mn, Ni and Fe having a valence of 2.

Co, Mn, Ni and Fe can be involved in charging and discharging reactions as they can be electrochemically redox reactions. As they have similar ionic radii, the transition metals in the crystal structure of sodium transition metal pyrophosphate For example, it may be substituted at the site of Fe), and a substitutional solid solution may be formed.

The cathode active material for a secondary battery according to an embodiment of the present invention may contain particulate sodium transition metal pyrophosphate. Specifically, the cathode active material for a secondary battery may contain a sodium transition metal pyrophosphate in particulate form having an average primary particle size of 500 nm to 10 mu m. Specifically, the cathode active material for a secondary battery may contain sodium transition metal pyrophosphate, which is composed of primary particles and is secondary particles having an average size of 10 μm to 500 μm.

The cathode active material for a secondary battery according to an embodiment of the present invention may further contain carbon. The carbon contained in the cathode active material may be any carbon used in a conventional secondary battery cathode active material, and examples thereof include acetylene black, graphite, soft carbon, and hard carbon.

In the cathode active material for a secondary battery according to an embodiment of the present invention, the sodium transition metal pyrophosphate contained in the cathode active material is in the form of a particle, and a coating layer coated with carbon may be formed on the particle surface.

In the cathode active material for a secondary battery according to an embodiment of the present invention, the cathode active material may further contain 1 to 43 parts by weight of carbon based on 100 parts by weight of the sodium transition metal pyrophosphate. At this time, the carbon contained in the cathode active material may be homogeneously mixed with the sodium transition metal pyrophosphate, and may exist as a coating layer coated on the surface of the sodium transition metal pyrophosphate particles.

The cathode active material for a secondary battery according to an embodiment of the present invention may have a capacity of 80 mAh / g or more under charge / discharge conditions of 1.7 / 4.0 V and 0.05 to 0.2 C (current density).

The cathode active material for a secondary battery according to an embodiment of the present invention may be a cathode active material for a sodium secondary battery. At this time, the sodium secondary battery may be a sodium secondary battery having molten Na as a cathode or a full-scale Na secondary battery.

The present invention includes a cathode for a sodium secondary battery including the above-described cathode active material for a secondary battery.

The positive electrode according to an embodiment of the present invention may include the positive electrode active material and the current collector described above, and the positive electrode active material layer coated or coated with the positive electrode active material may be formed on at least one surface of the current collector. In detail, in order to form the positive electrode active material layer, the positive electrode active material slurry may be applied or coated on the current collector, and the positive electrode active material slurry may be used in combination with the above-described positive electrode active material in combination with a dispersion medium, &Lt; / RTI &gt;

The present invention includes a sodium secondary battery having the above-described anode.

The sodium secondary battery according to an embodiment of the present invention may include a negative electrode containing sodium, a positive electrode having the positive electrode active material, and an electrolyte having an ionic conductivity with respect to sodium ion between the positive electrode and the negative electrode.

The sodium secondary battery according to an embodiment of the present invention includes a cathode containing sodium, a cathode containing the cathode active material, and a high-sodium sodium secondary battery in which both electrolytes are solid, and a sodium secondary battery And a sodium secondary battery having a positive electrode electrolyte together with a solid electrolyte (for example, NASICON), and may further include a separator if necessary. When the electrolyte (or liquid electrolyte) is provided, the anode and / or the cathode may be positioned in the electrolyte so that the sodium ions conducted in the solid electrolyte can be effectively transferred to the active material.

In a sodium secondary battery according to an embodiment of the present invention, the anode includes an anode active material containing sodium, and the electrolyte may include an organic solvent containing a sodium salt. Substantial For example, the cathode may be a sodium metal, and sodium salt contained in the electrolyte is _{NaAsF 6, NaPF 6, NaClO 4} , NaB (C 6 H 5), NaAlCl 4, NaBr, NaBF 4 , or to mixtures thereof And the organic solvent may contain ethylene carbonate, dimethyl carbonate, methylethyl carbonate, propylene carbonate, or a mixture thereof. However, it goes without saying that the present invention can not be limited by the kind of the negative electrode, the kind of the electrolyte, or the structure of the battery.

Hereinafter, a method for producing a sodium transition metal pyrophosphate for a cathode active material of a secondary battery will be described in detail.

A method for preparing a sodium transition metal pyrophosphate for a cathode active material of a secondary battery according to the present invention comprises the steps of: a) preparing a precursor raw material by mixing a sodium precursor, a metal precursor and a phosphoric acid precursor; b) heat treating the precursor material in an inert gas atmosphere.

In the method for preparing a sodium transition metal phosphate for a cathode active material of a secondary battery according to an embodiment of the present invention, step b) comprises: b1) heat treating the precursor material in an inert gas atmosphere at 200 to 400 ° C to prepare a first precursor material ; And b2) grinding the first precursor material; b3) heat treating the pulverized first precursor material in an inert gas atmosphere at 500 to 700 캜 to prepare a sodium transition metal pyrophosphate; . &Lt; / RTI &gt; At this time, it is needless to say that the crushing step (b4) in which the product obtained in the step b3) is physically crushed so as to have a suitable size for the cathode active material may be further performed.

In a method of preparing a sodium transition metal pyrophosphate for a cathode active material of a sodium secondary battery according to an embodiment of the present invention, the sodium precursor is sodium carbonate (Na ₂ CO ₃ ), sodium acetate (NaOCH ₂ CH ₃ ), sodium carbonate (Na ₂ CO ₃ ) (NaOH), and hydrates thereof. The metal precursor may comprise one or more selected materials from oxalates, acetates, carbonates and hydrates of the metals. Acid precursor is (NH _{4) 2} HPO _4, may be in the NH ₄ H ₂ PO _4, and H ₃ PO ₄ containing one or two or more selected materials.

In the method for preparing a sodium transition metal pyrophosphate for a sodium secondary battery positive electrode active material according to an embodiment of the present invention, the metal of the metal precursor may be an element selected from transition metals and one or more elements selected from Groups 12 to 14, The transition metal is at least one selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Tc, Ru, Th, Pd, Ag, Hf, Ta, W, Pt and Au, and groups 12 to 14 may include Zn, Al, Ga, In, Tl, Ge, Sn and Pb groups. Preferably, the metal of the metal precursor may be an element selected from at least one of Co, Ni, Fe, Mn, V, Cu, Ti, Al, Cr, Mo and Nb. More specifically, the metal of the metal precursor may be at least one selected from Co, Ni, Fe and Mn.

In the method for producing a sodium transition metal pyrophosphate for a cathode active material of a secondary battery according to an embodiment of the present invention, the molar ratio of Na: metal: phosphoric acid is 3 to 3.3: 2.2 to 2.5: 1 Precursor materials can be mixed.

In the preparation of the sodium transition metal pyrophosphate for a cathode active material of a secondary battery according to an embodiment of the present invention, the precursors may be mixed by the milling during the production of the precursor raw material, Can be carried out using conventional methods used for homogenous mixing and pulverization of powders such as attrition mills.

In the method for producing a sodium transition metal pyrophosphate for a cathode active material of a secondary battery according to an embodiment of the present invention, the produced raw material is heat-treated in an inert gas atmosphere containing argon, helium, neon, nitrogen or a mixed gas thereof.

In detail, the raw material is subjected to a low-temperature heat treatment at a temperature of 200 to 400 ° C in an inert gas atmosphere, followed by pulverizing a product obtained by a low-temperature heat treatment, and then subjected to a high-temperature heat treatment at 500 to 700 ° C in an inert gas atmosphere .

In the method for producing a sodium transition metal pyrophosphate for a cathode active material of a secondary battery according to an embodiment of the present invention, a precursor material is subjected to a two-stage heat treatment at a low temperature and a high temperature, and a product obtained by a low- By subjecting to milling and mixing again through milling, including milling or agitation, and then subjecting to a high temperature heat treatment, it is possible to produce a sodium transition metal pyrophosphate having excellent compositional uniformity and having a primary particle size in micrometer order have.

Hereinafter, the present invention will be described in detail with reference to practical examples. It should be understood that the present invention is not limited by the scope of the present invention, And the scope of the claims of the present invention.

(Production Example 1)

Preparation of sodium iron pyrophosphate

1.06 g of Na ₂ CO ₃ , 1.8 g of FeC ₂ O ₄ .2H ₂ O and 2.64 g (NH ₄ ) ₂ HPO ₄ were mixed and ball milled and then heat-treated at 300 ° C. for 6 hours in an argon gas atmosphere ). The primary heat-treated raw materials were ball milled, mixed again, and then heat-treated (secondary) in an argon gas atmosphere at 600 ° C for 6 hours to prepare sodium iron pyrophosphate.

 (Production Example 2)

Preparation of sodium manganese iron pyrophosphate

1.06 g Na ₂ CO ₃ , 0.9 g FeC ₂ O ₄ .2H ₂ O, 0.9 g MnC ₂ O ₄ .2H ₂ O and 2.64 g (NH ₄ ) ₂ HPO ₄ were mixed and ball milled, Followed by heat treatment (primary) in an argon gas atmosphere for 6 hours. The first heat treated raw materials were ball milled, mixed again, and then heat-treated (secondary) in an argon gas atmosphere at 600 ° C for 6 hours to prepare sodium manganese iron pyrophosphate.

(Production Example 3)

Preparation of sodium manganese pyrophosphate

1.06 g of Na ₂ CO ₃ , 1.8 g of MnC ₂ O ₄ .2H ₂ O and 2.64 g (NH ₄ ) ₂ HPO ₄ were mixed and ball milled and then heat-treated at 300 ° C. for 6 hours in an argon gas atmosphere ). The first heat treated raw materials were ball milled, mixed again, and then heat-treated (secondary) in an argon gas atmosphere at 600 ° C for 6 hours to prepare sodium manganese pyrophosphate.

(Production Example 4)

Preparation of sodium secondary battery

A binder (PVdF; polyvinylidene fluoride) was prepared by mixing the sodium transition metal pyrophosphate: conductive material (carbon black (super P)) and the binder (PVdF: polyvinylidene fluoride) prepared in Production Example 1, Production Example 2 or Production Example 3 at a weight ratio of 7: After the slurry was prepared by using N-methylpyrrolidone (NMP) solution, the prepared slurry was coated on an aluminum foil and dried in a 120 ° C vacuum oven for 10 hours to prepare a positive electrode. Of sodium metal with an anode prepared in the glove box to the opposite electrodes, by using an ethylene carbonate / diethylene carbonate (ethylenecarbonate / diethylenecarbonate) (1/1 volume ratio) containing 0.8M NaClO ₄ as an electrolyte was prepared in the cell.

1.7 (discharge) -4.0 (charge) V vs. &lt; RTI ID = 0.0 &gt; And charged and discharged at 0.05 and 0.2 C in the Na / Na ⁺ voltage range. The charge-discharge cycle was performed in the order of charge-discharge.

FIG. 1 is a scanning electron microscope (SEM) image of sodium iron pyrophosphate prepared in Preparation Example 1. FIG. 1 (a) is a high magnification scanning electron micrograph and FIG. 1 (b) is a low magnification scanning electron micrograph. As can be seen in FIG. 1, it was confirmed that sodium iron pyrophosphate was produced as a secondary particle on which primary particles having a size of several micrometers were formed.

Figure 2 Na _3.12 Fe _2.44 in Preparation Example 1 of Production Example 1 to the sodium transfer in a view showing an X- ray diffraction analysis of the metallic pyrophosphate, as can be seen in Figure 2, the triclinic crystal system prepared in Example 3 ( P ₂ O ₇ ) ₂ in Production Example 2, Na _3.12 (Fe _1.22 Mn _1.22 ) (P ₂ O ₇ ) _{2 in} Production Example ₂ and Na _3.12 Mn _2.44 (P ₂ O ₇ ) ₂ in Production Example 3 were produced. FIG. 3 shows that the sodium transition metal pyrophosphate prepared has a P-1 space group as a result of a structural simulation (GSAS rietveld refinement) based on the structure and measured composition by x-ray crystallization, It can be seen that a huge channel is formed in the [100] and [010] directions to provide a sink insertion path of sodium ions having a large ion radius.

FIG. 5 shows the results of evaluating the charge / discharge characteristics of the sodium secondary battery of Production Example 4 containing sodium iron pyrophosphate prepared in Production Example 1 as a cathode active material, showing 1.7 (discharge) -4.0 (charge) V vs. It is the result of charging / discharging at 0.05C in Na / Na ⁺ voltage range. As can be seen from the results of FIG. 5, after the sodium ion contained in the sodium iron pyrophosphate was removed immediately after the first charge / discharge, reversible sodium ion removal / insertion reaction was performed in which the sodium ion was inserted at the discharge And has a reversible capacity of about 85 mAh / g or more.

FIG. 6 is a diagram showing the reversible capacity according to the number of charge / discharge cycles of the sodium secondary battery of Production Example 4 containing sodium iron pyrophosphate prepared in Production Example 1 as a cathode active material, and shows 1.7 (discharge) -4.0 ) V vs. (C / 20 in Fig. 6) and 0.2C (C / 5 in Fig. 6) in the Na / Na ⁺ voltage range. As can be seen from FIG. 6, it can be seen that reversible insertion and desorption of sodium ions are performed smoothly, and that it has a stable charge / discharge characteristic even up to 60 cycles. In the charge / discharge cycle, the sodium iron pyrophosphate structure has two It can be seen that Na ⁺ reversibly intercalates / deintercalates.

7 is a graph showing the relationship between the current density of 1C (1C in Fig. 7) to 0.05C (C / 20 in Fig. 7) of the sodium secondary battery of Production Example 4 containing sodium iron pyrophosphate prepared in Production Example 1 as a cathode active material And shows the change of reversible capacity. As it can be seen in FIG. 7 0.1C (Fig. C / 10 6) in the current density may be seen having a reversible capacity 80mAhg ^-1 or more, at a current density of 1C 65mAhg (1C of FIG. ⁶⁾ at least ^one reversible capacity Is maintained.

FIG. 8 is a graph showing the crystal structure of the sodium iron pyrophosphate in a charged state through the Rietveld refinement using the GSAS program in order to confirm the structural change. FIG. 8 shows the Rietveld refinement crystal structure As a result of the analysis, it was confirmed that the triplet structure was maintained even when sodium ion was inserted or removed.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, Those skilled in the art will recognize that many modifications and variations are possible in light of the above teachings.

Accordingly, the spirit of the present invention should not be construed as being limited to the embodiments described, and all of the equivalents or equivalents of the claims, as well as the following claims, belong to the scope of the present invention .

Claims (11)

A positive electrode active material for a secondary battery comprising a sodium transition metal pyrophosphate satisfying the following formula (1).
(Formula 1)
Na _{3 .12-x 2} A ^c _{x 1} M ₁ ^a _y1 M ₂ ^b _y2 (P ₂ O ₇ ) ₂
(A and ^c stands for one or two or more selected element (A) from an alkali metal and an alkaline earth metal having a valence of 1 or 2. c in formula 1, x1 is a real 0≤x1≤0.5, x2 is c and the product of x1 (x2 = x1 * c), and, M ₁ ^a is a transition metal and 12 to 14 group having a valence of a mean one or more than one selected element (M _1), and M ₂ ^b is the valence of b This means the transition metal and 12 to 14 in the group of one or two or more selected element (M ₂₎ and having, a and b are the same or different from each other, a and b are independently 2 to an integer from 4 to each other, y1 is 0 < y1? 2.44, and y2 is a real number satisfying 0? Y2? The method according to claim 1,
Wherein the sodium transition metal pyrophosphate is a triclinic phase. The method according to claim 1,
Wherein the sodium transition metal pyrophosphate is a P-1 space group. The method according to claim 1,
The sodium transition metal pyrophosphate satisfies (y1 * a) + (y2 * b) = 4.88 in the formula (1). The method according to claim 1,
Wherein M 1 and M 2 are independently selected from the group consisting of Co, Ni, Fe, Mn, V, Cu, Ti, Al, Cr, Mo and Nb. 6. The method of claim 5,
Wherein A is one or more selected from Li, Na, Mg and Ca. The method according to claim 1,
The sodium transition metal pyrophosphate includes at least iron (Fe), manganese (Mn), or iron and manganese. The method according to claim 1,
Wherein the positive electrode active material has a capacity of 80 mAh / g or more under charge / discharge conditions of 1.7 V / 4.0 V and 0.05 C. 9. The compound according to any one of claims 1 to 8,
Wherein the positive electrode active material is a sodium secondary battery. 9. The compound according to any one of claims 1 to 8,
Wherein the positive electrode active material further contains carbon. A sodium secondary battery comprising a cathode active material according to any one of claims 1 to 8.

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