KR20140137821A - Positive Electrode Material for Sodium-Ion Batteries and Sodium-Ion Battery Having the Same - Google Patents

Abstract

Provided are a cathode material for a sodium secondary battery and a sodium secondary battery including the same. The cathode material includes a cathode active material represented by chemical formula 1; and a carbon layer applied to the surface thereof. [Chemical formula 1] Na(Cr_xM_1-x)O_2 wherein M is Ti, V, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Os, Ir, Pt or Au, and x ranges from 0.8 to 1.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a positive electrode material for a sodium secondary battery and a sodium secondary battery comprising the same. ≪

The present invention relates to a secondary battery, and more particularly, to a sodium secondary battery.

The secondary battery refers to a battery that can be charged repeatedly as well as discharged. In a typical lithium secondary battery of the secondary battery, lithium ions contained in the positive electrode active material are transferred to the negative electrode through the electrolyte and then inserted (filled) into the layered structure of the negative electrode active material. Then, lithium ions, which have been inserted into the layered structure of the negative electrode active material, It works through the principle of returning to the anode (discharging). Such a lithium secondary battery is currently commercialized and is being used as a small-sized power source such as a mobile phone, a notebook computer, and the like, and it is expected that the lithium secondary battery will also be usable as a large power source for a hybrid vehicle.

However, the composite metal oxide, which is mainly used as a cathode active material in a lithium secondary battery, contains a rare metal element such as lithium and may not meet the increase in demand. Accordingly, studies have been made on a sodium secondary battery using sodium, which is rich in supply and low in cost, as a cathode active material. As an example, Korean Laid-Open Patent Application No. 2012-0133300 discloses A _x MnPO ₄ F (A = Li or Na, 0 <x? 2) as a cathode active material.

However, it is known that the sodium anode materials developed so far still need to improve discharge capacity retention rate and speed characteristics.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a cathode material for a sodium secondary battery and a sodium secondary battery including the same, which have improved discharging capacity holding characteristics and speed characteristics.

According to an aspect of the present invention, there is provided a cathode material for a sodium secondary battery. The cathode material includes a cathode active material represented by the following formula (1) and a carbon layer coated on the surface of the cathode active material:

[Chemical Formula 1]

Na (Cr _x M _1-x ) O ₂

M is at least one element selected from the group consisting of Ti, V, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Hf, Ta, , Pt or Au, and x is 0.8 to 1.

The carbon layer may have a thickness of about 0.1 to 500 nm.

According to an aspect of the present invention, there is provided a method of manufacturing a cathode material for a sodium secondary battery. First, the sodium compound and the metal compound containing the chromium compound are mixed. The mixed metal compound is fired in an inert gas atmosphere to obtain the cathode active material of Formula 1. The cathode active material, the carbon precursor, and the organic solvent are mixed and heat-treated in a reducing atmosphere to obtain a cathode active material coated with a carbon layer.

The temperature during the firing process may be about 600 to 1200 degrees. The carbon precursor may be a pitch. The heat treatment may be performed at about 200 to 1500 degrees. The sodium compound is sodium carbonate, and the chromium compound may be chromium oxide.

According to an aspect of the present invention, there is provided a sodium secondary battery. The sodium secondary battery comprises a cathode including a cathode material containing a cathode active material represented by the above formula (1) and a carbon layer coated on the surface thereof, a cathode including a cathode active material capable of sodium intercalation, And an electrolyte positioned therebetween.

According to the present invention, by using the oxide of Formula 1 having a stable crystal structure as a cathode active material and uniformly coating a carbon layer on the surface of the cathode active material, the crystal structure of the cathode active material is further stabilized and at the same time, Can improve. As a result, the discharge capacity holding characteristic and the speed characteristic of the secondary battery can be improved.

1 is a flow chart illustrating a method of fabricating an anode in accordance with an embodiment of the present invention.
Figure 2 is a graph showing the XRD analysis of the powder according to the _second NaCrO NaCrO ₂ powder as in Preparation Example 1, the carbon coating in accordance with Preparation 2.
3a and 3b are photographs taken NaCrO ₂ powders according to the NaCrO ₂ powder as in Preparation Example 1 the carbon coating in accordance with Preparation Example 2, respectively.
Figures 4a and 4b are a TEM image taken NaCrO ₂ powders according to the NaCrO ₂ powder as in Preparation Example 1 the carbon coating in accordance with Preparation Example 2, respectively.
FIGS. 5A and 5B are graphs showing charging and discharging characteristics of a half-cell according to Production Example 4 and a half-cell according to Production Example 3, respectively.
6 is a graph showing the discharge capacity according to the number of cycles of the half-cell according to Production Example 4 and the half-cell according to Production Example 3. FIG.
7A is a graph showing a rate characteristic of a reversed phase according to Production Example 4. FIG. FIG. 7B is a graph showing a change in discharge capacity according to the number of cycles of the half-cell according to Production Example 4. FIG.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the embodiments described herein but may be embodied in other forms. Like reference numerals designate like elements throughout the specification.

In this specification, the term " on top of another layer " means that not only these layers are directly in contact but also another layer (s) is located between these layers.

A sodium secondary battery according to an embodiment of the present invention includes a positive electrode containing a positive electrode material containing a positive electrode active material and a carbon layer coated on the surface thereof, a negative electrode containing a negative electrode active material into which sodium can be inserted, As shown in FIG.

<Anode>

The cathode active material according to one embodiment of the present invention is represented by the following general formula (1).

[Chemical Formula 1]

Na (Cr _x M _{1 -x} ) O ₂

M is at least one element selected from the group consisting of Ti, V, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Hf, Ta, , Pt or Au, and x is 0.8 to 1.

The cathode active material represented by Chemical Formula 1 is a material exhibiting a layered rock salt structure, and Cr does not decrease oxidation water in an inert atmosphere and can maintain a relatively stable layer structure.

The carbon layer may be coated on the surface of the cathode active material represented by Formula 1 above. The carbon layer may have a uniform thickness of about 0.1 to 500 nm. The carbon layer not only stabilizes the layered structure of the cathode active material represented by Formula 1, that is, the crystal structure, but also improves the conductivity.

1 is a flow chart illustrating a method of fabricating an anode in accordance with an embodiment of the present invention.

Referring to FIG. 1, metal compounds, that is, a sodium compound and a chromium compound may be mixed (S1). At this time, the chromium-free transition metal compound can be further mixed. The molar ratio of the transition metal other than sodium, chromium, and chromium in the mixed metal compound may be 1: x: 1-x (x is 0.8 to 1). The chromium transition metal compound And may be Ti, V, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Os, Ir, have. The metal compounds may be metal oxides or compounds which can be metal oxides upon decomposition and / or oxidation, such as metal hydroxides, metal carbonates, metal nitrates, metal halides, or metal oxalates. Sodium compound is Na ₂ CO _3, NaHCO _3, or may be a Na ₂ 0 _2, chromium compound can be a Cr ₂ 0 _3. Chrome outer transition metal compound is _{TiO 2, Ti 2 O 3,} V 2 O 5, MnO 2, Mn 2 O 3, Mn 3 O 4, FeO, Fe 2 O 3, Fe 3 O 4, Fe (OH) O, Co ₃ O ₄ , Co (OH) ₂ , NiO _2, and the like.

The mixed metal compound is fired in an inert gas atmosphere to obtain the cathode active material of Formula 1 (S2). Specifically, the inert gas atmosphere may be an argon atmosphere or a nitrogen atmosphere. In addition, the firing temperature may be about 600 to 1200 degrees, specifically about 800 to 1000 degrees, more specifically about 900 degrees.

Thereafter, the cathode active material described in the above Formula 1 may be further pulverized. At this time, it is preferable that the positive electrode active material does not come into contact with moisture.

After mixing the cathode active material, the carbon precursor, and the organic solvent described in Formula 1, the cathode active material coated with the carbon layer may be obtained by performing heat treatment in a reducing atmosphere (S3). Specifically, the cathode active material may be contained in an organic solvent containing about 0.001 to 50 wt% of carbon precursor. The carbon precursor may be a pitch or a hydrocarbon based material such as polycrylic acid, polyacrylic acid, polyvinyl pyrrolidone, ascorbic acid, carboxylic acid, acid, acetic acid, acetylene black, adipic acid, malic acid, citric acid, acetic acid, glucose, or sucrose. The reducing atmosphere may be an inert gas atmosphere or a hydrogen atmosphere. In addition, the heat treatment temperature may be about 200 to 1500 degrees. The organic solvent may be NMP (N-methyl-2-pyrrolidone).

In the process of forming the cathode active material, the cathode active material coated with the carbon layer may be obtained by mixing the metal oxide and the carbon material. However, in this case, since the carbon material is present in the process of forming the crystal phase of the cathode active material by the reaction of the metal oxide, the cathode active material formed through this process may have a poor crystalline phase. However, in this embodiment, the crystal phase of the positive electrode active material can be kept in a good state by coating the carbon layer after obtaining the positive electrode active material having the crystalline phase in advance.

After the cathode active material coated with the carbon layer is obtained, the remaining solvent can be dried.

The cathode material may be obtained by mixing the cathode active material coated with the carbon layer, the conductive material, and the binder (S4). At this time, the conductive material may be a carbon material such as natural graphite, artificial graphite, coke, carbon black, carbon nanotube, and graphene. The binder may be a thermoplastic resin such as polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene, vinylidene fluoride copolymer, fluorine resin such as propylene hexafluoride, and / or polyolefin resin such as polyethylene or polypropylene .

The positive electrode material can be applied on the positive electrode collector to form the positive electrode (S5). The positive electrode current collector may be a conductive material such as Al, Ni, or stainless steel. The positive electrode material may be applied on the positive electrode collector by a method such as a pressing method using a paste or an organic solvent, applying the paste on a current collector, and pressing and fixing the paste. Examples of the organic solvent include amine-based solvents such as N, N-dimethylaminopropylamine and diethyltriamine; Ethers such as ethylene oxide and tetrahydrofuran; Ketone type such as methyl ethyl ketone; Esters such as methyl acetate; Aprotic polar solvent such as dimethylacetamide, N-methyl-2-pyrrolidone and the like. The paste may be applied on the positive electrode current collector using, for example, a gravure coating method, a slit die coating method, a knife coating method, or a spray coating method.

<Cathode>

The negative electrode active material may be a metal, a metal alloy, a metal oxide, a metal fluoride, a metal sulfide, and a natural graphite, an artificial graphite, a coke, a carbon black, a carbon nanotube, And a carbon material such as a pin.

A negative electrode material can be obtained by mixing a negative electrode active material, a conductive material, and a binder. At this time, the conductive material may be a carbon material such as natural graphite, artificial graphite, coke, carbon black, carbon nanotube, and graphene. The binder may be a thermoplastic resin such as polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene, vinylidene fluoride copolymer, fluorine resin such as propylene hexafluoride, and / or polyolefin resin such as polyethylene or polypropylene .

The anode material can be applied on the anode current collector to form the anode. The positive electrode current collector may be a conductive material such as Al, Ni, or stainless steel. The negative electrode material may be applied to the positive electrode current collector by press molding, or by making the paste using an organic solvent or the like, applying the paste to the current collector, and pressing and fixing the paste. Examples of the organic solvent include amine-based solvents such as N, N-dimethylaminopropylamine and diethyltriamine; Ethers such as ethylene oxide and tetrahydrofuran; Ketone type such as methyl ethyl ketone; Esters such as methyl acetate; Aprotic polar solvent such as dimethylacetamide, N-methyl-2-pyrrolidone and the like. The paste may be applied on the negative electrode current collector by, for example, a gravure coating method, a slit die coating method, a knife coating method, or a spray coating method.

<Electrolyte>

The electrolyte may be NaClO ₄ , NaPF ₆ , NaAsF ₆ , NaSbF ₆ , NaBF ₄ , NaCF ₃ SO ₃ , NaN (SO ₂ CF ₃ ) ₂ , lower aliphatic carboxylic acid sodium salt, NaAlCl ₄ , Mixtures may also be used. Among them, it is preferable to use an electrolyte containing fluorine. Further, the electrolyte may be dissolved in an organic solvent and used as a non-aqueous electrolyte. Examples of the organic solvent include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, isopropyl methyl carbonate, vinylene carbonate, 4- Carbonates such as trifluoromethyl-1,3-dioxolan-2-one and 1,2-di (methoxycarbonyloxy) ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropylmethylether, 2,2,3,3-tetrafluoropropyldifluoromethylether, tetrahydrofuran, 2-methyltetrahydro Ethers such as furan; Esters such as methyl formate, methyl acetate and? -Butyrolactone; Nitriles such as acetonitrile and butyronitrile; Amides such as N, N-dimethylformamide and N, N-dimethylacetamide; Carbamates such as 3-methyl-2-oxazolidone; Sulfur-containing compounds such as sulfolane, dimethylsulfoxide, and 1,3-propanesultone; Or an organic solvent in which a fluorine-substituted group is further introduced may be used.

Alternatively, a solid electrolyte may be used. The solid electrolyte may be an organic solid electrolyte such as a polymer compound of a polyethylene oxide type, a polymer compound containing at least one of a polyorganosiloxane chain and a polyoxyalkylene chain. A so-called gel type electrolyte in which a non-aqueous electrolyte is supported on the polymer compound may also be used. On the other _{hand, Na 2 S-SiS 2,} Na 2 S-GeS 2, NaTi 2 (PO 4) 3, NaFe 2 (PO 4) 3, Na 2 (SO 4) 3, Fe 2 (SO 4) 2 (PO 4 ), Fe ₂ (MoO ₄ ) _3, and the like may be used. In some cases, the safety of the sodium secondary battery can be enhanced by using these solid electrolytes. Further, the solid electrolyte may serve as a separator to be described later, in which case a separator may not be required.

<Separator>

A separator may be disposed between the anode and the cathode. Such a separator may be a material having a form such as a porous film made of a material such as a polyolefin resin such as polyethylene or polypropylene, a fluororesin or a nitrogen-containing aromatic polymer, a nonwoven fabric or a woven fabric. The thickness of the separator is preferably as thin as long as the mechanical strength is maintained in that the volume energy density of the battery is high and the internal resistance is small. The thickness of the separator may generally be about 5 to 200 mu m, and more specifically, 5 to 40 mu m.

<Manufacturing Method of Sodium Secondary Battery>

A negative electrode, a separator, and a negative electrode are laminated in this order to form an electrode group, if necessary, the electrode group is rolled up and stored in a battery can, and a sodium secondary battery can be manufactured by impregnating the electrode group with a non-aqueous electrolyte. Alternatively, the anode, the solid electrolyte, and the cathode may be laminated to form an electrode assembly, and if necessary, the electrode assembly may be rolled up and stored in a battery can to manufacture a sodium secondary battery.

Hereinafter, exemplary embodiments of the present invention will be described in order to facilitate understanding of the present invention. It should be understood, however, that the following examples are intended to aid in the understanding of the present invention and are not intended to limit the scope of the present invention.

[Experimental Examples; Examples]

Production Example 1: NaCrO ₂  Powder manufacturing

1 mol of sodium carbonate (Na ₂ CO ₃ ) and 1 mol of chromium oxide (Cr ₂ O ₃ ) were quantified and sufficiently mixed in the mortar. The mixture was placed in an alumina crucible and reacted in an argon atmosphere at 900 ° C. At this time, the obtained NaCrO ₂ powder was stored in a glove box filled with argon as soon as possible so as not to react with moisture.

Preparation Example 2: Carbon-coated NaCrO ₂  Powder manufacturing

NaCrO ₂ powder prepared in Preparation Example 1 was added to NMP (N-methyl-2-pyrrolidone) containing 10 wt% of pitch. After that, it is dried for about one day to remove NMP. The mixture of NaCrO ₂ powder and pitch with NMP removed was heat treated at 700 ° C in an argon atmosphere to coat carbon on the surface of NaCrO ₂ powder. The powder obtained in all procedures was stored in a glove box to avoid contact with moisture.

Production Example 3: NaCrO ₂  Manufacture of anode and cathode using powder

After mixing the NaCrO 2 powder, the conductive agent (Super-P, KS-6) and the binder (Poly vinylidene fluoride) prepared in Preparation Example 1 in an organic solvent (NMP (N-methyl-2-pyrrolidone) Coated on a current collector, and pressed to form a positive electrode.

Thereafter, metallic sodium was used as a negative electrode, and a non-aqueous electrolyte containing an electrolyte NaClO4 and an organic solvent propylene carbonate was used to produce a half-cell.

Production Example 4: Carbon-coated NaCrO ₂  Manufacture of cathode using powder

A positive electrode and a half-cell were fabricated in the same manner as in Production Example 3, except that the carbon-coated NaCrO ₂ powder prepared in Preparation Example 2 was used in place of the NaCrO ₂ powder prepared in Preparation Example 1.

Figure 2 is a graph showing the XRD analysis of the powder according to the _second NaCrO NaCrO ₂ powder as in Preparation Example 1, the carbon coating in accordance with Preparation 2.

Referring to FIG. 2, the carbon-coated NaCrO ₂ powder has the same crystal structure as that of the non-carbon-coated NaCrO ₂ powder. From these results, it can be seen that the carbon coating layer does not have a crystal structure.

3a and 3b are photographs taken NaCrO ₂ powders according to the NaCrO ₂ powder as in Preparation Example 1 the carbon coating in accordance with Preparation Example 2, respectively.

Referring to FIGS. 3A and 3B, it can be seen that the non-carbon coated NaCrO ₂ powder has a greenish color while the carbon-coated NaCrO ₂ powder has a black color.

Figures 4a and 4b are a TEM image taken NaCrO ₂ powders according to the NaCrO ₂ powder as in Preparation Example 1 the carbon coating in accordance with Preparation Example 2, respectively.

Referring to FIGS. 4A and 4B, it can be seen that the carbon-coated NaCrO ₂ powder has a uniform carbon coating layer of about 50 nm.

FIGS. 5A and 5B are graphs showing charging and discharging characteristics of a half-cell according to Production Example 4 and a half-cell according to Production Example 3, respectively. 6 is a graph showing the discharge capacity according to the number of cycles of the half-cell according to Production Example 4 and the half-cell according to Production Example 3. FIG. At this time, constant current charging was performed at a rate of 20 mA / g up to 3.6 V, and discharge was performed at a rate equal to the charging rate up to a constant current discharge of 2.0 V. The charge and discharge proceeded for a total of 50 cycles.

Referring to FIGS. 5A, 5B, and 6, the anode (FIG. 5B, FIG. 6) formed by using NaCrO ₂ powder not coated with carbon has a significantly reduced discharge capacity About 80%) is deteriorated in charge / discharge characteristics. On the other hand, the anode formed by using the carbon-coated NaCrO ₂ powder (FIG. 5A, FIG. 6) showed almost no change in the discharge capacity (initial value: 108 mAhg ^-1 ; 50 cycles And 105 mAhg ^-1 after the process, and the discharge capacity retention rate is about 97%).

7A is a graph showing a rate characteristic of a reversed phase according to Production Example 4. FIG. FIG. 7B is a graph showing a change in discharge capacity according to the number of cycles of the half-cell according to Production Example 4. FIG. At this time, constant current charging was carried out at the same rate of 10 mA / g up to 3.6 V. In each cycle, the constant current discharge was carried out up to 2.0 V while the C-rate was varied.

Referring to FIGS. 7A and 7B, in the course of a total of 30 cycles, discharging was performed at the same C-rate for each cycle, and as a result of increasing the C-rate (i.e., increasing the discharging rate) 50C-rate and about 98mAhg- ¹ , respectively.

As described above, the improvement of the discharge capacity holding characteristic and the speed characteristic is achieved by using NaCrO ₂ having a stable crystal structure as a cathode active material and uniformly coating a carbon layer on the surface of the cathode active material to further stabilize the crystal structure of the cathode active material And the conductivity is improved at the same time.

Similar to Production Example 2, similar results were obtained when the content of pitch was varied between 1 and 50 wt%, or when the mixture of NaCrO ₂ powder and pitch was heat treated at 750 ° C in an argon atmosphere to coat carbon.

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 exemplary embodiments, but, on the contrary, This is possible.

Claims (9)

1. A cathode material for a sodium secondary battery comprising a cathode active material represented by the following formula (1) and a carbon layer coated on the surface thereof:
[Chemical Formula 1]
Na (Cr _x M _1-x ) O ₂
In Formula 1,
M is at least one element selected from the group consisting of Ti, V, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Hf, Ta, , Ir, Pt or Au,
x is from 0.8 to 1. The method according to claim 1,
Wherein the carbon layer has a thickness of 0.1 to 500 nm. Mixing metal compounds containing a sodium compound and a chromium compound;
Calcining the mixed metal compound in an inert gas atmosphere to obtain a cathode active material according to Formula 1; And
And mixing the cathode active material, the carbon precursor, and the organic solvent to heat treatment in a reducing atmosphere to obtain a cathode active material coated with a carbon layer.
[Chemical Formula 1]
Na (Cr _x M _1-x ) O ₂
In Formula 1,
M is at least one element selected from the group consisting of Ti, V, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Os, ,
x is from 0.8 to 1. The method of claim 3,
Wherein the temperature in the firing step is 600 to 1200 DEG C. The method of claim 3,
Wherein the carbon precursor is a pitch. The method of claim 3,
Wherein the heat treatment is performed at 200 to 1500 degrees. The method of claim 3,
Wherein the sodium compound is sodium carbonate, and the chromium compound is chromium oxide. A positive electrode comprising a positive electrode material containing a positive electrode active material represented by the following formula (1) and a carbon layer coated on the surface thereof;
A negative electrode comprising a negative electrode material containing a negative electrode active material into which sodium can be inserted; And
And an electrolyte disposed between the positive electrode and the negative electrode.
[Chemical Formula 1]
Na (Cr _x M _1-x ) O ₂
In Formula 1,
M is at least one element selected from the group consisting of Ti, V, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Os, ,
x is from 0.8 to 1. 9. The method of claim 8,
Wherein the carbon layer has a thickness of 0.1 to 500 nm.

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