KR101498797B1 - Negative Electrode Material for Sodium-Ion Batteries and Sodium-Ion Battery Having the Same - Google Patents

Abstract

A negative electrode material for a sodium secondary battery and a sodium secondary battery comprising the same are provided. The negative electrode material comprises a negative active material of TiO ₂ having an anatase phase and a carbon layer coated on the surface of the negative active material.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a negative 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 power source for a cellular 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 such as a hybrid car, and the demand is expected to increase.

However, since the lithium secondary battery uses a rare metal element such as lithium, there is a possibility that it can not meet the demand increase. Accordingly, research is being conducted on a sodium secondary battery using sodium supplied in a rich and inexpensive manner.

In a sodium secondary battery, a carbon material such as graphite is used as an anode active material as in a lithium secondary battery. However, such a carbon material is difficult to insert or insert large sodium ions in comparison with lithium ions, and is insufficient to be used as a negative electrode active material in a sodium secondary battery. Accordingly, studies on a negative electrode active material suitable for a sodium secondary battery are underway (see Korean Patent Publication No. 2010-0113479).

However, it is known that the negative electrode materials of the sodium secondary battery developed so far still need to improve the discharge capacity retention rate and the speed characteristic.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a negative electrode material for a sodium secondary battery and a sodium secondary battery including the negative electrode material.

According to an aspect of the present invention, there is provided an anode material for a sodium secondary battery. The negative electrode material includes a negative active material which is TiO ₂ having an anatase phase and a carbon layer coated on the surface of the negative active material.

The negative electrode active material may be TiO ₂ nanowire, and the carbon layer may be 0.1 to 100 nm . ≪ / RTI >

According to an aspect of the present invention, there is provided a method of manufacturing a negative electrode material for a sodium secondary battery. First, TiO ₂ having an anatase phase is formed by hydrothermal synthesis using a titanium compound as a material. A mixture of TiO ₂ and the carbon precursor is heat treated in a reducing atmosphere to coat the carbon layer on the TiO ₂ .

The carbon precursor may be a pitch. The heat treatment may be performed at a temperature of about 200 to 1200 degrees . ≪ / RTI >

The TiO ₂ is subjected to a hydrothermal synthesis method of a material the titanium compound and the sodium Na ₂ Ti ₃ O forms a _7, Na ₂ Ti ₃ O by dispersing ₇ in a protic solvent by ion exchange of Na by H H ₂ Ti ₃ O ₇ , and then heat-treating H ₂ Ti ₃ O ₇ .

According to an aspect of the present invention, there is provided a sodium secondary battery. The sodium secondary battery comprises a negative electrode containing a negative electrode containing a negative electrode material containing a negative electrode active material TiO ₂ having an anatase phase and a carbon layer coated on the surface thereof, and a positive electrode containing a complex metal oxide containing Na. An electrolyte is disposed between the anode and the cathode. The TiO ₂ may have the form of nanowires. The carbon layer coated on the surface of the negative electrode active material may have a thickness of 0.1 to 100 nm.

According to the present invention, by using a negative electrode active material capable of maintaining a stable crystal structure during charging and discharging and uniformly coating a carbon layer on the surface of the negative active material, the crystal structure of the negative electrode active material is further stabilized, Can be improved. 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 manufacturing a cathode according to an embodiment of the present invention.
Figure 2 is a graph showing the XRD analysis results of the TiO ₂ nano-wires in accordance with the TiO ₂ nano-wire as in Preparation Example 1, the carbon coating in accordance with Preparation 2.
3a and 3b are photographs taken of TiO ₂ nano-wires in accordance with the TiO ₂ nano-wire as in Preparation Example 1, the carbon coating in accordance with Preparation Example 2, respectively.
Figures 4a and 4b are SEM photographs taken of TiO ₂ nano-wires in accordance with the TiO ₂ nano-wire as in Preparation Example 1, the carbon coating in accordance with Preparation Example 2, respectively.
Figures 5a and 5b are a TEM image taken the TiO ₂ nano-wires in accordance with the TiO ₂ nano-wire as in Preparation Example 1, the carbon coating in accordance with Preparation Example 2, respectively.
6A and 6B 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.
FIGS. 7A and 7B are graphs showing the rate characteristics of the reversal paper according to Production Example 4 and the reversal paper according to Production Example 3, respectively.

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 negative electrode containing a negative electrode material containing a negative electrode active material to which sodium can be inserted and to be inserted and a carbon layer coated on the surface of the negative electrode active material, And an electrolyte positioned therebetween.

<Cathode>

The negative electrode active material according to an embodiment of the present invention may be TiO ₂ . TiO ₂ may be anatase. TiO ₂ is abundant in resources, low cost and non-toxic, and can achieve high capacity and low operating voltage when used as an anode active material. Among these various phases of TiO ₂ (Brookite, Rutile, Anatase, TiO ₂ (B)), the anatase phase can maintain a stable structure even when lithium ions are repeatedly inserted / removed.

TiO ₂ can have the form of nanowires, TiO ₂ The nanowire has a larger contact surface area with the electrolyte than TiO ₂ having other particle shapes, so that the diffusion rate of sodium ions can be improved. In the present invention, the nanowire has a nanometer scale in diameter and a length longer than the diameter. Thus, nanowires may also include nanorods and nanotubes.

A uniformly coated carbon layer is placed on the TiO ₂ nanowire. In this case, the electric conductivity can be further improved and the rapid charge / discharge characteristics can be improved. The carbon layer may have a thickness of about 0.1 to 100 nm As shown in FIG.

1 is a flow chart illustrating a method of manufacturing a cathode according to an embodiment of the present invention.

First, anatase phase TiO ₂ nanowire is formed using a titanium compound as a material by hydrothermal synthesis (S1). Specifically, a Na ₂ Ti ₃ O ₇ nanowire is formed by a hydrothermal synthesis method using a titanium compound and a sodium compound as a material, and the Na ₂ Ti ₃ O ₇ nanowire is dispersed in a protonic solvent such as hydrochloric acid, and Na is ion-exchanged with H 2. As a result, H ₂ Ti ₃ O ₇ nanowires are formed. H ₂ Ti ₃ O ₇ nanowires are annealed in air to form anatase TiO ₂ nanowires. The hydrothermal synthesis can be carried out at a temperature of about 160 to 190 degrees for 10 to 96 hours.

At this time, the titanium compound may be a titanium salt, a titanium oxide, or a titanium hydroxide, and the sodium compound may be a sodium salt or a sodium hydroxide.

TiO ₂ nanowire, carbon precursor, and organic solvent are mixed and heat treatment is performed in a reducing atmosphere to obtain a carbon layer-coated TiO ₂ nanowire (S 2). Specifically, a TiO ₂ nanowire serving as a negative electrode active material may be contained in an organic solvent containing about 0.001 to 50 wt% of a carbon precursor. The carbon precursor may be a pitch or a hydrocarbon based material such as polyacrylic acid, polyvinyl pyrrolidone, ascorbic acid, carboxylic acid, adipic acid, acid, malic acid, citric acid, acetic acid, glucose, or sucrose. The reducing atmosphere may be an inert gas atmosphere or a hydrogen atmosphere. Further, the heat treatment temperature is About 200 to 1200 degrees, specifically about 600 to 800 degrees. The organic solvent may be NMP (N-methyl-2-pyrrolidone).

In the process of forming the negative electrode active material, the negative electrode active material coated with the carbon layer may be obtained by mixing the metal oxide and the carbon material. However, in this case, as the carbon material reacts with the metal oxide to form a crystal phase of the negative electrode active material, the negative electrode active material formed through this process may have a poor crystalline phase. In other words, it may be difficult to have a single crystal phase. Further, it is difficult to form a uniform carbon layer on the negative electrode active material. However, in this embodiment, by obtaining the negative active material having a (single) crystal phase in advance and coating the carbon layer, a homogeneous carbon layer can be obtained while keeping the (single) crystal phase of the negative active material in a good state.

The anode material can be obtained by mixing the carbon layer-coated TiO ₂ nanowire, the conductive material, and the binder (S3). 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 negative electrode material may be coated on the negative electrode collector to form a negative electrode (S4). The negative electrode current collector may be a conductive material such as Al, Ni, copper, or stainless steel. The negative electrode material may be applied to the negative electrode current collector by press molding, or by making the paste using an organic solvent or the like, applying the paste onto 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.

<Anode>

The cathode active material can be formed using a metal oxide, metal fluoride, metal oxyfluoride, metal, or the like, which is capable of intercalating and deintercalating Na ⁺ and exhibiting a high operating voltage. Specifically, the cathode active material may be NaFeO ₂ , NaNiO ₂ , NaCoO ₂ , NaMnO ₂ , for example, as a composite metal oxide containing Na.

The positive electrode material can be obtained by mixing the positive electrode active material, the conductive material, and the 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 positive electrode material can be coated on the positive electrode collector to form the positive electrode. The positive electrode current collector may be a conductive material such as Al, Ni, copper, 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.

<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: Anatase TiO ₂  Nanowire manufacturing

Titanium oxide (TiO ₂ ) and sodium hydroxide (NaOH) were placed in a Teflon-lined autoclave with distilled water and hydrothermal synthesis was carried out at 170 ° C. for 11 hours. After the completion of the synthesis, the precipitate (Na ₂ Ti ₃ O ₇ nanowire) was washed with distilled water several times to neutralize it, then dispersed in hydrochloric acid to exchange hydrogen and sodium ions. The ion-exchanged precipitate (H ₂ Ti ₃ O ₇ ) was dried in an oven at 80 ° C. The dried sample (H ₂ Ti ₃ O ₇ nanowire) was heat-treated at 600 ° C. in the air to obtain an anatase TiO ₂ nanowire powder.

Production Example 2: Carbon-coated TiO ₂  Nanowire manufacturing

The TiO ₂ nanowires prepared in Production Example 1 were placed in NMP (N-methyl-2-pyrrolidone) containing 5 wt% of pitch. After that, it is dried for about one day to remove NMP. The TiO ₂ nanowires with NMP removed were heat treated in an argon atmosphere to carbonize the pitches to coat carbon on the surface of the TiO ₂ nanowires. The heat treatment temperature was 700 ° C. The powder obtained in all procedures was stored in a glove box to avoid contact with moisture.

Production Example 3: TiO ₂  Manufacture of anode and cathode using nanowires

The TiO ₂ nanowire, the super-p, the ketjen black, and the binder (PVDF) prepared in Preparation Example 1 were mixed in an organic solvent (NMP), coated on a copper collector, .

Thereafter, a non-aqueous electrolyte containing a positive electrode made of metal sodium, an electrolyte NaClO ₄ and an organic solvent propylene carbonate was used to produce a half-cell.

Production Example 4: Carbon-coated TiO ₂  Manufacture of anode using nanowire

A negative electrode and a _half- cell were fabricated in the same manner as in Preparation Example 3, except that the carbon-coated TiO ₂ nanowire prepared in Preparation Example 2 was used in place of the TiO ₂ nanowire prepared in Preparation Example 1.

Figure 2 is a graph showing the XRD analysis results of the TiO ₂ nano-wires in accordance with the TiO ₂ nano-wire as in Preparation Example 1, the carbon coating in accordance with Preparation 2.

Referring to FIG. 2, it can be seen that the carbon-coated TiO ₂ nanowire also has the same crystal structure as the carbon-coated TiO ₂ nanowire, that is, an anatase phase. From these results, it can be seen that the TiO ₂ nanowire according to Production Example 1 has an anatase phase and the carbon coating layer does not have a crystal structure.

3a and 3b are photographs taken of TiO ₂ nano-wires in accordance with the TiO ₂ nano-wire as in Preparation Example 1, the carbon coating in accordance with Preparation Example 2, respectively.

3a and 3b. Referring to FIG TiO ₂ powder non-coated carbon nanowire, while striking the white TiO ₂ powder, carbon nanowire coating can be seen that the striking black.

Figures 4a and 4b deulyigo SEM pictures taken of TiO ₂ nanowires according to the carbon-coated TiO ₂ nano-wire as in Preparation Example 1 according to Preparation Example 2, respectively, Fig. 5a and 5b is a carbon coating in accordance with Preparation Example 2 a TiO ₂ nano-wires in accordance with the TiO ₂ nano-wire as in Preparation example 1 are each a TEM photography.

Referring to FIGS. 4A, 4B, 5A and 5B, the TiO ₂ nanowires are in the form of nanorods, and the carbon-coated TiO ₂ nanowires have a carbon coating layer having a uniform thickness of about 5 to 10 nm .

6A and 6B 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. At this time, constant current charging was performed at 10 mAg ^-1 until 3.0 V, and discharging was continued until the constant current discharge was performed at 0 V at the same rate as the charging rate. The charge and discharge proceeded for a total of 50 cycles.

When FIG. 6a and FIG. 6b, carbon-coated anode formed by using the TiO ₂ nano-wire (Fig. 6a) is formed using time, TiO ₂ nanowire that is not a carbon coating hayeoteul proceeds total of 50 charge-discharge cycles It can be seen that the change amount of the charge capacity is relatively small as compared with one negative electrode (FIG. 6B). This means that the negative electrode formed using the carbon-coated TiO ₂ nanowire exhibits relatively uniform charge-discharge characteristics.

FIGS. 7A and 7B are graphs showing the rate characteristics of the reversal paper according to Production Example 4 and the reversal paper according to Production Example 3, respectively. At this time, constant-current charging was performed at a constant C-rate up to 3.0 V, and the discharge was performed at 10 mAg ^-1 in each cycle until the constant current discharge reached 0V.

Referring to FIGS. 7A and 7B, it can be seen that the discharge characteristics are kept substantially uniform in a total of 32 cycles. However, the charging characteristic is a cathode formed using appear to be somewhat degraded As higher the charge and discharge rate, the carbon-coated anode formed by using the TiO ₂ nano-wire (Fig. 7a) is TiO ₂ nanowire uncoated carbon ( 7 (b)). As can be seen from FIG.

As described above, the improvement of the charge capacity holding characteristic and the speed characteristic is achieved by using the TiO ₂ nanowire having a stable crystal structure as the negative electrode active material and uniformly coating the carbon layer on the surface of the negative electrode active material, Which is a result of stabilizing and improving conductivity at the same time.

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 (10)

A negative electrode active material having an anatase phase and being a TiO ₂ nanorod having a peak showing a (103) plane, a peak showing a (004) plane and a peak showing a (112) ; And
And a carbon layer coated on the surface of the negative electrode active material. The method according to claim 1,
Wherein the carbon layer has a thickness of 0.1 to 100 nm. The method according to claim 1,
Wherein the carbon layer has a thickness of 5 to 10 nm. Forming a TiO ₂ nanorod having an anatase phase by hydrothermal synthesis using a titanium compound as a material; And
Heat treating the mixture of the TiO ₂ nanorods and the carbon precursor in a reducing atmosphere to coat the carbon layer on the TiO ₂ nanorods,
The TiO ₂ nanorod has a peak of the (103) plane, a peak of the (004) plane, and a peak of the (112) plane in the X-ray diffraction spectrum to form a TiO ₂ nanorod, A method for manufacturing a negative electrode material of a battery. 5. The method of claim 4,
Wherein the carbon precursor is a pitch. 5. The method of claim 4,
Wherein the heat treatment is performed at 200 to 1200 degrees. 5. The method of claim 4,
The step of forming the TiO &lt; ₂ &gt;
Hydrothermally synthesizing a titanium compound and a sodium compound to form Na ₂ Ti ₃ O ₇ ;
Dispersing Na ₂ Ti ₃ O ₇ in a protonic solvent to ion-exchange Na with H to form H ₂ Ti ₃ O ₇ ; And
H ₂ Ti ₃ O ₇ to form the TiO ₂ nanorods. A negative electrode active material having an anatase phase and being a TiO ₂ nanorod having a peak showing a (103) plane, a peak showing a (004) plane and a peak showing a (112) A negative electrode comprising a negative electrode material containing a carbon layer coated on a surface thereof;
A positive electrode containing a composite metal oxide containing Na; And
And an electrolyte disposed between the positive electrode and the negative electrode. 9. The method of claim 8,
The carbon layer coated on the surface of the negative electrode active material has a thickness of 0.1 to 100 nm Of the thickness of the sodium secondary battery. 9. The method of claim 8,
Wherein the carbon layer coated on the surface of the negative electrode active material has a thickness of 5 to 10 nm.

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