KR20170058673A - Active Materials For Na Secondary Batteries And Mehthods Thereof - Google Patents

Abstract

A positive electrode active material of a sodium secondary battery having good electrochemical characteristics is disclosed. The present invention relates to a layered sodium-transition metal oxide powder coated with a buffer layer; Sacrificial anode material; And a conductive material for a cathode active material for a sodium secondary battery. According to the present invention, it is possible to provide a cathode active material for a sodium secondary battery that reduces initial irreversibility and can provide a sodium secondary battery that does not deteriorate cycle characteristics of the battery due to structural changes even when exposed to air.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive active material for a sodium secondary battery,

The present invention relates to a sodium secondary battery, and more particularly, to a cathode active material of a sodium secondary battery having good electrochemical characteristics.

Lithium secondary batteries have been commercialized since the 1990s, functioning as core power sources for small IT equipment and power tools, and expanding the range of power sources for electric vehicles (EV, HEV, PHEV).

Lithium resources, the main material of lithium secondary batteries, are limited to the continental regions of South America, such as Argentina, Bolivia and Chile. As lithium demand soars, supply-demand imbalance, raw material price increases, and lithium-

On the other hand, sodium is rich in reserves and low in price, which is very advantageous in terms of raw material supply and demand.

Sodium-ion batteries have also been studied since the 1970's, but lithium batteries have not been commercialized before, and the need for non-lithium based Post-Li batteries has arisen and full-fledged research is underway on sodium ion batteries.

The sodium secondary battery has the same working principle as the lithium secondary battery, but has a similar structure. However, the secondary sodium secondary battery is far from the characteristics of the lithium secondary battery. However, it can be an innovative alternative to overcome the limitations of current lithium secondary battery market as an energy storage and conversion device based on advantages such as easy supply of resources and low cost.

As the cathode active material in the sodium secondary battery, an oxide-based material such as NaFePO ₄ and the like, Na ₃ V ₂ (PO ₄ ) ₃ , NaFePO ₄ Polyanion series such as Na _x TiS ₂ , sulfide series such as Na _x TiS ₂ , fluoride series such as FeF _3, and phosphate series materials such as NASICON are used. Examples of the anode active material include petroleum cokes, Carbon black, hard carbon, and the like are known.

There are double positive electrode active material is a layered structure NaMnO ₂ or ^{_{Na x [M 1 1 -y- z}} M 2 y M 3 z] O 2 + a (M = Fe, Mn, Cu , etc.) is mainly used, the The structure shows good performance of the battery because the diffusion of Na ions occurs quickly and easily in the crystal during charging and discharging.

However, since the oxide of the layered structure has high reactivity with carbon dioxide and moisture in air, the crystal structure changes due to the surrounding environment even after synthesis, which adversely affects the charging / discharging mechanism of the whole battery.

In addition, the oxide having a layered structure are for example, Na _x MnO ₂ and ^{_{Na x [M 1 1 -y- z}} M 2 y M 3 z] O 2 + _a In this structure there is composed of the following composition of Na 1, the half-cell (half cell), the Na ion in the negative electrode is sufficiently supplied even in such a structure. Therefore, the Na ion content in the cell during the cycle is insignificantly changed. However, in the case of the full cell, Since there is no source, after the Na ions migrate to the cathode on the anode side, SEI (solid electrolyte interface) is formed on the cathode side and then to the anode side. At this time, since the amount of ions recovered from the Na ions released from the anode is reduced in the initial stage, the charge and discharge are continued to increase the irreversibility.

In order to solve the problems of the prior art described above, it is an object of the present invention to provide a cathode active material for a sodium secondary battery which reduces initial irreversibility.

Another object of the present invention is to provide a sodium secondary battery which does not deteriorate cycle characteristics of a battery due to structural changes even when exposed to air.

It is another object of the present invention to provide a method for producing the above-described cathode active material for a sodium secondary battery.

Another object of the present invention is to provide a sodium secondary battery including the above-mentioned cathode active material.

According to an aspect of the present invention, there is provided a semiconductor device comprising: a layered sodium transition metal oxide powder coated with a buffer layer; Sacrificial anode material; And a conductive material for a cathode active material for a sodium secondary battery.

According to an embodiment of the present invention, the sacrificial anode material preferably includes a Na ion source. At this time, the sacrificial anode material releases a Na ion source and is removed. As an example, the sacrificial anode material may comprise NaN ₃ .

In the present invention, the sodium transition metal oxide powder may be NaMnO ₂ or Na _x [M ¹ ₁ -yz M ² _y M ³ _z ] O _{2+ a} wherein M ¹ , M ² and M ³ are transition metals, x + y + z = 1, 0? x <1, 0? y <1, 0? z <1, 0? a <0.1).

In an embodiment of the present invention, the buffer layer is made of FePO ₄ , FeP ₂ O ₇ , MnPO ₄ CO ₃ , FePO ₄ F, Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) and M _{2 +} P ₂ O ₇ ) ₂ (wherein 2/3? 7/8, M is Fe or Mn, or a ratio of Fe and Mn is 1: 1) .

According to one aspect of the present invention, it is possible to provide a cathode active material for a sodium secondary battery that reduces initial irreversibility.

In addition, according to another aspect of the present invention, it is possible to provide a sodium secondary battery which does not deteriorate the cycle characteristics of the battery due to structural changes even when exposed to air.

FIG. 1 is a diagram schematically showing an operation principle of an anode according to an embodiment of the present invention.
FIG. 2 is a photograph showing a result of a transmission electron microscope observation of an active material produced according to an embodiment of the present invention.
3 is a graph showing a result of charge / discharge capacity measurement of a coin cell manufactured according to an embodiment of the present invention.
FIG. 4 is a graph showing a charge / discharge capacity measurement result of a coin cell manufactured according to another embodiment of the present invention.
FIG. 5 is a graph showing a charge / discharge capacity measurement result of a coin cell manufactured according to another embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to the preferred embodiments of the present invention with reference to the drawings.

In the present invention, the cathode active material includes a layered structure oxide powder and a sacrificial anode. In the present invention, the layered structure oxide may include a sodium oxide containing a transition metal. The sodium oxide is preferably a transition metal such as NaMnO ₂ or Na _x [M ¹ ₁ -yz M ² _y M ³ _z ] O _{2 + a} (M = Fe, Mn, Cu, 0? X <1, 0? Y <1, 0? Z <1, 0? A <0.1).

In the present invention, the active material may include a buffer layer coated on the surface. The buffer layer may include a transition metal containing phosphorous such as FePO ₄ , FeP ₂ O ₇ , MnPO ₄ CO ₃ , FePO ₄ F, Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ), M _{2 + 2} O ₇ ) ₂ (where 2/3? 7/8, where M is Fe or Mn or a ratio of Fe and Mn is 1: 1).

In one embodiment of the present invention, the buffer layer-coated active material can be prepared by the following method.

First, the synthesis of sodium transition metal oxides such as Na _x MnO ₂ or ^{_{Na x [M 1 1 -y- z}} M 2 y M 3 z] O 2 + a. The synthesis of the sodium transition metal oxide may be carried out by solid phase synthesis, hydrothermal synthesis, coprecipitation or sol-gel synthesis.

After the sodium transition metal oxide is prepared, precursors such as Fe (NO ₃ ) ₃ and NH ₄ H ₂ PO ₄ are prepared as precursors of the phosphate containing transition metal such as transition metal oxide FePO ₄ . The precursor is dissolved in deionized water or a solvent such as ethylene carbonate, and the prepared sodium transition metal oxide is mixed and stirred. The mixed solution is dried after appropriately stirring and then heat-treated at a temperature of about 400 ° C. to prepare a transition metal phosphate coated with a buffer layer (FePO 4).

The sodium transition metal oxide of the sodium secondary battery has a high capacity, but has a disadvantage in that the cycle characteristics are poor due to reaction with the electrolytic solution. In the present invention, the buffer layer formed on the surface of the sodium transition metal oxide limits the direct reaction between the sodium transition metal oxide and the electrolyte. Further, the buffer layer functions as a buffer layer when Na ions flowing out to the cathode at the time of charging are re-introduced at the time of discharge.

Meanwhile, in the present invention, the anode may further include a sacrificial anode material. The sacrificial anode material acts as a Na ion source during the initial charge. Also, in the present invention, the sacrificial anode material may be any material that does not remain while acting as a Na ion source at the time of initial charging. For example, the sacrificial anode material may be NaN ₃ . NaN ₃ can act as a sacrificial anode material by releasing Na ions upon charging and vaporizing with a gas (N ₂ ). Similarly, Na ₂ O and Na ₂ S may also be used as sacrificial anodic materials.

FIG. 1 is a diagram schematically showing an operation principle of an anode according to an embodiment of the present invention.

Referring to FIG. 1 (a), the anode active material includes a mixture of a sodium transition metal oxide 10A coated with a buffer layer and a sacrificial anode material 20A. Illustratively, in this example, the composition of the sodium transition metal oxide (active material) is Na _2/3 Fe _1/3 Mn _2/3 O ₂ and the composition of the buffer layer is FePO ₄ .

As shown in Fig. 1 (b), at the time of initial charging, the NaN ₃ sacrificial anode material 20A supplies Na ions to the cathode and creates a vacancy 20B in its place. Also, at the initial charging, the surface buffer layer of the sodium transition metal oxide 10B may have a (Na, Fe) PO ₄ compound composition by solving Na ions exiting from the inside. Further, the inside of the sodium transition metal oxide 10B may have a Fe-Mn-O compound composition in which Fe from the surface buffer layer and Mn inside are dissolved. Thus, as shown in Fig. 1 (b), the sodium transition metal oxide 10B exists in the form of a compound having an inclined composition from the surface to the inside.

In the present invention, a part of the Na ions supplied from the sacrificial anode material 20A may migrate to the cathode while the other part may be dissolved in the surface buffer layer. Accordingly, in the present invention, the sacrificial anode material can cause Na ions dissolved in the buffer layer to reach the saturation state quickly.

Fig. 1 (c) is a view schematically showing the active material state of the anode at the time of discharge.

Referring to the drawing, Na ions supplied from the cathode are caused to flow inward through the surface buffer layer of the sodium transition metal oxide (10C). At this time, some of the supplied Na ions react on the surface of the sodium transition metal oxide (10C), so that the Na concentration of the surface buffer layer becomes rich during initial charging. Additional Na ions are incorporated within the transition metal oxide (10C).

In the present invention, Na ions supplied from the sacrificial anode material are solidified on the surface of the sodium transition metal oxide, so that Na ions introduced from the cathode can smoothly flow into the sodium transition metal oxide 10B during discharge.

Accordingly, the provided sacrificial anode material can reduce the difference in charge / discharge capacity due to the reaction between the buffer layer and Na ions, and can improve charge / discharge reversibility.

As described above, according to one aspect of the present invention, the active material includes a sodium transition metal oxide coated with a buffer layer, and the buffer layer functions as a layer for suppressing the reaction with the electrolyte. At this time, the active material of the present invention may include a sacrificial anode material in order to suppress a decrease in the initial charge capacity due to the reaction of the introduced buffer layer and Na ions. Also, in the present invention, the sacrificial anode material reduces the difference in capacity at the time of initial charge-discharge and ensures the reversibility of the charge-discharge cycle.

Also, according to an aspect of the present invention, the void of the sacrificial anode material lost by the initial charging results in increasing the reactive surface area of the active material, and as a result, the charge / discharge capacity can be increased.

Hereinafter, preferred embodiments of the present invention will be described.

&Lt; Coating of buffer layer &

Na _0.7 MnO ₂ powder coated with FePO 4 buffer layer was prepared. The powder was prepared in the following manner.

First it was prepared Na _{0 .7} MnO ₂ powder. Na _{0 .7} MnO ₂ powders were weighed, mixed and pulverized NaCO ₃ and Mn ₃ O ₄ powder was prepared in 900 for 10 hours to heat treatment in air.

An FePO ₄ buffer layer was formed on the surface of the prepared powder. For the formation of the buffer layer, Fe (NO ₃ ) ₃ was used as the Fe precursor and NH ₄ H ₂ PO ₄ was used as the P precursor. Fe (NO ₃ ) ₃ and NH ₄ H ₂ PO ₄ were dissolved in deionized water and Na _{0 .7} MnO ₂ powder was added and stirred. At this time, the raw material was blended so that the ratio of Na _0.7 MnO ₂ : FePO ₄ was 19: 1 by weight.

Then, it was dried in a dry oven for a sufficient time. Then, the obtained dried powder was heat-treated at a temperature of 400 ° C.

2 is a photograph showing a result of a transmission electron microscope observation of the produced active material. FIG. 2 (a) is a transmission electron microscope of the active material, and a coating layer formed on the particle surface can be confirmed. 2 (b) to 2 (f) show the results of analysis showing the distribution of Na, Mn, Fe, P and O corresponding to the position of the photograph of FIG. 2. From these figures, It can be seen from this that the FePO ₄ coating layer is formed on the powder surface.

&Lt; Example 1 >

A slurry was prepared by dissolving a conductive material (Super P black) and a binder (PVDF) in an NMP solvent with Na _{0 .7} MnO ₂ powder not coated with a buffer layer as an active material. The slurry thus prepared was applied to an aluminum foil and punched into a circular shape of? 14 mm to prepare a positive electrode.

A 2032 coin cell type half cell was fabricated using a Na metal plate as the counter electrode (auxiliary electrode). Glass fiber was used as a separator, and a 1M NaClO ₄ solution in which EC / PC was 1: 1 in volume ratio and NaClO ₄ was dissolved in a solvent containing 5 wt% FEC was used as an electrolyte.

The charge and discharge characteristics and cycle characteristics of the prepared coin cell were evaluated. The WBCS 3000 Battery Cycler System manufactured by Won a-tech Co., Ltd. was used as a measuring device, and the charge-discharge cut-off voltage was 2 to 3.8 V, the constant current density was 0.05 C, and 5 to 10 mAh / g.

3 is a graph showing a result of charge / discharge capacity measurement of a coin cell manufactured according to the present embodiment.

Referring to FIG. 3, it can be seen that the charge capacity (red dot) is smaller than the discharge capacity (blue dot) in the first cycle and shows high Coulomb efficiency in the initial cycle, but decreases in the subsequent cycle. It is also shown from Fig. 3 that the charge / discharge capacity gradually decreases as the cycle is repeated. That is, it can be seen that the coin cell manufactured according to this embodiment has a good initial capacity and irreversible capacity, but the life span decreases as the cycle progresses.

&Lt; Example 2 >

A half cell was fabricated in the same manner as in Example 1, except that the Na _{0 .7} MnO ₂ powder coated with the FePO 4 buffer layer was used as the cathode active material. The charge / discharge characteristics and cycle characteristics of the prepared coin cell were evaluated in the same manner as in Example 1.

4 is a graph showing a result of charge / discharge capacity measurement of a coin cell manufactured according to the present embodiment.

In the present embodiment, the cycle characteristics are improved as compared with those in FIG. 3, but still show poor initial irreversibility.

&Lt; Example 3 >

A half cell was prepared in the same manner as in Example 1, except that the active material was weighed so that the weight ratio of the Na _{0 .7} MnO ₂ powder coated with the FePO ₄ buffer layer to the NaN ₃ sacrificial anode material was 9: 1. The charge / discharge characteristics and cycle characteristics of the prepared coin cell were evaluated in the same manner as in Example 1.

5 is a graph showing a result of charge / discharge capacity measurement of a coin cell manufactured according to the present embodiment.

Referring to FIG. 5, it can be seen that the difference between the initial charging and discharging capacities is significantly reduced as compared with Embodiments 1 and 2, and the irreversibility is remarkably improved. Also, it can be seen that the charge / discharge capacity is maintained at a constant level as the cycle is repeated, and the Coulomb efficiency is maintained at a constant level regardless of the cycle.

Claims (6)

A layered sodium transition metal oxide powder coated with a buffer layer;
Sacrificial anode material; And
A cathode active material for a sodium secondary battery comprising a conductive material. The method according to claim 1,
Wherein the sacrificial anode material comprises a Na ion source. The method according to claim 1,
Wherein the sacrificial anode material releases and is removed from the Na ion source. The method of claim 3,
Wherein the sacrificial anode material comprises NaNO ₃ . The method according to claim 1,
The sodium transition metal oxide powder,
NaMnO ₂ or ^{_{Na x [M 1 1 -y- z}} M 2 y M 3 z] O 2 + a ( ^{where, M 1, M 2, M} 3 is a transition metal, x + y + z = 1 , 0≤x Lt; 1, 0? Y <1, 0? Z <1, 0? A <0.1). The method according to claim 1,
The buffer layer is _{FePO 4, FeP 2 O 7,} MnPO 4 CO 3, FePO 4 F, Fe 3 (PO 4) 2 (P 2 O 7) , and _{M 2 + α / 2 (P} 2 O 7) 2 ( wherein, 2/3?? 7/8, M is Fe or Mn, or a ratio of Fe and Mn is 1: 1). Active material.

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