KR20230102101A - Electrode active material including Ru for sodium secondary battery and sodium secondary battery comprising the same - Google Patents

Abstract

An electrode active material for a sodium secondary battery containing Ru and a sodium secondary battery including the same are provided. The cathode active material for a sodium secondary battery may be represented by Chemical Formula 1 below.
[Formula 1] Na _x [M _y Tm _1-yz Ru _z ] O _2-a A _a
In Formula 1, M is an alkaline earth metal element, TM is manganese (Mn), nickel (Ni), iron (Fe), cobalt (Co), or a combination thereof, A is N, O, F or S, , 0.5≤a≤0.7, 0.1≤x≤0.3, 0.05≤z≤0.25, and 0≤a≤0.5.

Description

Electrode active material for sodium secondary battery containing Ru and sodium secondary battery comprising the same {Electrode active material including Ru for sodium secondary battery and sodium secondary battery comprising the same}

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

A secondary battery refers to a battery that can be used repeatedly because it can be charged as well as discharged. In a typical lithium secondary battery among secondary batteries, lithium ions included in the positive electrode active material move to the negative electrode through the electrolyte, and then are inserted into the layered structure of the negative electrode active material (charging). It works through the principle of returning to the anode (discharge). These lithium secondary batteries are currently commercialized and used as small power sources such as mobile phones and notebook computers, and are expected to be used as large power sources such as hybrid vehicles, and the demand for them is expected to increase.

However, since composite metal oxides, which are mainly used as cathode active materials in lithium secondary batteries, contain rare metal elements such as lithium, there is a concern that they may not be able to meet the increasing demand. Accordingly, a next-generation secondary battery using an abundant and cheap metal as a cathode active material is being studied. As an example of such a next-generation secondary battery, a sodium secondary battery using sodium ions is being studied, and Korean Patent Publication No. 2012-0133300 discloses Na _x MnPO ₄ F (0 < x ≤ 2) as a cathode active material for a sodium secondary battery. are starting

Since the sodium secondary battery is charged and discharged by intercalation/deintercalation of relatively large sodium ions, it may be difficult to maintain structural stability of the active material during charging and discharging.

Accordingly, an object of the present invention is to provide a secondary battery with improved lifespan characteristics by improving the structural safety of an active material during charging and discharging.

The technical problems of the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned will be clearly understood by those skilled in the art from the following description.

In order to achieve the above object, one aspect of the present invention provides a cathode active material for a sodium secondary battery represented by Formula 1 below.

[Formula 1]

Na _x [M _y Tm _1-yz Ru _z ]O _2-a A _a

In Formula 1, M is an alkaline earth metal element, TM is manganese (Mn), nickel (Ni), iron (Fe), cobalt (Co), or a combination thereof, A is N, O, F or S, , 0.5≤a≤0.7, 0.1≤x≤0.3, 0.05≤z≤0.25, and 0≤a≤0.5.

The cathode active material for a sodium secondary battery has a hexagonal P2 structure, and a space group may be P6 ₃ /mmc.

The M may be Mg. y may be 0.15 to 0.28. The TM may be Mn. When Na ions are inserted/deintercalated from the cathode active material, Ru may be fixed in a tetravalent state. z may be 0.1 to 0.2.

Another aspect of the present invention to achieve the above object provides a sodium secondary battery. The sodium secondary battery includes a cathode including the cathode active material represented by Chemical Formula 1; a negative electrode disposed facing the positive electrode and including a negative electrode active material; and an electrolyte disposed between the anode and the cathode and conducting sodium ions.

During charging and discharging of the sodium secondary battery, Ru of the cathode active material is fixed to tetravalent, and oxidation-reduction of Tm ³⁺ /Tm ⁴⁺ and oxidation-reduction of O ^2- / ^O- may be used.

According to the present invention, the alkaline earth metal in the cathode active material for a sodium secondary battery can reduce Jahn-Teller distortion due to the change in the oxidation number of TM when intercalation/deintercalation of sodium ions occurs during charging and discharging of the battery. In addition, Ru does not participate in redox reactions during charging and discharging, maintains a +4 valent oxidation number, and has a strong attraction as a 4d transition metal, maintaining partial covalent bonding even during charging. As an alkaline earth metal, Mg may inhibit migration to the sodium layer. As a result, the distortion of Tm, specifically Mn, is suppressed to further improve structural stability during battery charging and discharging, and furthermore, the polarization phenomenon is suppressed so that the intercalation/desorption of sodium is not hindered and the diffusion coefficient of sodium ions is improved, even at high rates. It can show a high capacity retention rate.

1 is a schematic diagram showing a sodium secondary battery according to an embodiment of the present invention.
2 is a graph showing the results of Rietveld refinement analysis of the positive electrode active material obtained in Preparation Example 2 of the positive electrode active material.
3 shows SEM pictures and EDS elemental mapping results of the positive electrode active material obtained in Preparation Example 2 of the positive electrode active material.
4 is a charge/discharge graph of batteries according to Battery Preparation Example 2 and Battery Comparative Example.
5 shows a graph of GITT (galvanostatic intermittent titration technique) results of batteries according to Battery Preparation Example 2 and Battery Comparative Example, and the calculated value of the diffusion coefficient of sodium ions accordingly.
6 is a XANES graph of Mn K edge and Ru K edge of an active material during charging and discharging of batteries according to Battery Preparation Example and Battery Comparative Example.
7 is a charge/discharge graph of batteries according to Battery Manufacturing Examples 1 to 3;
8 is a charge/discharge graph of batteries according to Battery Preparation Examples 2 and 4;
9 is a graph showing rate characteristics of batteries according to Battery Preparation Example 2 and Battery Comparative Example.

Hereinafter, in order to explain the present invention in more detail, preferred embodiments according to the present invention will be described in more detail with reference to the accompanying drawings. However, the present invention is not limited to the embodiments described herein and may be embodied in other forms. Like reference numbers indicate like elements throughout the specification.

In this specification, when a layer is “on” another layer, it means that the layers are directly adjacent to each other, as well as having another layer(s) between them.

Cathode active material for sodium secondary battery

A cathode active material for a sodium secondary battery according to an embodiment of the present invention may be represented by Chemical Formula 1 below.

[Formula 1]

Na _x [M _y Tm _1-yz Ru _z ]O _2-a A _a

In Formula 1, M is an alkaline earth metal element, TM is manganese (Mn), nickel (Ni), iron (Fe), cobalt (Co), or a combination thereof, A is N, O, F or S, , 0.5≤a≤0.7, 0.1≤x≤0.3, 0.05≤z≤0.25, and 0≤a≤0.5.

The cathode active material for a sodium secondary battery has a crystal structure of a layered P2 structure, specifically a hexagonal P2 structure, and a space group of P6 ₃ /mmc. This layered structure has a structure in which sodium layers and [M _y Tm _1-yz Ru _z ]O _2-a A _a layers are alternately stacked.

The cathode active material for a sodium secondary battery may utilize oxidation-reduction of O ^2- / ^O- as well as oxidation-reduction of Tm ³⁺ /Tm ⁴⁺ when insertion/deintercalation of sodium ions occurs during charging and discharging of the battery.

The TM may be manganese (Mn). Mn is inexpensive compared to other transition metals and has the advantage of having excellent thermal stability. In addition, Mn may change its oxidation number to Mn ³⁺ ↔ Mn ⁴⁺ when sodium ions are intercalated/deintercalated during charging and discharging of the battery (oxidation number increases during charging and decreases during discharging). Accordingly, the capacity may be increased.

In the cathode active material for a sodium secondary battery, the alkaline earth metal is a metal having a divalent oxidation number, and may be, for example, magnesium (Mg). The alkaline earth metal may be substituted at the position of TM. The degree of substitution of the alkaline earth metal, that is, the y value may be 0.15 to 0.28, specifically 0.17 to 0.25, and more specifically, 0.18 to 0.22. As a result, when intercalation/deintercalation of sodium ions occurs during battery charge/discharge processes, Jahn-Teller distortion caused by a change in oxidation number of TM can be reduced. Accordingly, a stable layered structure is maintained during charging and discharging, so that a reversible phase transition is possible, thereby improving lifespan characteristics. Accordingly, a reversible phase transition may be possible even when an oxygen redox reaction is used for charging and discharging the battery, such as oxidizing oxygen in the active material by increasing the charge end voltage of the battery to 4V or higher. As a result, the capacity of the battery can be greatly improved.

On the other hand, when the active material does not contain Ru, when the battery is fully charged at 4V or more, the alkaline earth metal, that is, Mg moves to the sodium layer and [M _y Tm _1-yz Ru _z ]O _2-a In the A _a layer, distortion of Tm, specifically Mn, may occur. Because of this imbalance, polarization can occur, and intercalation/deintercalation of sodium can be hindered. As a result, a phenomenon in which the path difference of the voltage curve increases between the end of charge and the beginning of discharge, that is, the phenomenon of increase in voltage history may occur. Although this reaction is reversible, some alkaline earth metal, Mg, may remain in the sodium layer and reduce capacity retention.

However, in the active material according to Chemical Formula 1 according to an embodiment of the present invention, Tm is substituted with Ru, and Ru does not participate in redox reactions during charging and discharging, maintains a +4 valent oxidation number, and has a strong attractive force as a 4d transition metal to charge. As the partial covalent bonding is maintained even during the deposition, the movement of alkaline earth metal, that is, Mg, into the sodium layer may be inhibited. As a result, distortion of Tm, specifically Mn, is suppressed, thereby further improving structural stability during battery charging and discharging, and furthermore, polarization is suppressed, so that intercalation/deintercalation of sodium may not be hindered. In other words, when Ru is included, the diffusion coefficient of sodium ions is improved compared to when Ru is not included, so that a high capacity retention rate can be exhibited even at a high rate. The degree of substitution of Ru, that is, the z value may be 0.07 to 0.24, specifically 0.1 to 0.2, and more specifically, 0.15 to 0.23.

1 is a schematic diagram showing a sodium secondary battery according to an embodiment of the present invention.

Referring to FIG. 1, a sodium secondary battery 100 includes positive electrodes 10 and 50 including the positive electrode active material described above, negative electrodes 40 and 60 opposite to the positive electrode, and disposed between the positive electrode and the negative electrode. It includes an electrolyte and a separator (20).

anode

The cathode may include a cathode current collector 50 and a cathode active material layer 10 disposed thereon. The cathode current collector 50 may be a conductor such as Al, Ni, or stainless.

The positive electrode active material layer 10 may be formed by mixing a positive electrode active material, a conductive material, and a binder to obtain a positive electrode material, and then coating the positive electrode material on the positive electrode current collector 50 . The cathode active material may be particles having a composition represented by Chemical Formula 1 described above.

The conductive material may be a carbon material such as natural graphite, artificial graphite, cokes, carbon black, carbon nanotubes, or graphene. The conductive material may be contained in an amount of 2 to 20 parts by weight based on 100 parts by weight of the cathode active material. The binder is a thermoplastic resin, for example, a fluororesin such as polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene, a vinylidene fluoride copolymer, hexafluoropropylene, and/or a polyolefin resin such as polyethylene and polypropylene. can include The binder may be contained in an amount of 2 to 20 parts by weight based on 100 parts by weight of the positive electrode active material.

Applying the cathode material on the cathode current collector may use pressure molding or a method of making a paste using an organic solvent, etc., and then applying the paste on the current collector and press to fix it. Organic solvents include amines such as N,N-dimethylaminopropylamine and diethyltriamine; ethers such as ethylene oxide and tetrahydrofuran; ketone systems such as methyl ethyl ketone; esters such as methyl acetate; and an aprotic polar solvent such as dimethylacetamide and N-methyl-2-pyrrolidone. Applying the paste on the positive electrode current collector may be performed 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 may include an anode current collector 60 and an anode active material layer 40 disposed thereon. The anode current collector 60 may be copper foil.

The anode active material layer 40 includes an anode active material, which includes sodium metal, sodium alloy, metal oxide, metal fluoride, metal sulfide, and It may be formed using carbon materials such as natural graphite, artificial graphite, cokes, carbon black, carbon nanotubes, and graphene. When the anode active material is sodium metal foil, the anode current collector may be omitted.

The anode active material layer 40 may further include a conductive material and/or a binder. In this case, the conductive material may be a carbon material such as natural graphite, artificial graphite, cokes, carbon black, carbon nanotubes, or graphene. The binder is a thermoplastic resin, for example, a fluororesin such as polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene, vinylidene fluoride copolymer, hexafluoropropylene, and/or a polyolefin resin such as polyethylene and polypropylene. can include

electrolyte

The electrolyte may be a mixture of a salt of sodium ions and an organic solvent.

The sodium ion salt 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 , etc. A mixture of two or more of may be used. Among these, it may be preferable to use an electrolyte containing fluorine. As an organic solvent, for example, 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, pentafluoropropylmethyl ether, 2,2,3,3-tetrafluoropropyldifluoromethyl ether, tetrahydrofuran, 2-methyltetrahydro ethers such as furan; esters such as methyl formate, methyl acetate, and γ-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; Alternatively, a fluorine substituent may be introduced into the organic solvent described above.

Alternatively, a solid electrolyte may be used. The solid electrolyte may be an organic solid electrolyte such as a polyethylene oxide polymer compound, a polymer compound containing at least one of a polyorganosiloxane chain and a polyoxyalkylene chain. In addition, a so-called gel-type electrolyte in which a non-aqueous electrolyte is supported on a polymer compound can 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 ), and inorganic solid electrolytes such as Fe ₂ (MoO ₄ ) ₃ may also be used. There are cases in which the safety of a secondary battery can be further enhanced by using these solid electrolytes. In addition, there are cases where the solid electrolyte serves as a separator described later, and in that case, there are cases where a separator is not required.

separator

The separator 20 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 20 is preferably thinner as long as mechanical strength is maintained in that the volumetric energy density of the battery increases and the internal resistance decreases. The thickness of the separator 20 may be generally about 5 to 200 μm, and more specifically, about 5 to 40 μm.

Manufacturing method of secondary battery

A secondary battery may be manufactured by forming an electrode group by sequentially stacking a positive electrode, a separator, and a negative electrode, rolling the electrode group if necessary, storing it in a battery can, and impregnating the electrode group with a non-aqueous electrolyte. Alternatively, after forming an electrode group by stacking a positive electrode, a solid electrolyte, and a negative electrode, the secondary battery may be manufactured by rolling the electrode group and storing it in a battery can, if necessary.

Hereinafter, preferred experimental examples are presented to aid understanding of the present invention. However, the following experimental examples are only for helping understanding of the present invention, and the present invention is not limited by the following experimental examples.

[Experiments; Examples]

<Production Examples>

Cathode Active Material Preparation Examples 1 to 4 : Na _0.6 [Mg _y Mn _1-yz Ru _z ] O ₂

High-energy ball milling for 6 hours at a speed of 380 rpm using Na ₂ CO ₃ (adding 5 mol% compared to loss during heat treatment), MgO, Mn ₂ O ₃ and RuO ₂ according to the molar ratio with ethanol as a dispersing medium ), and the obtained dispersion was completely dried. After pelletizing the powder obtained by the above method, heat treatment was performed at 900° C. for 12 hours in an oxygen atmosphere. Thereafter, the powder was ground well to obtain a cathode active material powder having a composition of Na _0.6 [Mg _y Mn _1-yz Ru _z ] O ₂ .

Anode Preparation Examples 1 to 4

After quantifying the positive electrode active material obtained in the positive electrode active material preparation example, PVDF (Polyvinylidene fluoride) as a binder, and Super P as a conductive material in a weight ratio of 8: 1: 1, slurry was prepared using NMP, and a doctor blade was used to obtain a thickness of 100 um. It was applied to Al foil, which is a current collector, and then dried at 80 ° C. for 12 hours to obtain a positive electrode.

Battery Manufacturing Examples 1 to 4

In a glove box, the anode according to the cathode preparation example was used, metal sodium was used as a cathode, a glass filter was used as a separator, 0.5M NaPF ₆ as an electrolyte and propylene carbonate (PC) and fluoroethylene as an organic solvent An R2032 coin-type battery was prepared using a non-aqueous electrolyte solution containing carbonate (FEC) in a volume ratio of 98:2.

Table 1 below shows the composition of the cathode active material, the cathode, and the cathode active material in Examples 1 to 4 of the battery.

active material composition Na _0.6 [Mg _y Mn _1-yz Ru _z ] O ₂ y 1-y-z z Cathode active material preparation example 1 anode
Preparation Example 1 battery
Preparation Example 1 Na _0.6 [Mg _0.15 Mn _0.65 Ru _0.2 ] O ₂ 0.15 0.65 0.2 Cathode active material preparation example 2 anode
Preparation Example 2 battery
Preparation Example 2 Na _0.6 [Mg _0.2 Mn _0.6 Ru _0.2 ] O ₂ 0.2 0.6 0.2 Cathode active material preparation example 3 anode
Preparation Example 3 battery
Preparation Example 3 Na _0.6 [Mg _0.28 Mn _0.52 Ru _0.2 ] O ₂ 0.28 0.52 0.2 Cathode Active Material Production Example 4 anode
Production Example 4 battery
Production Example 4 Na _0.6 [Mg _0.2 Mn _0.7 Ru _0.1 ] O ₂ 0.2 0.7 0.1

<Comparative Examples>

Comparative Example of Cathode Active Material : Na _0.6 [Mg _0.2 Mn _0.8 ] O ₂

Na _0.6 [Mg _0.2 Mn _0.8 ]O ₂ was obtained by using the same method as in the positive electrode active material preparation example, except for using Na ₂ CO ₃ (added 5% compared to loss during heat treatment), MgO, and Mn ₂ O ₃ according to the molar ratio. A positive electrode active material powder having a composition was obtained.

Anode Comparative Example

A positive electrode was obtained in the same manner as in the positive electrode preparation example, except that the positive electrode active material obtained in the positive electrode active material comparative example was used.

Battery manufacturing example

A battery was obtained in the same manner as in Battery Preparation Example, except that the positive electrode obtained in the positive electrode comparative example was used.

2 is a graph showing the results of Rietveld refinement analysis of the positive electrode active material obtained in Preparation Example 2 of the positive electrode active material.

Referring to FIG. 2 , it can be seen that the cathode active material Na _0.6 [Mg _0.2 Mn _0.6 Ru _0.2 ] O ₂ obtained in Preparation Example 2 of the cathode active material has a hexagonal structure of space group P6 ₃ /mmc.

3 shows SEM pictures and EDS elemental mapping results of the positive electrode active material obtained in Preparation Example 2 of the positive electrode active material.

Referring to FIG. 3 , it is confirmed that sodium and transition metals are uniformly spread in the particles in the Na _0.6 [Mg _0.2 Mn _0.6 Ru _0.2 ]O ₂ active material, which may mean that element doping between materials is well performed.

4 is a charge/discharge graph of batteries according to Battery Preparation Example 2 and Battery Comparative Example. Specifically, charging was performed at 0.05C to 4.5V, and discharging was performed at 0.05C to 1.5V.

Referring to FIG. 4, in the case of a comparative battery including a positive electrode active material of Na _0.6 [Mg _0.2 Mn _0.8 ] O ₂ according to Comparative Positive Active Material Example, a discharge capacity of 204 mAh g ^-1 was shown, but according to Preparation Example 2 of positive electrode active material In the case of Battery Preparation Example 2 including the cathode active material of Na _0.6 [Mg _0.2 Mn _0.6 Ru _0.2 ]O ₂ , a discharge capacity of 210 mAh g ^-1 was shown.

Here, in the case of Battery Preparation Example 2 including the cathode active material of Na _0.6 [Mg _0.2 Mn _0.6 Ru _0.2 ]O ₂ , compared to the battery Comparative Example including the cathode active material of Na _0.6 [Mg _0.2 Mn _0.8 ]O ₂ , the charging voltage It can be seen that the degree of difference between the increasing path and the voltage decreasing path during discharge is reduced, that is, hysteresis is reduced.

In addition, the charge capacity of up to about 50 mAh g ^-1 during charging is due to the increase in the oxidation number of Mn (from +3 to +4), and then at the second plateau shown at about 4V or higher, the oxidation number of oxygen It was estimated to increase (from -2 to -1).

5 shows a graph of galvanostatic intermittent titration technique (GITT) results of batteries according to Battery Preparation Example 2 and Battery Comparative Example, and a sodium ion diffusion coefficient calculation value accordingly.

Referring to FIG. 5, the diffusion coefficient of sodium ions is higher in the case of the cathode active material of Na _0.6 [Mg _0.2 Mn _0.6 Ru _0.2 ] _{O 2 than} that of the cathode active material of Na _0.6 [Mg _0.2 Mn _0.8 ] O 2 . It was much higher at the end of charge and at the beginning of discharge. This may mean that intercalation/deintercalation of sodium ions in the cathode active material of Na _0.6 [Mg _0.2 Mn _0.6 Ru _0.2 ]O ₂ occurs better during charging and discharging of the battery.

6 is a XANES graph of Mn K edge and Ru K edge of an active material during charging and discharging of batteries according to Battery Preparation Example and Battery Comparative Example.

Referring to FIG. 6, in the case of the battery comparative example, Mn of the positive electrode active material Na _0.6 [Mg _0.2 Mn _0.8 ] O ₂ in the charging and discharging process reached +4 when charged to 4.5V and Mn reached +4 when discharged to 1.5V. The oxidation number of is understood to be close to +3. In the case of battery manufacturing example, Mn of the cathode active material Na _0.6 [Mg _0.2 Mn _0.6 Ru _0.2 ] O ₂ in the charging and discharging process also reaches +4 when charged to 4.5V, and the oxidation number of Mn reaches +4 when discharged to 1.5V. was understood to be close to 3. However, the initial oxidation number of Mn in the cathode active material Na _0.6 [Mg _0.2 Mn _0.6 Ru _0.2 ]O ₂ was lower than the initial oxidation number of Mn in Na _0.6 [Mg _0.2 Mn _0.8 ]O ₂ .

Meanwhile, in the case of Battery Manufacturing Example, it was confirmed that the oxidation number of Ru of the cathode active material Na _0.6 [Mg _0.2 Mn _0.6 Ru _0.2 ]O ₂ was fixed to +4 during the charging and discharging process.

7 is a charge/discharge graph of batteries according to Battery Manufacturing Examples 1 to 3; Specifically, charging was performed at 0.05C to 4.5V, and discharging was performed at 0.05C to 1.5V.

Referring to FIG. 7, although the capacity varies depending on the Mn redox that appears up to about 50 mAh g ^-1 and the oxygen redox thereafter, Battery Manufacturing Examples 1 in which the number of moles of Mg is 0.15 to 0.28 3 to 3 showed high capacity due to oxygen redox.

8 is a charge/discharge graph of batteries according to Battery Preparation Examples 2 and 4; Specifically, charging was performed at 0.05C to 4.5V, and discharging was performed at 0.05C to 1.5V.

Referring to FIG. 8, when the number of moles of Ru is 0.1 and 0.2, there is a slight difference in the hysteresis reduction range, but it can be seen that the reduction effect is shown in all ranges.

9 is a graph showing rate characteristics of batteries according to Battery Preparation Example 2 and Battery Comparative Example.

Referring to FIG. 9 , Na _0.6 [Mg _0.2 Mn _0.8 ] O ₂ according to Preparation Example 2 of the positive electrode active material, Na _0.6 [Mg _0.2 Mn _0.6 Ru _0.2 In the case of battery preparation example 2 including a cathode active material of ]O ₂ , it was confirmed that the capacity retention rate at 5C compared to 0.05C was improved from 18% to 35% by doping with Ru, showing a stable capacity retention rate even at high rates.

In the above, the present invention has been described in detail with preferred embodiments, but the present invention is not limited to the above embodiments, and various modifications and changes are made by those skilled in the art within the technical spirit and scope of the present invention. this is possible

Claims (14)

A cathode active material for a sodium secondary battery represented by Formula 1 below;
[Formula 1]
Na _x [M _y Tm _1-yz Ru _z ]O _2-a A _a
In Formula 1, M is an alkaline earth metal element, TM is manganese (Mn), nickel (Ni), iron (Fe), cobalt (Co), or a combination thereof, A is N, O, F or S, , 0.5≤a≤0.7, 0.1≤x≤0.3, 0.05≤z≤0.25, and 0≤a≤0.5. In claim 1,
The cathode active material for a sodium secondary battery has a hexagonal P2 structure, and the space group is P6 ₃ /mmc. In claim 1,
Wherein M is Mg, a cathode active material for a sodium secondary battery. In claim 1,
y is a cathode active material for a sodium secondary battery of 0.15 to 0.28. In claim 1,
The TM is a cathode active material for a sodium secondary battery of Mn. In claim 1,
A cathode active material for a sodium secondary battery in which Ru is tetravalently fixed when Na ions are inserted/deintercalated from the cathode active material. In claim 1,
z is a cathode active material for a sodium secondary battery of 0.1 to 0.2. A positive electrode including a positive electrode active material represented by Formula 1 below;
a negative electrode disposed facing the positive electrode and including a negative electrode active material; and
a sodium secondary battery disposed between the positive electrode and the negative electrode and including an electrolyte conducting sodium ions;
[Formula 1]
Na _x [M _y Tm _1-yz Ru _z ]O _2-a A _a
In Formula 1, M is an alkaline earth metal element, TM is manganese (Mn), nickel (Ni), iron (Fe), cobalt (Co), or a combination thereof, A is N, O, F or S, , 0.5≤a≤0.7, 0.1≤x≤0.3, 0.05≤z≤0.25, and 0≤a≤0.5. In claim 8,
A sodium secondary battery in which Ru of the positive electrode active material is fixed to tetravalent during charging and discharging of the sodium secondary battery, and oxidation-reduction of Tm ³⁺ /Tm ⁴⁺ and oxidation-reduction of O ^2- / ^O- are used. In claim 8,
The cathode active material has a hexagonal P2 structure, and the space group is P6 ₃ /mmc sodium secondary battery. In claim 8,
The M is a sodium secondary battery of Mg. In claim 8,
y is a sodium secondary battery of 0.15 to 0.28. In claim 8,
The TM is a sodium secondary battery of Mn. In claim 8,
z is a sodium secondary battery of 0.1 to 0.2.

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