KR20200032662A - Sodium-Based Electrode Active Material and Secondary Battery Having the Same - Google Patents

Abstract

Provided by the present invention are a sodium-based electrode active material and a secondary battery comprising the same. The electrode active material is represented by chemical formula 1, Na_zMn_(1-y-x)M^1_yM^2_xO_(2-α)A_α, and has a hexagonal P2 structure. In chemical formula 1, z is 0.45 to 0.8, M^1 is an alkaline earth metal or Zn having a divalent oxidation number, M^2 is Co, Ni, Ti, V, Cr, Fe, or Cu having a trivalent oxidation number, y is 0.1 to 0.3, x is 0.05 to 0.25, A is N, O, F, or S, and α is 0 to 0.1. The present invention provides improved stability.

Description

Sodium-Based Electrode Active Material and Secondary Battery Having the Same

The present invention relates to a secondary battery, and specifically, to a secondary battery including a sodium-based electrode active material.

Secondary battery refers to a battery that can be used repeatedly because it can be charged as well as discharge. The representative lithium secondary battery among the secondary batteries is lithium ions contained in the positive electrode active material is transferred to the negative electrode through the electrolyte and then inserted into the layered structure of the negative electrode active material (charge), and then the lithium ions inserted into the layered structure of the negative electrode active material again It works through the principle of returning to the anode (discharging). The lithium secondary battery is currently commercialized and used as a small power source for mobile phones and notebook computers, and is expected to be used as a large power source for hybrid vehicles, and the demand is expected to increase.

However, a composite metal oxide mainly used as a positive electrode active material in a lithium secondary battery contains a rare metal element such as lithium, and there is a fear that it cannot meet the increasing demand. Accordingly, research is being conducted on a sodium secondary battery using sodium having a high supply and inexpensive as a positive electrode active material. As an example, Republic of Korea Patent Publication No. 2012-0133300 discloses A _x MnPO ₄ F (A = Li or Na, 0 <x ≤ 2) as a positive electrode active material.

However, the sodium anode materials developed to date still do not have excellent structural stability, and it is known that a battery using the same needs to improve the discharge capacity retention rate and stability.

Accordingly, an object of the present invention is to provide an active material for a secondary battery having improved discharge capacity retention characteristics and stability, and a secondary battery including the same.

In order to achieve the above object, one aspect of the present invention provides an electrode active material. The electrode active material is represented by the following Chemical Formula 1, and has a hexagonal P2 structure.

[Formula 1]

Na _z Mn _1-yx M ¹ _y M ² _x O _2-α A _α

In Formula 1, z is 0.45 to 0.8, M ¹ is an alkaline earth metal or Zn having a divalent oxidation number, and M ² is Co, Ni, Ti, V, Cr, Fe, or Cu having a trivalent oxidation number, y is 0.1 to 0.3, x is 0.05 to 0.25, A is N, O, F, or S, and α is 0 to 0.1.

The active material represented by Chemical Formula 1 may be represented by Chemical Formula 2.

[Formula 2]

Na _z Mn _1-yx Mg _y Co _x O _2-α A _α

In Formula 2, A, x, y, z, and α are as defined in Formula 1.

In Formula 1 or 2, z is 0.5 to 0.6, y is 0.2 to 0.25, and x may be 0.2 to 0.25. The space group of the electrode active material may be P6 ₃ / mmc. In Formula 1 or 2, the Mn may be in a valence state of 3.5 to 4. Furthermore, the Mn may be in a valence state of 3.75 to 4.

In order to achieve the above object, another aspect of the present invention provides a secondary battery. The secondary battery is represented by Chemical Formula 1 and includes a positive electrode having an electrode active material having a hexagonal P2 structure as a positive electrode active material, a negative electrode containing a negative electrode active material, and an electrolyte disposed between the positive electrode and the negative electrode.

The charging voltage of the secondary battery may be 4 to 4.6 V. The positive electrode may further include a sodium salt.

According to the present invention, since the electrode active material represented by Chemical Formula 1 may have a valence of Mn greater than 3, crystal structure distortion due to the Jahn-Teller effect is alleviated, thereby improving the capacity retention rate of the secondary battery.

In addition, in the secondary battery using such a positive electrode active material, M ² specifically, 3d orbital of Co and 2p orbital of O may overlap, so that when charging at 4 V or higher, oxidation of oxygen may occur, and oxygen is stably reduced in the subsequent discharge process. The capacity can be further improved as reversible redox of oxygen occurs stably during charging and discharging.

In addition, M ^2, specifically, due to the doping of Co, the band gap of the positive electrode active material may be reduced compared to that before Co doping, and thus the sodium secondary battery using the same may be capable of high-speed charging and discharging.

In addition, M ² specifically, the activation barrier energy for diffusion of sodium ions may also be lowered due to the doping of Co, so that ion conductivity may also be improved.

1 shows a sodium secondary battery according to an embodiment of the present invention.
Figure 2a is an active material preparation examples 1 to 3 and Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ (x = 0, 0.05, 0.1, and 0.2) according to comparative examples of the active material Rietveld refinement (Rietveld refinement) It is a graph showing the results of analysis based on the space group P6 ₃ / mmc using the method.
Figure 2b is an XRD graph of Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ (x = 0.3) according to Preparation Example 4 of the active material.
Figure 3 shows the change in the lattice parameter values of Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ (x = 0, 0.05, 0.1, and 0.2) according to active material preparation examples 1 to 3 and active material comparative example It is a graph.
4 is FE-SEM (Field Emission Scanning Electron Microscope) images of Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ particles according to Active _Material Preparation Examples 1 to 3 and Active Material Comparative Example ((a) x = 0, (b) x = 0.05, (c) x = 0.1, and (d) x = 0.2).
5 are graphs showing charging and discharging characteristics in 1st, 2nd, 10th, 50th, and 100th cycles of each of the half cells according to the battery manufacturing examples 1 to 3 and the battery comparative example.
5B is a graph showing discharge capacity and coulomb efficiency according to the number of cycles of reverse cells according to Battery Manufacturing Examples 1 to 3 and Battery Comparative Example.
6 is a graph showing the rate characteristics and coulomb efficiency of the reverse cells according to the battery manufacturing examples 1 to 3 and the battery comparative example.
7 is a graph showing cycle characteristics at 5C of a half cell according to Battery Manufacturing Example 3;
8 shows ex-situ XANES (X-ray absorption near edge structure) spectra for Mn K-edge, Co K-edge, and O K-edge during electrochemical reactions of reverse cells according to Preparation Example 3 and Comparative Example. Shows.
9 is Na _0.5 [Mg _0.25 Mn _0.75-x Co _x ] O ₂ (x = 0), Na _0.5 [Mg _0.25 Mn _0.75-x Co _x ] O ₂ (x = 0.125), and Na _0.5 [Mg _0.25 Mn _It represents the expected DOS (projected Density of State) of O 2p, Mn 3d, Co 3d orbitals of _0.75-x Co _x ] O ₂ (x = 0.25).
FIG. 10 shows a graph (b) showing Na ₁ -Na ₂ diffusion path (a) and activation barrier energy for Na ⁺ diffusion in Na _0.5 [Mg _0.25 Mn _0.75-x Co _x ] O ₂ (x = 0.25). .

Hereinafter, preferred embodiments of the present invention will be described in more detail with reference to the accompanying drawings in order to describe the present invention in more detail. However, the present invention is not limited to the embodiments described herein and may be embodied in other forms. Throughout the specification, the same reference numbers indicate the same components.

Anode active material

The positive electrode active material according to an embodiment of the present invention is represented by the following formula (1).

[Formula 1]

Na _z Mn _1-yx M ¹ _y M ² _x O _2-α A _α

In Chemical Formula 1, z may be 0.45 to 0.8. As an example, z may be 0.5 to 0.7, specifically 0.5 to 0.65, or 0.5 to 0.6. M ¹ is a metal having a divalent oxidation number and may be an alkaline earth metal or Zn. The alkaline earth metal may be Be, Mg, Ca, Sr, or Ba, but may be Mg as an example. M ² may be Co, Ni, Ti, V, Cr, Fe, or Cu. M ² may have a trivalent oxidation number. y may be 0.1 to 0.3. Specifically, y may be 0.15 to 0.25, for example, 0.2 to 0.25. x may be 0.05 to 0.25. Specifically, x may be 0.1 to 0.25, further 0.15 to 0.23, and for example, 0.2 to 0.23. A may be N, O, F, or S, and α may be 0 to 0.1.

In one example, the positive electrode active material may be represented by the following formula (2).

[Formula 2]

Na _z Mn _1-yx Mg _y Co _x O _2-α A _α

In Formula 2, A, x, y, z, and α are as defined in Formula 1.

The positive electrode active material represented by Chemical Formula 1 or 2 may have a hexagonal crystal structure. Specifically, a sodium layer having a hexagonal P2 structure and a metal oxide layer are layered compounds alternately stacked, and a space group may be P6 ₃ / mmc.

In the original state (pristine) that did not participate in charge / discharge, the positive electrode active material was doped with an alkaline earth metal having a valence of +2 or Zn (ie, M ¹ ), as well as M ² having a valence of +3. , The valence state of Mn may be 3.5 to 4 days, for example, 3.75 to 4. As described above, in the case where the valence of Mn is greater than 3, the positive electrode active material can alleviate crystal structure distortion due to the Jahn-Teller effect, thereby improving the capacity retention rate of the secondary battery. In addition, the sodium secondary battery using such a positive electrode active material M ² specifically, 3d orbitals of Co and 2p orbitals of O may overlap, so that when charging at 4V or higher, oxidation of oxygen may occur, and oxygen is stably reduced in the subsequent discharge process. The capacity can be further improved as reversible redox of oxygen occurs stably during charging and discharging. In addition, M ^2, specifically, due to the doping of Co, the band gap of the positive electrode active material may be reduced compared to that before Co doping, and thus the sodium secondary battery using the same may be capable of high-speed charging and discharging. In addition, M ² specifically, the activation barrier energy for diffusion of sodium ions may also be lowered due to the doping of Co, so that ion conductivity may also be improved.

A secondary battery having such a high capacity characteristic can be used as a unit battery of a battery module that is a power source for a medium-to-large device. The medium-to-large device may include, for example, a power tool that is moved by being driven by an electric motor; An electric vehicle (EV) including a hybrid electric vehicle (HEV) and a plug-in hybrid electric vehicle (PHEV); Electric two-wheeled vehicles including E-bikes and E-scooters; Or it may include an electric golf cart (electric golf cart), but is not limited thereto.

Hereinafter, a method of manufacturing a positive electrode active material according to an embodiment of the present invention will be described.

First, a metal salt solution containing a sodium salt, a manganese salt, a salt of a first metal (in Formula 1, M ¹ ), and a salt of a second metal (in Formula 1, M ² ) may be prepared. The molar ratio of sodium, manganese, first metal (in Formula 1, M ¹ ), and second metal (in Formula 1, M ² ) in the metal salt solution is z: 1-yx: y: x (x, y, And z is as defined in Formula 1). The metal salt may be a metal carbonate, metal nitrate, or metal oxalate. The sodium salt may be NaNO ₃ , Na ₂ CO ₃ , or NaHCO ₃ , and the manganese salt may be Mn (NO ₃ ) ₂ . These metal salts may take the form of hydrates. The metal salt solution may contain distilled water as a solvent.

A chelating agent may be further added to the metal salt solution. The chelating agent may be selected from the group consisting of tartaric acid, urea, citric acid, formic acid, glycolic acid, polyacrylic acid, adipic acid, and glycine. The chelating agent may be contained in 10wt% to 30wt% by weight of the metal salt. Meanwhile, a crystal growth inhibitor may be further included in the metal salt solution. The crystal growth inhibitor may be a saccharide or a derivative thereof, for example, glucose, sucrose, or a derivative thereof. The crystal growth inhibitor may be contained in 1wt% to 10wt% by weight of the metal salt.

The metal salt solution can be sufficiently mixed by stirring.

Thereafter, the metal salt solution may be subjected to ultrasonic spray pyrolysis to obtain a solid powder. The ultrasonic spray pyrolysis is a method that has the advantage of obtaining a metal oxide having a pure composition for a short time and a lower temperature than the solid-phase method. Is how to do it. In the thermal decomposition process, metal salts and the like may be converted to metal oxides.

Thereafter, the solid powder may be heat treated in a dry air atmosphere to obtain a positive electrode active material. The dry air atmosphere may be a dried oxygen atmosphere of 15 vol.% To 100 vol.%, Specifically, a dried atmosphere containing oxygen of 20 vol.% To 100 vol.% And the rest of the inert gas. At this time, the inert gas may be nitrogen. The dry atmosphere in the present specification may mean an atmosphere that does not contain moisture. In the heat treatment process in this atmosphere, there is an advantage that can prevent the volatilization of sodium. Further, heat treatment may be performed at 800 ° C to 1000 ° C.

Sodium secondary battery

1 shows a sodium secondary battery according to an embodiment of the present invention.

Referring to FIG. 1, a sodium secondary battery includes a positive electrode 120 containing a positive electrode active material described above, a negative electrode 140 containing a negative electrode active material, and an electrolyte 160 positioned therebetween. The positive electrode 120 may be located on the positive electrode current collector 110, and the negative electrode 140 may be located on the negative electrode current collector 150. In addition, the positive electrode 120 and the negative electrode 140 may be separated by a separator 130, and an electrolyte 160 may be disposed between the positive electrode 120 and the negative electrode 140.

<Anode>

A positive electrode material may be obtained by mixing the positive electrode active material, a conductive material, and a binder described in Chemical Formula 1.

The positive electrode active material described in Chemical Formula 1 may have a stable crystal structure, and thus has a low degree of deterioration due to moisture and a lower operating voltage. However, the positive electrode active material described in Chemical Formula 1 has a molar ratio of sodium compared to transition metal (Mn, M ¹ , and M ² in Chemical Formula 1) less than 1, that is, z in Chemical Formula 1 exhibits a value less than 1, The content of sodium may be less than other positive electrode active materials with z of 1 or more. To compensate for this, sodium salt may be added to the positive electrode material. Na ions included in the sodium salt may be reduced in the initial charging process of the battery to serve as an additional Na source. In this case, the initial charging capacity of the sodium secondary battery may be improved to improve battery performance. The sodium salt is added in an amount of 1 to 20 parts by weight, specifically 3 to 20 parts by weight or 3 to 15 parts by weight, more specifically 3 to 12 parts by weight, and 5 to 7 parts by weight, for example, based on 100 parts by weight of the positive electrode active material. Can be.

The conductive material may be natural graphite, artificial graphite, coke, carbon black, carbon nanotubes, graphene, or other carbon materials. Binders include thermoplastic resins such as polyvinylidene fluoride, polytetrafluoroethylene, ethylene tetrafluoride, vinylidene fluoride copolymers, fluorine resins such as propylene hexafluoride, and / or polyolefin resins such as polyethylene and polypropylene. It can contain.

When the sodium salt is added, the conductive material may contain 2 to 9 parts by weight, specifically 4 to 7 parts by weight, and more specifically 5 to 6 parts by weight, based on 100 parts by weight of the positive electrode active material, and the binder may be 2 to 9 parts by weight with respect to 100 parts by weight of the positive electrode active material, specifically 4 to 7 parts by weight, and more specifically 5 to 6 parts by weight may be contained.

The positive electrode material may be coated on the positive electrode current collector to form the positive electrode. The positive electrode current collector may be a conductor such as Al, Ni, and stainless steel. Applying the positive electrode material on the positive electrode current collector may be a method of forming a paste using pressure molding or an organic solvent, and then applying the paste onto the current collector and pressing to fix it. Organic solvents include amines such as N, N-dimethylaminopropylamine and diethyltriamine; Ether systems such as ethylene oxide and tetrahydrofuran; Ketone systems such as methyl ethyl ketone; Ester systems such as methyl acetate; Aprotic polar solvents 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>

Cathode active materials are metals, metal alloys, metal oxides, metal fluorides, metal sulfides, and natural graphite, artificial graphite, coke, carbon black, carbon nanotubes, and graphene that can deintercalate or convert Na ions. It can also be formed using a carbon material such as a fin.

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 natural graphite, artificial graphite, coke, carbon black, carbon nanotubes, carbon materials such as graphene. Binders include thermoplastic resins such as polyvinylidene fluoride, polytetrafluoroethylene, ethylene tetrafluoride, vinylidene fluoride copolymers, fluorine resins such as propylene hexafluoride, and / or polyolefin resins such as polyethylene and polypropylene. It can contain.

The anode material may be coated on the anode current collector to form an anode. The positive electrode current collector may be a conductor such as Al, Ni, and stainless steel. To apply the negative electrode material on the positive electrode current collector, a method of forming a paste using pressure molding or an organic solvent, etc., and then applying the paste onto the current collector and pressing to fix it may be used. Organic solvents include amines such as N, N-dimethylaminopropylamine and diethyltriamine; Ether systems such as ethylene oxide and tetrahydrofuran; Ketone systems such as methyl ethyl ketone; Ester systems such as methyl acetate; Aprotic polar solvents such as dimethylacetamide and N-methyl-2-pyrrolidone. Applying the paste on the negative 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.

<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 sodium carboxylate, NaAlCl _4, etc. Mixtures can also be used. Among these, it is preferable to use an electrolyte containing fluorine. Further, the electrolyte can be dissolved in an organic solvent and used as a non-aqueous electrolyte. 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-oxazolidon; Sulfur-containing compounds such as sulfolane, dimethyl sulfoxide and 1,3-propanesultone; Alternatively, a fluorine substituent introduced into the above-described organic solvent may be used.

Alternatively, a solid electrolyte may be used. The solid electrolyte may be an organic solid electrolyte such as a polymer compound containing at least one of a polyethylene oxide-based polymer compound, a polyorganosiloxane chain or a polyoxyalkylene chain. Further, a so-called gel type electrolyte in which a nonaqueous electrolyte solution is supported on a polymer compound can also be used. Meanwhile, 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 Fe ₂ (MoO ₄ ) ₃ . In some cases, the safety of the sodium secondary battery may be improved by using these solid electrolytes. In addition, the solid electrolyte may serve as a separator to be described later, and in that case, a separator may not be required.

<Separator>

A separator may be disposed between the anode and the cathode. The separator may be a material having a form of a porous film made of a material such as polyolefin resin such as polyethylene or polypropylene, a fluorine resin, or an aromatic polymer containing nitrogen, a nonwoven fabric or a woven fabric. The thickness of the separator is preferably as thin as possible as long as mechanical strength is maintained, in that the bulk energy density of the battery becomes high and the internal resistance becomes small. The thickness of the separator may generally be on the order of 5 to 200 μm, and more specifically 5 to 40 μm.

<Method for manufacturing sodium secondary battery>

A positive electrode, a separator, and a negative electrode may be stacked in order to form an electrode group, and if necessary, the electrode group may be rolled and stored in a battery can, and a sodium secondary battery may be prepared by impregnating the electrode group with a non-aqueous electrolyte. Alternatively, a positive electrode, a solid electrolyte, and a negative electrode may be stacked to form an electrode group, and then, if necessary, the electrode group may be rolled and stored in a battery can to produce a sodium secondary battery.

The sodium secondary battery may be driven with a charging voltage of 4 to 5V, for example, 4.2 to 4.6V, and a discharge voltage of 1 to 1.5V. When the charging voltage is 4 or more and 4.2V or more, oxygen may be reversibly oxidized from O ² to O ^-1 in the positive electrode active material represented by Chemical Formula 1 or 2. In addition, M ² specifically, the 3d orbital of Co and the 2p orbital of O may overlap, such that oxidation of oxygen in the charging process and reduction of oxygen in a subsequent discharge process may appear stably, the anode represented by Chemical Formula 1 or 2 The sodium secondary battery having an active material may exhibit excellent capacity retention rate with a large increase in capacity.

Hereinafter, a preferred experimental example (example) is presented to help understanding of the present invention. However, the following experimental examples are only to aid the understanding of the present invention, and the present invention is not limited by the following experimental examples.

[Experimental Examples; Examples]

Active material manufacturing examples

Active material preparation examples 1 to 4: Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂  (x = 0.05, 0.1, 0.2, and 0.3) Preparation

Sodium nitrate (NaNO ₃ , 99%, 3,000), Manganese (II) nitrate tetrahydrate, Mn (NO ₃ ) ₂ · 4H ₂ O, ≥97%, Sigma-Aldrich), magnesium nitrate 6 Hydrate (Magnesium nitrate hexahydrate, Mg (NO ₃ ) ₂ · 6H ₂ O, ≥98.0%, Sigma-Aldrich), Cobalt (II) nitrate hexahydrate, Co (NO ₃ ) ₂ · 6H ₂ O, ≥ 98.0%, Sigma-Aldrich), citric acid (≥99.5, Junsei), and sucrose (Sucrose, ≥99.5, 3,000) were dissolved in distilled water and stirred for more than 12 hours using a magnetic bar to mix well. The citric acid was used in a ratio of 0.2 times the weight of the nitrates, and the sucrose was 0.05 times the weight of the nitrates. The stirred solution was sprayed through a nozzle of an ultrasonic spray into a quartz tube maintained at 400 ° C at a constant rate to obtain a solid powder. After pelletizing the solid powder at a constant pressure, it was placed in an alumina crucible, and in a dry air atmosphere containing 21 vol.% O ₂ and 79 vol.% N ₂ , the alumina crucible was at a rate of 5 ° C./min. After heating, the mixture was maintained at 870 ° C for 10 hours, and then slowly cooled to 30 ° C at a rate of 3 ° C / min, P2 type Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ (Preparation Example 1: x = 0.05, Preparation Example 2: x = 0.1, Preparation Example 3: x = 0.2, Preparation Example 4: x = 0.3).

Comparative example of active material: Na _0.6 [Mg _0.2 Mn _0.8 ] O ₂  Produce

P2 type Na _0.6 [Mg _0.2 Mn _0.8 ] O ₂ was prepared by using the same method as the active material preparation example 1, except that cobalt nitrate hexahydrate was not used.

Figure 2a is an active material preparation examples 1 to 3 and Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ (x = 0, 0.05, 0.1, and 0.2) according to comparative examples of the active material Rietveld refinement (Rietveld refinement) It is a graph showing the results of analysis based on the space group P6 ₃ / mmc using the method. Figure 2b is an XRD graph of Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ (x = 0.3) according to Preparation Example 4 of the active material.

Referring to Figure 2a, X observed in both Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ (x = 0, 0.05, 0.1, and 0.2) according to the active material preparation examples 1 to 3 and the active material comparative example It can be seen that the X-ray diffraction (XRD) pattern was in good agreement with the calculated pattern of the hexagonal P2 structure space group P6 ₃ / mmc, and no impurities were detected.

Referring to FIG. 2B, the X-ray diffraction (XRD) pattern observed at Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ (x = 0.3) according to Preparation Example 4 is hexagonal P2 structure space group P6 _{Although it} partially coincides with the calculated pattern of ₃ / mmc, it can be seen that a peak due to impurities is observed as indicated. This means that impurities are generated according to the increase in the Co content, and these impurities do not contribute to the capacity and act as a resistance, so that the secondary battery using this as an active material has secondary capacity and rate characteristics using the active material preparation examples 1 to 3 It may not be as good as a battery.

Figure 3 shows the change in the lattice parameter values of Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ (x = 0, 0.05, 0.1, and 0.2) according to active material preparation examples 1 to 3 and active material comparative example It is a graph.

Referring to FIG. 3, it can be seen that a decrease in the lattice parameter values appears as the content of Co increases. Considering the ion radius of Co ³⁺ (0.545 0.5), Mn ³⁺ (0.645 Å), and Mn ⁴⁺ (0.53 Å), the reduction of these lattice parameter values is such that Co ³⁺ selects Mn ³⁺ with the same oxidation number. It was presumed to have been incorporated while replacing with.

4 is FE-SEM (Field Emission Scanning Electron Microscope) images of Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ particles according to Active _Material Preparation Examples 1 to 3 and Active Material Comparative Example ((a) x = 0, (b) x = 0.05, (c) x = 0.1, and (d) x = 0.2).

4, Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ particles ((a) x = 0, (b) x = 0.05, according to active material preparation examples 1 to 3 and an active material comparative example) (c) x = 0.1, and (d) x = 0.2) all showed the shape of the secondary particles aggregated by the primary particles, and the difference in particle morphology was not large.

Battery manufacturing examples

Battery Preparation Example 1 Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂  Preparation of anode and half-cell using (x = 0.05)

Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ (x = 0.05) powder, conductive material (Super-P), and binder (Poly vinylidene fluoride) prepared in Active Material Preparation Example 1 of 8: 1: 1 After mixing in an organic solvent (NMP (N-Methyl-2-Pyrrolidone)) in a weight ratio to form a slurry, the slurry was cast on an aluminum current collector to form an anode.

Thereafter, metal sodium was used as a cathode, a glass filter was used as a separator, and electrolyte 0.5 M NaPF ₆ and an organic solvent propylene carbonate (PC, 98 vol.%) And fluoroethylene carbonate (FEC, 2 vol.%) Were included. A non-aqueous electrolyte was prepared using a non-aqueous electrolyte.

Battery Manufacturing Example 2: Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂  Preparation of anode and half-cell using (x = 0.1)

Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ prepared in Preparation Example 1 (x = 0.05) Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ prepared in Active Example 2 instead of powder (x = 0.1) A positive electrode and a half cell were prepared using the same method as in Battery Preparation Example 1, except that powder was used.

Battery Preparation Example 3: Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂  Preparation of anode and half-cell using (x = 0.2)

Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ prepared in Active Example 1 (x = 0.05) Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ prepared in Active Example 3 instead of powder (x = 0.2) A positive electrode and a half cell were prepared using the same method as in Battery Preparation Example 1, except that powder was used.

Battery Comparative Example: Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂  Preparation of anode and half-cell using (x = 0)

Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ prepared in Preparation Example 1 (x = 0.05) Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ prepared in Comparative Example active material instead of powder ( x = 0) A positive electrode and a half cell were prepared using the same method as in Battery Preparation Example 1, except that powder was used.

5 are graphs showing charging and discharging characteristics in 1st, 2nd, 10th, 50th, and 100th cycles of each of the half cells according to the battery manufacturing examples 1 to 3 and the battery comparative example, and FIG. 5b is the battery It is a graph showing discharge capacity and coulomb efficiency according to the number of cycles of the reverse cells according to Manufacturing Examples 1 to 3 and the battery comparative example. At this time, the charging was performed by constant current charging at 0.1C (26 mAg ^-1 ) up to 4.6V, and the discharge was performed at a rate equal to the charging speed and constant current discharge up to 1.5V.

5A, when x is 0, 0.05, 0.1, and 0.2 in the positive electrode active material Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ , initial discharge of 183, 202, 207, and 214 mAhg ⁻¹ The dose is shown. Co was shown that the former (x = 0) after the contrast Co is doped (x = 0.05, 0.1, 0.2 ) reduction in the charge capacity in less than 4V is doped, which Mn ^3, which may be oxidized to Mn ⁴⁺ during charging ⁺ Due to dilution of concentration. However, in the case of Na _0.6 [Mg _0.2 Mn _0.6 Co _0.2 ] O ₂ having an average oxidation state of Mn of 4 ⁺ , the charging capacity observed at less than 4V is considered as Co ³⁺ when considering that Mn ⁴⁺ can no longer be oxidized. May have occurred because it was oxidized to Co ⁴⁺ .

5A and 5B, the capacity achieved at 4 V or higher was higher when Co was doped than that of the Co-free electrode, which clearly shows that Co substitution improved the cycling performance of the half cell. Semi-inverted papers with anodes with x of 0.0, 0.05, 0.1 and 0.2 retained 36%, 45%, 67% and 87% of the initial capacity, respectively, for 100 cycles.

6 is a graph showing the rate characteristics and coulomb efficiency of the reverse cells according to the battery manufacturing examples 1 to 3 and the battery comparative example. At this time, the charging was performed with constant current charging up to 4.6 V, and the discharge was performed with constant current discharge up to 1.5 V at the same rate as the charging speed.

Referring to FIG. 6, when x is 0.2 in Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ , a capacity of 107 mAhg ⁻¹ was exhibited even at 7C, compared to 214 mAhg ⁻¹ at 0.1C. It corresponds to 50%, and when x is 0.05 and 0.1 in Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ , it represents 40% of the capacity at initial 0.1 C at 7C. On the other hand, Na _0.6 [Mg _0.2 Mn _0.8 ] O ₂ showed only 30% of the capacity at the initial 0.1 C at 7C. As described above, it can be seen that replacing Mn with Co can provide excellent speed performance.

7 is a graph showing cycle characteristics at 5C of a half cell according to Battery Manufacturing Example 3;

Referring to FIG. 7, excellent long-term cycling stability, such as showing a capacity retention rate of 72% or more for 1000 cycles at 5C (1.3Ag ^-1 ) when x is 0.2 in Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ electrode Showed.

8 shows ex-situ XANES (X-ray absorption near edge structure) spectra for Mn K-edge, Co K-edge, and O K-edge during electrochemical reactions of reverse cells according to Preparation Example 3 and Comparative Example. Shows.

Referring to FIG. 8, when first looking at the Mn K-edge spectrum of the pristine state, the average oxidation state of Mn is +3.75 in Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ (x = 0, comparative example), Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ (x = 0.2, Preparation Example 3) was found to increase as the content of Co increased to +4.

In the case of Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ (x = 0, comparative example), when charging 4.6V, the manganese was oxidized from 3.75+ to 4+, and the spectrum shifted toward a slightly higher energy, and 1.5V It can be seen that when discharged, it moved to a lower energy than the pristine, but remained higher than Mn ³⁺ . On the other hand, the half-charging paper according to the comparative example had an initial charge capacity of 120mAhg ^-1 , considering that the maximum oxidation state of Mn is 4+, the oxidation state of Mn is oxidized from 3.75+ to 4+, and oxygen It was estimated as a result of oxidation from O ² to O ^-1 . In addition, it was estimated that the initial discharge capacity was 183 mAhg ^{-1 and} oxygen was reduced from O ^1- to O ^-2 and Mn was reduced from Mn ⁴⁺ to Mn ^3.5+ (a).

In the case of Na _0.6 [Mg _0.2 Mn _0.8-x Co _x ] O ₂ (x = 0.2, Preparation Example 3), since the oxidation state of Mn in the initial state is 4+, there is no change in the Mn K-edge spectrum during charging (b ), Co oxidation from Co ³⁺ to Co ⁴⁺ was observed in the Co K-edge spectrum (c). In addition, a new peak appears at 530 eV when charging at 4.6 V compared to when charging at 4.15 V in the O K-edge spectrum, which is a stable voltage (voltage plateau) at 4 V or higher in a graph showing charge and discharge characteristics for Preparation Example 3 of FIG. 5A. ), And was estimated as a result of the oxidation of oxygen from O ² to O ^-1 (d). From these results, the initial charging capacity of the half ^- cell according to Preparation Example 3 is estimated as a result of oxidation of Co ³⁺ to Co ⁴⁺ at less than 4 V and oxygen oxidation of O ² to O ^1- at more than 4 V. Became. In addition, the initial discharge capacity of 214 mAhg ^-1 is reduced from Co ⁴⁺ to Co ³⁺ , and oxygen is reversibly reduced from O ^1- to O ^2- and Mn is reduced from Mn ⁴⁺ to Mn ^3.62+ . It was estimated as a result of the results (b, c, d).

9 shows Na _0.5 [Mg _0.25 Mn _0.75-x Co _x ] O ₂ (x = 0), Na _0.5 [Mg _0.25 Mn _0.75-x Co _x ] O ₂ (x = 0.125), and Na _0.5 [Mg _0.25 Mn _It represents the expected DOS (projected Density of State) of O 2p, Mn 3d, Co 3d orbitals of _0.75-x Co _x ] O ₂ (x = 0.25). Expected DOS was obtained using HSE06 (Heyd-Scuseria-Ernzerhof hybrid functional).

9, Na _0.5 [Mg _0.25 Mn _0.75-x Co _x ] O ₂ (x = 0), Na _0.5 [Mg _0.25 Mn _0.75-x Co _x ] O ₂ (x = 0.125), and Na _0.5 [ Electron-occupied state near Fermi level (E _F ) at both Mg _0.25 Mn _0.75-x Co _x ] O ₂ (x = 0.25) is shown to be affected by ps of O 2p than Mn 3d From this, it can be seen that O ² / O ^1- redox reaction is possible by substitution of Mg. In addition, Mn is replaced with Co as in Na _0.5 [Mg _0.25 Mn _0.75-x Co _x ] O ₂ (x = 0.125), and Na _0.5 [Mg _0.25 Mn _0.75-x Co _x ] O ₂ (x = 0.25). And it can be seen that the band gap is significantly lowered as the substitution amount increases. It can be estimated that this improves conductivity and enables smooth charge transfer at high rates. On the other hand, O 2p and Co 3d of Na _0.5 [Mg _0.25 Mn _0.75-x Co _x ] O ₂ (x = 0.125), and Na _0.5 [Mg _0.25 Mn _0.75-x Co _x ] O ₂ (x = 0.25) High orbital stability can also be obtained at high rates, as orbital overlap can result in high solids and also a reduction in bandgap, resulting in smooth charge transfer at high rates.

10 shows a graph (b) of Na ₁ -Na ₂ diffusion pathway (a) and activation barrier energy for Na ⁺ diffusion in Na _0.5 [Mg _0.25 Mn _0.75-x Co _x ] O ₂ (x = 0.25). . At this time, the activation barrier energy was obtained through NEB (nudged elastic band) calculation.

Referring to FIG. 10, in the Na _0.5 [Mg _0.25 Mn _0.75-x Co _x ] O ₂ (x = 0.25) structure, Na ⁺ ion diffusion in the ab plane requires an activation barrier energy of about 538 meV, which is Na _0.5 [ This is a reduced value compared to Mg _0.25 Mn _0.75-x Co _x ] O ₂ (x = 0). Through this, when the Mn is replaced with Co, the ionic conductivity can also be improved, which can be seen as an excellent power capability.

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

Claims (9)

Electrode active material represented by the following formula (1), hexagonal P2 structure:
[Formula 1]
Na _z Mn _1-yx M ¹ _y M ² _x O _2-α A _α
In Chemical Formula 1,
z is 0.45 to 0.8,
M ¹ is an alkaline earth metal having divalent oxidation water or Zn,
M ² is Co, Ni, Ti, V, Cr, Fe, or Cu having trivalent oxidation number,
y is 0.1 to 0.3,
x is 0.05 to 0.25,
A is N, O, F, or S,
α is 0 to 0.1. According to claim 1,
The active material represented by Formula 1 is an electrode active material represented by Formula 2:
[Formula 2]
Na _z Mn _1-yx Mg _y Co _x O _2-α A _α
In Formula 2, A, x, y, z, and α are as defined in Formula 1. According to claim 2,
z is 0.5 to 0.6,
y is 0.2 to 0.25,
x is 0.2 to 0.25 electrode active material. According to claim 1,
The space group of the electrode active material is P6 ₃ / mmc electrode active material. According to claim 1,
The Mn is an electrode active material in a valence state of 3.5 to 4. According to claim 4,
The Mn is an electrode active material in a valence state of 3.75 to 4. A positive electrode represented by the following Chemical Formula 1 and comprising an electrode active material having a hexagonal P2 structure as a positive electrode active material;
A negative electrode containing a negative electrode active material; And
A secondary battery comprising an electrolyte disposed between the positive electrode and the negative electrode.
[Formula 1]
Na _z Mn _1-yx M ¹ _y M ² _x O _2-α A _α
In Chemical Formula 1,
z is 0.45 to 0.8,
M ¹ is an alkaline earth metal having divalent oxidation water or Zn,
M ² is Co, Ni, Ti, V, Cr, Fe, or Cu having trivalent oxidation number,
y is 0.1 to 0.3,
x is 0.05 to 0.25,
A is N, O, F, or S,
α is 0 to 0.1. The method of claim 7,
The secondary battery has a charging voltage of 4 to 4.6 V. The method of claim 7,
The anode is a secondary battery further comprising a sodium salt.

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