KR102633443B1 - Positive active material having tunnel structure for sodium secondary battery and the sodium secondary battery comprising the same - Google Patents

Abstract

It relates to a cathode active material for a sodium secondary battery that has a tunnel structure and has a structure in which at least one of sodium and transition metal in sodium-transition metal oxide is substituted with lithium.

Description

Positive active material having tunnel structure for sodium secondary battery and the sodium secondary battery comprising the same}

The present invention relates to a positive electrode active material including a tunnel structure and a sodium secondary battery including the same.

With the development of portable mobile electronic devices such as smartphones, MP3 players, and tablet PCs, the demand for secondary batteries capable of storing electrical energy is explosively increasing. In particular, with the emergence of electric vehicles, mid- to large-sized energy storage systems, and portable devices requiring high energy density, demand for lithium secondary batteries is increasing.

Lithium secondary batteries have safety issues due to the high reactivity of lithium, and since lithium is an expensive element, various studies are being conducted to solve this problem.

Among these solutions, sodium secondary batteries are being researched. Compared to lithium secondary batteries, sodium secondary batteries are eco-friendly, have excellent price competitiveness, and have high energy storage characteristics, so they are actively used for medium to large-sized batteries such as for power storage and electric vehicles. It is being studied.

On the other hand, in the case of sodium secondary batteries, the lifespan characteristics are poor compared to lithium secondary batteries due to the transition of the crystal structure due to the change in the oxidation number of the central metal during charging and discharging, and the discharge capacity is low due to the high content of inactive sodium in the crystal structure. There is a downside.

Therefore, there is still a need for a positive electrode active material that can provide a sodium secondary battery with improved discharge capacity and lifespan characteristics.

(Prior patent) Republic of Korea Patent Publication No. 10-2016-0066638

The purpose of the present invention is to provide a cathode active material for a sodium secondary battery having a tunnel structure and a novel structure formed by substituting lithium from sodium or a transition metal, and a sodium secondary battery containing the same.

In addition, another object of the present invention is to provide a cathode active material with a mixture of tunnel structure and spinel structure and improved electrochemical properties, and a sodium secondary battery containing the same.

The technical problems to be solved by the present invention are not limited to those described above.

In order to solve the above technical problem, the present invention provides a positive electrode active material and a sodium secondary battery containing the same.

Embodiments of the present invention include a cathode active material for a sodium secondary battery that has a tunnel structure and has a structure in which at least one of sodium and transition metal in sodium-transition metal oxide is substituted with lithium.

In one embodiment, it further includes a spinel structure and may be represented by one or more of the following Chemical Formulas 1 to 3.

[Formula 1] [Na _xy Li _y ]TMO ₂

[Formula 2] Na _x [TM _z Li _1-z ]O ₂

[Formula 3] [Na _xy Li _y ][TM _z Li _1-z ]O ₂

In Formulas 1 to 3, TM is one or more elements selected from the group consisting of Mn, Co, Ni, V, and Ti, and 0<x≤1, 0<y≤0.5, 0<z≤1.

In one embodiment, the positive electrode active material includes secondary particles composed of a group of a plurality of primary particles, and the primary particles include both a tunnel structure and a spinel structure, and as the substitution amount of lithium increases, the spinel structure increases. It may include:

In one embodiment, the electronic structure of the positive electrode active material is stabilized by oxidizing Mn ³⁺ to Mn ⁴⁺ by substituting sodium or a transition metal with lithium, but as the amount of lithium substitution increases, the Mn ³⁺ changes to Mn ⁴⁺ . ⁺ may include an increase in the amount of oxidation.

In one embodiment, the positive electrode active material may include less than 50% of Mn ³⁺ relative to the total concentration of Mn ³⁺ and Mn ⁴⁺ .

In one embodiment, in a sodium secondary battery employing a positive electrode containing the positive electrode active material, the capacity retention after charging and discharging 1000 times at a constant current of 0.1C rate in a voltage range of 2.0 to 3.8V with respect to sodium metal is 80. It may be more than %.

In one embodiment, the tunnel structure may include a tunnel structure having an S-shaped cross-section and a polygonal tunnel structure having a smaller area than the S-shaped tunnel structure.

In one embodiment, the positive electrode active material includes secondary particles composed of a plurality of primary particles, and the primary particles are provided in the form of nanorods having a longer length in the c-axis direction, Secondary particles may have a specific surface area (BET) of 1.3 m2/g or more.

In one embodiment, in the powder When lithium replaces sodium, the intensity of the diffraction peak of (111) relative to (200) may increase as the amount of lithium substitution increases.

In one embodiment, the transition metal is made of manganese, and according to a thermal stability evaluation method using a differential scanning calorimeter, the maximum exothermic peak temperature (T _o ) of the cathode active material made of sodium-manganese oxide is The maximum heating peak temperature (T) of the cathode active material having a structure in which at least one of sodium and manganese is substituted with lithium is higher than 9℃, and the calorific value (H _o ) of the cathode active material made of sodium-manganese oxide is higher than sodium. -The calorific value (H _o ) of a cathode active material having a structure in which at least one of sodium and manganese in manganese oxide is substituted with lithium may be less than 90%.

According to another aspect of the present invention, embodiments of the present invention include a positive electrode containing the above-described positive electrode active material for a sodium secondary battery; cathode; and an electrolyte solution containing sodium ions.

According to the present invention as described above, a manganese-based cathode active material for a sodium secondary battery having a tunnel structure is used to further include a spinel structure, thereby suppressing the Jahn-Teller effect of manganese, thereby improving the cathode active material for a sodium secondary battery based on manganese. A cathode active material exhibiting electrochemical properties and a sodium secondary battery containing the same can be provided.

1 is a diagram schematically showing a positive electrode active material according to an embodiment of the present invention.
Figure 2a is a diagram showing the tunnel structure of a positive electrode active material according to an embodiment of the present invention.
Figure 2b is a graph showing the deconvoluted peak of the sodium cathode active material according to an embodiment of the present invention and the conventional NMO-based sodium cathode active material.
Figure 3 is a graph confirming rate characteristics using half cells for Comparative Examples 1 to 3.
Figure 4 is a graph of the entire (a) and partial enlargement (b) of the XRD patterns for Examples 4, 5, and 8.
Figure 5 is a graph (a) confirming rate characteristics using half cells for Example 1 and Examples 5 to 8, and a graph (b) showing cycle characteristics of Examples 1, 7, and 8.
Figure 6 is a graph of the entire (a) and partial enlargement (b) of the XRD patterns for Examples 2, 10, and 11.
Figure 7 is a graph (a) confirming rate characteristics using half cells for Example 2 and Examples 9 to 11, and a graph (b) showing cycle characteristics of Examples 2, 10, and 11.
Figure 8 is a graph (a) confirming the rate characteristics using half cells for Example 3 and Examples 12 to 15, and a graph (b) showing the cycle characteristics of Example 3 and Examples 12 to 15.
Figure 9 is an SEM image of Example 1 and Comparative Example 1.
Figure 10 is an XRD graph of Examples 1 to 3 and Comparative Example 1.
Figure 11 is an enlarged view between 37.1° and 37.6° in the XRD patterns of Examples 1 to 3 and Comparative Example 1 (a), and shows the representative crystal structure (top) of LiMn ₂ O ₄ and Na _0.44 MnO ₂ This is a drawing showing the crystal structure (bottom) (b), and the XRD pattern for a wide range (c).
Figure 12 is a diagram showing the Mn 2p3/2 spectrum (a) and N2 adsorption and desorption curves (showing pore size distribution) of Example 3 and Comparative Example 1 (b).
Figure 13 is an SEM image of Example 3.
Figure 14 is a TEM image of the primary particles of Example 3 and Comparative Example 1.
Figure 15 shows the TEM image (a), EDS mapping image (b), SAED pattern (c), HRTEM and FFT image (d) of Example 3, and the SAED pattern (e), HRTEM and FFT image of Comparative Example 1 ( f).
Figure 16 shows the results of DSC (Differential scanning calorimetry) of Example 3 and Comparative Example 1.
Figure 17 shows the results of confirming the electrochemical performance of positive electrode active materials according to comparative examples and examples.
Figure 18 shows the results of confirming the cycle characteristics of positive electrode active materials according to comparative examples and examples.
Figure 19 shows the results of evaluating electrochemical performance in the anode using Example 3 and a full cell using hard carbon as the cathode.
Figure 20 shows the results of confirming the electrode reaction rate and thermal stability of the positive electrode active materials according to Examples 1 to 3 and Comparative Example 1.
Figure 21 shows an XRD graph and SEM image for comparing the cycle performance of Example 3 and Comparative Example 1.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. However, the technical idea of the present invention is not limited to the embodiments described herein and may be embodied in other forms. Rather, the embodiments introduced herein are provided so that the disclosure will be thorough and complete and so that the spirit of the invention can be sufficiently conveyed to those skilled in the art.

In this specification, when an element is referred to as being on another element, it means that it may be formed directly on the other element or that a third element may be interposed between them. Additionally, in the drawings, the thicknesses of films and regions are exaggerated for effective explanation of technical content.

Additionally, in various embodiments of the present specification, terms such as first, second, and third are used to describe various components, but these components should not be limited by these terms. These terms are merely used to distinguish one component from another. Accordingly, what is referred to as a first component in one embodiment may be referred to as a second component in another embodiment. Each embodiment described and illustrated herein also includes its complementary embodiment. Additionally, in this specification, 'and/or' is used to mean including at least one of the components listed before and after.

In the specification, singular expressions include plural expressions unless the context clearly dictates otherwise. In addition, terms such as "include" or "have" are intended to designate the presence of features, numbers, steps, components, or a combination thereof described in the specification, but are not intended to indicate the presence of one or more other features, numbers, steps, or components. It should not be understood as excluding the possibility of the presence or addition of elements or combinations thereof.

Additionally, in the following description of the present invention, if it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the gist of the present invention, the detailed description will be omitted.

In addition, in the present application, the ratio of the first crystal structure in a specific part is higher than the ratio of the second crystal structure, meaning that the specific part includes both the first crystal structure and the second crystal structure, but the specific part includes both the first crystal structure and the second crystal structure. It is interpreted to mean that the ratio of the first crystal structure is higher than the ratio of the second crystal structure, as well as that the specific part has only the first crystal structure.

Additionally, in the present application, the crystal system refers to triclinic, monoclinic, orthorhombic, tetragonal, trigonal or rhombohedral, and hexagonal. , and can be composed of 7 cubic systems.

In addition, in this application specification, “mol%” refers to the content of any metal contained in the cathode active material or cathode active material precursor, assuming that the sum of the remaining metals excluding sodium and oxygen in the cathode active material or precursor is 100%. It is interpreted in the sense that it represents.

1 is a diagram schematically showing a positive electrode active material according to an embodiment of the present invention. Figure 2 is a diagram showing the tunnel structure of a positive electrode active material according to an embodiment of the present invention.

This embodiment includes a cathode active material for a sodium secondary battery that has a tunnel structure and has a structure in which at least one of sodium and transition metal in sodium-transition metal oxide is substituted with lithium.

In addition, the positive electrode active material further includes a spinel structure and may be represented by one or more of the following Chemical Formulas 1 to 3.

[Formula 1]

[Na _xy Li _y ]TMO ₂

[Formula 2]

Na _x [TM _z Li _1-z ]O ₂

[Formula 3]

[Na _xy Li _y ][TM _z Li _1-z ]O ₂

In Formulas 1 to 3, TM is one or more elements selected from the group consisting of Mn, Co, Ni, V, and Ti, and 0<x≤1, 0<y≤0.5, 0<z≤1.

In the cathode active material for sodium secondary batteries, a spinel structure can be formed by substitution of lithium in sodium-transition metal oxide. The degree to which the spinel structure is formed and the tunnel structure and spinel structure are determined by the degree of substitution by lithium. It may affect structural stability. At this time, the spinel structure formed by substituting the lithium for sodium, the spinel structure formed by substituting the lithium for a transition metal, and the spinel structure formed by substituting the lithium for both sodium and the transition metal have a relationship with the tunnel structure. may be provided differently, and the degree and size of the spinel structure formed are also different from each other.

In the positive electrode active material according to this embodiment, in Formulas 1 to 3, x, y, and z are provided within the above-mentioned ranges, so that the positive electrode active material for sodium secondary batteries can be formed with a spinel structure in the tunnel structure to improve structural stability. there is. Specifically, the TM may be Mn.

Sodium-transition metal oxide _{( Na} This is because the dispersion state of cations varies due to the repulsion between TM (transition metal) and TM (transition metal).

Among them, the tunnel-type sodium secondary battery anode material is known to have a general structure that is stable even in aqueous electrolyte and can be used in both aqueous and non-aqueous sodium secondary batteries. The representative tunnel-type sodium manganese cathode active material crystallographically has 5 manganese positions (2 are trivalent and 3 are tetravalent) and 3 sodium positions. The structural frame consists of edge-sharing MnO octahedral double and triple linear chains and single chains of edge-sharing MnO square pyramids. Each chain is aligned parallel to the c-axis and connected to a nearby chain through sharing edges of the polyhedron, resulting in two types of tunnel structures.

Oxides using Mn as a transition metal have a relative advantage over other metal-based layered oxides in terms of competitiveness such as capacity, price competitiveness, reserves, and eco-friendliness. On the other hand, tunnel-type sodium-manganese oxide exhibits high capacity, but exhibits low lifespan characteristics due to structural collapse and amorphization due to repetitive stress. Additionally, these materials tend to experience Jahn-Teller distortion, which causes severe material degradation during charge/discharge, which originates from Mn ³⁺ ions (t _2g ³ e _g ¹ ). Jahn-Teller distortion is a phenomenon triggered by the crystal field stabilization energy. In a molecular orbital with a specific number of d electrons and spin state, the t _2g and e _g electronic states are d _x2-y2 , d _z2 , d _xy , d _xz , This refers to the phenomenon of separation into d _yz . At this time, the Z-axis length of the crystal becomes longer or shorter than the

On the other hand, the positive electrode active material according to the present embodiment may further include a spinel structure by substituting lithium in the sodium atomic layer, the manganese atomic layer, or the sodium and manganese atomic layers, thereby preventing the Jahn-Teller effect, resulting in sodium secondary The lifespan characteristics of the battery can be improved.

Specifically, in Formulas 1 to 3, x may be 0.44, y may be 0.044 to 0.132, and z may be 0.95 to 0.99. More specifically, x may be 0.44, y may be 0.022 to 0.308, and z may be 0.01 to 0.1. For example, the positive electrode active material is [Na _0.22 Li _0.22 ][Mn]O _2, [Na _0.308 Li _0.132 ][Mn]O _2, [Na _0.396 Li _0.044 ][Mn]O _2, [Na _0.44 ][Mn _0.95 Li _0.05 ]O _2, [Na _0.44 ][Mn _0.97 Li _0.03 ]O _2, [Na _0.44 ][Mn _0.95 Li _0.05 ]O _2, [Na _0.44 ][Mn _0.97 Li _0.03 ]O _2, [Na _0.44 ][Mn _0.99 Li _0.01 ]O _2, and It may be any one or more of [Na _0.396 Li _0.044 ][Mn _0.97 Li _0.03 ]O ₂ .

Referring to Figures 1 and 2, the positive electrode active material according to this embodiment includes secondary particles 10 composed of a group of a plurality of primary particles 100, and the primary particles 100 have a tunnel structure and a spinel structure. Can include all. Additionally, the lithium may be substituted with at least one of sodium and a transition metal, and as the substitution amount of lithium increases, the spinel structure may increase. The primary particle 100 is provided in the form of a nanorod with a longer length in the c-axis direction, and the secondary particle 10 may be provided with a specific surface area (BET) of 1.3 m2/g or more. there is. The positive electrode active material may include secondary particles 10 having a substantially spherical shape, and the secondary particles 10 may be formed by aggregating a plurality of primary particles 100. Among the primary particles 100, the primary particles 100 located on the surface of the secondary particles 10 may be provided in the form of nanorods.

In this way, the secondary particle 10, which is composed of a group of a plurality of primary particles 100, may have a different specific surface area depending on the structure, crystallinity, arrangement, etc. of the primary particle 100, and the surface of the secondary particle 10 The primary particles 100 provided in the unit are provided in the form of nanorods formed to be long in the C-axis direction, thereby increasing the specific surface area of the secondary particles 10. The specific surface area of the secondary particle 10 is 1.3 m2/g or more, specifically 1.5 m2/g or more, or 2 m2/g or more, or 2 m2/g or more to 5 m2/g or less, or 2.08 m2/g. It may be g.

The primary particle 100 has a tunnel structure composed of edge-sharing MnO5 pyramids and MnO6 octahedra, and the edge-sharing MnO5 pyramid is the entire Mn ³⁺ constituting the positive electrode active material. It is composed of half of Mn ³⁺ , and the MnO6 octahedron may be composed of Mn ³⁺ remaining after being used in the edge-sharing MnO5 pyramid and all of Mn ⁴⁺ constituting the positive electrode active material. The polyhedron made of the edge-sharing MnO5 pyramid and MnO6 octahedron may include an S-shaped tunnel and a polygonal tunnel with a cross-section having a smaller area than the S-shaped tunnel. The polygonal tunnel may be a pentagonal tunnel (see Figure 2).

For example, the S-shaped tunnel has two Na and Na placed inside to form a sodium diffusion channel, while the polygonal tunnel (e.g., pentagon) is entirely filled with Na, making insertion and release of sodium impossible. do. The S-shaped tunnel has a wide size and an open structure, so that it can accommodate changes in the volume of the positive electrode active material during the electrochemical charging and discharging process and improve cycle performance. That is, the positive electrode active material according to the present embodiment has an S-shaped tunnel with a relatively large space, so that a buffer space is provided so that the increase and decrease of the positive electrode active material occurring during the charging and discharging process does not act as an external force generated internally. By providing this, it is possible to prevent collapse of the structure even when performing a long-term charge/discharge cycle.

In other words, in a typical cathode active material made only of sodium, the polygonal tunnel cannot function as a diffusion channel for sodium. On the other hand, the positive electrode active material according to this embodiment forms a 3D spinel framework of edge-sharing LiMn2O4 formed by lithium occupying tetrahedral sites in the MnO6 octahedron forming the polygonal tunnel. Aligned MnO6 sheets within Na0.44MnO2 can form a spinel structure, activating reaction sites and subsequently providing diffusion channels for ions. The spinel structure allows ions to diffuse more easily because the ion path is formed in three dimensions.

In this embodiment, the S-shaped tunnel structure forms a diffusion channel for sodium ions, while the polygonal tunnel structure maintains the internal space of the S-shaped tunnel structure and acts as a support framework that maintains the overall structure of the positive electrode active material. It can function.

The positive electrode active material according to this embodiment has a heterogeneous structure including both a tunnel structure and a spinel structure by substitution of lithium, and can exhibit improved rate characteristics by promoting the movement of sodium ions during the charge and discharge process.

Figure 2b is a graph showing the deconvoluted peak of the sodium cathode active material according to an embodiment of the present invention and the conventional NMO-based sodium cathode active material.

In Figure 2b, the cathode active material according to this embodiment is a cathode active material ([NL][ML]O) in which lithium is substituted on both sides of the sodium and transition metal sites, and the conventional NMO-based cathode active material is a cathode active material containing sodium and transition metal. It refers to a positive electrode active material consisting only of lithium and not substituted with lithium.

It was confirmed that the sodium cathode active material according to an embodiment of the present invention showed a deconvoluted peak of 55.6 eV in the XPS spectrum of Li 1s, whereby the lithium was replaced with sodium or a transition metal to form a tunnel structure. This means that it is effectively substituted in the crystal lattice. The positive electrode active material can be separated into two parts at 641.5 eV and 642.8 eV in the Mn 2p3/2 spectrum, meaning Mn ³⁺ and Mn ⁴⁺ , respectively. On the other hand, it was confirmed that the conventional cathode active material not substituted by lithium showed a peak showing the opposite pattern to that of the sodium cathode active material according to the embodiment of the present invention.

The electronic structure of the positive electrode active material may be stabilized by substituting sodium or a transition metal with lithium and oxidizing Mn ³⁺ to Mn ⁴⁺ . As the substitution amount of lithium increases, the amount of oxidation of Mn ³⁺ to Mn ⁴⁺ may increase. Additionally, the positive electrode active material may contain less than 50% of Mn ³⁺ relative to the total concentration of Mn ³⁺ and Mn ⁴⁺ . The positive electrode active material according to this embodiment includes sodium-transition metal oxide with a tunnel structure, and can form a spinel structure by being substituted for any one or more of the sodium and transition metal. Here, the spinel structure according to this embodiment maintains the concentration ratio of Mn ³⁺ and Mn ⁴⁺ in the above-described range, thereby ensuring efficient movement of ions within the positive electrode active material as a whole by being arranged approximately uniformly around the tunnel structure. It is possible to provide a structure provided with an ion diffusion channel performed by.

In the positive electrode active material according to this embodiment, in the powder When lithium replaces sodium, the intensity of the diffraction peak of (111) relative to (200) may increase as the amount of lithium substitution increases.

In the positive electrode active material according to this embodiment, the transition metal may be made of manganese. At this time, according to the thermal stability evaluation method using a differential scanning calorimeter, the maximum exothermic peak temperature (T _o ) of the positive electrode active material made of sodium-manganese oxide is determined by determining whether at least one of sodium and manganese in the sodium-manganese oxide is substituted with lithium. The maximum exothermic peak temperature (T) of the cathode active material having the structure can be higher than 9℃. In addition, the calorific value (H _o ) of a cathode active material having a structure in which at least one of sodium and manganese in sodium-manganese oxide is substituted with lithium is 90% or less compared to the calorific value (H _o ) of a cathode active material made of sodium-manganese oxide. It may appear as

The cathode active material has at least one of sodium and transition metal replaced by lithium, so that the tunnel structure further includes a spinel structure, and the stability of the crystal structure is improved, thereby improving thermal stability.

The sodium secondary battery according to the present invention includes a positive electrode, a negative electrode, an electrolyte, and a separator containing the above-described positive electrode active material.

The anode, cathode, and separator can be wound or folded and accommodated inside the battery case. Organic electrolyte can be injected into the battery case, and it is sealed with a cap assembly. The battery case may be cylindrical, prismatic, thin-film, etc., but is not limited thereto.

A separator may be disposed between the anode and the cathode to form a battery structure. The battery structure may have a coin cell structure, and may be formed by being stacked in a bi-cell structure, then impregnated with an organic electrolyte solution, and the obtained result is accommodated in a pouch and sealed.

Below, each configuration will be described in detail.

The positive electrode includes a current collector and a positive electrode active material layer formed on the current collector. The positive electrode active material layer includes the positive electrode active material according to an embodiment of the present invention described above.

Additionally, the positive electrode active material may have a coating layer on its surface, or the positive electrode active material compound and a compound having a coating layer may be mixed and used. The coating layer may include, as a coating element compound, an oxide, a hydroxide, an oxyhydroxide of a coating element, an oxycarbonate of a coating element, or a hydroxycarbonate of a coating element. The compounds that make up these coating layers may be amorphous or crystalline. Coating elements included in the coating layer may include Sb, Mg, Al, Co, K, Ka, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or mixtures thereof. The coating layer formation process may use a method (for example, spray coating, dipping, etc.) that does not adversely affect the physical properties of the positive electrode active material by using these elements in the compound.

The positive electrode active material layer may further include a binder and a conductive material.

The binder serves to adhere the positive electrode active material particles to each other and to the current collector, and representative examples include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, and polyvinyl alcohol. Chloride, carboxylated polyvinylchloride, polyvinylfluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene- Butadiene rubber, acrylated styrene-butadiene rubber, polyacrylic acid, epoxy resin, nylon, etc. can be used, but are not limited thereto.

The conductive material is used to provide conductivity to the electrode, and in the battery being constructed, any electronically conductive material can be used as long as it does not cause chemical change. Examples include natural graphite, artificial graphite, carbon black, acetylene black, and Ketjen. Metal powders such as black, carbon fiber, copper, nickel, aluminum, and silver, metal fibers, etc. can be used, and conductive materials such as polyphenylene derivatives can be used one type or in a mixture of one or more types.

Al may be used as the current collector, but is not limited thereto.

The negative electrode includes a current collector and a negative electrode active material layer located on the current collector.

The negative electrode active material layer may include sodium metal, a sodium metal-based alloy, a sodium intercalating compound, or a carbon-based material, but is not necessarily limited to these, and can be used as a negative electrode active material in the art. Anything that contains or can absorb/release sodium is possible. Here, the sodium metal-based alloy may include, for example, an alloy of sodium with aluminum, tin, indium, calcium, titanium, vanadium, etc., but is not limited thereto.

The negative electrode active material layer can generally be made of metallic sodium with a thickness of 3 to 500 μm, and can be used in various forms such as films, sheets, foils, nets, porous materials, foams, and non-woven fabrics.

When using a negative electrode active material other than sodium metal or sodium alloy, a carbon-based material having a graphene structure, etc. can be used. A mixed cathode of materials such as graphite or graphitized carbon, a mixed cathode of a carbon-based material and a metal or alloy, or a composite cathode can be used. Carbon-based materials include natural graphite, which can electrochemically absorb and release sodium ions, artificial graphite, mesophase carbon, expanded graphite, carbon fiber, vapor-grown carbon fiber, pitch-based carbonaceous material, needle coke, petroleum coke, Carbonaceous materials such as polyacrylonitrile-based carbon fiber and carbon black, or amorphous carbon materials synthesized by thermal decomposition of 5- or 6-membered ring cyclic hydrocarbons or cyclic oxygen-containing organic compounds may be used.

The negative electrode active material layer may also include a binder and, optionally, may further include a conductive material.

The binder serves to adhere the negative electrode active material particles to each other well and to attach the negative electrode active material to the current collector. Representative examples thereof include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, polyvinyl chloride, and carboxylic acid. Silized polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, arcl. Related styrene-butadiene rubber, polyacrylic acid, epoxy resin, nylon, etc. can be used, but are not limited thereto.

The conductive material is used to provide conductivity to the electrode, and in the battery being constructed, any electronically conductive material can be used as long as it does not cause chemical change. Examples include natural graphite, artificial graphite, carbon black, and carbon fiber. Conductive materials including carbon-based materials, metal powders such as copper, nickel, aluminum, and silver, metal materials such as metal fibers, conductive polymers such as polyphenylene derivatives, or mixtures thereof can be used.

The current collector may be copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, or a combination thereof.

The positive electrode and the negative electrode are manufactured by mixing an active material, a conductive material, and a binder in a solvent to prepare an active material composition, and applying this composition to a current collector. Since this electrode manufacturing method is widely known in the art, detailed description will be omitted in this specification. The solvent may be N-methylpyrrolidone, but is not limited thereto.

The electrolyte may be a liquid electrolyte solution, and the liquid electrolyte solution may include a sodium salt and an aqueous solution.

Sodium salts that can be used in the sodium secondary battery of the present invention include NaClO ₄ , NaPF ₆ , NaSbF ₆ , NaBF ₄ , NaCF ₃ SO ₃ , NaN(SO ₂ CF ₃ ) ₂ , NaCl, NaC ₂ H ₃ O ₂ , Na ₂ SO ₄ , NaNO ₃ , NaBH ₄ , NaOH lower aliphatic carboxylic acid sodium salt, NaAlC _{14 ,} etc., and a mixture of two or more of these may be used.

The separator separates the cathode from the anode and provides a passage for sodium ions to move, and any type commonly used in sodium secondary batteries can be used. That is, one that has low resistance to ion movement in the electrolyte and has excellent electrolyte moisturizing ability can be used. For example, it is selected from glass fiber, polyester, Teflon, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), or a combination thereof, and may be in the form of non-woven or woven fabric. For example, in sodium secondary batteries, polyolefin-based polymer separators such as polyethylene and polypropylene are mainly used, and coated separators containing ceramic components or polymer materials may be used to ensure heat resistance or mechanical strength, and can optionally be single- or multi-layered. It can be used as a structure.

Hereinafter, examples and comparative examples of the present invention will be described. However, the following examples are only preferred examples of the present invention and the scope of the present invention is not limited by the following examples.

1. Manufacturing of cathode active material

Comparative Example 1 (Na _0.44 MnO ₂ ; NMO)

0.0139 mole of Sodium acetate, with 5% excess sodium, CH ₃ COONa, Aladdin, AR, 0.03 mole of Manganese acetate tetrahydrate, (Mn(CH ₃ COO) ₂ 4H ₂ O, Aladdin , AR) and citric acid (C ₆ H ₈ O ₇ , Sigma-Aldrich, AR) were mixed using ball milling at 220 rpm for 24 hours to obtain a mixed solid powder. Here, citric acid It was used at a ratio of 0.2 times the sum of the weight of silver acetate (sodium acetate and manganese acetate tetrahydrate). The prepared mixed solid powder was dried at 80 ° C and then calcined at 950 ° C for 10 hours in an air atmosphere. It was manufactured from positive electrode active material powder (NMO powder) and is shown in Table 1.

Comparative Example 2 (Na _0.44 MnO ₂ ; NMO-900)

Cathode active material powder was prepared in the same manner as in Comparative Example 1, except that the sintering temperature was set to 900°C, and is shown in Table 1.

Comparative Example 3 (Na _0.44 MnO ₂ ; NMO-1000)

Cathode active material powder was prepared in the same manner as in Comparative Example 1, except that the sintering temperature was set to 1000°C, and is shown in Table 1.

Example 1 ([Na _0.308 Li _0.132 ]MnO ₂ ; [NL]MO)

0.0097 mol of Sodium acetate (with 5% excess sodium, CH ₃ COONa, Aladdin, AR), 0.004 mol of Lithium nitrate (with 2% excess lithium, LiNO ₃ , Aladdin, AR), 0.03 mol of Manganese acetate tetrahydrate (Mn(CH ₃ COO) ₂ 4H ₂ O, Aladdin, AR) and citric acid (C ₆ H ₈ O ₇ , Sigma-Aldrich, AR) were prepared by ball milling (Ball). -Milling) was mixed at 220 rpm for 24 hours to obtain a mixed solid powder. Here, citric acid was used at a ratio of 0.2 times the sum of the weight of acetates (sodium acetate and manganese acetate tetrahydrate). Preparation The mixed solid powder was dried at 80°C and then calcined at 950°C for 10 hours in an air atmosphere to prepare positive electrode active material powder ([NL]MO powder), which is shown in Table 1.

Example 2 (Na _0.44 [Mn _0.97 Li _0.03 ]O ₂ ; N[ML]O)

0.0139 mol of Sodium acetate (with 5% excess sodium, CH ₃ COONa, Aladdin, AR), 0.001 mol of Lithium nitrate (with 2% excess lithium, LiNO ₃ , Aladdin, AR), 0.029 mol of Manganese acetate tetrahydrate (Mn(CH ₃ COO) ₂ 4H ₂ O, Aladdin, AR) and citric acid (C ₆ H ₈ O ₇ , Sigma-Aldrich, AR) were prepared by ball milling (Ball). Cathode active material powder (N[ML]O powder) was prepared in the same manner as in Example 1, except that mixed solid powder was obtained by mixing at 220 rpm for 24 hours using milling), and this is shown in Table 1. .

Example 3 ([Na _0.308 Li _0.132 ][Mn _0.97 Li _0.03 ]O ₂ ; [NL][ML]O)

0.0125 mol of Sodium acetate (with 5% excess sodium, CH ₃ COONa, Aladdin, AR), 0.0023 mol of Lithium nitrate (with 2% excess lithium, LiNO ₃ , Aladdin, AR), 0.029 mol of Manganese acetate tetrahydrate (Mn(CH ₃ COO) ₂ 4H ₂ O, Aladdin, AR) and citric acid (C ₆ H ₈ O ₇ , Sigma-Aldrich, AR) were prepared by ball milling (Ball). Cathode active material powder ([NL][ML]O powder) was prepared in the same manner as in Example 1, except that mixed solid powder was obtained by mixing at 220 rpm for 24 hours using milling), and it is shown in Table 1. indicated.

Examples 4 to 15

Cathode active material powder ([NL][ML]O powder) was prepared in the same manner as in Example 1, except for using sodium acetate, lithium nitrate, and manganese acetate tetrahydrate as shown in Table 2 below, and is shown in Table 1. It was.

Category 1 Category 2 chemical formula Na(mol) Li(mol)-Na substitution Mn(mol) Li(mol)-Mn substitution Li(mol)-total Comparative Example 1 NMO Na _0.44 MnO ₂ 0.44 0 One 0 0 Comparative Example 2 NMO-900 Na _0.44 MnO ₂ 0.44 0 One 0 0 Comparative Example 3 NMO-1000 Na _0.44 MnO ₂ 0.44 0 One 0 0 Example 1 [NL]MO [Na _0.308 Li _0.132 ]MnO ₂ 0.308 0.132 One 0 0.132 Example 2 N[ML]O Na _0.44 [Mn _0.97 Li _0.03 ]O ₂ 0.44 0 0.97 0.03 0.03 Example 3 [NL][ML]O [Na _0.308 Li _0.132 ][Mn _0.97 Li _0.03 ]O ₂ 0.308 0.132 0.97 0.03 0.162 Example 4 [NL]MO-0.308 [Na _0.132 Li _0.308 ]MnO ₂ 0.132 0.308 One 0 0.308 Example 5 [NL]MO-0.22 [Na _0.22 Li _0.22 ]MnO ₂ 0.22 0.22 One 0 0.22 Example 6 [NL]MO-0.176 [Na _0.264 Li _0.176 ]MnO ₂ 0.264 0.176 One 0 0.176 Example 7 [NL]MO-0.088 [Na _0.352 Li _0.088 ]MnO ₂ 0.352 0.088 One 0 0.088 Example 8 [NL]MO-0.044 [Na _0.396 Li _0.044 ]MnO ₂ 0.396 0.044 One 0 0.044 Example 9 N[ML]O-0.1 Na _0.44 [Mn _0.9 Li _0.1 ]O ₂ 0.44 0 0.9 0.1 0.1 Example 10 N[ML]O-0.05 Na _0.44 [Mn _0.95 Li _0.05 ]O ₂ 0.44 0 0.95 0.05 0.05 Example 11 N[ML]O-0.01 Na _0.44 [Mn _0.99 Li _0.01 ]O ₂ 0.44 0 0.99 0.01 0.01 Example 12 [NL][ML]O-0.088 [Na _0.352 Li _0.088 ][Mn _0.97 Li _0.03 ]O ₂ 0.352 0.088 0.97 0.03 0.118 Example 13 [NL][ML]O-0.066 [Na _0.374 Li _0.066 ][Mn _0.97 Li _0.03 ]O ₂ 0.374 0.066 0.97 0.03 0.096 Example 14 [NL][ML]O-0.044 [Na _0.396 Li _0.044 ][Mn _0.97 Li _0.03 ]O ₂ 0.396 0.044 0.97 0.03 0.074 Example 15 [NL][ML]O-0.022 [Na _0.418 Li _0.022 ][Mn _0.97 Li _0.03 ]O ₂ 0.418 0.022 0.97 0.03 0.052

Category 1 Category 2 chemical formula Sodium acetate (mol) Lithium nitrate (mol) Manganese acetate (mol) Firing temperature (℃) Comparative Example 1 NMO Na _0.44 MnO ₂ 0.0139 0 0.03 950 Comparative Example 2 NMO-900 Na _0.44 MnO ₂ 0.0139 0 0.03 900 Comparative Example 3 NMO-1000 Na _0.44 MnO ₂ 0.0139 0 0.03 1000 Example 1 [NL]MO [Na _0.308 Li _0.132 ]MnO ₂ 0.0097 0.004 0.03 950 Example 2 N[ML]O Na _0.44 [Mn _0.97 Li _0.03 ]O ₂ 0.0139 0.001 0.029 950 Example 3 [NL][ML]O [Na _0.308 Li _0.132 ][Mn _0.97 Li _0.03 ]O ₂ 0.0125 0.005 0.029 950 Example 4 [NL]MO-0.308 [Na _0.132 Li _0.308 ]MnO ₂ 0.00417 0.010 0.03 950 Example 5 [NL]MO-0.22 [Na _0.22 Li _0.22 ]MnO ₂ 0.00695 0.007 0.03 950 Example 6 [NL]MO-0.176 [Na _0.264 Li _0.176 ]MnO ₂ 0.00834 0.006 0.03 950 Example 7 [NL]MO-0.088 [Na _0.352 Li _0.088 ]MnO ₂ 0.01112 0.003 0.03 950 Example 8 [NL]MO-0.044 [Na _0.396 Li _0.044 ]MnO ₂ 0.01251 0.015 0.03 950 Example 9 N[ML]O-0.1 Na _0.44 [Mn _0.9 Li _0.1 ]O ₂ 0.0139 0.003 0.027 950 Example 10 N[ML]O-0.05 Na _0.44 [Mn _0.95 Li _0.05 ]O ₂ 0.0139 0.002 0.0285 950 Example 11 N[ML]O-0.01 Na _0.44 [Mn _0.99 Li _0.01 ]O ₂ 0.0139 0.000 0.0297 950 Example 12 [NL][ML]O-0.088 [Na _0.352 Li _0.088 ][Mn _0.97 Li _0.03 ]O ₂ 0.01112 0.004 0.029 950 Example 13 [NL][ML]O-0.066 [Na _0.374 Li _0.066 ][Mn _0.97 Li _0.03 ]O ₂ 0.011815 0.003 0.029 950 Example 14 [NL][ML]O-0.044 [Na _0.396 Li _0.044 ][Mn _0.97 Li _0.03 ]O ₂ 0.01251 0.002 0.029 950 Example 15 [NL][ML]O-0.022 [Na _0.418 Li _0.022 ][Mn _0.97 Li _0.03 ]O ₂ 0.013205 0.002 0.029 950

2. Half-cell and full-cell manufacturing using positive electrode active materials according to Examples and Comparative Examples

Half cells and full cells were manufactured using the positive electrode active materials according to the above-described examples and comparative examples.

For the positive electrode, 80 wt% of positive electrode active material, 10 wt% of conductive carbon material (Super P), and 10 wt% of PVdF binder are dispersed in N-methyl pyrrolidone (NMP) and then mixed for 30 minutes to form a positive electrode slurry. did. The prepared positive electrode slurry was applied on aluminum foil, roll pressed, and dried at 110°C for 12 hours under vacuum to prepare a positive electrode.

When manufacturing a half cell using the manufactured cathode active material, the cathode is manufactured by coating the cathode made of slurry on aluminum foil so that the loading level of the cathode active material is 5 mg/cm2 (aluminum foil coated with cathode active material) When sampled in a 1㎠ square, the weight of the positive electrode active material alone in the positive electrode composition is 5 mg), and the electrolyte contains 80 μl of 1.0M LiPF6 EC/DEC = 1/1 (v/v) ( EC: ethylene carbonate, DEC: diethyl carbonate) was used, and a glass fiber isolation membrane (Whatman GF/F CAT No. 1825-150) was used. The half cell was manufactured as a 2032-coin type half cell (hereinafter referred to as coin cell) using Na _o as the cathode.

When manufacturing a full cell using the manufactured cathode active material, the cathode is manufactured by coating the cathode made of slurry on aluminum foil so that the loading level of the cathode active material is 8.5 mg/cm2, and a cathode prepared in a size corresponding to the manufactured cathode is prepared. Hard carbon was used as the cathode. The electrolyte used was EC/DEC=1/1(v/v) (EC: ethylene carbonate, DEC: diethyl carbonate) containing 80㎕ of 1.0M LiPF6. In a pouch-shaped battery case, a positive electrode, a separator (Celgard, 2320 model), and a negative electrode were stacked and sealed with the prepared electrolyte solution to produce a pouch-shaped full cell.

3. Evaluation of examples and preparation examples

(1) Confirmation of capacity and cycle characteristics using half cell

The manufactured half-cell was charged to 4.3V and discharged to 2.7V at a constant current of 0.5C (1C is 180 mA/g) at 30°C, and 100 cycles were performed under the same conditions as the charge and discharge test to confirm capacity retention ( below, 2.7V-4.3V).

(2) Confirmation of capacity and cycle characteristics using full cells

Using the manufactured full cell, a cycle was performed at 3.0V (discharge voltage) and 4.2V (charge voltage) at 25°C with a constant current of 1C to confirm capacity and capacity retention.

(3) Confirmation of microstructure of cathode active material

For the positive electrode active materials according to examples and manufacturing examples, scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and transmission Microstructure was analyzed using transmission electron microscopy (TEM), etc., and thermal stability was confirmed using thermogravimetry (TG) and differential scanning calorimetry (DSC).

In the following, the microstructure, surface properties, and electrochemical properties of the positive electrode active materials according to Examples and Comparative Examples were confirmed.

Table 3 below shows discharge capacity and cycle characteristics according to examples and preparation examples. Figure 3 is a graph confirming rate characteristics using half cells for Comparative Examples 1 to 3. Figure 4 is a graph of the entire (a) and partial enlargement (b) of the XRD patterns for Examples 4, 5, and 8. Figure 5 is a graph (a) confirming rate characteristics using half cells for Example 1 and Examples 5 to 8, and a graph (b) showing cycle characteristics of Examples 1, 7, and 8. Figure 6 is a graph of the entire (a) and partial enlargement (b) of the XRD patterns for Examples 2, 10, and 11. Figure 7 is a graph (a) confirming rate characteristics using half cells for Example 2 and Examples 9 to 11, and a graph (b) showing cycle characteristics of Examples 2, 10, and 11. Figure 8 is a graph (a) confirming the rate characteristics using half cells for Example 3 and Examples 12 to 15, and a graph (b) showing the cycle characteristics of Example 3 and Examples 12 to 15.

Category 1 Category 2 chemical formula 0.1C discharge (mAh/g) 5C discharge (mAh/g) Cycle capacity retention (%) Comparative Example 1 NMO Na _0.44 MnO ₂ 119 95 84.1 @ ^108th Comparative Example 2 NMO-900 Na _0.44 MnO ₂ 115.4 101.1 - Example 1 [NL]MO [Na _0.308 Li _0.132 ]MnO ₂ 120 118 70.5 @ ^300th Example 2 N[ML]O Na _0.44 [Mn _0.97 Li _0.03 ]O ₂ 116 107 82.6 @ ^300th Example 3 [NL][ML]O [Na _0.308 Li _0.132 ][Mn _0.97 Li _0.03 ]O ₂ 118 115 91.2 @ ^300th Example 9 N[ML]O-0.1 Na _0.44 [Mn _0.9 Li _0.1 ]O ₂ 93.9 79.3 - Example 10 N[ML]O-0.05 Na _0.44 [Mn _0.95 Li _0.05 ]O ₂ 107.7 92.1 71.0 @ 311 ^th Example 11 N[ML]O-0.01 Na _0.44 [Mn _0.99 Li _0.01 ]O ₂ 118.6 94.8 83.8 @ ^185th Example 12 [NL][ML]O-0.088 [Na _0.352 Li _0.088 ][Mn _0.97 Li _0.03 ]O ₂ 110.8 115.6 92.2 @ ^100th Example 13 [NL][ML]O-0.066 [Na _0.374 Li _0.066 ][Mn _0.97 Li _0.03 ]O ₂ 111.9 114.5 90.3 @ ^150th Example 14 [NL][ML]O-0.044 [Na _0.396 Li _0.044 ][Mn _0.97 Li _0.03 ]O ₂ 112.3 114.4 95.4 @ ^200th Example 15 [NL][ML]O-0.022 [Na _0.418 Li _0.022 ][Mn _0.97 Li _0.03 ]O ₂ 112.1 110.1 95.4 @ ^200th

Referring to Table 3 and Figure 3, in Comparative Examples 1 to 3, the sintering temperatures of the cathode active material made of sodium-manganese oxide without substitution of lithium were changed to 950°C, 900°C, and 1000°C, respectively. As a result of checking the capacity at different C-rates when discharging at 0.1C, 0.5C, 1C, 2C, and 5C, it was confirmed that Comparative Example 1 was similar at low rates, but was superior at higher rates. Therefore, when comparing the performance of the following examples, Comparative Example 1, which is the cathode active material with the best performance, was used as a standard, and the performance evaluation was performed by keeping the firing temperature of the examples the same as Comparative Example 1.

Referring to Table 3, it can be seen that while there is no significant difference in 0.1C discharge between Examples 1 to 3 and Comparative Example 1, Examples 1 to 3 are superior to Comparative Example 1 in high rate 5C discharge. there was. In addition, as a result of checking the discharge capacity maintenance rate after performing a cycle at 5C current, the discharge capacity maintenance rate at 108 cycles in Comparative Example 1 and the discharge capacity retention rate at 300 cycles in Examples 1 to 3 were similar values. I was able to confirm.

In the cathode active material consisting of only a tunnel structure as in Comparative Example 1, a part of the sodium atomic layer was replaced with lyrium as in Examples 1 to 3 (Example 1), or a part of the manganese atomic layer was replaced (Example Example 2) and when replacing part of the sodium atomic layer and the manganese atomic layer (Example 3), a spinel structure is further formed, and Examples 1 to 3 are positive electrode active materials in which the spinel structure and the tunnel structure are mixed together. It was confirmed that both electrochemical characteristics and cycle characteristics at high rates were superior compared to Comparative Example 1.

This is because the cathode active material, which has a tunnel structure made of sodium and manganese as in Comparative Example 1, has a problem in that the cycle characteristics of the battery are continuously deteriorated due to the Jahn-Teller effect of manganese during the charging and discharging process. On the other hand, in the case of Examples 1 to 3, lithium is substituted in part of the atomic layer of sodium or manganese, so the positive electrode active material is a tunnel structure with a mixed spinel structure, thereby suppressing the Jahn-Teller effect, thereby suppressing the sodium secondary It was confirmed that battery performance was improved.

Referring to FIG. 4, Examples 4, 5, and 8 are for positive electrode active materials containing both a tunnel structure and a spinel structure by substitution of lithium as described above, and the Bragg angle (2θ) of the X-ray diffraction pattern is It was confirmed that two diffraction peaks of (200) and (111) appeared in the range of 17° to 22°. On the other hand, when lithium is not substituted and only the tunnel structure is formed (Comparative Example 1, see Figure 9 below), only a diffraction peak of (200) appears in the range of angle (2θ) of 17° to 22°. In other words, the diffraction peak of (111) is caused by substitution of lithium at the sodium site. As the amount of lithium substitution increases, the intensity of the diffraction peak of (200) is almost similar, while the intensity of the diffraction peak of (111) gradually increases. I was able to confirm. In addition, the diffraction peak of (140) occurring at an angle (2θ) of 16.5° and the diffraction peak of (130) occurring at an angle (2θ) of 14° gradually increase as the lithium content increases. It was confirmed that there was a shift in direction.

Figure 5 shows a positive electrode active material in which lithium substitutes sodium. Examples 1, 7, and 8, in which the substitution amount of lithium is 0.132, 0.088, and 0.044, respectively, all similarly show excellent high-rate capacity characteristics, while the substitution amount of lithium is 0.132, 0.088, and 0.044, respectively. It was confirmed that Examples 5, 6, and 7, which were 0.22, 0.264, and 0.308, respectively, showed relatively low characteristics. In other words, the spinel structure that occurs when lithium replaces sodium atoms improves the rate characteristics, but when the substitution amount of lithium is too large, the capacity is low, as shown in Examples 5, 6, and 7. In addition, in the case of Examples 1, 7, and 8, it was confirmed that the 1C cycle characteristics were also excellent.

Referring to FIG. 6, when lithium replaces manganese, in Examples 1 and 11, where the substitution amount is small, only a diffraction peak of (200) appears in the angle (2θ) range of 17° to 22°, but the substitution amount is In this relatively large Example 10, it was confirmed that two diffraction peaks of (200) and (111) appeared. In other words, it can be confirmed that when lithium substitutes manganese, a spinel structure occurs more based on a larger substitution amount than when lithium substitutes sodium.

Figure 7 shows a positive electrode active material in which lithium is substituted for manganese. Examples 11 and 2, in which lithium substitution amounts are 0.01 and 0.03, respectively, show excellent characteristics at high rates, but as Examples 10 and 9 progress, the lithium decreases. It was confirmed that as the substitution amount increased, the capacity at high rates gradually decreased. In addition, as a result of confirmation by 1C cycle, it was confirmed that Example 2 was the best.

Figure 8 shows a positive electrode active material in which lithium substitutes both sodium and manganese. As reviewed in Figure 6 above, when manganese is substituted, the substitution amount of lithium for manganese is fixed similar to Example 2, which has the best performance. And the capacity characteristics at each rate were confirmed by varying the amount of sodium substitution. Comparing Examples 3, 12, 13, 14, and 15 with lithium substitution amounts of 0.132, 0.088, 0.066, 0.044, and 0.022, respectively, Examples 14 and 15 are relatively superior at high rates, but these results are 10C The results were confirmed at very high rates up to and 15C, and it was confirmed that the results were almost similar below 5C. That is, it was confirmed in Figure 5 that excellent capacity characteristics were exhibited even at high rates, similar to the substitution amounts of lithium confirmed in the case of replacing only sodium (0.132, 0.088, and 0.044 in Examples 1, 7, and 8). On the other hand, when examining the 1C cycle characteristics together, it was confirmed that Examples 14 and 15 were the best. This is believed to affect cycle characteristics depending on the total amount of lithium substitution when lithium is substituted for both sodium and manganese.

As mentioned above, compared to the case where lithium is not substituted, the positive electrode active material in which lithium substitutes sodium or manganese and has a spinel structure shows relatively excellent results in high rate characteristics, while the case where the substitution amount of lithium is within a certain range is the best. It was confirmed that the total content of lithium substituted for sodium and manganese had an effect on the high rate characteristics.

Therefore, in the following, based on a sintering temperature of 950°C, Comparative Example 1 in which lithium was not substituted (NMO, Na _0.44 MnO ₂ ) and Example 1 in which lithium replaced only sodium ([NL]MO, [Na _0.308 Li _0.132 ] MnO ₂ ), Example 2 in which lithium replaced only manganese (N[ML]O, Na _0.44 [Mn _0.97 Li _0.03 ]O ₂ ), and Example 3 in which lithium replaced both sodium and manganese ([Na _0.308 Li _0.132 ][Mn _0.97 Li _0.03 ]O ₂ ), the substitution amount of lithium was fixed and each performance was compared. That is, Examples 1 and 2 were selected as representative cathode active materials with the best performance, and Example 3 was selected as a cathode active material in which the amount of lithium substituted for sodium and manganese was the same as Examples 1 and 2.

Figure 9 is an SEM image of Example 1 and Comparative Example 1, and Figure 10 is an XRD graph of Examples 1 to 3 and Comparative Example 1. Figure 11 is an enlarged view between 37.1° and 37.6° in the XRD patterns of Examples 1 to 3 and Comparative Example 1 (a), and shows the representative crystal structure (top) of LiMn ₂ O ₄ and Na _0.44 MnO ₂ This is a drawing showing the crystal structure (bottom) (b), and the XRD pattern for a wide range (c). Figure 12 is a diagram showing the Mn 2p3/2 spectrum (a) and N2 adsorption and desorption curves (showing pore size distribution) of Example 3 and Comparative Example 1 (b). Figure 13 is an SEM image of Example 3.

Referring to Figure 9, Example 3 ([NL][Ml]O) is a positive electrode active material containing both a spinel structure and a tunnel structure, and Comparative Example 1 (NMO) is a positive electrode active material consisting of only a tunnel structure, Example 3 And when comparing the primary particles in Comparative Example 1, it can be seen that Example 3 is provided in the form of a nanorod with a longer length in the c-axis direction.

In general, Na _0.44 MnO ₂ consisting of only a tunnel structure has an orthorhombic structure with a space group Pbam , and LiMn ₂ O ₄ consisting of only a spinel structure has a cubic spinel structure having a space group Fd3m . structure) (see (b) of FIG. 10).

Figures 10 and 11 (a) show _{the powder} It can be confirmed that NMO) appears in an orthorhombic structure with the previously reported Pbam .

Example 2 is a transition metal substituted with lithium, and only a diffraction peak of (200) appears in the Bragg angle (2θ) of the X-ray diffraction pattern in the range of 17° to 22°, but depending on lithium substitution, Comparative Example 1 It can be seen that the characteristic peak appears at a slightly lower 2θ angle compared to . As in Example 2, when lithium substitutes only the transition metal, it can be seen that both (200) and (111) diffraction peaks appear when the substitution amount of lithium increases more than when lithium substitutes sodium (Figure 6). In Example 2, the amount of lithium substitution was small, so the diffraction peak of (111) was not indexed, but this was believed to be because the presence of a spinel structure was only a trace amount. On the other hand, in the case of Example 2, not only the spinel structure but also the lithium ion (Li+) possesses strong polarity, and as a result, it is believed that lithium mainly occupies the manganese site and expands the sodium diffusion channel.

In the powder X-ray diffraction pattern of the positive electrode active material of Example 1 and Example 3, the Bragg angle (2θ) of the It can be confirmed that lithium is substituted on the sodium site ([NL] MO), forming a typical cubic spinel structure of LiMn ₂ O ₄ with a space group of Fd3m.

0.76 Å)을 가진 결정 격자의 리튬이 나트륨(

1.02Å)로 치환되었음을 의미한다. 또한, 실시예 3([NL][ML]O)의 결정구조는 (200) 및 (111)의 회절피크가 나타나나 (111)의 회절피크는 리튬이 나트륨만을 치환한 양극활물질인 실시예 1보다 작은 강도로 나타나고, 또한 비교예 1(NMO)과 유사한 결정구조를 가지면서 프레임워크 구조가 유지됨을 확인할 수 있었다(도 11(b) 참조).In Examples 1 and 3, a mixed precursor of Na _0.44 MnO ₂ precursor and Li _0.44 MnO ₂ precursor was prepared by adjusting the content of Na/Li raw materials, and then the prepared Li _0.44 MnO ₂ precursor was calcined at 500 ° C. or higher. In this case, it is partially oxidized to form a spinel structure while maintaining the tunnel structure, forming a heterogeneous structure of tunnel structure/spinel structure. In addition, it was confirmed that the diffraction peak of [NL]MO appeared at a higher 2θ angle, thereby reducing the lattice parameter. This is a relatively small atomic radius (

Lithium disodium (0.76 Å) crystal lattice

It means that it has been replaced with 1.02Å). In addition, the crystal structure of Example 3 ([NL][ML]O) shows diffraction peaks of (200) and (111), but the diffraction peak of (111) is Example 1, which is a positive electrode active material in which lithium replaces only sodium. It was confirmed that the framework structure was maintained while appearing at a lower intensity and having a crystal structure similar to Comparative Example 1 (NMO) (see FIG. 11(b)).

Referring to Rietveld refinement and lattice parameters, the positive electrode active material of Example 3 ([NL][ML]O) has a ratio of Na _0.44 MnO ₂ /LiMn ₂ O ₄ of 92.4: You can confirm that it appears as 7.6.

In Figure 11 (b), the upper part shows the LiMn ₂ O ₄ crystal structure, and the lower part shows the Na _0.44 MnO ₂ crystal structure. The crystal structure of Na _0.44 MnO ₂ in the tunnel structure can be defined by the total Mn ³⁺ and total Mn ⁴⁺ contained in the Na _0.44 MnO ₂ . For example, the tunnel structure consists of an edge-sharing MnO5 pyramid containing half of the total Mn ³⁺ of Na _0.44 MnO ₂ and an MnO6 octahedron containing all of the Mn ⁴⁺ and the remaining half of the Mn ³⁺ . These edge-sharing MnO5 pyramids and MnO6 octahedra form large-sized S-shaped tunnels and relatively small-sized pentagonal tunnels, where two sodiums, Na1 and Na2, are provided in the S-shaped tunnels, and one sodium, Na3, is located in the above-mentioned S-shaped tunnels. Provided within a pentagonal tunnel (total of 3 sodium). Here, the S-shaped tunnel acts as a sodium diffusion channel by providing a movement path for Na1 and Na2 due to its large space, so the S-shaped tunnel has an open structure and improves cycle performance by accommodating volume changes during the electrochemical charging and discharging process. It can be improved.

In a typical cathode active material made of sodium-manganese oxide, the tunnel structure allows insertion/release of sodium through a one-dimensional channel. On the other hand, the positive electrode active material according to this embodiment can form a 3D spinel framework of LiMn ₂ O ₄ through edge sharing in the MnO 6 octahedron, where lithium occupies the tetrahedral site. Accordingly, the aligned MnO6 sheet of Na _0.44 MnO ₂ forms the framework of the spinel structure, thereby activating the reaction site and providing more ion diffusion channels.

That is, the positive electrode active materials of Examples 1 and 3 have a heterogeneous structure of tunnel structure and spinel structure, and lithium is substituted with one or more of sodium and transition metals to improve the sodium movement speed and increase the sodium diffusion coefficient.

It was confirmed that a deconvoluted peak of 55.6 eV existed in the XPS spectrum of Li 1s confirmed by It was confirmed that the substitution was efficient (see (c) of FIG. 11).

Figure 12(a) shows the Mn 2p3/2 spectrum for each of Example 3 and Comparative Example 1, which is separated into two configurations at 641.5 eV and 642.8 eV, which are Mn ³⁺ (641.5 eV) and Mn ^4, respectively. ⁺ means (642.8eV). The concentration of Mn ³⁺ was 47.3% in Example 3 ([NL][ML]O) and 50.6% in Comparative Example 1 (NMO), confirming that the concentration of Mn 3+ in Example 3 was lower than that in Comparative Example 1. This means that Mn ^3+, which has a high spin, is oxidized to JT-inactive Mn ⁴⁺ as Mn is replaced by Li, which has a low valence, due to the charge compensation mechanism. Referring to the standard Gibbs free energy of Li2O (-561.2 kJ/mol) and MnO2 (-465.2 kJ/mol), the binding energy of Li-O is stronger than that of Mn-O, Structural stability can be improved.

That is, in the case of Example 3, structural rearrangement of the MnO6 octahedron can be prevented and imbalance of Mn ions can be prevented by the charge compensation mechanism and the strong binding energy of Li-O.

Figure 12(b) shows the N2 adsorption-desorption isotherm, and when compared with the general IUPAC IV isotherm, it was confirmed that the positive electrode active materials of Example 3 and Comparative Example 1 had a mesoporous structure. Referring to the SEM image, the isotherm profile of the positive electrode is mainly due to primary particles in the form of stacked nanorods, and in Brunauer-Emmett-Teller (BET) surface area, the positive electrode active material of Example 3 ([NL][ML]O) (2.08 ㎡/g) was confirmed to be larger than the positive electrode active material (1.05 ㎡/g) of Comparative Example 1 (NMO).

During the process of manufacturing the positive electrode active material, nitrate is decomposed during the firing process, and as a result, the positive electrode active material has mesopores. At this time, the surface area of the positive electrode active material increases, and the increased surface area increases the movement of sodium ions. As shown in Figure 12(b), it was confirmed that the size of the mesopore was 15 to 40 nm.

1 μm 너비와

220 nm 두께의 나노로드 형태를 갖는 일차입자가 비교적 균일하게 분포함을 확인할 수 있었다. 또한, 리튬이 치환된 경우에도 양극활물질을 구성하는 일차입자의 대략 유사하게 유지되므로, 리튬의 첨가는 일차입자의 형상에서는 큰 영향을 미치지는 않는 것으로 판단된다(도 9 참조).Figure 13 is an SEM image of Example 3,

1 μm width and

It was confirmed that primary particles in the form of nanorods with a thickness of 220 nm were relatively uniformly distributed. In addition, even when lithium is substituted, the primary particles constituting the positive electrode active material remain approximately similar, so it is judged that the addition of lithium does not have a significant effect on the shape of the primary particles (see FIG. 9).

Figure 14 is a TEM image of the primary particles of Example 3 and Comparative Example 1. Figure 15 shows the TEM image (a), EDS mapping image (b), SAED pattern (c), HRTEM and FFT image (d) of Example 3, and the SAED pattern (e), HRTEM and FFT image of Comparative Example 1 ( f).

Referring to FIGS. 14 and 15, the EDS mapping (energy dispersive spectrometer mapping) image obtained from TEM (FIG. 15(b)) confirms that Na, Mn, and O are uniformly dispersed in the primary particles. On the other hand, it is difficult to detect lithium (Li) in Example 3 ([NL][ML]O) because the electron beam scattering power is weak and the total content is low. The selected area electron diffraction (SAED) pattern for the [010] direction shows (200) and (001) crystal planes representing single crystals, resulting in the crystals growing along the [001] direction (c-axis direction) and producing excellent sodium It can be confirmed that it is provided in the form of a nanorod that provides a diffusion channel for ions.

The atomic-scale crystal structures of Example 3 ([NL][ML]O) and Comparative Example 1 (NMO) were confirmed using high-resolution TEM (HRTEM). As in Example 3 ([NL][ML]O), the tunnel structure and spinel structure are grown and combined inside (Figure 15 (d)). Figure 15 (d-I) shows a tunnel structure with (200) and (001) lattice planes on the [010] crystal zone axis by FFT (fast Fourier transform), and Figure 15 (d-II) shows It shows a spinel structure with (111) and (220) crystal planes by FFT, that is, a heterogeneous structure of tunnel structure/spinel structure is formed. On the other hand, Comparative Example 1 (NMO) shows a uniform lattice with interplanar spacing of 4.55 and 2.84 Å, corresponding to the orthorhombic (200) and (001) lattice planes along the [010] crystal band axis (Figure 15) see f)). Since the positive electrode active material of Example 3 includes a tunnel structure on the (200) plane and a spinel structure on the (111) plane, it is believed that electrochemical performance is improved by promoting diffusion of sodium ions.

That is, the positive electrode active material in which lithium replaces sodium or manganese, as in Examples 1 to 3, is manufactured by a simple solid-state synthesis method using high temperature sintering, and has relatively less Mn ³⁺ compared to Comparative Example 1. It was confirmed that it was included in quantity.

Figure 16 shows the results of DSC (Differential scanning calorimetry) of Example 3 and Comparative Example 1. In Figure 17, each was evaluated using an anode charged at 4V and impregnated in an electrolyte solution.

282.7℃이고, 실시예 3은

291.1℃였다. 또한, 발열량(H_o)은 비교예 1(910.5J/g)에 비하여 실시예 3(852.9J/g)은 대략 90% 이하로 더 열적으로 안정함을 확인할 수 있었다.Referring to FIG. 16, according to the thermal stability evaluation method using a differential scanning calorimeter, the maximum exothermic peak temperature (T _o ) of the cathode active material made of sodium-manganese oxide in Comparative Example 1 was It was confirmed that the maximum exothermic peak temperature (T) of the positive electrode active material having a structure in which at least one of sodium and manganese was substituted with lithium was higher than 9°C. Specifically, Comparative Example 1 was

282.7℃, Example 3

It was 291.1℃. In addition, it was confirmed that the calorific value (H _o ) of Example 3 (852.9 J/g) was approximately 90% or less compared to Comparative Example 1 (910.5 J/g), making it more thermally stable.

The thermal stability of the positive electrode active material is related to oxygen release and differentiation of the electrolyte. When lithium is substituted as in Example 3, the maximum exothermic peak temperature is relatively reduced and the amount of heat generated is reduced compared to Comparative Example 1, which is due to the This is believed to be because substitution prevents thermal runaway and suppresses structural decomposition by suppressing side reactions between the positive electrode active material and the electrolyte.

Figure 17 shows the results of confirming the electrochemical performance of positive electrode active materials according to comparative examples and examples. Figure 18 shows the results of confirming the cycle characteristics of positive electrode active materials according to comparative examples and examples. Specifically, (a) of Figure 17 is a dQ/dV plot for the first 4 cycles evaluated with the coin-type half-cell of Comparative Example 1, and (b) is the first cycle evaluated with the coin-type half-cell of Example 3. This is a dQ/dV plot for 4 cycles, and (c) is the result of confirming the characteristics of Examples 1 to 3 and Comparative Example 1 by rate. Figure 18 (a) shows the results of confirming the performance of 300 cycles with 1C of Examples 1 to 3 and Comparative Example 1, and (b) shows the performance from 2 cycles to 300 cycles with 1C of Comparative Example 1 and Example 3. This is the progressed voltage profile, (c) is the result of confirming the coulombic efficiency for 1C of Examples 1 to 3 and Comparative Example 1, and (d) is the result of confirming the 1200 cycle of Examples 1 to 3 and Comparative Example 1. It is a result.

Referring to Figures 17 (a) and (b), in the half-cell of sodium ions for Comparative Example 1 and Example 3, the dQ/dV plot of the first 4 cycles shows more than 6 reversible redox peaks, Here, a multi-step Mn ^{3 +} /Mn ⁴⁺ reaction is included. Example 3 can confirm that an additional oxidation peak occurs at approximately 3.7V, indicating that lithium ions (Li ⁺ ) are released from LiMn ₂ O ₄ . On the other hand, this additional oxidation peak disappears after two cycles, which means that LiMn2O4 undergoes a phase transition during the process of insertion/expulsion of sodium ions (Na+). During the sodiation process, which is the charging process of the first cycle of a sodium battery, sodium ions are inserted into the empty space of the spinel structure Li _x Mn ₂ O ₄ . At this time, compared to Comparative Example 1, in the spinel structure of Example 3, a slight disodium reaction occurred during the discharge process of the first cycle in order to accommodate sodium ions with a relatively larger atomic radius. This causes the reduction peak of the positive electrode active material of Example 3 to move in the negative direction. That is, in Example 3, excellent capacity at low current densities and a slight shift in the voltage platform are related to insertion/expulsion of sodium ions in LiMn ₂ O ₄ and activation of the positive electrode active material. The dual structure, which includes both the tunnel structure (Na _0.44 MnO ₂ ) and the spinel structure (LiMn ₂ O ₄ ), directly connects the 3D channel in the spinel structure and the 1D channel in the tunnel structure, and shortens the diffusion channel for sodium ions. It can exhibit high redox reaction rates. Therefore, Example 3 shows a high capacity exceeding 119.6 mAh/g at 0.1 C after the first 4 cycles and shows a further improved sodium ion storage capacity compared to Comparative Example 1.

In Example 3, despite the increase in current density and the decrease in capacity due to electrochemical polarization, the discharge capacities were 117.6 mAh/g, 116.8 mAh/g, and 114.8 mAh at rates of 0.1, 0.5, 1, 2, 5, and 10 C, respectively. /g, 111.7mAh/g, 106.9mAh/g, and 101.2mAh/g, and it was confirmed that the performance was excellent at 97.0mAh/g (0.1C capacity) at very high rates such as 15C (1.8A/g). It was confirmed that it exhibited excellent characteristics (equivalent to 82.5%). In addition, when the cycle is performed at a discharge capacity of 15 C and then again at 0.5 C, the maximum discharge capacity is recovered due to activation of the spinel structure, where Example 1 ([NL] MO) recovers to 123.0 mAh/g. .

Figure 18(a) shows the results of performing 300 cycles at 1C, and it can be seen that Examples 1 to 3 exhibit superior cycle characteristics compared to Comparative Example 1. The positive electrode active material of Comparative Example 1, which consists only of a conventional tunnel structure, has poor life characteristics because the lattice collapses when repeated cycles of sodium ion insertion/expulsion are performed. In Comparative Example 1 (NMO), the discharge capacity decreased as the cycle was performed, and was 82.8 mAh/g after 180 cycles, while Examples 1 to 3 showed excellent characteristics compared to Comparative Example 1. In addition, it was confirmed that Example 1 was the best at 118.4 mAh/g at the beginning of the ([NL]MO) cycle, but gradually decreased to 83.5 mAh/g at 300 cycles.

Example 2 (N[ML]O) has excellent capacity retention of 82.6% after 300 cycles, but the capacity is relatively low at 88.3 mAh/g. In Example 3 ([NL][ML]O), it was confirmed that the capacity retention after 300 cycles was 91.2% and the capacity was 104.3 mAh/g, maintaining excellent reversible capacity, which indicates that Example 3 had excellent structural stability. This is believed to be because it is a positive electrode active material with . In addition, referring to the voltage profile in (b) of FIG. 18, Example 3 ([NL][ML]O) is a cathode active material in which lithium is co-doped by substituting both sodium and manganese on both sides. As a result, it can be confirmed that the voltage profile in the process of 300 cycles shows several plateaus and steps showing various phase changes, showing low electrochemical polarization and excellent return capacity. . In addition, Example 3 ([NL][ML]O) shows an optimal coulombic efficiency of 99.8%, which shows higher structural stability, capacity, and cell efficiency compared to sodium-manganese oxide, which only has a tunnel structure due to substitution of lithium. ) was confirmed to exhibit excellent charging and discharging reversibility (see Figure 18 (c)). In addition, as a result of confirming the long-term lifespan characteristics at high rates, it was confirmed that even after 1200 cycles at 10C, it showed a high capacity retention of 81.0% at 78.9mAh/g. At 10C 1200 cycles, the retention capacities of Examples 1 and 2 were 12.4% and 66.2%, respectively, and Comparative Example 1 had already rapidly decreased to 23.3% at 1000 cycles and could not proceed to 1200 cycles.

Figure 19 shows the results of evaluating electrochemical performance in the anode using Example 3 and a full cell using hard carbon as the cathode. Specifically, in Figure 19, (a) is the voltage profile of the full cell, (b) is the CV curve of the full cell at 0.1 mV/s, (c) is the result of performing 200 cycles at 1C, and (d) These are the capacity characteristics by rate, and (e) is the result of performing a cycle at C.

Referring to Figure 19, it can be seen that after 230 cycles, it shows a capacity of 130.4 mAh/g and a capacity retention of 77.9%, showing stable cycle characteristics. Hard carbon is a hard carbon, and considering the generally low initial coulombic efficiency when using hard carbon, in the full cell using Example 3, the capacity ratio of the cathode to the anode is approximately 1.2 (evaluated based on the mass of the anode). It was confirmed that it was. In addition, in Figure 19 (b), the CV profile during the initial 1 to 5 cycles of the full cell is shown at 0.1 mV/s within the potential range of 1.5-3.9 V.

Although the peak is slightly shifted in the initial cycle due to the formation of a solid-electrolyte interphase (SEI) film and electrolyte decomposition, the redox peaks that appear in six pairs are similar to those of Example 3 ([NL][ML]O). This is due to the insertion/release of sodium ions (Na+) between carbons (see Figure 19 (a)).

The charge/discharge profile in (c) of FIG. 19 shows that the sodium ion full cell has excellent reversibility and low polarization in the process of performing 5 to 200 cycles at 1C. The full cell has an energy density of 329.1Wh/kg and shows excellent cycle characteristics at 105.0mAh/g after 200 cycles. In addition, referring to (d) of Figure 19, the rate characteristics of the full cell are 118.4 mAh/g, 114.9 mAh/g, 114.2 mAh/g, and 113.0 mAh/ at 0.1C, 0.2C, 0.5C, 1C, and 2C, respectively. g and a reversible capacity of 110.9 mAh/g. In addition, after performing 80 cycles at various rates, it was confirmed that excellent reversible performance was achieved at 107.7 mAh/g with a high coulombic efficiency of 99.9%.

Referring to (e) of FIG. 19, 71.5 mAh/g was maintained even after 900 cycles at 2C, and the capacity loss was approximately less than 0.043% per cycle, confirming long-term cycle stability. On the other hand, it is believed that the slight decrease in capacity in this full cell can be eliminated by improving the design of the anode, cathode, and electrolyte.

Example 3 ([NL][ML]O), both half-cell and full-cell, showed excellent electrochemical and cycle characteristics, in which lithium was substituted and introduced into the crystal lattice, and a heterogeneous cell containing both a tunnel structure and a spinel structure was introduced. It is believed that this is because the structure was formed. Here, substitution of lithium is a component of the transition metal layer and is believed to improve the structural stability of the positive electrode active material, stabilize the charge-ordering state, and prevent M3+ imbalance. In addition, the heterogeneous structure of the tunnel structure and spinel structure can provide a diffusion channel for sodium ions, promoting the movement of ions and improving rate-specific capacity and reaction speed. Additionally, the stack of primary particles forming a conductive network can increase the contact interface of the anode/electrolyte and promote continuous movement of electrons/ions.

Figure 20 shows the results of confirming the electrode reaction rate and thermal stability of the positive electrode active materials according to Examples 1 to 3 and Comparative Example 1. In Figure 20, (a) is the GITT curve and the calculated diffusion coefficient of sodium ions, (b) is the CV curve of Example 3 ([NL][ML]O) at various scan speeds, and (c) is Capacity of Example 3 ([NL][ML]O) at different scan speeds, (d) is a graph of peak current for v ^½ , and (e) is DSC in the state of 4V charging and impregnation with electrolyte. It is a result. Figure 20(f) shows the resistance after 0, 50, 100, and 200 cycles, and (g) shows the result of comparing Rct. Table 4 below shows the results showing Rct per cycle.

Category 1 Category 2 0cycle
(Rct, Ω) 50cycle
(Rct, Ω) 100cycle
(Rct, Ω) 100cycle
(Rct, Ω) Comparative Example 1 NMO 136.2 161.4 183.8 290.1 Example 1 [NL]MO 66.2 129.5 159 184.8 Example 2 N[ML]O 81.9 73.8 92 105.6 Example 3 [NL][ML]O 56.8 62.7 69.8 67.3

1.6 Х 10^-10㎠/s로, 비교예 1(NMO)의

5.8 Х 10^-11㎠/s에 비해 높은 값을 갖는다. 실시예 3은 실시예 1, 2와 비교예 1에 비하여 가장 낮은 오버포텐셜(overpotential)을 갖고, 이는 가장 낮은 전기화학적 분극과 반응속도가 가장 빠름을 의미하고, 이는 높은 나트륨 저장능에 기인한다. 여기서, 오버포텐셜은 컷오프 전압(cutoff voltage)과 안정 전압(stable voltage) 사이의 전압 차이가 동일한 경우를 의미한다.In Figure 20 (a), the charge/discharge GITT (galvanostatic intermittent titration technique) curves of Examples 1 to 3 and Comparative Example 1 in a 2-cycle process at 0.05C are shown. The minimum values of sodium ion diffusion coefficients (Na+ diffusion coefficients, DNa) in Examples 1 to 3 and Comparative Example 1 indicate a 70% to 80% disodium state and a 20% to 30% sodium ion state, which indicates an active It involves the diffusion of natrium ions during their insertion into the material or their release from the active material. DNA of Example 3 ([NL][ML]O) was

At 1.6 Х 10 ^-10 cm2/s, of Comparative Example 1 (NMO)

It has a higher value compared to 5.8 Х 10 ^-11 ㎠/s. Example 3 has the lowest overpotential compared to Examples 1 and 2 and Comparative Example 1, which means the lowest electrochemical polarization and the fastest reaction rate, which is due to the high sodium storage capacity. Here, overpotential refers to the case where the voltage difference between the cutoff voltage and the stable voltage is the same.

In Figure 20 (b), the CV curve of Example 3 ([NL][ML]O) was confirmed at various scan rates between 0.1 and 1.5 mV/s. Here, the redox current increases rapidly as the scan rate increases, while a stable potential difference between each redox peak represents the minimum polarization of the anode. Additionally, the value of (b) can evaluate the capacitance effect or diffusion control capacitance by fitting the slope of log (i) and log (v). where i and v represent peak current and scan rate, respectively.

Typically, when the b-value is approximately 0.5, it means an ion diffusion control step, and when it is 1.0, it means a surface adhesion/desorption control step. Each b- of Comparative Example 1, Example 1, Example 2, and Example 3 The values are 0.83, 0.86, 0.84, and 0.83, which means that the electrochemical capacity and insertion/ejection reaction influence together during the sodium oxidation reaction. The specific contribution can be expressed as equation (1) below.

Equation (1) i(V) = k ₁ v + k ₂ v ^1/2

Here, k ₁ v is the electrochemical capacitance contribution, and k ₂ v ^1/2 represents the diffusion control gear. k ₁ and k ₂ are constants obtained by fitting i/ v ^1/2 and v ^1/2 2.

Figure 20(c) shows the approximate calculation of the capacitance contribution of Example 3 ([NL][ML]O) at different scan speeds, and at a low scan speed (0.1 mV/s), Example 3 Approximately 49.6% of the total current response to is determined by surface electrosorption. As the scan rate increased, the electrochemical capacitive contribution was confirmed to reach a maximum of 79.0% at 1.5 mV/s, indicating that the pseudocapacitive contribution to the overall capacitance is important at high scan rates. . That is, the speed performance of Example 3 ([NL][ML]O) is improved by the high pseudocapacitance contribution.

1.4 Х 10^-10㎠/s이고, 이는 GITT를 사용하여 얻은 결과와 일치하는데, 이는 리튬의 치환에 의하여 이온의 확산 속도가 향상하였음을 의미한다. 반면, 비교예 1(NMO)의 평균 DNa는

7.9 Х 10^-11㎠/s로 매우 낮게 나타남을 확인할 수 있었다. 이는 실시예 3([NL][ML]O)에서는 나트륨 이온의 확산을 향상시키는 3D 채널을 제공하는 터널구조 /스피넬구조의 이종구조에 의하는 것으로 판단된다. The average DNa is calculated by the Randles-Sevcik equation based on a linear fit for i _p (peak current) and v ^1/2 (square root of scan speed) as shown, and the larger the slope, the larger the DNa. The average DNA of Example 3 ([NL][ML]O) was

1.4 Х 10 ^-10 cm2/s, which is consistent with the results obtained using GITT, which means that the diffusion rate of ions was improved by the substitution of lithium. On the other hand, the average DNA of Comparative Example 1 (NMO) was

It was confirmed that it was very low at 7.9 Х 10 ^-11 ㎠/s. This is believed to be due to the heterogeneous structure of tunnel structure/spinel structure in Example 3 ([NL][ML]O), which provides a 3D channel that improves diffusion of sodium ions.

Examples 1 to 3 and Comparative Example 1 all show larger diffusion coefficients in the O3 and R3 peaks, but smaller values in the O4 and R4 regions, indicating the transfer of different sodium ions in the insertion/release steps of different sodium ions. represents dynamics. Example 3 ([NL][ML]O) shows the largest DNa in the O3 and R3 regions, which means that the reaction rate increased by substitution of lithium.

In Figures 20 (f) and (g), charge transfer resistance due to lithium substitution was examined using electrochemical impedance spectra (EIS). Open circuit voltage before cycling and Nyquist impedance for Comparative Example 1 (NMO) and Example 3 ([NL][ML]O) in the fully disodiumized state after 50, 100 and 200 cycles. It was confirmed and fit to different equivalent circuit models.

The semicircles appearing in the high-frequency and mid-frequency regions represent SEI resistance (RSEI) and charge-transfer resistance (Rct), respectively, and the sloping line in the low-frequency region represents the concentration of sodium ions in Na0.44MnO2 particles. It is related to proliferation. The Rct value for Example 3 ([NL][ML]O) in the fresh state was 56.8Ω, meaning that the transfer rate of sodium ions was relatively increased compared to other positive electrode active materials. Here, high Rct acts as the main cause of capacity degradation.

Comparative Example 1 (NMO) showed a higher Rct value than Examples 1 to 3 in which lithium was substituted, and the Rct of Example 3 ([NL][ML]O) hardly changed during the cycle. You can confirm that it is maintained. In other words, this means that charge transfer resistance is minimized by substitution of lithium, thereby effectively improving the lifespan characteristics of the positive electrode active material.

282.7℃(발열피크)에서 910.5J/g이고, 실시예 3은

291.1℃(발열피크)에서 852.9J/g로 나타났다. DSC 결과는 양극활물질에서의 산소방출과 전해질의 분해와 관련된 결과로, 리튬의 치환에 의하여 전극과 전해질 사이의 부반응이 억제되고 이에 의하여 열폭주를 방지함을 확인할 수 있었다.In (e) of Figure 20, the thermal stability is the result of differential scanning calorimetry (DSC) of Example 3 ([NL][ML]O) and Comparative Example 1 (NMO) in a state impregnated with electrolyte and charged at 4.0 V. am. Comparative Example 1 is

It is 910.5J/g at 282.7℃ (exothermic peak), and Example 3 is

It was found to be 852.9J/g at 291.1℃ (exothermic peak). The DSC results were related to the release of oxygen from the positive electrode active material and the decomposition of the electrolyte, and it was confirmed that the substitution of lithium suppressed side reactions between the electrode and the electrolyte, thereby preventing thermal runaway.

Figure 21 shows an XRD graph and SEM image for comparing the cycle performance of Example 3 and Comparative Example 1. Specifically, Figure 21 compares the anodes using Example 3 and Comparative Example 1, respectively. (a) is Ex situ (stored with Mylar film to prevent exposure), (b) is the lattice parameters a, b, c and unit cell volume in various states, (c) is the XRD graph after 200 cycles, (d) ) is the SEM image after 200 cycles.

In Figure 21 (a), as a result of confirmation in the fresh state and the fully-desodiated state, most of the diffraction peaks during the emission of sodium ions are relative to those in the fresh state. It appeared at a higher 2θ angle, which is believed to be because the electrostatic repulsion between adjacent oxygen atoms was reduced after the release of sodium ions, causing the unit cell to shrink.

0.66 mol)의 과도한 삽입이 발생함을 의미한다. 주요피크의 쉬프트는 나트륨 이온의 삽입/방출 과정에서 발생하는 격자 파라미터의 변화를 발생시킴을 확인할 수 있었다 (도 21의 (b) 참조).When sodium ions are released in Comparative Example 1 (NMO), a new Na ₂ Mn ₅ O ₁₀ phase is observed, which is believed to be due to incomplete disodiumization and the formation of an intermediate phase. On the other hand, in Example 3 ([NL][ML]O), no new peaks occurred during primary charging and the main diffraction peak appeared at a relatively high 2θ angle, meaning that the active material was undergoing a solution reaction. In the fully-sodiated state, all major diffraction peaks associated with tunnel-structured Na _x MnO ₂ appear at relatively low 2θ angles, and sodium ions (

This means that excessive insertion of 0.66 mol) occurs. It was confirmed that the shift of the main peak causes a change in the lattice parameter that occurs during the insertion/ejection process of sodium ions (see (b) of FIG. 21).

The volume change (6.28%) of Example 3 ([NL][ML]O) was found to be much lower than the volume change (10.72%) of Comparative Example 1 (NMO), which is due to the sodiumization/dehydration due to substitution of lithium. It is believed to prevent unit cells from expanding during the sodiumization process. This reduction in volume change serves as a major factor in improving the long-term lifespan characteristics of the anode.

In Example 3 ([NL][ML]O), the diffraction peak of (111) resulting from LiMn2O4, which is a spinel structure, is shifted to a higher 2θ angle in the disodium state, and is restored to its original state after discharge. It showed good structural reversibility. On the other hand, in Example 3 ([NL][ML]O, a slight dislocation occurs in the spinel structure, and the diffraction peak intensity of Li _x Mn ₂ O ₄ of the spinel structure decreases, but the impurity phase ) does not occur, which means that Example 3 ([NL][ML]O) has excellent structural stability. That is, when lithium is substituted on the manganese site, a heterogeneous structure of tunnel structure/spinel structure is formed. It was confirmed that the structural reversibility of Example 3 ([NL][ML]O) was improved, the extraction rate of sodium ions was increased, and cycle stability was improved.

25 °에서 넓은 피크는 마일러지 필름임). 실시예 3([NL][ML]O)의 결정상은 1C 200사이클 후 여전히 터널구조인 Na_xMnO₂가 유지되는 반면, 비교예 1(NMO)을 이용한 양극에서는 Na_0.7MnO₂ 상의 3.28%는 200 사이클 후 초기 터널구조의 Na_0.44MnO₂ 상이 복구되지 않고 구조적 비가역이 형성됨을 확인할 수 있었다. 이러한 구조적 비가역의 발생은 나트륨 이온의 이동 속도능을 저하시키고, 사이클 과장에서 용량저하를 발생시킨다.In the XRD analysis of Figure 21(c), the electrochemical behavior of Comparative Example 1 (NMO) and Example 3 ([NL][ML]O) during the sodiumization/disodiumization process was confirmed (

The broad peak at 25° is the mileage film). The crystal _phase of Example 3 ₍ [NL _] [ML]O) still maintains the tunnel _structure of Na It was confirmed that after 200 cycles, the Na _0.44 MnO ₂ phase of the initial tunnel structure was not recovered and structural irreversibility was formed. The occurrence of such structural irreversibility reduces the movement speed of sodium ions and causes capacity reduction during cycle exaggeration.

Figures 21 (d) and (e) are SEM images of the anodes of Comparative Example 1 (NMO) and Example 3 ([NL][ML]O) after 200 cycles of 1C, respectively, of Comparative Example 1 (NMO). In this case, it was confirmed that the primary particles were not able to maintain their shape and the nanorod shape was pulverized or aggregated, whereas in Example 3 ([NL][ML]O), the nanorod shape was maintained, showing excellent structural stability even during the charging and discharging process. It was confirmed that the reversibility of the crystal phase was observed.

In Example 3 ([NL][ML]O), the width and length of the primary particle may range from nano to micron. For example, the primary particle may have a width of nanometer to facilitate the movement of sodium ions. It was confirmed that the strain occurring during the insertion/release process of sodium ions was alleviated. That is, Example 3 ([NL][ML]O), unlike Comparative Example 1 (NMO), maintains the nano-sized shape of the primary particles, thereby mitigating structural changes during the long-term cycle process and reducing the amount of energy generated during charge and discharge. Excellent structural reversibility improves lifespan characteristics.

Above, the present invention has been described in detail using preferred embodiments, but the scope of the present invention is not limited to the specific embodiments and should be interpreted in accordance with the appended claims. Additionally, those skilled in the art should understand that many modifications and variations are possible without departing from the scope of the present invention.

Claims (11)

In the cathode active material for sodium secondary batteries,
It has a tunnel structure and includes a sodium-transition metal oxide structure in which at least one of sodium and transition metal is substituted with lithium,
The positive electrode active material includes Mn ³⁺ in an amount of less than 50% relative to the total concentration of Mn ³⁺ and Mn ⁴⁺ ,
The cathode active material further includes a spinel structure, and is a cathode active material for a sodium secondary battery represented by one or more of the following Chemical Formulas 1 to 3.
[Formula 1]
[Na _xy Li _y ]TMO ₂
[Formula 2]
Na _x [TM _z Li _1-z ]O ₂
[Formula 3]
[Na _xy Li _y ][TM _z Li _1-z ]O ₂
In Formulas 1 to 3, TM is Mn, 0<x≤1, 0<y≤0.5, 0<z≤1. delete According to paragraph 1,
The positive electrode active material includes secondary particles composed of a plurality of primary particles,
The primary particles include both tunnel structures and spinel structures,
A positive active material for a sodium secondary battery, wherein the spinel structure increases as the amount of lithium substitution increases. According to paragraph 3,
The positive electrode active material stabilizes the electronic structure by substituting sodium or a transition metal with lithium to oxidize Mn ³⁺ to Mn ⁴⁺ , but as the amount of lithium substitution increases, the amount of oxidation of Mn ³⁺ to Mn ⁴⁺ increases. A cathode active material for a sodium secondary battery containing: delete According to paragraph 1,
In a sodium secondary battery employing a positive electrode containing the above positive electrode active material, a positive electrode for a sodium secondary battery having a capacity retention of more than 80% after charging and discharging 1000 times at a constant current of 0.1C rate in a voltage range of 2.0 to 3.8V with respect to sodium metal. Active material. According to paragraph 1,
The tunnel structure is a cathode active material for a sodium secondary battery including a tunnel structure having an S-shaped cross-section and a polygonal tunnel structure having a smaller area than the S-shaped tunnel structure. According to paragraph 1,
The positive electrode active material includes secondary particles composed of a plurality of primary particles,
The primary particle includes a nanorod shape with a longer length in the C-axis direction,
The secondary particle is a cathode active material for a sodium secondary battery having a specific surface area (BET) of 1.3 m2/g or more. According to paragraph 1,
In the powder
A cathode active material for a sodium secondary battery in which, when lithium replaces sodium, the intensity of the diffraction peak of (111) relative to (200) increases as the amount of lithium substitution increases. According to paragraph 1,
The transition metal consists of manganese,
According to the thermal stability evaluation method using differential scanning calorimetry,
The maximum exothermic peak temperature (T _o ) of a cathode active material made of sodium-manganese oxide is It appears higher than 9℃,
The calorific value (H _o ) of a cathode active material having a structure in which at least one of sodium and manganese in sodium-manganese oxide is substituted with lithium is less than 90% of the calorific value (H _o ) of a cathode active material made of sodium-manganese oxide. Cathode active material for sodium secondary batteries. A positive electrode containing the positive electrode active material for a sodium secondary battery according to any one of claims 1, 3, 4, and 6 to 10;
cathode; and
A sodium secondary battery containing an electrolyte solution containing sodium ions.