WO2013129848A1

WO2013129848A1 - Sodium secondary battery

Info

Publication number: WO2013129848A1
Application number: PCT/KR2013/001599
Authority: WO
Inventors: Jeong Soo Kim; Young Shol Kim; Je Hyun Chae; Hee Young Sun
Original assignee: Sk Innovation Co.,Ltd.; Ceramatec, Inc.
Priority date: 2012-02-27
Filing date: 2013-02-27
Publication date: 2013-09-06

Abstract

There is provided a sodium secondary battery, including: an anode containing sodium; a cathode containing a sodium salt, a first polymer, and a cathode active material allowing reversible intercalation/deintercalation of sodium ions; and a solid electrolyte provided between the anode and the cathode and having sodium ion conductivity, wherein the first polymer contained in the cathode is melted at an operating temperature of the sodium secondary battery and forms a complex together with the sodium salt.

Description

SODIUM SECONDARY BATTERY

The following disclosure relates to a sodium secondary battery, and in particular, to a sodium secondary battery having a low operating temperature, a small change in resistance, excellent reversibility and increased energy density, and stable charging and discharging characteristics.

In general, batteries may be classified into a first battery, which is disposable, and a secondary battery, which is rechargeable several times. Between these, the secondary battery has been popularized as a necessary energy source of portable electronic devices such as a notebook, a camcorder, and a cellular phone since the secondary battery can be used several times.

Recently, the shape and size of the secondary battery are changed depending on the use purposes ranging from a large capacity battery used for energy storage and a medium capacity battery applied in transport vehicles to a small capacity battery used as a power source for a portable device, and thus the use range thereof is expanding.

This secondary battery is constituted of an anode, a cathode, an electrolyte, and current collectors. A reduction reaction by electrons generated at the anode occurs at the cathode. The current collectors serve to supply the electrons, which are generated from the anode at the discharging time of the battery, to a cathode active material, or supply the electrons, which are supplied from the cathode at the charging time of the battery, to an anode active material.

As to a sodium secondary battery of the secondary battery, as shown in Korean Patent Laid-Open Publication No. 1996-0002926, molten metal sodium (Na) is used for an anode, and sulfur (S) is used for a cathode, and the anode and the cathode are separated from each other by a solid electrolyte tube made of alumina or ceramic having excellent selective permeability to sodium ions, so that the sodium ions can pass through the solid electrolyte tube.

The sodium secondary battery has excellent competitiveness in view of the price and quantity due to the use of sodium extracted from salt. In addition, the sodium secondary battery is cheap and light, and energy density thereof is two to four times larger than that of a battery using lithium ions.

For these reasons, the sodium secondary battery is cheaper and has the longer power conservation time than the existing secondary batteries, and thus, the sodium secondary battery is becoming the next generation storage medium capable of efficiently storing a large amount of power when it is utilized as a secondary battery for storing renewable energy such as solar light, wind power, or the like.

However, in the case of using molten sulfur as an anode, a high operating temperature of about 350℃ or higher is required. Furthermore, products therefrom interrupt addition reactions by sulfur, which may decrease utilizability of an active material, and cycle characteristics of battery may be degraded. Hence, studies on a cathode material for substituting sulfur therefor have been continued.

An embodiment of the present invention is directed to providing a sodium secondary battery having a low operating temperature, a small change in resistance, excellent reversibility and increased energy density, and stable charging and discharging characteristics, as compared with a sodium-sulfur battery of the prior art.

In one general aspect, there is provided a sodium secondary battery, including: an anode containing sodium; a cathode containing a sodium salt, a first polymer, and a cathode active material allowing reversible intercalation/deintercalation of sodium ions; and a solid electrolyte provided between the anode and the cathode and having sodium ion conductivity, wherein the first polymer contained in the cathode is melted at an operating temperature of the sodium secondary battery and forms a complex together with the sodium salt.

The cathode may further contain a second polymer of which a solid state is maintained at the operating temperature of the sodium secondary battery, and the second polymer may be obtained by in-situ cross-linking polymer precursors contained in the cathode, at the cathode, due to operation of the sodium secondary battery.

The first polymer that constitutes an amorphous phase in the operating state of the sodium secondary battery and forms a complex together with the sodium salt may include one or two or more selected from polyethylene oxide (PEO), polyphenylene oxide (PPO), polymethylene oxide (PMO), poly(ethylene glycol) diacrylate (PEGDA), and (poly(ethylene glycol)) (PEG).

In the sodium secondary battery according to one embodiment of the present invention, the cathode active material allowing reversible intercalation/deintercalation of sodium (sodium ions) may include sodium transition metal oxide, sodium transition metal phosphate, or a mixture thereof.

The polymer precursors cross-linked due to operation of the sodium secondary battery may contain one or two or more materials selected from divinyl benzene, acrylate, acrylamide, glutaraldehyde, benzoxazine, and poly(amic acid). The acrylamide may include methylene bis-acrylamide, N-isopropylacrylamide, or a mixture thereof, and the acrylate may include mono-acrylate, bisacrylate, triacrylate, or a mixture thereof.

The sodium salt forming a complex together with the first polymer may include one or two or more selected from salts defined by Formula 1 below:

(Formula 1)

NaX

(where, X is I, ClO₄, PF₆, BF₄, CF₃SO₃, or N(CF₃SO₂)₂)

A molar ratio of the sodium salt and a repeating unit of the first polymer, which form a complex, may be 1:2 to 50.

Contents of the sodium salt and the first polymer contained in the cathode may be 5 to 100 parts by weight based on 100 parts by weight of a cathode active material.

In the case where the cathode contains a second polymer, the cathode may contain 1 to 50 parts by weight of the polymer precursors based on 100 parts by weight of the cathode active material.

The cathode may further contain inorganic particles, as well as the sodium salt, the first polymer, and the cathode active material, and may further contain inorganic particles, as well as the sodium salt, the first polymer, the cathode active material, and the second polymer.

The inorganic particle contained in the cathode may be one or two or more selected from Al₂O₃, TiO₂, SiO₂, BaTiO₃ and PbTiO₃, and the cathode may contain 0.1 to 40 parts by weight of the inorganic particles based on 100 parts by weight of the cathode active material.

The sodium secondary battery according to the present invention has a small change in resistance and excellent reversibility at the time adsorption/desorption of sodium ions due to a sodium complex oxide based cathode active material where intercalation/deintercalation of sodium ions occur; has very fast charging/discharging rates by containing the first polymer that forms an amorphous phase in the operating state of the battery and the sodium salt, together with the cathode active material in the cathode; and maintains stable characteristics by maintaining the state where the cathode active material and the first polymer are contacted with each other with a wide specific surface area. Further, as the polymer precursors are cross-linked with each other in the cathode to form the second polymer during operation of the secondary battery, strength of the cathode can be enhanced even without preventing contact between the first polymer of which a phase change between amorphous and crystalline phases occurs and the cathode active material, and generation of stress due to the phase change of the first polymer can be fundamentally prevented.

FIG. 1 shows test results of charging and discharging characteristics of a sodium secondary battery manufactured according to one embodiment of the present invention; and

FIG. 2 shows test results of charging and discharging cycle characteristics of the sodium secondary battery manufactured according to one embodiment of the present invention.

Hereinafter, a sodium secondary battery of the present invention will be described in detail with reference to the accompanying drawings. The drawings exemplified below are provided by way of examples so that the spirit of the present invention can be sufficiently transferred to those skilled in the art to which the present invention pertains. Therefore, the prevent invention is not limited to the drawings set forth below, and may be embodied in different forms, and the drawings set forth below may be exaggerated in order to clarify the spirit of the present invention. Also, like reference numerals denote like elements throughout the specification.

Here, unless indicated otherwise, the terms used in the specification including technical and scientific terms have the same meaning as those that are usually understood by those who skilled in the art to which the present invention pertains, and detailed description of the known functions and constitutions that may obscure the gist of the present invention will be omitted.

A sodium secondary battery according to the present invention includes an anode containing sodium, a cathode active material, a cathode containing a first polymer and a sodium salt, and a solid electrolyte provided between the anode and the cathode and having sodium ion conductivity.

In the sodium secondary battery according to the present invention, the cathode contains a first polymer forming an amorphous phase at an operating temperature of the sodium secondary battery and a sodium salt forming a complex together with the first polymer, as well as a cathode active material for allowing reversible intercalation/deintercalation of sodium ions.

That is to say, the first polymer contained in the cathode is melted in an amorphous phase at the operating temperature of the sodium secondary battery to form a complex together with the sodium salt.

In the sodium secondary battery according to an embodiment of the present invention, the first polymer contained in the cathode serves to conduct the sodium ions due to segmental movement of a polymer chain while a heteroatom such as oxygen located on a main chain of the polymer interacts with a sodium ion.

This property of the polymer of forming a complex together with sodium ions and the ion conductivity of the polymer due to segmental movement are rarely expressed in a crystalline phase. For example, crystalline phase polyethylene oxide has ion conductivity of only 10^-8S/cm or less.

The sodium secondary battery according to the present invention contains a first polymer that forms a complex together with the sodium salt contained in the cathode, and constitutes an amorphous molten phase at an operating temperature of the sodium secondary battery, and thus, ion conductivity of sodium ions due to segmental movement of the polymer can be significantly improved. For example, the operating temperature of the sodium secondary battery may be 80 to 350℃, substantially, 98 to 250℃, and more substantially, 98 to 200℃.

As to the sodium secondary battery according to an embodiment of the present invention, in the operating state of the sodium secondary battery, including the molten sodium for the anode, which is warmed to the operating temperature at which at the least sodium contained in the anode is melted and then operated, ion conductivity of sodium ions due to segmental movement of the first polymer can be significantly improved by containing the first polymer for forming an amorphous phase and the sodium salt in the cathode. For example, polyethylene oxide exhibits sodium ion conductivity of 10^-3S/cm at a temperature of 97.5℃, a melting point of sodium.

The sodium salt contained together with the first polymer in the cathode forms a complex together with the first polymer melted in the operating state of the sodium secondary battery. As such, the cathode contains the complex between the amorphous phase first polymer and the sodium salt, thereby securing a movement route of sodium ions having a sufficient amount such that intercalation/deintercalation of sodium ions can be promoted.

As described above, the cathode of the sodium secondary battery contains a cathode active material for allowing intercalation/deintercalation of sodium ions, and the first polymer of forming an amorphous phase in the operating state of the sodium secondary battery and the sodium salt, and thus, sodium ions inflowing through the solid electrolyte are conducted at a very fast rate, thereby enabling stable and very fast charging and discharging.

As the sodium secondary battery is changed from the operating state to a non-operating state, that is, from a state at which the sodium secondary battery is warmed to the operating temperature to a state at which the sodium secondary battery is cooled to below the melting point of sodium, the first polymer contained in the cathode is phase-changed into a solid phase to thereby serve as a binder that binds the cathode active material contained in the cathode to a current collector. As the sodium secondary battery is changed from the non-operating state to the operating state, the first polymer serving as a binder serves as an electrolyte that conducts the sodium ions.

That is, as the operating state of the sodium secondary battery is changed, the first polymer contained in the cathode alternately serves as a binder that binds the cathode active material to the current collector and as an electrolyte that conducts the sodium ions.

Therefore, even though charging and discharging of the battery are repeated and change in the operating state is repeated for a long time, deterioration in agglomeration of the cathode active material or desorption of the cathode active material from the current collector can be prevented, and a state where the cathode active material and the first polymer are contacted with each other with a wide specific surface area can be maintained.

In the sodium secondary battery according to an embodiment of the present invention, as the cathode active material contained in the cathode, any material that allows reversible intercalation/deintercalation of sodium ions may be used. Preferably, sodium transition metal oxide, sodium transition metal phosphate, or a mixture thereof may be included in order to perform easy adsorption/desorption of sodium ions to achieve a less change in resistance and excellent reversibility, and increase energy density.

In the sodium secondary battery according to an embodiment of the present invention, as the first polymer contained in the cathode in order to form an amorphous phase at the operating temperature of the secondary battery, form a complex together with the sodium salt, and effectively conduct the sodium ions by segmental movement, one or two or more selected from polyethylene oxide (PEO), polyphenylene oxide (PPO), polymethylene oxide (PMO), poly(ethylene glycol) diacrylate (PEGDA), and (poly(ethylene glycol)) (PEG) may be preferable. The first polymer has preferably a number average molecular weight of 100,000 to 1,000,000.

In the sodium secondary battery according to an embodiment of the present invention, as the sodium salt that forms a complex together with the first polymer forming an amorphous phase at an operating temperature of the secondary battery to thereby improve conductivity of sodium ions, one or two or more selected from salts defined by Formula 1 below may be preferable.

[Formula 1]

NaX

In Formula 1, X is I, ClO₄, PF₆, BF₄, CF₃SO₃, or N(CF₃SO₂)₂.

In the sodium secondary battery according to an embodiment of the present invention, a molar ratio of the sodium salt and a repeating unit of the first polymer contained in the cathode may be 1 (sodium salt): 2 to 50 (repeating unit of the first polymer). The reason is that ion conductivity of sodium ions is proportional to a concentration of the sodium salt, but the concentration thereof is too high, the sodium ions are coupled or associated with counter negative ions so that ion conductivity is rather reduced.

In the sodium secondary battery according to an embodiment of the present invention, contents of the sodium salt and the first polymer contained in the cathode may be 5 to 100 parts by weight based on 100 parts by weight of a cathode active material. The reason is that the first polymer forming a complex together with the sodium salt is uniformly adsorbed onto a surface of the cathode active material even without reducing density of the cathode active material and electric conductivity of the cathode, to thereby fix particles in a physically stable manner.

In the sodium secondary battery according to an embodiment of the present invention, the cathode may further contain a second polymer, of which a solid phase is maintained at the operating temperature of the sodium secondary battery, together with the sodium salt, the first polymer, and the cathode active material.

The second polymer contained in the cathode may serve to prevent deterioration in physical strength of the cathode due to the first polymer that constitutes an amorphous phase at the operating time of the secondary battery, and fix particles of the cathode active material in a physically stable manner.

In the sodium secondary battery according to an embodiment of the present invention, the second polymer, of which a solid phase is maintained at the operating time of the battery, may be obtained by in-situ cross-linking polymer precursors contained in the cathode at the cathode due to the operation of the sodium secondary battery.

That is, as the cathode fabricated by coating the polymer precursors together with the sodium salt, the first polymer, and the cathode active material on the current collector is warmed to the operating temperature for operating the secondary battery, and then operated, the polymer precursors are cross-linked at the cathode of the secondary battery under the operation, thereby forming the second polymer.

As described above, in the sodium secondary battery according to an embodiment of the present invention, as the polymer precursors polymerize to form the second polymer in the cathode of the secondary battery under the operation due to operating of the sodium secondary battery, the cathode active material is stably fixed even without preventing contact between the first polymer that is phase-changed to an amorphous phase and the cathode active material, and thus, strength of the cathode can be enhanced.

Further, depending on the operating state, volume change due to phase change of the first polymer, which results from a phase change between the amorphous phase and the crystalline phase of the first polymer, may cause physical stress at the cathode. Here, the second polymer is in-situ formed in the cathode under the operation, and thus, generation of stress due to the phase change of the first polymer can be prevented. Therefore, lifespan and stability of the sodium secondary battery can be significantly improved.

In the sodium secondary battery according to an embodiment of the present invention, in order to in-situ form the second polymer, of which a solid phase is maintained at the operating time of the secondary battery, in the cathode of the secondary battery, examples of the polymer precursor may include one or two or more materials selected from divinyl benzene, acrylate, acrylamide, glutaraldehyde, benzoxazine, and poly(amic acid). Here, examples of acrylamide may include methylene bis-acrylamide, N-isopropylacrylamide, or a mixture thereof, and examples of acrylate may include mono-acrylate, bisacrylate, triacrylate, or a mixture thereof.

In the sodium secondary battery according to an embodiment of the present invention, the cathode preferably contains 1 to 50 parts by weight of polymer precursors based on 100 parts by weight of a cathode active material. This content enables physical strength of the cathode to be enhanced by the second polymer while preventing reduction in density of the cathode active material and electric conductivity of the cathode.

In the sodium secondary battery according to an embodiment of the present invention, the cathode may further contain an inorganic particle, together with the sodium salt, the first polymer, and the cathode active material, or the sodium salt, the first polymer, the cathode active material, and the second polymer obtained by cross-linking the polymer precursors (polymer precursors before the secondary battery is warmed to the initial operating temperature).

The inorganic particle contained in the cathode can improve conductivity of sodium ions due to a complex of the first polymer and the sodium salt, and enhance mechanical strength of the cathode.

In the case where the cathode further contains the inorganic particle, together with the sodium salt, the first polymer, the cathode active material, and the second polymer obtained by cross-linking of the polymer precursors, binding between a cathode current collector and a cathode active material, between the cathode active material and the cathode active material, and between the cathode active material and the inorganic particle occurs by the second polymer formed by cross-linking the polymer precursors in the cathode under the operation. Therefore, large-area contacts between the cathode active material and the amorphous first polymer and between the inorganic particle and the amorphous first polymer are possible, and thus, conductivity due to the inorganic particle can be maximally improved and mechanical strength due to the inorganic particle can be enhanced.

In the sodium secondary battery according to an embodiment of the present invention, the inorganic particle may be one or two or more materials selected from Al₂O₃, TiO₂, SiO₂, BaTiO₃, and PbTiO₃. The inorganic particle may have an average particle size (diameter) of 0.1μm to 10μm.

In the sodium secondary battery according to an embodiment of the present invention, the cathode may contain 0.1 to 40 parts by weight of inorganic particles based on 100 parts by weight of the cathode active material in order to improve mechanical strength and conductivity and maintain high density of the cathode active material.

In the sodium secondary battery according to an embodiment of the present invention, any material that can selective conductivity to the sodium ions may be used as the solid electrolyte provided between the cathode and the anode, and, for selective conductivity to the sodium ions, examples of the solid electrolyte may include a solid electrolyte commonly used in a battery field. Examples of the solid electrolyte may include a Na super ionic conductor (NaSICON), β-alumina, β˝-alumina, or a laminate thereof.

The sodium secondary battery according to an embodiment of the present invention may include metal sodium of forming a molten phase at the operating time of the battery for the cathode, and examples of the sodium secondary battery may include a secondary battery including a cathode electrolytic liquid separated from a cathode by a solid electrolyte and impregnating the cathode therewith.

Examples of the sodium secondary battery according to an embodiment of the present invention may include an all solid-state secondary battery where solid phases of a cathode and an electrolyte are all maintained at the operating time of the battery.

FIGS. 1 and 2 are graphs showing the measurement results of electrical properties of a sodium secondary battery according to the present invention. Specifically, the sodium secondary battery was manufactured by including a cathode having a current collector (Al) on which a cathode active material layer is formed, a sodium ion conductive solid electrolyte of NaSICON, and an anode of metal sodium. Here, the cathode active material layer contained Na_3.32Fe_2.34(P₂O₇)₂ as a cathode active material, NaBF₄ as sodium salt, and polyethylene oxide (PEO) as a first polymer while the mole ratio of NaBF₄ : ethylene oxide (repeating unit) was 1:3 and the weight of NaBF₄ and polyethylene oxide per 100g of Na_3.32Fe_2.34(P₂O₇)₂ was 68g. The test was conducted at a temperature of 120℃ under the conditions of a charge/discharge voltage of 1.7~3.4V and constant current. As a result of the test, as shown in FIG. 1, at the beginning, the discharge flat voltage was 2.7V at C/5, but after 20 cycles, the discharge flat voltage increased to 2.9V. In addition, it may be seen from FIG. 2 that charging and discharging cycle characteristics were stable and excellent without the reduction in capacitance, and thus the amorphous phase change occurs at the operating temperature in the first polymer to thereby facilitating the movement of sodium positive ions, which can induce a smooth and fast reaction with the active material in the cathode.

Claims

A sodium secondary battery, comprising:

an anode containing sodium;

a cathode containing a sodium salt, a first polymer, and a cathode active material allowing reversible intercalation/deintercalation of sodium ions; and

a solid electrolyte provided between the anode and the cathode and having sodium ion conductivity,

wherein the first polymer contained in the cathode is melted at an operating temperature of the sodium secondary battery and forms a complex together with the sodium salt.
The sodium secondary battery of claim 1, wherein the cathode further contains a second polymer of which a solid state is maintained at the operating temperature of the sodium secondary battery.
The sodium secondary battery of claim 2, wherein the second polymer is obtained by in-situ cross-linking polymer precursors contained in the cathode, at the cathode, due to operation of the sodium secondary battery.
The sodium secondary battery of claim 1, wherein the first polymer is one or two or more selected from polyethylene oxide (PEO), polyphenylene oxide (PPO), polymethylene oxide (PMO), poly(ethylene glycol) diacrylate (PEGDA), and (poly(ethylene glycol)) (PEG).
The sodium secondary battery of claim 1, wherein the cathode active material is sodium transition metal oxide, sodium transition metal phosphate, or a mixture thereof.
The sodium secondary battery of claim 1, wherein the polymer precursors contain one or two or more materials selected from divinyl benzene, acrylate, acrylamide, glutaraldehyde, benzoxazine, and poly(amic acid).
The sodium secondary battery of claim 4, wherein the sodium salt is one or two or more selected from salts defined by Formula 1 below:

(Formula 1)

NaX

(where, X is I, ClO₄, PF₆, BF₄, CF₃SO₃, or N(CF₃SO₂)₂).
The sodium secondary battery of any one of claims 1 to 7, wherein a molar ratio of the sodium salt : a repeating unit of the first polymer is 1: 2 to 50.
The sodium secondary battery of claim 8, wherein the contents of the sodium salt and the first polymer contained in the cathode is 5 to 100 parts by weight based on 100 parts by weight of a cathode active material.
The sodium secondary battery of any one of claims 2 to 7, wherein the cathode contains 1 to 50 parts by weight of the polymer precursors based on 100 parts by weight of the cathode active material.
The sodium secondary battery of any one of claims 1 to 7, wherein the cathode further contains inorganic particles.
The sodium secondary battery of claim 11, wherein the cathode contains 0.1 to 40 parts by weight of the inorganic particles based on 100 parts by weight of the cathode active material.
The sodium secondary battery of claim 11, wherein the inorganic particle is one or two or more selected from Al₂O₃, TiO₂, SiO₂, BaTiO₃ and PbTiO₃.