JP2000040513A - Nonaqueous electrolyte secondary battery and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low cost nonaqueous electrolyte secondary battery that has a high cycle characteristic and contributes to resource savings. SOLUTION: This nonaqueous electrolyte secondary battery uses a cubic garnet crystal expressed by a formula Na3M2Fe3O12 (M is an element capable of taking a form of the oxidation number + 6) as a precursor, and also uses a lithium containing composite oxide expressed by a formula (Na1-xLix)yM2Fe3 O12 (0.3<=x<=1.0, 2.5<=y<=3.0) made by replacing a part or all of Na of the precursor with Li as its positive electrode active material or negative electrode active material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly, to a non-aqueous electrolyte secondary battery using a novel electrode active material.

[0002]

2. Description of the Related Art Various materials have been put to practical use as electrode active materials for non-aqueous electrolyte secondary batteries. Examples of the positive electrode active material include metal chalcogenides such as TiS ₂ , MoS ₂ and NbSe ₃ , oxides such as MnO ₂ , MoO ₃ and V ₂ O _5, and lithium-containing materials such as LiCoO ₂ , LiNiO ₂ and LiMn ₂ O _4. There are metal oxides and the like.

Examples of the negative electrode active material include lithium metal, alloys such as Al capable of storing and releasing lithium, and carbon-based materials that store lithium ions between layers.

[0004]

The performance required for the electrode active material of the non-aqueous electrolyte secondary battery is cyclability. It is desired that this performance has little deterioration due to overcharge / overdischarge and that there is little change in capacity and potential even after repeated charge and discharge. It is known that the cyclability greatly depends on the crystal structure of the electrode active material. In order to obtain good cyclability, whether the crystal structure does not change due to charge and discharge, or if the change has little effect on cyclability,
Alternatively, a substance is required in which the crystal structure destroyed by the first charge is maintained in the same structure thereafter. In the conventional electrode active material, the above-mentioned positive electrode active material is overcharged /
The crystal structure is easily broken by overdischarge. In addition, alloy-based and carbon-based materials have the same problem as the negative electrode active material.

On the other hand, in addition to the demand for non-aqueous electrolyte secondary batteries to improve the performance of the batteries themselves such as cycle characteristics, the use of materials abundant in nature contributes to resource saving, Attempts have also been made to reduce the cost of the battery itself.

[0006] The inventor first paid attention to a cubic garnet crystal known to have a stable crystal structure. In addition, iron (F
e) was selected. As a result of examining various substances based on these two requirements, the following general formula (Na _1-x Li _x ) _y
M ₂ Fe ₃ O ₁₂ (provided that 0.3 ≦ x ≦ 1.0, ₂ .
It has been found that the novel electrode active material represented by (5 ≦ y ≦ 3.0) can be applied to a nonaqueous electrolyte secondary battery.

The present invention has been made based on this finding, and it is an object of the present invention to provide a non-aqueous electrolyte secondary battery which has high cyclability, contributes to resource saving, and can achieve low cost.

[0008]

The non-aqueous electrolyte secondary battery according to the first invention has a general formula of Na ₃ M ₂ Fe ₃ O ₁₂ (M
Is a cubic garnet crystal represented by the formula (Na _1-x Li) in which a part or all of Na of the precursor is replaced with Li by using a cubic garnet crystal represented by the following formula: _x ) _{yM 2} Fe ₃ O ₁₂ (0.3 ≦ x
≦ 1.0, 2.5 ≦ y ≦ 3.0) are used as the positive electrode active material or the negative electrode active material.

A second invention is a non-aqueous electrolyte secondary battery in which M is Te. In the first or second invention, a non-aqueous electrolyte secondary battery in which the value of x is 1 is defined as a third invention.

A fourth invention is a method for manufacturing a non-aqueous electrolyte secondary battery according to the second invention, wherein Na ₃ Te ₂ Fe ₃
The O ₁₂ cubic garnet crystal represented was further comprising the step of generating a lithium composite oxide represented by the ion exchange method in _{(Na 1-x Li x)} y Te 2 Fe 3 O 12. According to a fifth invention, Na ₃ T is used as the ion exchange method.
A method for producing a non-aqueous electrolyte secondary battery including a step of melting and reacting a mixture of e ₂ Fe ₃ O ₁₂ and LiNO ₃ .

In a sixth aspect of the present invention, the Na ₃ M ₂ Fe ₃ O is used.
_{For the} precursor Na and Fe raw materials represented by ₁₂ ,
a A non-aqueous electrolyte secondary battery manufacturing method using Na and Fe of aFeO ₂ . In addition, the precursor Na ₃ Te
_{As a} seventh invention, a method for producing a non-aqueous electrolyte secondary battery including a step of sintering ₂ Fe ₃ O ₁₂ by subjecting a mixture of NaFeO ₂ and TeO ₂ to heat treatment at 670 ° C. or more and 750 ° C. or less as a seventh invention I have.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION === Evaluation Battery === An evaluation battery was manufactured to evaluate the characteristics of the nonaqueous electrolyte secondary battery according to the present invention. FIG. 1 shows an embodiment of the evaluation battery in a side sectional view. The battery for evaluation 1 is a battery case 10 made of a stainless steel sheet having resistance to organic electrolyte.
And a sealing plate 11 made of the same material and sealed with a polypropylene gasket 12. Each member constituting the battery is housed inside the battery 1. Specifically, the constituent members include a current collector 13 formed of a nickel mesh or the like formed on the bottom of the battery case 10, a positive electrode mixture 14 formed on the current collector 13 by pressing, and Metal lithium 15 which is laminated on the electrode mixture 14 and serves as a negative electrode material, and a microporous separator 16 made of polypropylene for insulating both electrodes are included. Thus, the sealing plate 11 placed on the metallic lithium 15 becomes the negative electrode side, and the battery case 10 becomes the positive electrode side. Further, a 1 N solution of LiPF ₆ dissolved in an equal volume mixed solvent of ethylene carbonate and diethyl carbonate is sealed as an electrolytic solution in the battery container. The external dimensions of the evaluation battery 1 are 24 mm in diameter and 5 mm in height.

The electrode mixture 14 is an important requirement of the nonaqueous electrolyte secondary battery of the present invention, and has the general formula (Na _1-x Li _x )
_y M ₂ Fe ₃ O ₁₂ (M is an element capable of taking an oxidation number + 6 state, 0.3 ≦ x ≦ 1.0, 2.5 ≦ y ≦ 3.
The lithium-containing composite oxide represented by 0) is used as an electrode active material. In the present embodiment, this electrode active material is 76 wt.
%, Acetylene black 20 wt% as a conductive material, and polytetrafluoroethylene 4 wt% as a binder are mixed to form an electrode mixture. Then, after the electrode mixture 14 is pressure-formed on the current collector 13, the electrode mixture 14 is pressed by a vacuum dryer.
It dried under reduced pressure of 150C for 6 hours at a pressure of ^10-3 atm to obtain a positive electrode.

The above-mentioned electrode active material has a potential of about 2.5 V, and can be used for both a positive electrode and a negative electrode depending on the electrode active material on the counter electrode side to be combined. When used for the positive electrode, lithium metal or the like is used for the negative electrode as in the case of the battery for evaluation. At this time, the battery operates as a battery having an average voltage of about 2.5 V. On the other hand, when the above-mentioned electrode active material is used for the negative electrode, LiCoO ₂ or LiNiO _2
A transition metal oxide containing lithium such as ₂ is used. In this case, it operates as a battery having an average voltage of about 1.5 V.

The battery for evaluation in the present embodiment uses this electrode active material for the positive electrode, and uses the metal lithium alone as the negative electrode, so that the characteristics of only the electrode active material concerned can be accurately evaluated. . Next, the electrode active material will be described.

=== Electrode active material === (Na _1-x Li _x ) _y M ₂ Fe ₃ O ₁₂ (M is an element capable of taking an oxidation number + 6, 0 <x ≦ 1.0,
The electrode active material of the lithium-containing composite oxide represented by 2.5 ≦ y ≦ 3.0) is obtained by synthesizing a precursor having a cubic garnet-type structure represented by Na _y M ₂ Fe ₃ O ₁₂ and then adding sodium. It is obtained by exchanging (Na) ions for lithium (Li) ions. In this embodiment, tellurium (Te) is adopted as M, and (Na _1-x Li _x ) ₃ Te
₂ Fe ₃ O ₁₂ was used as the electrode active material in the electrode mixture. This electrode active material is specifically produced through the following production process.

Production of Precursor Having Cubic Garnet Structure First, Na ₃ Te ₂ Fe ₃ O ₁₂ having a cubic garnet structure was produced as a precursor in order to obtain a target lithium-containing composite oxide serving as an electrode active material. I do. for that reason,
Na ₂ CO ₃ , Fe ₂ O ₃ , and TeO ₂ were dry-mixed with Na ₂ CO ₃ : Fe ₂ O ₃ : TeO ₂ = 3: 3: 4 (molar ratio), respectively, and the mixture was subjected to isostatic pressing to 1000 k.
Pelletize by applying a pressure of g / cm ² . When this pellet is fired at 750 ° C. for 20 hours in an air atmosphere,
₃ Te ₂ Fe ₃ O ₁₂ is obtained.

(Na_1-xLi_x)₃Te₂Fe₃O
₁₂Exchanging the precursor obtained by the method of
With (Na_1-xLi_x)₃Te₂Fe₃O₁₂Get.
The method of ion exchange is as follows:
How to treat the precursor and lithium chloride hexanol solution
A method of treating a precursor in a well-known method is well known. Book
In the embodiment, the former method can perform ion exchange more efficiently.
The law is adopted. Specifically, generated by the method of
The obtained precursor is ground to about 20 μm or less by a ball mill.
Crush / classify. For 1 mol of this powdery precursor, 10
Molar amount of LiNO₃Mix. Put this mixture in platinum
Melting reaction is caused by heating under air atmosphere using
The desired electrode active material (Na_1-xLi_x)₃Te₂F
e₃O ₁₂Get. In this embodiment, in this melting reaction,
Ion exchange by changing the heat treatment conditions (temperature / time)
The switching rate x is controlled. In this embodiment, the electrode active material
Types of evaluation batteries having different values of x in
(Batteries A to D) were prepared and the characteristics were compared.

=== Ion Exchange Conditions for Evaluation Cells A to D === <Battery A> A mixture of a precursor and LiNO ₃ was heated at 400 ° C./1.
Melting reaction was performed for 2 hours to perform ion exchange. As a result of analyzing the composition after ion exchange by ICP emission spectroscopy, it was found that all of the sodium ions in the precursor were exchanged for lithium ions, resulting in Li ₃ Te ₂ Fe ₃ O ₁₂ (x = 1). all right. This lithium-containing composite oxide was applied to an electrode mixture as an electrode active material, to produce a battery A.

<Battery B> The ion exchange treatment conditions were set to 30
0 ° C./6 hours. As a result, (Na _0.7 Li
An electrode active material having a composition of _0.3 ) ₃ Te ₂ Fe ₃ O ₁₂ (x = 0.3) was obtained. Battery B was produced using this electrode active material.

<Battery C> The ion exchange treatment conditions were set to 30
0 ° C./2 hours. As a result, (Na _0.8 Li
_0.2 ) ₃ Te ₂ Fe ₃ O ₁₂ (x = 0.2) was obtained.
Battery C was produced using the electrode active material having this composition.

[0022] As <Battery D> comparative, without ion exchange precursor _Na 3 were classified _{Te 2 Fe 3 O 1 2 (} x =
0) was applied to an electrode mixture as it was, to obtain a battery D. Table 1
Shows the ion exchange processing conditions and the like in the batteries A to D.

[0023]

[Table 1]

=== Characteristic Comparison Results === After aging the batteries A to E manufactured under the above conditions at room temperature for one week, the current was 0.2 mA / cm ² , the charging voltage was 4.3 V, and the discharging was 4.3 V. The charging / discharging test was performed by setting the voltage throughout the test to 2.0 V.

FIG. 2 shows the charge capacity when charging and discharging under the above conditions are repeated. The horizontal axis of the graph is the number of times of charging and discharging, and the vertical axis is the capacity. Batteries A and B have sufficient capacity as batteries, and their capacities are hardly deteriorated even after charging and discharging 20 times. Also, battery C,
It can be seen that D does not function as a battery. Therefore, it was confirmed that the battery operates effectively when x ≧ 0.3. FIG. 3 shows the capacity-potential characteristics at the time of charging and discharging of the battery A for reference.

=== Modification Example of Precursor Generation Method === In the above embodiment, in order to generate precursor Na ₃ Te ₂ Fe ₃ O ₁₂ , Na ₂ CO ₃ , Fe ₂ O ₃ , and TeO _{2 are used.} These three substances are used as starting materials, but one substance, NaFeO ₂ , can also be used for Na and Fe. Thereby, the storage space for the raw material can be reduced. In addition, a phase other than the desired crystal phase (cubic garnet structure) was slightly formed in the formation by the above three substances, and about 5% of a different phase component was mixed in the measurement by X-ray diffraction.

Therefore, Na ₃ T is used by using NaFeO _3.
e ₂ Fe ₃ O ₁₂ was produced, and the production rate of a precursor having a desired structure was examined. The actual production process involves NaFeO
₃ : Dry mixing with TeO ₂ = 3: 2 (molar ratio), and the mixture is pelletized by applying a pressure of 1000 kg / cm ^{2 with} a hydrostatic press. This pellet is heat-treated in an air atmosphere to produce Na ₃ Te ₂ Fe ₃ O ₁₂ .
In addition to TeO ₂ , TeO ₃ , Te carbonate and the like can be considered as Te raw materials.

In the above process, the yield of Na ₃ Te ₂ Fe ₃ O ₁₂ having a cubic garnet structure when the heat treatment conditions (temperature / time) are changed is shown in Table 2 below.

[0029]

[Table 2] In the heat treatment under the condition 3 in the above table, the sample after firing was in a semi-molten state, and Na ₄ TeO ₅ was a main component.

=== Others === Electrode active material (Na _1-x Li _x ) _{y In} M ₂ Fe ₃ O ₁₂ , the value of y is the time when the battery was manufactured, and is the value before charging and discharging. is there. The garnet crystal, which is a precursor, has y =
However, when the electrode active material is formed by ion exchange, a defect or the like occurs in the crystal structure, and actually 2.5 ≦ y ≦ 3.0.
Is known. In addition, when operating as a battery, 2.
5 ≦ y ≦ 4.

As for M, in addition to Te, molybdenum M
o (molybdenum) and W (tungsten). T
Since the nonaqueous electrolyte secondary battery using the electrode active material containing e has operated effectively, the lithium-containing composite oxide with M as Mo or W is used as the electrode active material for the nonaqueous electrolyte secondary battery. You can easily imagine what you can do.

[0032]

EFFECT OF THE INVENTION The general formula (Na _1-x Li _x ) _y M ₂ Fe
_The electrode active material represented by ₃ O ₁₂ is generated using a garnet crystal having a stable crystal structure as a precursor. Therefore, a non-aqueous electrolyte secondary battery using this electrode active material for a positive electrode or a negative electrode can obtain excellent cycle characteristics.

Further, iron which is abundant in nature is used as a main component. Therefore, it is possible to contribute to resource saving and achieve a reduction in cost of the battery.

By using Te as M, a non-aqueous electrolyte secondary battery that operates effectively can be obtained. Furthermore, x
By setting = 1, a high-performance non-aqueous electrolyte secondary battery with further improved characteristics such as capacity and cycleability can be achieved.

The electrode active material can be obtained by a simple treatment in which the precursor is melted and reacted with lithium nitrate. Therefore, the non-aqueous electrolyte 2 of the present invention can be produced without increasing the cost.
A secondary battery can be manufactured.

Further, by using NaFeO ₂ as a raw material of Na and Fe contained in the precursor, it is not necessary to separately prepare a raw material containing Na and Fe. Therefore, a series of processes in which the raw materials are uniformly mixed at a predetermined ratio and the generation process is performed becomes simpler than when individual raw materials are prepared. Therefore, productivity is improved. There is also an effect that the storage space for raw materials can be reduced.

The precursor Na ₃ Te ₂ Fe ₃ O _{12 is} prepared by a simple process of heat-treating a mixture of NaFeO ₂ and TeO _2.
Can be easily obtained, and by setting the condition of the heat treatment to 650 ° C. or more and 750 ° C., a precursor having a desired crystal phase can be obtained at 10 ° C.
It can be obtained at a production rate of 0%, and can prevent the performance of the battery from deteriorating due to contamination by impurities. In addition, since there is no need to separate impurities, and the efficiency of use of raw materials is improved, manufacturing costs can be reduced.

[Brief description of the drawings]

FIG. 1 is a side sectional view of an evaluation battery for evaluating characteristics of a nonaqueous electrolyte secondary battery of the present invention.

FIG. 2 is a graph showing the relationship between the number of times of charge / discharge and the charge / discharge capacity of the battery for evaluation.

FIG. 3

FIG. 3 is a capacity-potential characteristic diagram of an evaluation battery A.

[Explanation of symbols]

 1 evaluation battery 14 electrode mixture 15 metallic lithium

Claims (7)

[Claims] 1. A compound of the general formula Na₃M₂Fe₃O₁₂(M is an acid
Element that can assume the state of chemical number +6)
Using a cubic garnet crystal as a precursor, Na of the precursor
A general formula (Na) obtained by partially or entirely substituting Li
_1-xLi_x) _yM₂Fe₃O₁₂(0.3 ≦ x ≦ 1.
0, 2.5 ≦ y ≦ 3.0)
Use of oxides as positive or negative electrode active materials
And a non-aqueous electrolyte secondary battery. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein M is Te. 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the value of x is 1. 4. The method for producing a nonaqueous electrolyte secondary battery according to claim 2, wherein the cubic garnet crystal represented by Na ₃ Te ₂ Fe ₃ O ₁₂ is treated by an ion exchange method. A method for producing a non-aqueous electrolyte secondary battery, comprising a step of producing a lithium composite oxide represented by Na _1-x Li _x ) _y Te ₂ Fe ₃ O ₁₂ . 5. The method for producing a non-aqueous electrolyte secondary battery according to claim 4, further comprising a step of melting and reacting a mixture of Na ₃ Te ₂ Fe ₃ O ₁₂ and LiNO ₃ as the ion exchange method. Method. 6. The non-aqueous electrolyte 2 according to claim 1 or 2.
A method for manufacturing a secondary battery, comprising a step of using Na and Fe contained in NaFeO ₂ as raw materials of Na and Fe contained in the precursor to generate the precursor, wherein the non-aqueous electrolyte 2 Manufacturing method of secondary battery. 7. The method for producing a non-aqueous electrolyte secondary battery according to claim 2, wherein the precursor Na ₃ Te ₂ Fe ₃ O is used.
_{12. A} method for producing a non-aqueous electrolyte secondary battery, comprising a step of sintering the mixture of No. ₁₂ and NaFeO ₂ and TeO ₂ by heat treatment at 670 ° C. or more and 750 ° C. or less.