JP2019091528A - Electrode active material for sodium ion secondary battery and method of producing the same - Google Patents

Abstract

To provide an electrode active material for a sodium ion secondary battery, high in capacity and excellent in cycle characteristics.SOLUTION: An electrode active material for a sodium ion secondary battery contains a sodium-containing complex oxide having an O3-type layered rock-salt structure and represented by general formula (1): x[NaM1O]-(1-x)[M2O] (where, 0.5≤x≤0.9, -0.1≤α≤0.1 and -0.1≤β≤0.1) or a sodium-containing complex oxide having a cationic irregular sequence-type rock-salt structure and represented by general formula (2): y[NaM2O]-(1-y)[NaM1O] (where, 0.1≤y≤0.4, -0.1≤δ≤0.1 and -0.1≤γ≤0.1), or general formula (3): NaM1O(-0.1≤ε≤0.1), M1 containing Mn and M2 containing Ti.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an electrode active material for a sodium ion secondary battery including a sodium-containing composite oxide, and a method for producing the same.

In place of lithium ion secondary batteries, development of sodium ion secondary batteries using abundant resources and inexpensive sodium as a main raw material is underway. As an electrode active material for a sodium ion secondary battery, research on sodium-containing composite oxides such as NaCrO ₂ is active, but a sufficiently high capacity can not be obtained.

  On the other hand, research on sodium-containing composite oxides containing abundant resources of Mn or Ti as main elements is also active, but there is room for improvement in terms of capacity or cycle characteristics (Non-Patent Documents 1 and 2). Furthermore, attempts have also been made to achieve high capacity using solid phase redox reactions of oxide ions (Non-patent Documents 3 and 4).

Journal of the Electrochemical Society, 158, A1307-A1312 (2011) Chemical Communications, 51, 8480-8483 (2015) Nature Materials, 12, 228 (2012) Electrochemistry Communications, 53, 29 (2015)

  One of the objects of the present invention is to provide a novel electrode active material for a sodium ion secondary battery having high capacity and excellent cycle characteristics.

One aspect of the present invention has an O3 type layered rock salt structure, and has a general formula (1): x [NaM1O2 _-α ]-(1-x) [M2O2 _-β ] (where 0.5 ≦ x ≦ 0.9, −0.1 ≦ α ≦ 0.1 and −0.1 ≦ β ≦ 0.1), M 1 contains Mn, M 2 contains Ti, and contains sodium-containing composite oxide The present invention relates to an electrode active material for a sodium ion secondary battery.

Another aspect of the present invention is a cationic disordered rock salt structure having a general formula (2): y [Na ₂ M ₂ O ₃ -γ]-(1-y) [NaM 1 O _{2 -δ} ] (where 0) 1 ≦ y ≦ 0.5, −0.1 ≦ δ ≦ 0.1, −0.1 ≦ γ ≦ 0.1), or the general formula (3): NaM ₁ O ₂ −ε (−0.1 ≦ ε) The present invention relates to an electrode active material for a sodium ion secondary battery, including a sodium-containing composite oxide represented by ≦ 0.1, wherein M 1 contains Mn and M 2 contains Ti.

Yet another aspect of the present invention is a solid phase reaction comprising calcining a first raw material containing Na, M 1 and M 2, M 1 containing Mn, and M 2 containing Ti, the general formula (1): x [NaM 1 O _{2 — α} ]-(1-x) [M ₂ O _2-β ] (where 0.5 ≦ x ≦ 0.9, −0.1 ≦ α ≦ 0.1 and −0.1 ≦ β ≦ 0.1) The present invention relates to a method for producing an electrode active material for a sodium ion secondary battery, which comprises the step of preparing a sodium-containing composite oxide having an O 3 -type layered rock salt structure.

Yet another aspect of the present invention is a method of mechanical milling a second raw material comprising Na ₂ M ₂ O ₃ and NaM ₁ O ₂ , M 1 containing Mn, and M 2 containing Ti, general formula (2): y [Na _{2 M2O 3-γ] - (1} -y) [NaM1O 2-δ] ( however, 0.1 ≦ y ≦ 0.5, -0.1 ≦ δ ≦ 0.1, -0.1 ≦ γ ≦ 0. The present invention relates to a method for producing an electrode active material for a sodium ion secondary battery, having a step of preparing a sodium-containing composite oxide represented by 1) and having a rock salt type crystal structure of a cation disordered array type.

Yet another aspect of the present invention includes a NaM1O ₂ of O3 type layered structure, M1 is by mechanical milling of the third raw material containing Mn, the general formula _{(3): NaM1O 2-ε} (-0.1 ≦ The present invention relates to a method for producing an electrode active material for a sodium ion secondary battery, which has a step of preparing a sodium-containing composite oxide having a rock salt type crystal structure represented by ε ≦ 0.1 and having a cation irregular arrangement type.

  ADVANTAGE OF THE INVENTION According to the electrode active material for sodium ion secondary batteries concerning this invention, provision of the sodium ion secondary battery excellent in high capacity | capacitance and cycling characteristics is attained.

It is a longitudinal cross-sectional view of an example of the sodium ion secondary battery concerning the embodiment of the present invention. It is a figure which shows the XRD pattern of the sodium containing complex oxide represented by General formula (1), and a reference oxide. It is a figure which shows the charging / discharging characteristic of the electrode active material containing the sodium containing complex oxide represented by General formula (1). Is a diagram showing charge-discharge characteristics of NaMnO _2. NaMnTiO is a diagram showing charge-discharge characteristics of the _4. It is a figure which shows the cycling characteristics of the electrode active material containing sodium containing complex oxide represented by General formula (1). It is a figure which shows the relationship of the synthetic | combination temperature of a sodium containing complex oxide represented by General formula (1), and cycling characteristics. It is a figure which shows the XRD pattern of sodium containing complex oxide represented by General formula (2), and a reference oxide. It is a figure which shows the charging / discharging characteristic of the electrode active material containing sodium-containing complex oxide represented by General formula (2). It is a figure which shows the cycling characteristics of the electrode active material containing sodium-containing complex oxide represented by General formula (2). It is a figure which shows the XRD pattern of the sodium containing complex oxide represented by General formula (3), and a reference oxide. It is a figure which shows the charging / discharging characteristic of the electrode active material containing sodium-containing complex oxide represented by General formula (3). It is a figure which shows the cycling characteristics of the electrode active material containing sodium containing complex oxide represented by General formula (3).

DESCRIPTION OF EMBODIMENTS OF THE INVENTION
First, the contents of the embodiment of the present invention will be listed and described.
[1] An electrode active material for a sodium ion secondary battery according to the present invention has an O 3 type layered rock salt structure, and has a general formula (1): x [NaM 1 O _{2 -α} ]-(1 -x) [M ₂ O _2- sodium-containing composite oxide (hereinafter referred to as sodium) represented by _{β 1} ] (wherein 0.5 ≦ x ≦ 0.9, −0.1 ≦ α ≦ 0.1 and −0.1 ≦ β ≦ 0.1) Containing complex oxide (1)). M1 contains Mn and M2 contains Ti. According to the sodium-containing composite oxide (1), for example, it is possible to obtain an electrode active material for a sodium ion secondary battery, which has better cycle characteristics than NaMnO ₂ of O 3 type layered rock salt structure and higher capacity than NaMnTiO ₄ It is. The sodium-containing composite oxide (1) is typically represented by the general formula (1a): xNaMnO _2- (1-x) TiO ₂ .

[2] Another electrode active material for a sodium ion secondary battery according to the present invention has a cation irregularly arranged rock salt structure, and has a general formula (2): y [Na ₂ M ₂ O ₃ -γ]-(1- y) sodium-containing composite represented by [NaM ₁ O _{2 -δ} ] (where, 0.1 ≦ y ≦ 0.5, −0.1 ≦ δ ≦ 0.1, −0.1 ≦ γ ≦ 0.1) The oxide (hereinafter also referred to as sodium-containing composite oxide (2)) is included. M1 contains Mn and M2 contains Ti. According to the sodium-containing composite oxide (2), it is possible to obtain, for example, an electrode active material for a sodium ion secondary battery which has better cycle characteristics than Na ₂ TiO ₃ or NaMnO _{2 having} an O 3 type layered rock salt structure. The sodium-containing composite oxide (2) is typically represented by the general formula (2a): yNa ₂ TiO _3- (1-y) NaMnO ₂ .

[3] Another electrode active material for a sodium ion secondary battery according to the present invention has a cation irregularly arranged rock salt structure, and is represented by the general formula (3): NaM1O _2-ε (-0.1 ≦ ε ≦ The sodium-containing composite oxide represented by 0.1) (hereinafter also referred to as sodium-containing composite oxide (3)) is included. M1 contains Mn. According to the sodium-containing composite oxide (3), it is possible to obtain, for example, an electrode active material for a sodium ion secondary battery which is more excellent in cycle characteristics than NaMnO _{2 having} an O 3 -type layered rock salt structure. The sodium-containing composite oxide (3) is typically represented by the general formula (3a): NaMnO ₂ .

[4] The sodium-containing composite oxide (2) is preferably capable of exhibiting a capacity by solid phase redox reaction of oxide ions. Thereby, it is possible to obtain a particularly high capacity electrode active material for a sodium ion secondary battery. The sodium-containing composite oxide (2) is more cyclic than an oxide having a similar crystal structure (for example, xNa ₃ NbO _4- δ-(1-x) NaMnO _2-γ ) exhibiting a solid phase redox reaction of oxide ions It has excellent characteristics.

  [5] From the viewpoint of obtaining better cycle characteristics, the proportion of Mn in M1 is preferably 50 mol% or more. Moreover, it is preferable that the ratio of Ti to M2 is 50 mol% or more from a viewpoint similar to [6].

[7] Next, a first method of producing an electrode active material for a sodium ion secondary battery according to an embodiment of the present invention includes Na, M1 and M2, M1 includes Mn, and M2 includes Ti. 1 Raw material is calcined by a solid phase reaction: General formula (1): x [NaM 1 O _{2 -α} ]-(1-x) [M ₂ O _2-β ] (where 0.5 ≦ x ≦ 0.9, −0 (1) having a step of preparing a sodium-containing composite oxide represented by 1 ≦ α ≦ 0.1 and −0.1 ≦ β ≦ 0.1) and having an O 3 type layered rock salt structure. [8] The first raw material preferably contains Na ₂ CO ₃ , TiO ₂ and Mn ₂ O ₃ .

[9] Next, a second method of producing an electrode active material for a sodium ion secondary battery according to an embodiment of the present invention includes Na ₂ M ₂ O ₃ and NaM ₁ O ₂ , M 1 contains Mn, and M 2 contains Ti. The second raw material is mechanically milled to obtain the general formula (2): y [Na ₂ M ₂ O ₃ -γ]-(1-y) [NaM 1 O _{2 -δ} ] (where 0.1 ≦ y ≦ 0.5). , -0.1δδ ≦ 0.1, -0.1 ≦ γ ≦ 0.1), and preparing a sodium-containing composite oxide having a rock salt type crystal structure of the cation disordered array type. . [10] The second raw material preferably contains Na ₂ TiO ₃ and NaMnO ₂ .

[11] Next, a third method of preparation of a sodium ion secondary battery electrode active material according to an embodiment of the present invention includes a NaM1O ₂ of O3 type layered structure, M1 is mechanical the third material containing Mn By milling, a sodium-containing composite oxide represented by the general formula (3): NaM1O _2-.epsilon. (-0.1.ltoreq..epsilon..ltoreq.0.1) and having a rock salt type crystal structure of cation disordered array type is prepared. Process. [12] The third raw material preferably contains NaMnO ₂ .
Details of Embodiments of the Invention

  Next, the embodiment of the present invention will be described more specifically. The present invention is not limited to these exemplifications, but is shown by the appended claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. .

<Electrode active material>
<Sodium-containing complex oxide (1)>
The sodium-containing composite oxide (1) containing M, Mn, and M2 is represented by the general formula (1): x [NaM1O2 _-α ]-(1-x) [M2O2 _-β ]; It has an O 3 -type layered rock salt structure and is excellent in cycle characteristics. A sodium-containing composite oxide containing Mn tends to have insufficient electrochemical characteristics while having excellent electrochemical activity. On the other hand, by combining Mn and Ti, good cycle characteristics can be expressed.

  M1 contains Mn as an essential element, but may contain a third element other than Mn. Examples of the third element other than Mn include Nb and Mo. From the viewpoint of resource price, capacity, etc., the amount of Mn in M1 may be, for example, 50 mol% or more, preferably 80 mol% or more, and more preferably 90 mol% or more.

  M2 contains Ti as an essential element, but may contain a third element other than Ti. Examples of the third element other than Ti include Al and the like. From the viewpoint of resource price, capacity, etc., the amount of Ti in M2 may be, for example, 50 mol% or more, preferably 80 mol% or more, and more preferably 90 mol% or more.

The general formula (1) does not mean a mixture of NaM1O _2-α and M2O _2-β but represents a compound containing Na, M1 and M2. That is, the general formula (1) is a compound of x mole NaM1O _2-α and (1-x) mole M2O _2-β, and is a compound having a crystal structure specific to the complex. expressing.

  In addition, similarly to the general formula (2), not a mixture but a compound having a specific crystal structure is expressed. The specific crystal structure according to the general formula (1) or (2) means an O3 type layered rock salt structure or a cation irregularly arranged rock salt structure.

x is the molar ratio of NaM 1 O _2-α to M ₂ O _2-β when the sodium-containing composite oxide (1) is regarded as a complex of NaM ₁ O _2-α and M ₂ O _2-β There is. The range of x is 0.5 ≦ x ≦ 0.9, and if it is less than 0.5, high capacity can not be obtained, and if it exceeds 0.9, the effect of complexing M1 and M2 becomes weak. It becomes difficult to obtain good cycle characteristics. From the viewpoint of the balance between capacity and cycle characteristics, the range of x is preferably 0.6 ≦ x ≦ 0.9, and more preferably 0.75 ≦ x ≦ 0.9.

α and β are real numbers indicating the deficiency or excess of oxygen based on NaM ₁ O ₂ and M ₂ O _2, and are not particularly limited, but usually −0.1 ≦ α ≦ 0.1 and −0.1 It is considered to be within the range of ≦ β ≦ 0.1.

<Sodium-containing complex oxide (2)>
Formula _{(2): y [Na 2} M2O 3-γ] - (1-y) is represented by _{[NaM1O 2-δ],} M1 comprises Mn, M2 is sodium-containing composite oxide containing Ti (2) Has a cation irregularly arranged rock salt structure and expresses high capacity. Unlike the crystal structures of NaMnO ₂ and Na ₂ TiO _{3 in} the cation irregularly arranged rock salt structure, the cationic species Na, M 1 and M 2 are irregularly arranged.

  Ti oxide may not be able to obtain high electrochemical activity, but when it is complexed with a manganese compound, it develops its capacity by solid phase redox reaction of oxide ions. Furthermore, good cycle characteristics are also exhibited by combining Mn and Ti.

In the general formula (2), M1 may contain Mn as an essential element, and may contain a third element other than Mn as in the sodium-containing composite oxide (1). M2 may contain Ti as an essential element, and may contain a third element other than Ti as in the sodium-containing composite oxide (1). From the viewpoint of resource price, capacity, etc., the amount of Mn in M1 may be, for example, 50 mol% or more, preferably 80 mol% or more, and more preferably 90 mol% or more. From the same point of view, the amount of Ti in M2 may be, for example, 50 mol% or more, preferably 80 mol% or more, and more preferably 90 mol% or more. Further, the general formula (2) does not mean a mixture of Na ₂ M ₂ O ₃ -γ and NaM ₁ O _{2 -δ} , but like the general formula (1), a compound or complex containing Na, M 1 and M 2 It represents an object.

  A typical XRD peak of the sodium-containing composite oxide (2) is observed, for example, in the range of (111) plane, 2θ = 38 ° to 41 ° observed in the range of 2θ = 32 ° to 36 ° ( It is a peak of (202) plane observed in the range of 200) plane, 2θ = 54 ° to 59 °. The crystal structure of the sodium-containing composite oxide (2) is assigned to the space group Fm-3 m.

  Sodium contained in the sodium-containing composite oxide (2) is released from the sodium-containing composite oxide (2) with Faraday reaction in the range of, for example, 2.0 V to 4.0 V with respect to metallic sodium, It is stored in the sodium-containing composite oxide (2) along with the Faraday reaction in the potential window area. At this time, the redox reaction in which Mn is involved and the redox reaction in which oxide ions are involved are interchanged in a potential range of approximately 3 V to 3.5 V. In the high potential region, it is considered that the redox reaction of the oxide ion is in progress.

y is sodium-containing composite oxide (2), the molar content ratio of Na ₂ M2O _3-gamma and NaM1O _2-δ when captured and composite of Na ₂ M2O _3-gamma and NaM1O _2-δ The ratio is shown. The range of y is 0.1 ≦ y ≦ 0.5, and if it is less than 0.1, the effect of combining M1 and M2 becomes weak, making it difficult to obtain good cycle characteristics, 0.5 If it exceeds, you can not obtain high capacity. From the viewpoint of balance between capacity and cycle characteristics, the range of y is preferably 0.2 ≦ y ≦ 0.4, and more preferably 0.2 ≦ y ≦ 0.35.

γ and δ are real numbers indicating the deficiency or excess of oxygen based on Na ₂ M ₂ O ₃ and NaM ₁ O ₂ , and are not particularly limited, but usually −0.1 ≦ γ ≦ 0.1 and −0. It is considered to be within the range of 1 ≦ δ ≦ 0.1.

<Sodium-containing composite oxide (3)>
A sodium-containing composite oxide (3) having a cation irregularly arranged rock salt structure and represented by the general formula (3): NaM ₁ O ₂ -ε, wherein M 1 contains Mn is more than the NaMnO ₂ of the O 3 type layered rock salt structure Excellent in cycle characteristics. This is considered to be due to the fact that the grain boundaries are increased by the reduction of the grain size due to the irregular arrangement of the cations, and the diffusion of sodium ions is more likely to occur.

  M1 contains Mn as an essential element, but may contain a third element other than Mn. Examples of the third element other than Mn include Nb, Mo, Al and the like. From the viewpoint of resource price, capacity, etc., the amount of Mn in M1 may be, for example, 50 mol% or more, preferably 80 mol% or more, and more preferably 90 mol% or more.

ε is a real number indicating the loss or excess amount of oxygen based on NaM ₁ O ₂ , and is not particularly limited, but usually considered to be within the range of −0.1 ≦ ε ≦ 0.1.

(Average primary particle size)
The average primary particle diameter of the sodium-containing composite oxides (1) to (3) is not particularly limited, but is, for example, 0.1 μm or more and preferably 0.2 μm or more from the viewpoint of sufficiently developing a rock salt type crystal structure. And 0.3 μm or more are more preferable. The average primary particle diameter is, for example, 5 μm or less, preferably 3 μm or less, and more preferably 2 μm or less. The average primary particle diameter is an average of the maximum diameters of the selected primary particles by arbitrarily selecting 10 to 30 primary particles in a scanning electron microscope (SEM) image of sodium-containing composite oxides (1) to (3) It should be determined as a value.

<Method of producing sodium-containing composite oxide (1) (first production method)>
The first production method comprises Na, M 1 and M 2, M 1 contains Mn, and M 2 is a solid phase reaction in which a first raw material containing Ti is calcined to obtain a compound represented by the general formula (1): x [NaM 1 O _{2 -α} ] -(1-x) [M ₂ O _2-β ] (where, 0.5 ≦ x ≦ 0.9, −0.1 ≦ α ≦ 0.1 and −0.1 ≦ β ≦ 0.1), and And B. preparing a sodium-containing composite oxide having an O 3 -type layered rock salt structure.

  As a 1st raw material, the mixture of the compound containing at least 1 sort (s) selected from the group which consists of Na, M1, and M2 can be used. As the compound, oxides, hydroxides, oxyhydroxides, hydrogencarbonates, carbonates, nitrates, sulfates, halides, organic acid salts (eg, oxalates) and the like are suitable. The raw materials are prepared by appropriately combining and mixing them.

The first raw material may be, for example, a mixture containing a sodium compound, an M1 compound, and an M2 compound. The sodium compound can be used as _{Na 2 CO 3, NaHCO 3,} Na 2 O. Above all, handling of Na ₂ CO ₃ is easy. As the M1 compound, Mn ₂ O ₃ , MnO ₂ or the like can be used. As the M2 compound, Na ₂ TiO ₃ , TiO ₂ or the like can be used. Among them, the first raw material preferably contains Na ₂ CO ₃ , Mn ₂ O ₃ and TiO ₂ .

  The first raw materials are preferably mixed by mechanical milling. Mechanical milling is a method of mixing materials by repeatedly applying shear force to materials using collision energy of balls, beads, stirring blades and the like in an inert gas atmosphere. Mechanical milling also allows mixing in atomic order. As an apparatus which performs mechanical milling, stirring apparatuses, such as a ball mill, a bead mill, a stamp mill, a jet mill, can be used. The mixing may be dry or wet. A sodium-containing composite having a layered rock salt structure by calcinating the mixed first raw material in an atmosphere of air, oxygen or an inert gas, for example, at a calcination temperature of 700 ° C. to 1100 ° C., preferably 800 ° C. to 1000 ° C. Oxides can be synthesized.

  The firing temperature is an average temperature during a period excluding a temperature raising process and a temperature lowering process (hereinafter, a main firing period). Therefore, the temperature may be lower than 700 ° C. or higher than 1100 ° C. momentarily or for a short time during the main baking period. However, it is preferable that the temperature of the firing atmosphere be maintained in the above temperature range for 80% or more of the main firing period. The main firing period is, for example, 6 hours to 48 hours.

As the inert gas, at least one selected from nitrogen, argon, helium and the like can be used. Air, the pressure of the oxygen or the inert gas atmosphere is not particularly restricted as long as it is for example ^{0.9 × 10 5 Pa~1.1 × 10 5} Pa.

  Prior to the above-described firing, the raw material may be temporarily fired in a temperature range of 400 ° C. or more and less than 600 ° C. in the same atmosphere as described above. By pre-baking, volatile components (for example, water) contained in the raw material can be gently removed. Thereby, crystal growth in the subsequent main firing period is facilitated to proceed.

<Method of producing sodium-containing composite oxide (2) (second method of production)>
The second production method contains Na ₂ M ₂ O ₃ and NaM ₁ O ₂ , M 1 contains Mn, and M 2 has a general formula (2): y [Na ₂ M ₂ O ₃ by mechanical milling a second raw material containing Ti. _-γ ]-(1-y) [NaM1O2 _-δ ] (where 0.1 ≦ y ≦ 0.5, −0.1 ≦ δ ≦ 0.1, −0.1 ≦ γ ≦ 0.1) And a step of preparing a sodium-containing composite oxide having a rock salt-type crystal structure of the cation irregular arrangement type.

Although the second raw material is not particularly limited, for example, a mixture of Na ₂ M ₂ O ₃ and NaM ₁ O ₂ can be used. Among them, the second raw material preferably contains Na ₂ TiO ₃ and NaMnO ₂ . The second raw material is mixed by mechanical milling. The mixing time by mechanical milling is preferably 10 to 60 hours, more preferably 30 to 50 hours.

<Method of producing sodium-containing composite oxide (3) (third production method)>
Third manufacturing method comprises NaM1O ₂ of O3 type layered structure, M1 is by mechanical milling of the third raw material containing Mn, the general formula _{(3): NaM1O 2-ε} (-0.1 ≦ ε ≦ And a step of preparing a sodium-containing composite oxide having a rock salt-type crystal structure represented by 0.1) and having a cation disordered array type. The third raw material preferably contains NaMnO ₂ . The third raw material is mixed by mechanical milling. The mixing time by mechanical milling is preferably 10 to 60 hours, more preferably 30 to 50 hours.

<Sodium ion secondary battery>
Next, the sodium ion secondary battery will be described. A sodium ion secondary battery comprises a positive electrode containing the above electrode active material, a negative electrode, and a non-aqueous electrolyte having sodium ion conductivity. FIG. 1 is a longitudinal sectional view schematically showing a sodium ion secondary battery 100 according to an embodiment of the present invention. The sodium ion secondary battery 100 includes a stacked electrode group, a non-aqueous electrolyte (not shown), and a rectangular battery case 10 that accommodates these. The battery case 10 is made of, for example, aluminum, and is configured of a bottomed container body 12 whose upper portion is opened, and a lid 13 which closes the upper portion opening.

  At the center of the lid 13 is provided a safety valve 16 for releasing gas generated inside when the internal pressure of the battery case 10 rises. With the safety valve 16 at the center, an external negative electrode terminal 14 penetrating the lid 13 is provided on one side of the lid 13, and at a position near the other side of the lid 13, the outside penetrating the lid 13 A positive terminal is provided.

  The stacked electrode group is constituted by a plurality of sheet-like positive electrodes 2 and a plurality of negative electrodes 3 and a plurality of separators 1 interposed therebetween. A positive electrode lead piece 2 a is formed at one end of each positive electrode 2. The positive electrode lead pieces 2 a of the plurality of positive electrodes 2 are bundled and connected to an external positive electrode terminal provided on the lid 13 of the battery case 10. Similarly, at one end of each negative electrode 3, a negative electrode lead piece 3 a is formed. The negative electrode lead pieces 3 a of the plurality of negative electrodes 3 are bundled and connected to the external negative electrode terminal 14 provided on the lid 13 of the battery case 10.

  Each of the external positive electrode terminal and the external negative electrode terminal 14 has a columnar shape, and at least a portion exposed to the outside has a screw groove. A nut 7 is fitted in a screw groove of each terminal, and the nut 7 is fixed to the lid 13 by rotating the nut 7. A collar 8 is provided at a portion of each terminal housed in the battery case 10, and the collar 8 is provided with an O-ring gasket 9 on the inner surface of the lid 13 by the rotation of the nut 7. It is fixed through.

(Positive electrode)
The positive electrode includes a positive electrode current collector and a positive electrode active material (or positive electrode mixture) carried on the positive electrode current collector. The positive electrode current collector may be a metal foil or a metal porous body. As a material of the positive electrode current collector, aluminum, an aluminum alloy and the like are preferable from the viewpoint of stability at the positive electrode potential.

As the positive electrode active material, any of sodium-containing composite oxides (1) to (3) may be used alone or in combination, and these may be used in combination with storage and release (or insertion and removal of other sodium ions). You may use together with the material (following and 2nd positive electrode active material) to separate. The second positive electrode active material is not particularly limited, but sodium chromite (NaCrO ₂ ), sodium nickel manganate (NaNi _0.5 Mn _0.5 O ₂ , Na _2/3 Ti _1/6 Ni _1/3 Mn _1/2 O ₂ ), sodium iron cobaltate (such as NaFe _0.5 Co _0.5 O ₂ ), sodium ferromanganate (such as Na _2/3 Fe _1/3 Mn _2/3 O ₂ ), and the like.

  The positive electrode mixture can further contain a conductive aid and / or a binder in addition to the positive electrode active material. The positive electrode is obtained by coating or filling a positive electrode current collector with a positive electrode mixture, drying it, and rolling the dried product in the thickness direction as necessary. The positive electrode mixture is usually used in the form of a slurry containing a dispersion medium.

  As a conductive support agent, carbon black, graphite, carbon fiber etc. are mentioned. The binder may, for example, be a fluorine resin, a polyolefin resin, a rubbery polymer, a polyamide resin, a polyimide resin (polyamide imide etc.) or a cellulose ether. As a dispersion medium, water etc. are used other than organic solvents, such as N- methyl -2- pyrrolidone (NMP: N- methyl -2- pyrolidone), for example.

(Negative electrode)
The negative electrode includes a negative electrode current collector and a negative electrode active material (or negative electrode mixture) carried on the negative electrode current collector. Like the positive electrode current collector, the negative electrode current collector may be a metal foil or a metal porous body. The material of the negative electrode current collector is preferably copper, a copper alloy, nickel, a nickel alloy, stainless steel or the like because it does not form an alloy with sodium and is stable at the negative electrode potential.

As the negative electrode active material, at least one of a material that reversibly occludes and releases (or inserts and desorbs) sodium ions, a material that alloys with sodium, and the like can be used. Specifically, metals such as sodium, titanium, zinc, indium, tin, silicon or alloys or compounds thereof; carbon materials can be exemplified. Examples of the metal compound include lithium titanate (such as Li ₂ Ti ₃ O ₇ and Li ₄ Ti ₅ O ₁₂ ) and sodium titanate (such as Na ₂ Ti ₃ O ₇ and Na ₄ Ti ₅ O ₁₂ ). Examples of carbon materials include graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), and the like.

  The negative electrode can be formed, for example, by applying or filling a negative electrode mixture containing a negative electrode active material to a negative electrode current collector in the same manner as in the case of a positive electrode, drying it, and rolling the dried material in the thickness direction. The negative electrode active material may be pre-doped with sodium ions, if necessary.

(Separator)
As the separator, a resin microporous membrane, non-woven fabric, etc. can be used. Examples of the material of the separator include polyolefin resin, polyphenylene sulfide resin, polyamide resin, and polyimide resin.

(Non-aqueous electrolyte)
The non-aqueous electrolyte is not particularly limited as long as it has sodium ion conductivity. For example, an organic electrolyte in which a sodium salt is dissolved in an organic solvent can be used. At this time, the concentration of the sodium salt in the non-aqueous electrolyte may be, for example, 0.3 to 3 mol / liter. Alternatively, an ionic liquid containing sodium ions and anions may be used. The “ionic liquid” is a salt in a molten state (molten salt).

The type of anion constituting the sodium salt is not particularly limited, but hexafluorophosphate ion (PF ₆ ⁻ ), tetrafluoroborate ion (BF ₄ ⁻ ), perchlorate ion (ClO ₄ ⁻ ), bis (oxalato) At least one of borate ion (B (C ₂ O ₄ ) ₂ ⁻ ), trifluoromethane sulfonate ion (CF ₃ SO ₃ ⁻ ), bissulfonylamide anion and the like is used.

  As the organic solvent, cyclic carbonates such as ethylene carbonate, propylene carbonate and butylene carbonate; linear carbonates such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate; cyclic carbonates such as γ-butyrolactone and the like can be preferably used. These are used in combination as appropriate.

  The ionic liquid may further contain a second cation in addition to the sodium ion (first cation). The second cation is preferably an organic cation, and more preferably a nitrogen-containing onium cation.

Hereinafter, the present invention will be described in more detail based on examples. However, the following examples do not limit the present invention.
Example 1
In the present example, the sodium-containing composite oxide (1) was prepared in the following manner.
Na ₂ CO ₃ , TiO ₂ (anatase type) and Mn ₂ O ₃ were mixed at a predetermined molar ratio, and mixed in a ball mill of 300 rpm for 5 hours to obtain a first raw material. The obtained first raw material is formed into pellets, which are calcined at 800 ° C. for 12 hours in an inert atmosphere, and represented by xNaMnO _2- (1-x) TiO ₂ , x = 0.67, 0.75 , 0.8, 0.9 four kinds of sodium containing complex oxides (1) were obtained. At this time, the composition formula of the sodium-containing composite oxide can be approximately expressed as follows.

Na _0.67 Mn _0.67 Ti _0.33 O ₂ (x = 0.67)
Na _0.75 Mn _0.75 Ti _0.25 O ₂ (x = 0.75)
Na _0.8 Mn _0.8 Ti _0.2 O ₂ (x = 0.8)
Na _0.9 Mn _0.9 Ti _0.1 O ₂ (x = 0.9)

  Next, the synthesized sodium-containing composite oxide (1) and acetylene black as a conductive additive are mixed at a mass ratio of 9: 1, mixed in a ball mill of 300 rpm for 6 hours, and mixed with sodium-containing composite oxide ( 1) and the carbon material were compounded.

  Next, a sodium-containing composite oxide (1) complexed with a carbon material and polyvinylidene fluoride as a binder are mixed at a mass ratio of 93: 7, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) is dispersed. A slurry containing as a medium was prepared. The obtained slurry was applied to one side of a 20 μm thick aluminum foil serving as a current collector. The thickness of the coating was 40 μm. The coated film was sufficiently dried and then punched out with an aluminum foil to obtain a coin-shaped electrode having a diameter of 1.0 cm.

Next, a coin-shaped cell was produced using a metallic sodium foil as a counter electrode and a non-aqueous electrolyte having sodium ion conductivity. The sodium ion conductive non-aqueous electrolyte, a volume ratio of ethylene carbonate (EC) and dimethyl carbonate (DMC) 1: was used as the a NaPF ₆ dissolved at 1 mol / L concentration in a mixed solvent of 1. The coin-shaped cell was charged and discharged at a constant current of 10 mA / g at a lower limit voltage of 1.2 V, an upper limit voltage of 4.5 V, and the cycle characteristics were evaluated.

2 shows an XRD pattern of the sodium-containing composite oxide (1) and the reference oxide (NaMnO _2, NaMnTiO _4). Sodium-containing composite oxide (1) it may be understood to have the same O3 type layered rock salt structure and NaMnO _2. It can also be understood that the smaller the x value, the closer to the tunnel crystal structure of NaMnTiO ₄ .

The measurement conditions of XRD are shown below.
X-ray: CuKα
Voltage-current: 40kV-20mA
Measurement angle range: 2θ = 10-70 °
Step: 0.02 °
Scanning speed: 6 ° / min

FIG. 3 shows charge / discharge characteristics of a coin-type cell using sodium-containing composite oxide (1) as an electrode active material. Further, FIG. 4 shows charge / discharge characteristics of a coin-type cell using NaMnO ₂ as an electrode active material. Further, FIG. 5 shows charge / discharge characteristics of a coin-type cell using NaMnTiO ₄ as an electrode active material. FIG. 6 summarizes the cycle characteristics of a coin cell in which the sodium-containing composite oxide (1) is an electrode active material.

It can be understood from FIG. 4 that although NaMnO ₂ can express a high capacity, there is room for improvement in the cycle characteristics. Further, it can be understood from FIG. 5 that although NaMnTiO ₄ has excellent cycle characteristics, there is room for improvement in capacity. On the other hand, it can be understood that the sodium-containing composite oxide (1) has high capacity and excellent cycle characteristics, and has the merits of NaMnO ₂ and NaMnTiO ₄ . The larger the value of x, the higher the capacity, and it can be seen that in the range of x = 0.75 to 0.9, both high capacity and good cycle characteristics can be achieved in a particularly balanced manner.

Example 2
Next, a sodium-containing composite oxide (x = 0.8) was synthesized and evaluated in the same manner as in Example 1 except that the firing temperature of the pellets of the first raw material was changed to 1000 ° C. FIG. 7 shows the cycle characteristics of the sodium-containing composite oxide (1) at baking temperatures of 1000 ° C. and 800 ° C. in comparison. It can be understood from FIG. 7 that the capacity can be further increased by raising the firing temperature.

Example 3
In the present example, the sodium-containing composite oxide (2) was prepared by mechanical milling in the following manner.
Na ₂ CO ₃ and TiO ₂ (anatase type) are mixed at a predetermined molar ratio, mixed in a ball mill of 300 rpm for 5 hours, calcined in air at 800 ° C. for 12 hours, gradually cooled, the second raw material Na ₂ TiO ₃ was obtained as a part of In addition, Na ₂ CO ₃ and Mn ₂ O ₃ are mixed at a predetermined molar ratio, mixed for 5 hours in a ball mill of 300 rpm, calcined in Ar at 850 ° C. for 2 hours, and gradually cooled, the second raw material NaMnO ₂ was obtained as part of

Next, Na ₂ TiO ₃ and NaMnO ₂ are mixed at a predetermined molar ratio, mechanically milled in a ball mill at 600 rpm for 48 hours, and expressed as yNa ₂ TiO _3- (1-y) NaMnO ₂ , y = 0 Three types of sodium-containing composite oxides (2) of 22 (2/9), 0.33 (1/3) and 0.5 (1/2) were obtained. At this time, the composition formula of the sodium-containing composite oxide can be approximately expressed as follows.

Na _1.1 Mn _0.7 Ti _0.2 O ₂ (y = _2/9 )
Na _1.14 Mn _0.57 Ti _0.29 O ₂ (y = 1/3)
Na _1.2 Mn _0.4 Ti _0.4 O ₂ (y = 1/2)

  Next, as in Example 1, the synthesized sodium-containing composite oxide (2) was complexed with a carbon material. Next, using a sodium-containing composite oxide (2) complexed with a carbon material, a coin-shaped electrode having a diameter of 1.0 cm was obtained as in Example 1. Then, a coin-shaped cell was produced in the same manner as in Example 1, and charge and discharge were performed with a lower limit voltage of 1.2 V, an upper limit voltage of 4.3 V or 4.5 V, and a constant current of 10 mA / g to evaluate cycle characteristics.

FIG. 8 shows XRD patterns of the sodium-containing composite oxide (2) and the reference oxides (NaMnO ₂ , Na ₂ TiO ₃ ) measured under the same conditions as in Example 1. It can be understood from FIG. 8 that the sodium-containing composite oxide (2) has a cation irregularly-ordered rock salt structure which is different from NaMnO ₂ and Na ₂ TiO ₃ .

  FIG. 9 shows charge / discharge characteristics of a coin-type battery using the sodium-containing composite oxide (2) as an electrode active material. From FIG. 9, it can be understood that as the y value is smaller, a higher capacity can be obtained up to y = 2/9, and a better cycle characteristic can be obtained around y = 1/3.

FIG. 10 shows cycle characteristics of a coin-type battery using sodium-containing composite oxide (2) as an electrode active material. Further, FIG. 10 also shows cycle characteristics of a coin-type battery in which a sodium-containing composite oxide having a similar crystal structure and a composition formula of about Na _1.3 Nb _0.3 Mn _0.4 O ₂ is used as an electrode active material. It has been found that Na _1.3 Nb _0.3 Mn _0.4 O ₂ develops high capacity by solid phase redox reaction of oxide ions. From FIG. 10, according to the sodium-containing composite oxide (2), in addition to the fact that a high capacity comparable to Na _1.3 Nb _0.3 Mn _0.4 O ₂ is obtained, a sodium ion secondary battery having more excellent cycle characteristics is obtained. I know that I can do it.

Example 4
In the present example, the sodium-containing composite oxide (3) was prepared by mechanical milling in the following manner.
NaMnO ₂ of the O 3 type layered rock salt structure prepared in the same manner as in Example 3 is mechanically milled with a ball mill at 600 rpm for 48 hours to obtain NaMnO ₂ (sodium-containing composite oxide (3)) of cation disordered ordered rock salt structure. The

  Next, in the same manner as Example 1, the sodium-containing composite oxide (3) was complexed with the carbon material. Next, using a sodium-containing composite oxide (3) complexed with a carbon material, a coin-shaped electrode having a diameter of 1.0 cm was obtained as in Example 1. Then, a coin type cell was produced in the same manner as in Example 1, and charge and discharge were performed at a constant current of lower limit voltage 1.2 V, upper limit voltage 4.5 V, 10 mA / g, and cycle characteristics were evaluated.

FIG. 11 shows the XRD patterns of the sodium-containing composite oxide (3) and the reference oxide (NaMnO _{2 of the} O3 type layered rock salt structure) measured under the same conditions as in Example 1. It can be understood from FIG. 11 that the sodium-containing composite oxide (3) has a cation irregularly arranged rock salt structure.

FIG. 12 shows charge / discharge characteristics of a coin-type battery using sodium-containing composite oxide (3) (NaMnO ₂ -MM) as an electrode active material. Further, FIG. 12 also shows cycle characteristics of a coin-type battery using an O 3 -type layered rock salt structure NaMnO ₂ (As-Prepared) as an electrode active material. From FIG. 12, it can be seen that according to the sodium-containing composite oxide (3), it is possible to obtain a sodium ion secondary battery having higher capacity and better cycle characteristics than the O 3 -type layered rock salt structure NaMnO ₂ .

FIG. 13 shows the cycle characteristics of a coin battery in which the sodium-containing composite oxide (3) is used as the electrode active material. From FIG. 13, it can be seen that according to the sodium-containing composite oxide (3), it is possible to obtain a sodium ion secondary battery having better cycle characteristics than the O 3 -type layered rock salt structure NaMnO ₂ .

  The electrode active material for a sodium ion secondary battery according to the present invention is useful as a material of a sodium ion secondary battery having high capacity and good cycle characteristics.

  1: Separator, 2: Positive electrode, 2a: Positive electrode lead piece, 3: Negative electrode, 3a: Negative electrode lead piece, 7: Nut, 8: Flange, 9: Gasket, 10: Battery case, 12: Container body, 13: Lid Body, 14: External negative terminal, 16: Safety valve, 100: Sodium ion secondary battery

Claims (11)

Has O3 type layered rock salt structure,
General formula (1): x [NaM1O2- [ _alpha ]]-(1-x) [M2O2- [ _beta ]] (wherein 0.5 <x <0.9, -0.1 <[alpha] <0.1 and- 0.1 ≦ β ≦ 0.1),
An electrode active material for a sodium ion secondary battery, including a sodium-containing composite oxide in which M1 contains Mn and M2 contains Ti. Has a cation irregularly arranged rock salt structure,
Formula _{(2): y [Na 2} M2O 3-γ] - (1-y) [NaM1O 2-δ] ( however, 0.1 ≦ y ≦ 0.5, -0.1 ≦ δ ≦ 0.1 , −0.1 ≦ γ ≦ 0.1), or General Formula (3): NaM ₁ O ₂ −ε (−0.1 ≦ ε ≦ 0.1),
An electrode active material for a sodium ion secondary battery, including a sodium-containing composite oxide in which M1 contains Mn and M2 contains Ti.   The electrode active material for a sodium ion secondary battery according to claim 2, wherein the sodium-containing composite oxide represented by the general formula (2) expresses a capacity by a solid phase redox reaction of an oxide ion.   The electrode active material for a sodium ion secondary battery according to any one of claims 1 to 3, wherein the proportion of Mn in M1 is 50 mol% or more.   The electrode active material for a sodium ion secondary battery according to any one of claims 1 to 3, wherein the ratio of Ti to M2 is 50 mol% or more. Na, M1 and M2 containing, M1 containing Mn, M2 by the solid phase reaction of firing the first raw material containing Ti,
General formula (1): x [NaM1O2- [ _alpha ]]-(1-x) [M2O2- [ _beta ]] (wherein 0.5 <x <0.9, -0.1 <[alpha] <0.1 and- The manufacturing method of the electrode active material for sodium ion secondary batteries which has the process of preparing the sodium containing complex oxide which is represented by 0.1 <= (beta) <= 0.1 and which has an O3 type layered rock salt structure. Wherein the first material comprises Na ₂ CO _3, TiO ₂ and Mn ₂ O _3, the manufacturing method of the sodium ion secondary battery electrode active material according to claim 6. By mechanical milling a second material containing Na ₂ M ₂ O ₃ and NaM ₁ O ₂ , M 1 containing Mn and M 2 containing
Formula _{(2): y [Na 2} M2O 3-γ] - (1-y) [NaM1O 2-δ] ( however, 0.1 ≦ y ≦ 0.5, -0.1 ≦ δ ≦ 0.1 , 0.1 ≦ γ ≦ 0.1), and has a step of preparing a sodium-containing composite oxide having a rock salt type crystal structure of a cation irregular arrangement type, for an electrode active material for a sodium ion secondary battery Production method. It said second material comprises Na ₂ TiO ₃ and NaMnO _2, a manufacturing method of a sodium ion secondary battery electrode active material according to claim 8. By mechanical milling of a third raw material containing Mn and containing NaM ₁ O ₂ of O 3 type layered structure,
Having a step of preparing a sodium-containing composite oxide represented by the general formula (3): NaM ₁ O ₂ -ε (−0.1 ≦ ε ≦ 0.1) and having a rock salt type crystal structure of cation irregular arrangement type, The manufacturing method of the electrode active material for sodium ion secondary batteries. The method of manufacturing an electrode active material for a sodium ion secondary battery according to claim 10, wherein the third raw material contains NaMnO ₂ .