JP5988223B2 - Sodium ion battery - Google Patents

Description

  The present invention relates to a sodium ion battery.

  Lithium ion batteries are widely used in portable electronic devices such as laptop computers and mobile phones, and are being put to practical use as secondary batteries for next-generation electric vehicles and hybrid vehicles.

  However, lithium is expensive, and resources are extremely unevenly distributed in South America, and there is concern about a stable supply of raw materials. For this reason, sodium ion batteries using abundant resources and inexpensive sodium salts instead of lithium salts have been proposed as alternatives to lithium ion batteries.

As a positive electrode active material used for a sodium ion battery, many positive electrode active materials used for a lithium ion battery can be used as they are. For example, iron phosphate represented by FePO ₄ has a space group of trigonal P321, and the FeO ₄ tetrahedron and PO ₄ tetrahedron form a vertex shared skeleton, so the bottleneck of guest cation diffusion is large, It can function as an intercalation host not only for lithium ions having a small ion radius but also for sodium ions having a large ion radius (Patent Document 1).

  However, there are few reports on the negative electrode active material used in the lithium ion battery, and the charge / discharge operation has been confirmed in the carbon-based negative electrode active material. For example, a carbon material obtained by using resorcinol as a raw material and carbonizing this is known as a negative electrode active material for a lithium ion battery (Non-Patent Document 1 and Non-Patent Document 2).

JP 2007-12491 A

Electrochemical and Solid-state Letters, 8 (4) A222- A225 (2005) Electrochimica Acta 47 (2002) 3303-3307

However, in the above-described conventional carbon-based negative electrode active material, the potential at which sodium intercalates with carbon as the negative electrode active material during charging is 0.3 V (vs Li / Li ⁺ ), which is the potential at which Na metal precipitates. When overvoltage is taken into consideration, sodium metal may be deposited on the negative electrode active material during charging. Sodium is a highly reactive metal, and in the unlikely event that the negative electrode active material on which sodium is deposited touches the atmosphere, there is a risk that it will come into contact with moisture or water in the atmosphere and generate intense heat generation. For this reason, the practical use of sodium ion batteries has been delayed.

  The present invention has been made in view of the above-described conventional situation, and the negative electrode active material for a sodium ion battery in which the potential at the time of charging is higher than the potential at which sodium precipitates and the risk of sodium metal precipitation at the time of charging is low. Providing a sodium ion battery is a problem to be solved.

In order to solve the above-described conventional problems, the present inventors have examined whether Li ₄ Ti ₅ O ₁₂ , which is known as an electrode active material of a lithium ion battery, can be applied to an electrode active material of a sodium ion battery. Li ₄ Ti ₅ O ₁₂ is capable of sodium intercalation-deintercalation, and its redox potential is higher than that of a carbon-based negative electrode active material, and thus has attracted attention as being highly safe. However, since the ion radius of sodium ions is considerably larger than the ion radius of lithium ions, it is difficult to intercalate sodium ions into Li ₄ Ti ₅ O ₁₂ , and Li ₄ Ti ₅ as a negative electrode active material for sodium ion batteries. The application of O ₁₂ was not considered and the operation was not confirmed. The present inventors have already filed an application for an electrolytic solution for a sodium ion battery having a wide potential window to which a nitrile compound has been added (Japanese Patent Application No. 2008-322913), but if this electrolytic solution is used, Li ₄ Ti ₅ The inventors have found that O ₁₂ can be reliably operated as a negative electrode active material, and have completed the present invention.

That is, the electrode active material for a sodium ion battery of the present invention is a sodium ion battery in which a positive electrode including a positive electrode active material and a negative electrode including a negative electrode active material are in contact with an electrolyte solution in which a sodium salt is dissolved in an organic solvent. An electrode active material to be used, which is characterized by being made of Li ₄ Ti ₅ O ₁₂ .

The sodium ion battery of the present invention is a sodium ion battery in which a positive electrode including a positive electrode active material and a negative electrode including a negative electrode active material are in contact with an electrolyte solution in which a sodium salt is dissolved in an organic solvent, The positive electrode active material or the negative electrode active material is made of Li ₄ Ti ₅ O ₁₂ .

  The sodium ion battery of the present invention includes an organic solvent of an electrolytic solution, a chain saturated hydrocarbon dinitrile compound in which a nitrile group is bonded to both ends of the chain saturated hydrocarbon compound, and a nitrile at at least one terminal of the chain ether compound. It is preferable that at least one nitrile compound among a chain ether dinitrile compound and a cyanoacetate group to which a group is bonded and at least one of a cyclic carbonate ester, a cyclic carboxylic acid ester and a chain carbonate ester are included.

  In the sodium ion battery of the present invention, preferably, at least one of a chain saturated hydrocarbon dinitrile compound having a nitrile group bonded to both ends of the chain saturated hydrocarbon compound as an organic solvent of the electrolytic solution, or a chain ether compound. At least one nitrile compound among a chain ether nitrile compound having a nitrile group bonded thereto and a cyanoacetic acid ester. These nitrile compounds serve to widen the potential window. Further, it is preferable that at least one of a cyclic carbonate, a cyclic carboxylic acid ester and a chain carbonate is contained as an organic solvent. These esters serve to lower the viscosity of the high-viscosity nitrile compound and increase the specific conductivity.

  Therefore, according to the sodium ion battery of the present invention, it is possible to use, as the positive electrode active material, a substance that does not easily decompose even when the electrolyte is at a high potential and reaches a positive potential region where charge and discharge are high.

  The reason why the electrolyte of the sodium ion battery of the present invention has a wide potential window despite the fact that it contains a cyclic carbonate ester, a cyclic carboxylic acid ester, or a chain carbonate ester that does not have a very wide potential window. Although it is not necessarily clear, it is considered as follows. That is, it is presumed that the chain carbonic acid ester plays a role of increasing the specific conductivity in order to lower the viscosity. In addition, as is known in the art, cyclic carbonates and cyclic carboxylic esters can pass sodium ions while improving reduction resistance by forming a protective film called SEI on the carbon negative electrode. The characteristic which can be given can be provided. For this reason, it is estimated that the effect can be exerted on the enlargement of the negative and positive potential windows.

  As the cyclic carbonate, ethylene carbonate or propylene carbonate can be used. Moreover, (gamma) -butyrolactone can be used as cyclic carboxylic acid ester. Further, dimethyl carbonate or diethyl carbonate can be used as the chain carbonate.

Examples of the sodium salt include NaClO ₄ , NaPF ₆ , NaBF ₄ , NaCF ₃ SO ₃ , NaN (CF ₃ SO ₂ ) ₂ , NaN (FSO ₂ ) ₂ , NaN (C ₂ F ₅ SO ₂ ) ₂ , NaC ( CF ₃ SO ₂ ) ₃ can be used. These sodium salts may be used alone or in combination of two or more kinds. Particularly preferred is NaPF _6.

It is the electric potential-current curve of the electrolyte solution of Test Examples 1-5 and Comparative Example 1. 6 is a potential-current curve of electrolytic solutions of Test Example 6 and Comparative Example 1. 6 is a potential-current curve of electrolytic solutions of Test Examples 7 and 8 and Comparative Example 1. It is the electric potential-current curve of the electrode pellet used in the Example. It is sectional drawing of the battery case used for the sodium ion battery of the Example. It is sectional drawing of the sodium ion battery of an Example. It is a graph which shows the charging / discharging characteristic of the sodium ion battery of an Example. Is a graph showing the charge-discharge characteristics of NaNi _0.5 Fe _0.5 O _2. Is a graph showing the charge-discharge characteristics of NaNi _0.5 Ti _0.5 O _2. Is a graph showing the charge-discharge characteristics of NaFePO _4.

As described above, the present inventors have developed an electrolytic solution for a sodium ion battery having a wide potential window particularly on the positive potential side, and have filed a patent application therefor (Japanese Patent Application No. 2008-322913). And if the electrolytic solution is used, it is already used as a positive electrode active material a high potential redox positive electrode active material that exists in a region where the potential for charging exceeds 5.1 V (vs. Li / Li ⁺ ). I have confirmed. Examples of such a high potential redox positive electrode active material include NaMPO ₄ and NaMPO ₄ F (M = Ni, Co, Mn). These positive electrode active materials and the electrode active material for a sodium ion battery of the present invention In combination with the electrolyte for sodium ion batteries described in Japanese Patent Application No. 2008-322913, a sodium ion battery having a high energy density and a large capacity can be obtained.

In Japanese Patent Application No. 2008-322913, Test Examples 1 to 8 and Comparative Example 1 in which a sodium ion battery electrolyte is embodied are described as follows.

  Test Examples 1 to 5 are test examples including a chain saturated hydrocarbon dinitrile compound in which both ends of the chain saturated hydrocarbon compound are substituted with nitrile groups as an organic solvent. Test Example 6 is a test example including a chain ether dinitrile compound in which both ends of a chain ether compound are substituted with nitrile groups as an organic solvent. Furthermore, Test Examples 7 and 8 are test examples containing cyanoacetic acid ester as the organic solvent. Table 1 shows the compositions of the electrolyte solutions of each test example and comparative example.

(Test Example 1)
In Test Example 1, a solvent in which succinonitrile, ethylene carbonate (EC), and dimethyl carbonate (DMC) were mixed at a volume ratio of 50:25:25 was used as the organic solvent. Sodium fluorophosphate (NaPF ₆ ) was dissolved to a concentration of 0.5 mol / L to obtain a sodium ion battery electrolyte.

(Test Example 2)
In Test Example 2, a solvent in which glutaronitrile, ethylene carbonate (EC), and dimethyl carbonate (DMC) were mixed as an organic solvent in a volume ratio of 50:25:25 was used. Others are the same as in Test Example 1, and a description thereof is omitted.

(Test Example 3)
In Test Example 3, a solvent in which adiponitrile, ethylene carbonate (EC), and dimethyl carbonate (DMC) were mixed at a volume ratio of 50:25:25 was used as the organic solvent. Others are the same as in Test Example 1, and a description thereof is omitted.

(Test Example 4)
In Test Example 4, a solvent obtained by mixing sebacononitrile, ethylene carbonate (EC), and dimethyl carbonate (DMC) at a volume ratio of 50:25:25 was used as the organic solvent. Others are the same as in Test Example 1, and a description thereof is omitted.

(Test Example 5)
In Test Example 5, a solvent in which dodecanedinitrile, ethylene carbonate (EC), and dimethyl carbonate (DMC) were mixed at a volume ratio of 50:25:25 was used as the organic solvent. Others are the same as in Test Example 1, and a description thereof is omitted.

(Test Example 6)
In Test Example 6, a solvent in which oxypropionitrile, ethylene carbonate (EC), and dimethyl carbonate (DMC) were mixed at a volume ratio of 50:25:25 was used as the organic solvent. Others are the same as in Test Example 1, and a description thereof is omitted.

(Test Example 7)
In Test Example 7, a solvent in which methyl cyanoacetate, ethylene carbonate (EC), and dimethyl carbonate (DMC) were mixed at a volume ratio of 50:25:25 was used as the organic solvent. Others are the same as in Test Example 1, and a description thereof is omitted.

(Test Example 8)
In Test Example 8, a solvent obtained by mixing butyl cyanoacetate, ethylene carbonate (EC), and dimethyl carbonate (DMC) in a volume ratio of 50:25:25 was used as the organic solvent. Others are the same as in Test Example 1, and a description thereof is omitted.

(Comparative Example 1)
In Comparative Example 1, a solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 50:50 was used as the organic solvent. The concentration of sodium hexafluorophosphate (NaPF ₆ ) was 1.0 mol / L. Others are the same as in Test Example 1, and a description thereof is omitted.

-Evaluation-
(Measurement of potential-current curve)
With respect to the electrolyte solutions for sodium ion batteries of Test Examples 1 to 8 and Comparative Example 1 prepared as described above, potential-current curves were measured. A potentiogalvanostat was used for measurement, glassy carbon was used for the working electrode, and a platinum net was used for the counter electrode. Further, metallic lithium was used for the reference electrode. The potential was swept at a rate of 5 mV / sec, and a potential-current curve was measured. The results are shown in FIGS.

As shown in FIG. 1, the organic solvent is composed of ethylene dicarbonate and dimethyl carbonate. In Comparative Example 1 that does not contain a nitrile compound, it becomes 50 μA / cm ² at 5.8 V, and rises greatly at higher voltages. On the other hand, in Test Examples 1 to 5 using various chain saturated hydrocarbon dinitrile compounds, 50 μA / cm ² is not exceeded unless a higher potential is reached (the current value at 50 μA / cm ² is 6. 5V, 7.0V in Test Example 2, 7.5V in Test Example 3, 7.2V in Test Example 4, and 7.2V in Test Example 5.

Further, as shown in FIG. 2, the organic solvent contains a chain ether dinitrile compound in which both ends of the chain ether compound are substituted with nitrile groups, and further contains an ethylene dicarbonate and dimethyl carbonate. Then, the voltage at which the current became 50 μA / cm ² was 6.8 V, and it was found that the electrolyte was stable up to a high voltage.

  Moreover, as shown in FIG. 3, in the electrolyte solutions of Test Examples 7 and 8 in which the organic solvent contains cyanoacetate and further contains ethylene dicarbonate and dimethyl carbonate, the organic solvent consists of ethylene dicarbonate and dimethyl carbonate. Compared with Comparative Example 1, it was found that the current value at a voltage of 6.2 V or higher was small, and the decomposition resistance of the electrolytic solution at a high voltage was excellent.

  From the above results, when the electrolytes of Test Examples 1 to 8 are used, a high potential redox positive electrode active material that exists in a region where the voltage for charging exceeds 6 V is used as the positive electrode active material of the sodium ion battery. As a result, the battery voltage and energy density are high, and a large-capacity sodium ion battery can be obtained.

-Reference example-
For reference, instead of NaPF ₆ which is the electrolyte in the case of Test Examples 1 to 8, electrolyte solutions for lithium ion batteries in which LiPF ₆ is dissolved in an organic solvent are prepared with various compositions. Similarly, a potential-current curve was measured. Table 2 collectively shows the electrode potential at the predetermined current density obtained from the potential-current curve, including the cases of Test Examples 1 to 8 in which NaPF ₆ is added as an electrolyte. In this table, the composition of the organic solvent was (nitriles) :( solvent other than nitriles) = 50: 50 (volume ratio). The mixing ratio when two types of solvents other than nitriles were mixed was 1: 1 (volume ratio).

From Table 2, in the case of an electrolytic solution for a lithium ion battery using LiPF ₆ as an electrolyte, the chain saturated hydrocarbon dinitrile compound in which a nitrile group is bonded to both ends of the chain saturated hydrocarbon compound, the end of the chain ether compound. At least one nitrile compound of chain ether nitrile compound and cyanoacetate having at least one nitrile group bonded thereto and at least one of cyclic carbonate, cyclic carboxylic acid ester and chain carbonate It can be seen that the potential window widens in the positive direction.

Here, in the case of the leftmost column of Table 2 (that is, the electrolyte is LiPF ₆ and nitriles: EC: DMC = 50: 25: 25) and the rightmost column of Table 2 (that is, When the electrolyte is NaPF ₆ and compared with the case of nitriles: EC: DMC = 50: 25: 25), the range of the potential window is as long as the composition of the organic solvent is the same even if the electrolyte is different. It can be seen that the tendency is almost the same. That is, the range of the potential window is approximately determined by the composition of the organic solvent regardless of the type of metal ions in the electrolyte. Therefore, even when the electrolyte is NaPF ₆ , as in the case of LiPF ₆ , the chain saturated hydrocarbon dinitrile compound or the chain ether compound in which the nitrile group is bonded to both ends of the chain saturated hydrocarbon compound is used. Included are at least one nitrile compound of a chain ether nitrile compound and a cyanoacetate ester having a nitrile group bonded to at least one end, and at least one of a cyclic carbonate ester, a cyclic carboxylate ester and a chain carbonate ester. It can be seen that the potential window widens in the positive direction.

If the above-mentioned electrolytic solution containing a nitrile compound is used, Li ₄ Ti ₅ O ₁₂ can be operated as an electrode active material. Hereinafter, specific embodiments will be described in detail.
As lithium titanate Li ₄ Ti ₅ O ₁₂ , XA-106 manufactured by Ishihara Sangyo was used. The characteristics are shown in Table 3.

<Manufacture of electrode material pellets>
The Li ₄ Ti ₅ O ₁₂ powder, acetylene black, and polytetrafluoroethylene (PTFE) were weighed in a ratio (weight ratio) of 80: 25: 5, mixed in a mortar, and then disked by a hot press method. Shaped electrode material pellets were molded.

-Evaluation-
(CV measurement)
For the negative electrode of the example produced as described above, cyclic voltammetry was measured under the following conditions.
The electrolytic solution used was a solution in which NaPF ₆ was dissolved in a mixed organic solvent of ethylene carbonate (EC): dimethyl carbonate (DMC): sebaconitrile = 25 :: 25: 50 to a concentration of 0.5 mol / L. The disc-shaped electrode material pellet produced as described above was immersed in this electrolytic solution, and Na metal was immersed as a counter electrode and a reference electrode. The potential sweep rate was 5 mV / sec, and the potential sweep was performed in the range of 0.4 to 2.0 V (0.7 to 2.3 V in terms of lithium reference electrode) with respect to the sodium reference electrode.

As a result, as shown in FIG. 4, a clear oxidation / reduction current flows between 0.4 and 2.0 V with respect to the sodium reference electrode, and Li ₄ Ti ₅ O ₁₂ contained in the disk-shaped electrode material pellet. Was found to be chargeable / dischargeable as an electrode active material. The oxidation-reduction potential appears in the vicinity of 0.9 V with respect to the sodium reference electrode (1.2 V with respect to the lithium reference electrode), and this potential is a potential at which lithium ions of Li ₄ Ti ₅ O ₁₂ are desorbed. Since it is lower than (1.55 V with respect to lithium reference electrode), it was found that it can operate sufficiently as an electrode active material of a sodium ion battery. In addition, since this potential is higher than 0.3 V (vs. lithium reference electrode), which is the potential at which sodium metal is deposited, when this Li ₄ Ti ₅ O ₁₂ is used as the negative electrode active material, sodium is deposited during the charging reaction. A sodium ion battery that can be prevented and is excellent in safety can be configured.

<Example of sodium ion battery>
Prepare a disk-shaped electrode material pellet prepared as described above and a sodium pellet obtained by punching a metal sodium plate in the same shape, and use a coin cell made of stainless steel (SUS316L model 2032) as a battery container. Produced. That is, as shown in FIG. 5, a bottomed cylindrical electrode can 11 made of SUS316L and a bottomed cylindrical flat electrode cap 12 made of SUS316L were prepared.

Then, as shown in FIG. 6, the electrode can 11 is filled with a spacer 13 made of SUS316L, a sodium pellet 14, and a separator 15. On the other hand, a wave washer 16 made of SUS316L, a spacer 17 made of SUS316L, and an electrode material pellet 18 are filled in the electrode cap 12. And after putting a nitrile compound containing electrolyte in the electrode can 11, the electrode cap 12 is mounted via the insulating gasket 19, and it seals, and it is set as a sodium ion battery. Here, the nitrile compound-containing electrolyte was an electrolyte having the following composition.
This is a solution in which NaPF ₆ is dissolved in a mixed solvent of ethylene carbonate: dimethyl carbonate: sebaconitrile = 25: 25: 50 so as to be 0.5 mol / L.

-Evaluation-
(Charge / discharge characteristics measurement)
About the sodium ion battery comprised as mentioned above, the charging / discharging characteristic in a constant current was measured. The discharge rate was 0.05 C, and the relationship between the amount of discharge electricity and the negative electrode potential was determined. As a result, as shown in FIG. 7, it was found that charging / discharging was sufficiently possible.

In the sodium ion battery of the example, the positive electrode active material is Li ₄ Ti ₅ O ₁₂ and the negative electrode active material is sodium metal, but a positive electrode that operates at about 5 V (vs lithium reference electrode) instead of sodium metal (for example, 4 If .8V NaCoPO4 or the like is used, Li ₄ Ti ₅ O ₁₂ acts as a negative electrode active material, and the electromotive force is increased (for example, in the case of NaCoPO ₄ 4.8-1.2 = 3.6 V). In addition, as a positive electrode active material for a sodium ion battery, NaNi _0.5 Fe _0.5 O ₂ , NaNi _0.5 Ti _0.5 O ₂ , NaFePO _4, or the like can be used (see FIGS. 8 to 10). Source: Seminar material “Technical Information Association: Higher capacity, improved safety and material trends in lithium secondary batteries”

<Sodium ion battery>
The present invention is applied to a sodium ion battery.
Here, the sodium ion battery includes an electrolytic solution, a positive electrode, a negative electrode, a separator, and a case.

(Electrolyte)
The electrolytic solution contains Na salt (electrolyte) and an organic solvent.
As the Na salt, those conventionally known as Na salts for Na ion batteries can be used. For example, for example, NaClO ₄ , NaPF ₆ , NaBF ₄ , NaCF ₃ SO ₃ , NaN (CF ₃ SO ₂ ) ₂ , NaN (FSO ₂ ) ₂ , NaN (C ₂ F ₅ SO ₂ ) ₂ , NaC (CF ₃ SO ₂ ) ₃ etc. are mentioned. The mixing ratio of the solvent and the solute is not particularly limited and is appropriately set according to the purpose.
Moreover, it is also preferable to add about 0.1 to 3% of various additives (for example, vinylene carbonate, fluoroethylene carbonate, ethylene sulfite). Thereby, a corrosion-resistant film is formed on the negative electrode side, and the corrosion resistance is improved.

As the organic solvent, a general one used for a Na ion battery can be adopted. As such an organic solvent, one kind or two or more kinds selected from cyclic carbonates, cyclic carboxylic acid esters and chain carbonates are preferable. More preferably, a cyclic carbonate and a chain carbonate are used in combination. Specifically, it is particularly preferable to use ethylene carbonate and dimethyl carbonate in combination. The blending ratio of both is not particularly limited. As the cyclic carboxylic acid ester, γ-butyrolactone or propylene carbonate can be used.
As the chain carbonate, in addition to dimethyl carbonate, diethyl carbonate or ethyl methyl carbonate can be used.
Furthermore, a nitrile compound can be used as an organic solvent. Here, as the nitrile compound, a chain saturated hydrocarbon dinitrile compound in which a nitrile group is bonded to both ends of a chain saturated hydrocarbon compound, a chain ether in which a nitrile group is bonded to at least one terminal of a chain ether compound. Mention may be made of at least one nitrile compound among nitrile compounds and cyanoacetic acid esters.

Examples of the chain saturated hydrocarbon dinitrile compound in which nitrile groups are bonded to both ends of the chain saturated hydrocarbon compound include succinonitrile NC (CH ₂ ) ₂ CN, glutaronitrile NC (CH ₂ ) ₃ CN, adiponitrile In addition to linear dinitrile compounds such as NC (CH ₂ ) ₄ CN, sebaconitrile NC (CH ₂ ) ₈ CN, dodecanedinitrile NC (CH ₂ ) ₁₀ CN, 2-methylglutaronitrile NCCH (CH ₃ ) _CH 2 _CH 2 CN may have a branched as such. These chain saturated hydrocarbon dinitrile compounds are not particularly limited in carbon number, but are preferably 7 to 20. More preferably, it is 10-12.

Examples of the chain ether nitrile compound in which a nitrile group is bonded to at least one end of the chain ether compound include oxydipropionitrile NCCH ₂ CH ₂ —O—CH ₂ CH ₂ CN and 3-methoxypropionitrile CH _{3. -O-CH} 2 CH 2 CN, and the like. These chain ether nitrile compounds are not particularly limited in carbon number, but are preferably 20 or less.
Examples of cyanoacetic acid esters include methyl cyanoacetate, ethyl cyanoacetate, propyl cyanoacetate, and butyl cyanoacetate. These cyanoacetic acid esters are not particularly limited in carbon number, but are preferably 20 or less.

These nitrile compounds have the effect of expanding the potential window particularly in the positive direction in the electrolytic solution.
A dinitrile compound is preferable from the viewpoint of the action of expanding the potential window. Of these, the use of sebacononitrile is more preferable.
However, since a nitrile compound has high viscosity, it is preferable to use together with the above-mentioned chain carbonate ester, cyclic carbonate ester, and / or cyclic carboxylic acid ester. More preferably, a nitrile compound, a chain carbonate ester and a cyclic carbonate ester are used in combination. Dimethyl carbonate can be employed as the chain carbonate, and ethylene carbonate can be employed as the cyclic carbonate.
In this case, the blending ratio of the nitrile compound in the whole organic solvent is preferably 1 to 90% by volume. More preferably, it is 15-70 volume%, More preferably, it is 30-50 volume%.

  The concentration of Na salt is 0.01 mol / L or more, which is lower than the saturated state. If the concentration of Na salt is less than 0.01 mol / L, ion conduction due to Na ions is reduced, and the electric resistance of the electrolytic solution is increased, which is not preferable. On the other hand, exceeding the saturated state is not preferable because a dissolved Na salt is precipitated due to environmental changes such as temperature.

(Positive electrode)
The positive electrode includes a positive electrode active material and a current collector.
The positive electrode active material refers to a “substance that can be reversibly oxidized and reduced by charge and discharge as a positive electrode of a secondary battery”. Further, the positive electrode active material of the sodium ion battery is required to be a material capable of reversibly intercalating and deintercalating sodium ions.
Examples of such a positive electrode active material include NaFeO ₂ , NaNiO ₂ , NaCoO ₂ , NaMnO ₂ , NaFe _1-x M ¹ _x O ₂ , and NaNi _1-x M ¹ _{x described in} JP2009-129741A. O ₂ , NaCo _1-x M ¹ _x O ₂ , NaMn _1-x M ¹ _x O ₂ (where M ¹ is one or more elements selected from the group consisting of trivalent metals, and 0 ≦ x <0 .5)) and the like. Among these, a composite oxide mainly containing iron and sodium, and using a composite oxide having a hexagonal crystal structure as a positive electrode active material, a high discharge voltage can be obtained. A secondary battery having a high density can be obtained.

More preferably, the positive electrode active material is a composite oxide mainly containing iron and sodium, has a hexagonal crystal structure, and has an interplanar spacing of 2 in the X-ray diffraction analysis of the composite oxide. A complex oxide having a value obtained by dividing the intensity of a peak of 20 angstroms by the intensity of a peak of 5.36 angstroms in plane spacing is 2 or less. In heating a metal compound mixture containing a sodium compound and an iron compound in a temperature range of 400 ° C. or more and 900 ° C. or less, the atmosphere is heated as an inert atmosphere in a temperature range of less than 100 ° C. during the temperature increase. Is preferred.
Moreover, what doped the transition metal atom of these compounds with the other metal atom may be used. The dopant is not particularly limited as long as the electrochemical characteristics can be changed in the oxidation-reduction reaction.

(Current collector)
A current collector is a conductive substrate carrying a positive electrode active material.
The molding material for the current collector of the positive electrode is required to be stable during charging. In particular, when using a phosphate-based positive electrode active material having an olivine crystal structure with a high oxidation-reduction potential, it is necessary to use a conductive metal material having excellent corrosion resistance such as Al, Ti, Ni, SUS304, SUS316, and SUS316L. preferable.
In addition, a material obtained by coating a conductive metal material such as Al or Ti with conductive DLC (diamond-like carbon) by a known method can also be used. Here, the conductive diamond-like carbon is carbon having an amorphous structure in which both diamond bonds (SP ₃ hybrid orbital bonds between carbons) and graphite bonds (SP ₂ hybrid orbital bonds between carbons) are mixed. Among them, the one whose conductivity is 1000 Ωcm or less. However, in addition to the amorphous structure, those having a phase composed of a crystal structure partially composed of a graphite structure (that is, a hexagonal crystal structure composed of SP ₂ hybrid orbital bonds) and thereby exhibiting conductivity are also included. . Diamond-like carbon having properties intermediate between graphite and diamond adjusts the conductivity by adjusting the ratio of SP ₂ hybrid orbital bonds and SP ₃ hybrid orbital bonds of the carbon atoms constituting diamond-like carbon during film formation. be able to.
Of course, you may coat | cover the said corrosion-resistant electroconductive metal material with electroconductive DLC.
The shape and structure of the current collector can be arbitrarily designed according to the structure of the positive electrode active material and the battery.

(Pretreatment of positive electrode)
The positive electrode for a sodium ion battery is subjected to an immersion treatment step of immersing the positive electrode in a pretreatment electrolyte solution in which a sodium salt is dissolved in an organic solvent containing 1% by volume or more of a nitrile compound before being incorporated in the sodium ion battery, Furthermore, a positive voltage processing step of applying a positive voltage to the electrodes can be performed. The electrode thus pretreated has a wide potential window even when used in an electrolyte solution containing no nitrile compound or an electrolyte solution with a small amount of nitrile compound added. It becomes difficult to disassemble. The reason why the electrode has such a wide potential window is presumed to be that a corrosion-resistant film containing nitrogen as a component is formed on the electrode.

(Negative electrode)
The negative electrode includes a negative electrode active material and a current collector.
The negative electrode active material is a “substance that allows sodium ions to enter and exit as a negative electrode of a secondary battery and reversibly repeat oxidation and reduction”. In the present invention, Li ₄ Ti ₅ O ₁₂ is used.

  The current collector can be formed of a general-purpose conductive metal material, such as Cu, Al, Ni, Ti, SUS304, SUS316, and SUS316L. However, it is necessary to appropriately select the current collector according to the Na salt in the electrolytic solution.

(Electroconductive member for positive electrode)
Some positive electrode active materials have low electrical conductivity. Therefore, it is preferable to provide a sufficient electron conduction path between the positive electrode active material and the current collector by interposing a conductive electron conduction member.
Here, the form of the electron conducting member is not particularly limited as long as an electron conducting path can be formed between the positive electrode active material and the current collector. For example, carbon black such as acetylene black, graphite powder, diamond-like carbon, glassy Conductive powder (conductive aid) such as carbon can be used. Diamond-like carbon and glassy carbon have a much wider potential window than carbon black and graphite, and are excellent in corrosion resistance when a high potential is applied, and therefore can be suitably used. Moreover, it is also preferable that metal particles for electrochemical reaction are supported on these conductive assistants. Examples of the metal include Pt, Ru, Rh, Pd, Ag, Ir, Au, Ni, and Cu. These may be used alone or an alloy thereof.
As the electron conductive material, a conductive film (such as a DLC film) covering the positive electrode active material or a conductive thin film (such as a gold thin film) in which the positive electrode active material is embedded can be used.
In particular, since the conductivity of the positive electrode active material itself and / or its surface coating is small, if it is simply supported on the current collector and does not function as the positive electrode of a sodium ion battery, the positive electrode active material For performance evaluation, this can be physically driven into a conductive thin film such as gold with a hammer or the like to form the positive electrode of the battery.

(Electroconductive member for negative electrode)
The thing similar to the electron conductive member for positive electrodes can be used.

(Separator)
The separator is immersed in the electrolytic solution, separates the positive electrode and the negative electrode to prevent a short circuit therebetween, and allows the passage of Na ions.
Examples of such separators include porous films made of polyolefin resins such as polyethylene and polypropylene.

(Case)
The case is formed of a material having corrosion resistance against the electrolytic solution. The shape can be arbitrarily designed according to the intended use of the battery.
When the case also serves as a current collector or is electrically coupled to the current collector, the case is formed of the same or the same material as the current collector forming material of each electrode.

  The present invention is not limited to the description of the embodiment of the invention. Various modifications may be included in the present invention as long as those skilled in the art can easily conceive without departing from the description of the scope of claims.

DESCRIPTION OF SYMBOLS 11 ... Electrode can 12 ... Electrode cap 13, 17 ... Spacer 14 ... Sodium pellet 15 ... Separator 16 ... Wave washer 18 ... Electrode material pellet 19 ... Insulation gasket

Claims (2)

A sodium ion battery in which a positive electrode including a positive electrode active material and a negative electrode including a negative electrode active material are in contact with an electrolyte solution in which a sodium salt is dissolved in an organic solvent,
The negative active material viewed contains a _Li 4 _Ti 5 _{O 12,}
The organic solvent of the electrolyte includes a chain saturated hydrocarbon dinitrile compound in which a nitrile group is bonded to both ends of the chain saturated hydrocarbon compound, and a chain formula in which a nitrile group is bonded to at least one end of the chain ether compound. At least one nitrile compound of an ether nitrile compound and a cyanoacetic acid ester;
At least one of cyclic carbonate, cyclic carboxylic acid ester and chain carbonate,
Contains
A sodium ion battery, wherein a blending ratio of the nitrile compound in the whole organic solvent is 1 to 90% by volume. The sodium salts are NaClO ₄ , NaPF ₆ , NaBF ₄ , NaCF ₃ SO ₃ , NaN (CF ₃ SO ₂ ) ₂ , NaN (FSO ₂ ) ₂ , NaN (C ₂ F ₅ SO ₂ ) ₂ , NaC (CF ₃ SO _2. The sodium ion battery according to claim 1, wherein at least one of ₂ ) and ₃ is included.

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