JP2013182741A - Separator and electrochemical cell including the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for an electrochemical cell, capable of withstanding a high voltage exceeding 4.5 V, and an electrochemical cell including the separator.SOLUTION: A separator 10 is inserted between a positive electrode active material pellet 14 and a lithium negative electrode active material 18 so as to be brought into contact with both electrodes. The separator 10 is formed of two porous films one of which in contact with the positive electrode active material pellet 14 comprises a porous PTFE film 10a and the other of which in contact with on a side of the lithium negative electrode active material 18 comprises a PE film 10b. A phosphoric acid-based positive electrode material such as LiNiPOand LiCoPOand an olivine fluoride-based positive electrode material having high charging voltage, such as LiNiPOF and LiCoPOF, can be used as a positive electrode active material constituting the positive electrode active material pellet 14.

Description

  The present invention relates to a separator that can be suitably used for an electrochemical cell that operates at a high voltage, and an electrochemical cell using the separator.

  In electrochemical cells such as Li-ion batteries, Na-ion batteries, and capacitors, a porous insulating film called a separator is used so as to allow passage of ions while preventing a short circuit between the positive electrode and the negative electrode. As a material for such a separator, a porous film made of polyethylene (hereinafter referred to as “PE”) or polypropylene (hereinafter referred to as “PP”) is used.

  However, in recent years, in the field of electrochemical cells, it has been required to obtain as high a voltage as possible with a single cell, and positive electrode materials, negative electrode materials, and electrolytic solutions that enable higher voltages have been developed. With the development of such new electrode materials and electrolytes, separators that can withstand high voltages are also required.

  On the other hand, Patent Document 1 and Patent Document 2 have a description relating to a separator having a multilayer structure.

JP 2006-261093 A Japanese Patent Laid-Open No. 10-302749

  In a separator such as a conventional lithium ion battery, when the voltage of a single cell exceeds 4.5 V, an extra reaction current unrelated to normal charging flows or the battery deteriorates. There has been a problem of short circuit between the negative electrode and the positive electrode.

  This invention is made | formed in view of the said conventional situation, and makes it the problem which should be solved to provide the separator for electrochemical cells which can endure a high voltage.

  In order to find a separator material that can withstand a high voltage exceeding 4.5 V, the present inventors have conducted extensive tests and research on various separator materials. However, PP has a limit of about 4.3V. For this reason, fluorine resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF) were also investigated as materials having excellent chemical stability, but it was found that they deteriorate on the side in contact with the negative electrode. .

  Therefore, the present inventors use a separator made of different materials for the positive electrode side and the negative electrode side, use a material with high oxidation resistance on the positive electrode side where strong oxidizing power acts, and on the negative electrode side where strong reducing action acts. As a result, it has been possible to design a multilayer separator using a material that can withstand contact with a metal that is highly oxidizable such as lithium metal and sodium metal. As a result of further intensive studies, the present invention was completed.

  That is, the electrochemical cell of the present invention is an electrochemical cell in which a separator is inserted in close contact between a positive electrode and a negative electrode, and the separator is a porous multilayer film. Of these, the film in contact with the positive electrode side is made of polytetrafluoroethylene, and the film in contact with the negative electrode side of the multilayer film is made of polyethylene or polypropylene.

In the electrochemical cell of the present invention, the porous separator is a multilayer film, and the film in contact with the positive electrode side where strong oxidizing power acts on the multilayer film is a chain saturated hydrocarbon having high oxidation resistance. In this case, a polymer in which hydrogen is completely substituted with fluorine is used. For this reason, the positive electrode side surface of the separator is not discolored or deteriorated even at a high voltage exceeding 4.5V. Among the multilayer films, the polymer in contact with the negative electrode side on which a strong reducing action acts is made of a polymer composed of chain saturated hydrocarbons that can withstand highly reducing metals such as lithium and sodium that are deposited by a low potential. ing. For this reason, the negative electrode side surface of the separator is not decomposed or deteriorated.
Therefore, according to the electrochemical cell of this invention, it becomes an electrochemical cell in which the function of a separator does not fall easily also to a high voltage.

Examples of the polymer in which hydrogen of the chain saturated hydrocarbon is completely substituted with fluorine include polytetrafluoroethylene, polyhexafluoropropylene, tetrafluoroethylene-hexafluoropropylene copolymer, and the like. Moreover, the material which mixed these may be sufficient.
On the other hand, examples of the polymer composed of chain saturated hydrocarbon include polyethylene, polypropylene, and ethylene-propylene copolymer. Moreover, the material which mixed these may be sufficient.

The type of electrochemical cell is not particularly limited as long as it is an electrochemical cell in which a separator is inserted in close contact between a positive electrode and a negative electrode. Examples of such electrochemical cells include alkali metal ion batteries such as Li ion batteries and Na ion batteries, capacitors, and the like. When the electrochemical cell is an alkali metal ion battery, it is possible to use a positive electrode material capable of obtaining a high discharge voltage such as LiCoPO ₄ or NaFeO _2, so that the voltage and energy density of the battery can be increased. .

  Further, from the viewpoint of further improving the withstand voltage characteristics of the separator, it is also preferable to include an insulating ceramic in the separator. Examples of such an insulating ceramic include alumina powder and silica powder.

Potential at the LiBF ₄ solution in the electrode formed by molding DLC powder PTFE powder as a binder - a current curve. It is a perspective view of the separator of an embodiment. It is sectional drawing of the battery case used for the lithium ion battery of embodiment. It is sectional drawing of the lithium ion battery of embodiment.

  The separator of the present invention is immersed in an electrolytic solution, separates the positive electrode and the negative electrode to prevent a short circuit therebetween, and allows the passage of ions. The separator is a multilayer film. As a film in contact with the positive electrode side on which strong oxidizing power acts, a film made of a polymer in which hydrogen of chain saturated hydrocarbon is completely substituted with fluorine (for example, PTFE) having high oxidation resistance is used. Yes. On the other hand, a film in contact with the negative electrode side on which a strong reducing action acts in a multilayer film is a polymer (for example, polyethylene or polypropylene) made of a chain saturated hydrocarbon that can withstand a highly reducing metal such as lithium and sodium deposited by a low potential. ) Is used. For this reason, even if the voltage exceeds 4.5 V, the positive electrode side surface of the separator is not discolored or deteriorated, and the negative electrode side surface of the separator is not decomposed or deteriorated.

(Experimental example 1)
In order to investigate how high a potential is stably present when a porous polytetrafluoroethylene film is used as a separator material for an electrochemical cell, Experimental Example 1 shown below was performed. That is, diamond-like carbon (DLC) powder (average particle size 0.03 μm) and PTFE powder were mixed at a weight ratio of 80:20, and a disk-shaped electrode of 8 mmφ was produced by hot pressing.

Measurement of potential-current curve The electrode of Experimental Example 1 produced as described above was subjected to potential scanning in an electrolyte for a lithium ion battery, and a potential-current curve was measured. The electrolytic solution was prepared as follows. That is, a mixed solution was prepared so that ethylene carbonate, dimethyl carbonate, and sebaconitrile were in a volume ratio of 25:25:50, and a solution in which LiBF ₄ was dissolved as a lithium electrolyte so as to be 1 mol / L was used as the electrolytic solution. And placed in a three-electrode electrolytic cell container. In the measurement of the potential-current curve, the above electrode (area 1.0 cm ² ) was used as a working electrode, a platinum net as a counter electrode, and Li metal as a reference electrode. The sweep speed was 5 mV / sec.

• Measurement results The measurement results are shown in FIG. Diamond-like carbon is known to have a wide potential window, but in the electrode of Experimental Example 1 in which diamond-like carbon powder was molded using PTFE as a binder, around 6.7 V with respect to the (Li / Li ⁺ ) electrode. Almost no oxidation current flowed. In addition, after the measurement of the potential-current curve, no change was observed before and after potential scanning even when the surface of the electrode was observed with the naked eye, an optical microscope, or FESEM. From this result, it was found that PTFE was not oxidized to a high potential of 6.7 V with respect to the (Li / Li ⁺ ) electrode and could exist stably.

Hereinafter, embodiments of a separator of the present invention and a lithium secondary battery using the separator will be described.
<Preparation of separator>
As shown in FIG. 2, the separator 10 of the embodiment is configured by laminating a circular PTFE film 10a and a circular PE film 10b having the same diameter. The PTFE membrane 10a is a porous membrane made of polytetraethylene (for example, LSWP03700, LSWP09025 made by Nihon Millipore), and the PE membrane 10b is a porous membrane made of polyethylene (for example, PP single layer: 2400, 2500 made by Celgard). PE single layer: Celgard's symbolic single layer PE separator, 3-layer PP / PE / PP membrane: Celgard's, 2320). Here, the PTFE film 10a and the PE film 10b may be simply overlapped, or may be integrated by thermocompression bonding while maintaining porosity by calendaring or the like. Further, a PP (polypropylene) porous film may be used instead of the PE (polyethylene) porous film. Furthermore, from the viewpoint of further improving the withstand voltage characteristics of the separator, the separator may contain an insulating ceramic such as alumina powder or silica powder.

<Production of lithium ion battery>
As an electrochemical cell using the separator of the present invention, a production example of a lithium ion battery is shown below.
First, as shown in FIG. 3, a bottomed cylindrical titanium or nickel positive electrode can 11 and a bottomed cylindrical and flat titanium or nickel negative electrode cap 12 are prepared. The positive electrode can 11 and the negative electrode cap 12 constitute a battery case.

Next, as shown in FIG. 4, the positive electrode can 11 is filled with a titanium or nickel spacer 13, a positive electrode active material pellet 14, a PTFE film 10a, and a PE film 10b in this order. On the other hand, the negative electrode cap 12 is filled with a wave washer 16 made of titanium or nickel, a spacer 17 made of titanium or nickel, and a lithium negative electrode active material 18. And after putting electrolyte solution in the positive electrode can 11, the negative electrode cap 12 is mounted and crimped | sealed through the insulating gasket 19, and it is set as a lithium ion battery.
Examples of the positive electrode active material constituting the positive electrode active material pellet 14 include Li _1-x NiPO ₄ having an even higher charging voltage in addition to an oxidized positive electrode active material such as Li _1-x CoO ₂ and Li _1-x NiO ₂ . Examples thereof include phosphoric acid-based positive electrode active materials such as Li _1-x CoPO ₄ , olivine fluoride-based positive electrode active materials such as Li _2-x NiPO ₄ F and Li _2-x CoPO ₄ F, and FeF ₃ having a perovskite structure. Further, lithium as the negative electrode active material constituting the negative electrode active material 18, artificial graphite, natural graphite, various carbon materials and lithium titanate _{(Li 4} Ti ₅ O _12), such as hard carbon, _{H 2 Ti} 12 _{O 25,} such as _{H 2 Ti 6 O 13, Fe} 2 O 3 and the like.

In the lithium ion battery of the embodiment configured as described above, the PTFE film 10a having high oxidation resistance is used as a film in contact with the positive electrode active material pellet 14 (that is, positive electrode) side on which a strong oxidizing action works. In addition, a PE film 10b that can sufficiently withstand the reducing power of lithium is used as a film that is in contact with the lithium negative electrode active material 18 (that is, the negative electrode) side on which a strong reducing action acts. For this reason, even if the voltage exceeds 4.5 V, the positive electrode side surface of the separator is not discolored or deteriorated, and the negative electrode side surface of the separator is not decomposed or deteriorated. Therefore, phosphoric acid-based positive electrode active materials such as Li _1-x NiPO ₄ and Li _1-x CoPO ₄ having a high charging voltage, and olivine fluorides such as Li _2−x NiPO ₄ F and Li _2−x CoPO ₄ F Even in a lithium ion battery using a positive electrode active material or the like, the separator 10 does not deteriorate.

  The separator of the present invention can be applied to alkali metal secondary batteries such as lithium ion batteries and sodium ion batteries shown below.

-Lithium ion battery-
Here, the lithium ion battery includes an electrolytic solution, a positive electrode, a negative electrode, a separator, and a case.

(Electrolyte)
The electrolytic solution contains a Li salt (electrolyte) and an organic solvent.
(1) As the Li salt, a general Li salt for a Li ion battery can be used. For example, LiPF ₆ (lithium hexafluorophosphate), LiBF ₄ (lithium tetrafluoroborate), LiTFSI (lithium bis (trifluoromethanesulfonyl) imide), LiTFS (lithium trifluoromethanesulfonate), LiBETI (lithium bis ( Pentafluoroethanesulfonyl) imide), LiB ₁₂ F _12-x H _x , and borate anions containing B, such as LiBOB (Lithium bis [oxalate] borate), LiBCB (Lithium bis [croconato] borate), LiCSB ( Li bis [croconato salicylato] borate), LiBSB (Li bis [salicylato (2-)] borate), LiBBB (Li bis [1,2-benzenediolato (2-)] borate), LiBNB (Li bis [2,3- naphtalene-diolato (2-)-O, O ′] borate), LiBBPB (Libis [2,3-biphenyldiolato (2-)-O, O ′] borate), or two or more thereof can be used. Among these, the use of a lithium salt that can be used even when the oxidation-reduction potential of the positive electrode is 4.5 V or more is particularly preferable because the high voltage resistance characteristic of the separator of the present invention can be effectively exhibited. As such an electrolyte, it is preferable to use at least one of LiPF ₆ , LiBF ₄ and LiBOB. Further, when LiTFSI, LiTFS, or LiBETI is used, it is preferable to add at least one of LiPF _6, LiBF ₄ and LiBOB.

As the organic solvent, a general solvent used for a Li ion battery can be adopted. Such an organic solvent is preferably one or more selected from cyclic carbonates, cyclic carboxylic acid esters and chain carbonates. 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, propylene carbonate (PC), or the like can be used. As the chain carbonate, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and the like can be used. Furthermore, a nitrile compound can be used as the 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, or a chain cyano 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 ether 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 NC (CH ₂ ) ₄ CN, Pimeonitrile NC (CH ₂ ) ₅ CN, Suberonitrile NC (CH ₂ ) ₆ CN, Sebacononitrile NC (CH ₂ ) ₈ CN, Dodecandinitrile NC (CH ₂ ) ₁₀ CN, etc. In addition to the chain dinitrile compound, it may have a branch such as 2-methylglutaronitrile NCCH (CH ₃ ) CH ₂ CH ₂ CN. These chain saturated hydrocarbon dinitrile compounds are not particularly limited in carbon number, but are preferably 20 or less. More preferably, it is 7-12.

Examples of the chain cyanoether 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 cyano ether compounds are not particularly limited in carbon number, but are preferably 20 or less.
Further, when a chain cyanoether compound is used in the electrolytic solution, it is also preferable to add BF ₃ . BF ₃ exhibits properties as a strong Lewis acid because a boron atom is bonded to three fluorine atoms having a high electronegativity. For this reason, the vacant orbit of BF ₃ is strongly coordinated to the oxygen lone pair present in the ether molecule to form a complex, and the electron density of oxygen decreases. As a result, the oxidation resistance of the ether molecules is improved, and the electrolytic solution has a wide potential window. In particular, if BF ₃ is added to a cyanoether compound having two or more nitrile groups, the electron withdrawing effect of the nitrile group is increased, and the potential window can be further widened.
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 5-70 volume%, More preferably, it is 10-50 volume%.

  In addition, it is also preferable to add various additives (for example, vinylene carbonate, fluoroethylene carbonate, ethylene sulfite, propane sultone) to the electrolyte solution in an amount of about 0.1 to 3% by weight. Thereby, a corrosion-resistant film is formed on the negative electrode side, and the corrosion resistance is improved.

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

  In addition, it is preferable that the water content in the electrolytic solution is as small as possible. This is because when the water content is high, the lithium ion electrolyte is difficult to dissolve, and as a result of the lithium ions, the specific conductivity of the electrolytic solution is lowered, and the internal resistance and overvoltage of the battery are increased. For this reason, in the adjustment of the electrolytic solution, it is preferable that the water content of the solvent be 100 ppm or less with a dehydrating agent such as zeolite. More preferred is 50 ppm or less, and most preferred is 30 ppm or less.

(Positive electrode)
The positive electrode includes a positive electrode active material and a current collector.
(Positive electrode active material)
The positive electrode active material refers to “a material in which lithium is inserted / extracted in the crystal structure at a higher potential than the negative electrode, and oxidation / reduction is performed accordingly”.
Examples of the positive electrode active material include (1) oxides, (2) phosphates having an olivine crystal structure, (3) olivine fluorides, and (4) metal fluorides.

(1) Oxide-based 1-1 specific substances As oxide-based materials, Li _1-x CoO ₂ (x = 0 to 1: layered structure), Li _1-x NiO ₂ (x = 0 to ₁ : layered structure) ), Li _1-x Mn ₂ O ₄ (x = 0 to 1: spinel structure), Li _2−y MnO ₃ (y = 0 to 2) and their solid solutions (herein, solid solutions are those of the above oxide series) In the positive electrode active material, it refers to a material in which metal atoms are mixed in a free ratio. Here, x varies depending on lithium intercalation-deintercalation accompanying charging and discharging. Moreover, what doped the metal atom of these with the other metal atom is also contained. The dopant is not particularly limited as long as the electrochemical characteristics can be changed in the oxidation-reduction reaction. For example, one or more of Li, Mg, Al, Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, and Mo can be used.
1-2 Characteristics The general discharge potential of this positive electrode active material is less than 5 V (vs Li / Li ⁺ ). However, LiNi _0.5 Mn _1.5 O ₄ partially substituted with Ni in the LiMn ₂ O ₄ system has a discharge potential of 4.7 V, and takes into account overvoltage when performing rapid charging, and 5 V May require a charging voltage exceeding. Further, since LiCoMnO ₄ starts with a discharge voltage of about 5.2V, the charge voltage also exceeds 5V. Such a positive electrode active material having a high charging voltage can be suitably used because it can effectively exhibit the high voltage resistance characteristic of the separator of the present invention. In addition, oxide systems generally decompose at less than 300 ° C., and have a relatively large exothermic reaction as oxygen is generated. Therefore, a control circuit that does not cause overcharging is required.

(2) Phosphate-based 2-1 specific substance having olivine-type crystal structure As phosphate-based substances having olivine-type crystal structure, Li _1-x NiPO ₄ (x = 0 to ₁ ), Li _1-x CoPO ₄ (x = 0 to ₁ ), Li _1-x MnPO ₄ (x = 0 to ₁ ), Li _1-x FePO ₄ (x = 0 to 1) and their solid solutions (herein the solid solution is the above-mentioned phosphorus In the acid salt positive electrode active material, it refers to a material in which metal atoms are mixed in a free ratio. Here, x varies depending on lithium intercalation-deintercalation accompanying charging and discharging. Moreover, what doped the metal atom of these with the other metal atom is also contained. The dopant is not particularly limited as long as the electrochemical characteristics can be changed in the oxidation-reduction reaction. For example, one or more of Mg, Al, Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, and Mo can be used (see JP 2008-130525 A).
2-2 Characteristics The oxidation-reduction potential of this positive electrode active material is attracting attention because, unlike the above oxide system, if it is less than 300 ° C., the exothermic reaction is small, oxygen is not generated, and safety is high. Of the phosphate systems, the LiCoPO ₄ system has a discharge potential of about 4.8 V, and an electrolyte having a withstand voltage of 5 V or more is required for rapid charging. It is suggested that the discharge potential of LiNiPO ₄ is 5.2 V (vs Li / Li ⁺ ).

(3) Olivine fluoride system 3-1 Specific substances Li _2-x NiPO ₄ F (x = 0 to 2), Li _2-x CoPO ₄ F (x = 0 to 2) are known, and other Li _{2 -x MnPO 4 F (x = 0~2} ), Li 2-x FePO 4 F (x = 0~2) are considered. Here, x varies depending on lithium intercalation-deintercalation accompanying charging and discharging.
Further, these solid solutions (herein, the solid solution refers to a material in which metal atoms are mixed in a free ratio in the olivine fluoride-based positive electrode active material) can also be mentioned. Furthermore, the thing which doped the metal atom of these with another metal atom is also contained. The dopant is not particularly limited as long as the electrochemical characteristics can be changed in the oxidation-reduction reaction. For example, one or more of Mg, Al, Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, and Mo can be used.
3-2 Characteristics Similar to the olivine system, the redox potential of this positive electrode active material is different from that of the above oxide system. The effect of battery ignition is considered to be small, and is attracting attention in terms of safety. Moreover, the electric capacity density (mAh / g) of a battery can be made higher than the said phosphate system (refer Unexamined-Japanese-Patent No. 2003-229126). However, for example, the Li ₂ CoPO ₄ F system has an average discharge potential of about 4.8 V, and an electrolytic solution having a withstand voltage of 5 V or more is required for rapid charging. Further, the discharge potential of the Li ₂ NiPO ₄ F system is about 5.3 V (vs Li / Li ⁺ ), and an electrolytic solution having a withstand voltage at 5 V or more is required.

(4) FeF ₃ having a metal fluoride 4-1 specific materials and properties perovskite structure. This compound has a theoretical capacity density of 220 mAh / g when one Li ion is inserted per molecule, and the theoretical capacity density of a phosphate-based positive electrode active material having a conventional olivine type crystal structure (for example, LiFePO ₄ Has the advantage of having an energy density greater than 170 mAh / g).
Further, as a positive electrode active material having a discharge potential and energy density (Wh / kg) larger than that of FeF ₃ , (Li _x K _y Na _1-xy ) nMF ₃ (wherein x, y, and n are 0 or more) 1 or less, M is any one of Mn, Co and Ni, and a part of M is one or more of Mg, Cu, Co, Mn, Al, Cr, Ga, V and In. Which may be substituted). This positive electrode active material has a fluorine-based perovskite structure, and M is Mn, Co, or Ni as a constituent element. In the fully charged state, it becomes MF ₃ and M is considered to be a +3 state. In a complete discharge state, n = 1 and M is considered to be a +2 state. In this positive electrode active material, a redox wave is observed at a high potential of 4 V (vs Li / Li ⁺ ) or higher in cyclic voltammetry, and the positive electrode active material has a high discharge potential exceeding 4 V (vs Li / Li ⁺ ). This value is higher than the FeF ₃ discharge voltage ^{3.3V (vs Li / Li +)} , the energy density is positive electrode active material larger than FeF _3. Furthermore, since not only lithium ions but also sodium ions can enter and exit, it can be driven as a positive electrode active material of a lithium ion battery or a sodium ion battery.
(5) Others In addition, lithium-free FeF ₃ , conjugated polymers using organic conductive materials, chevrel phase compounds, and the like can also be used. In addition, transition metal chalcogenides, vanadium oxides and lithium salts thereof, niobium oxides and lithium salts thereof, and a plurality of different positive electrode active materials may be used in combination.
The average particle diameter of the positive electrode active material particles is not particularly limited, but is preferably 10 nm to 30 μm.

(Current collector for positive electrode)
The positive electrode 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 and olivine fluoride-based positive electrode active material having an olivine-type crystal structure with a high redox potential, it is preferable to use a material excellent in corrosion resistance.
For example, when LiPF ₆ or LiBF ₄ is used as the electrolyte, austenitic stainless steel, Ni, Al, Ti, or the like can be used, but it is preferable to select them appropriately in consideration of the operating potential of the positive electrode active material to be used. For example, when LiPF ₆ is used as the electrolyte, it can be used even at 6 V with respect to the Li / Li ⁺ electrode. However, when LiBF ₄ is used as the electrolyte, SUS304 is 5.8 V or less with respect to the Li / Li ⁺ electrode. It can be used only when charge / discharge is possible. More preferable are SUS316, SUS316L and SUS317 to which molybdenum is added for improving corrosion resistance. Further, when LiTFSI is used as the electrolyte, it is preferable that LiPF ₆ coexists in order to form a corrosion-resistant film on the surface of the positive electrode current collector. LiBETI and LiTFS are the same as in LiTFSI.
In addition, a conductive corrosion-resistant film made of one or more of conductive DLC (diamond-like carbon), glassy carbon, gold, and platinum is formed on a conductive metal material such as Al, Ni, titanium, and austenitic stainless steel. Can also be used as a current collector. When the electrolyte is a lithium salt such as LiBF ₄ or LiPF ₆ that easily forms a fluoride film, a thick fluoride film is formed on the aluminum and the corrosion resistance is improved, but the electronic conductivity is lowered, and consequently The increase in output accompanying the increase in ohmic overvoltage is impeded. If the conductive metal material such as Al is coated with the conductive DLC, the fluoride film is generated only in a very small area of the defective portion of the conductive DLC. For this reason, even if the voltage is increased, the decrease in electron conductivity is negligible, and it is possible to prevent the decrease in output due to the increased voltage, which is a concern.
Here, the conductive diamond-like carbon is carbon having an amorphous structure in which both bonds of 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 lithium ion battery performs an immersion treatment step of immersing the positive electrode in a pretreatment electrolytic solution in which a lithium salt is dissolved in an organic solvent containing 1% by volume or more of a nitrile compound before being incorporated in the lithium ion battery, Further, a positive voltage processing step for applying a positive voltage to the electrode is performed. The electrode thus pretreated has a wide potential window even when used in an electrolyte solution containing no nitrile compound or in a lithium ion battery using an electrolyte solution with a small amount of nitrile compound added. It becomes difficult to disassemble (see Japanese Patent Application No. 2009-180007). 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.
(Negative electrode active material)
The negative electrode active material refers to “a material in which lithium is inserted / extracted in the crystal structure at a lower potential than the positive electrode, and oxidation / reduction is performed accordingly”.
Examples of the negative electrode active material include various carbon materials such as artificial graphite, natural graphite, and hard carbon, lithium titanate (Li ₄ Ti ₅ O ₁₂ ), H ₂ Ti ₁₂ O ₂₅ , H ₂ Ti ₆ O ₁₃ , Fe ₂ O ₃ etc. are mentioned. Moreover, the composite material which mixed these suitably can also be mentioned. Furthermore, there may be mentioned Si fine particles and Si thin films, and fine particles and thin films in which these Si are Si-based alloys such as Si—Ni, Si—Cu, Si—Nb, Si—Zn, Si—Sn. Further, SiO oxide, Si-SiO ₂ composite, Si-SiO ₂ - can be given complex, etc., such as carbon.

(Current collector for negative electrode)
The current collector for the negative electrode can be formed of a general-purpose conductive metal material, Cu, Al, Ni, Ti, austenitic stainless steel, or the like.
However, when a nitrile compound is used for the electrolytic solution (including combined use with other organic solvents), it is necessary to select appropriately according to the Li salt in the electrolytic solution. That is, when LiPF ₆ or LiBF ₄ is used as the electrolyte, austenitic stainless steel, Ni, Al, Ti, or the like can be used. However, it is necessary to select appropriately according to the operating potential of the negative electrode active material to be used. In the case of using carbon or Si as the negative electrode active material, when LiBF ₄ is used as the electrolyte, a current collector made of Al, Ni, Ti, austenitic stainless or the like other than Cu can be used. In the case where lithium titanate or a Fe ₂ O ₃ based compound is used as the negative electrode active material, all of the above materials containing Cu are applicable. On the other hand, when LiPF _{6 is} used as the electrolyte, Al, Ni and Ti are preferable, and austenitic stainless steel and Cu are not preferable. In addition, when LiTFSI, LiBETI, or LiTFS is used as the electrolyte, any of Ni, Ti, Al, Cu, and austenitic stainless steel can be used.

(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 fine particles are supported on these conductive assistants. Examples of the metal fine particles include Pt, Au, Ni and the like. These may be used alone or an alloy thereof.
Further, conductive diamond-like carbon may be attached to the particles made of the positive electrode active material by a dry plating method. In this case, the conductive diamond-like carbon plays the role of a conductive auxiliary agent, and the electron conductivity that is a characteristic necessary for the positive electrode for the secondary battery is imparted. Furthermore, since conductive diamond-like carbon has a wide potential window and is excellent in durability against a high potential, a positive electrode active material having a high energy density in which a charging reaction is performed at a high potential can be effectively used. Similarly, glassy carbon can be used instead of diamond-like carbon.
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 Li ₂ NiPO ₄ F-based positive electrode active material has low conductivity of its own and / or its surface coating, it does not function as a positive electrode of a lithium ion battery if it is simply supported on a current collector. There is a case. In order to evaluate the performance of the Li ₂ NiPO ₄ F-based positive electrode active material, it 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.
Here, the Li ₂ NiPO ₄ F-based positive electrode active material refers to Li ₂ NiPO ₄ F and a material appropriately doped with dopant.

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

(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 using an electrolytic solution in which a lithium salt is dissolved, a base material made of austenitic stainless steel, a case made of Ti, Ni and / or Al can be used. However, there are cases where it is necessary to select appropriately depending on the operating potential of the positive electrode and negative electrode active material to be used.
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 case may be an Al laminate film in which an Al foil is covered with polyethylene (PE) or polypropylene (PP). The shape can be arbitrarily designed according to the intended use of the battery.

-Sodium ion battery-
The present invention can also be applied to sodium ion batteries.
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 , NaB ₁₂ F _12-x H _x , and borate anions containing B, such as NaBOB (Sodium bis [oxalate] borate), LiBCB (Sodium bis [croconato] borate), NaCSB (Sodium bis [croconato] salicylato] borate), NaBSB (Sodium bis [salicylato (2-)] borate), NaBBB (Sodium bis [1,2-benzenediolato (2-)] borate), NaBNB (Sodium bis [2,3-naphtalene-diolato ( 2-)-O, O ′] borate), NaBBPB (Sodium bis [2,3-biphenyldiolato (2-)-O, O ′] borate) and the like. 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 various additives (for example, vinylene carbonate, fluoroethylene carbonate, ethylene sulfite, propane sultone) about 0.1 to 3%. Thereby, it forms with a corrosion-resistant film | membrane by the negative electrode side, and corrosion resistance improves.

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 NC (CH ₂ ) ₄ CN, Pimeonitrile NC (CH ₂ ) ₅ CN, Suberonitrile NC (CH ₂ ) ₆ CN, Sebacononitrile NC (CH ₂ ) ₈ CN, Dodecandinitrile NC (CH ₂ ) ₁₀ CN, etc. In addition to the chain dinitrile compound, it may have a branch such as 2-methylglutaronitrile NCCH (CH ₃ ) CH ₂ CH ₂ CN. 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 positive electrode active materials 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 JP-A-2009-129741. 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.
Further, (Li _x Na _(3-x) ) _n Fe ₂ F ₇ (wherein n is a number of 0 or more and 1 or less, x is a number of 0 or more and 3 or less, and a part of Fe is Mg, A compound represented by one or more of Ni, Cu, Co, Mn, Al, Cr, Ga, V, and In may also be used as the positive electrode active material.
This positive electrode active material has a high discharge potential of about 4 V (vs Li / Li ⁺ ). The theoretical capacity density is Li _x Na _(3-x ) which is considered to be the theoretically highest discharge state from the state of Fe ₂ F _{7 which} is theoretically considered to be the highest charged state (ie, the state where n = 0). _{) It} shows a high value of 190 mAh / g as a lithium ion base up to the state of Fe ₂ F ₇ (that is, the state of n = 1). For this reason, the theoretical capacity density in consideration of voltage and electric capacity density is 700 Wh / kg (= 3.7 V × 190 mAh / g), and a high capacity density can be expected. This value is about 1.23 times the theoretical capacity density of LiFePO ₄ 570 Wh / kg (= 3.4 V × 170 mAh / g). Moreover, since sodium ions can be used as the incoming and outgoing alkali metals in addition to lithium ions, they can be used not only as positive electrode active materials for lithium ion batteries but also as positive electrode active materials for sodium ion batteries.
Metal fluorides can also be used as the positive electrode active material for sodium ion batteries. An example is FeF ₃ having a perovskite structure. This compound has a theoretical capacity density of 220 mAh / g when one Li ion is inserted per molecule, and the theoretical capacity density of a phosphate-based positive electrode active material having a conventional olivine type crystal structure (for example, LiFePO ₄ Has the advantage of having an energy density greater than 170 mAh / g).
Further, as a positive electrode active material having a discharge potential and energy density (Wh / kg) larger than that of FeF ₃ , (Li _x K _y Na _1-xy ) nMF ₃ (wherein x, y, and n are 0 or more) 1 or less, M is any one of Mn, Co and Ni, and a part of M is one or more of Mg, Cu, Co, Mn, Al, Cr, Ga, V and In. Which may be substituted). This positive electrode active material has a fluorine-based perovskite structure, and M is Mn, Co, or Ni as a constituent element. In the fully charged state, it becomes MF ₃ and M is considered to be a +3 state. In a complete discharge state, n = 1 and M is considered to be a +2 state. Since not only sodium ions but also lithium ions can enter and exit, it can be driven as a positive electrode active material for lithium ion batteries and sodium ion batteries.

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.
(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 electrode 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.

  For the current collector, the positive electrode electron conductive member, the negative electrode electron conductive member, and the case, the materials described in the description of the lithium ion battery can be similarly used.

  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 10 ... Separator (10a ... PTFE film | membrane, 10b ... PE film | membrane), 11 ... Positive electrode can, 12 ... Negative electrode cap, 14 ... Positive electrode active material pellet (positive electrode), 18 ... Lithium negative electrode active material (negative electrode)

Claims (3)

An electrochemical cell in which a separator is inserted in close contact between a positive electrode and a negative electrode,
The separator is a multilayer film having porosity, and the film in contact with the positive electrode side of the multilayer film is made of polytetrafluoroethylene, and the film in contact with the negative electrode side of the multilayer film is made of polyethylene or polypropylene. An electrochemical cell characterized by that.   The electrochemical cell according to claim 1, wherein an insulating ceramic powder is contained in the separator.   The electrochemical cell according to claim 1 or 2, wherein a voltage applied between the positive electrode and the negative electrode is 4.5 V or more.

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