JP5672432B2 - Positive electrode for secondary battery - Google Patents

Description

  The present invention relates to a positive electrode for a secondary battery that can realize a high discharge voltage and is unlikely to have a reduced charge amount even after repeated charge and discharge.

Conventionally, lithium cobalt oxide (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), solid solutions thereof, lithium manganate (LiMn ₂ O ₄ ), and the like have been used as positive electrode active materials for lithium ion batteries. In order to further increase the energy density, a search for a new positive electrode active material is underway.

For example, _Li 2 NiPO ₄ F in Patent Documents 1 and _2, LiNiPO _4, LiCoPO 4 and _Li 2 CoPO ₄ F has been proposed as a high positive electrode active material energy density. If these positive electrode active materials having a large energy density are used in a lithium ion battery, theoretically, a lithium ion battery having a large charge capacity should be obtained.

On the other hand, various positive electrode active materials have been found for sodium ion batteries. For example, NaFeO ₂ , NaNiO ₂ , NaCoO ₂ , NaMnO ₂ , NaFe _1-x M ¹ _x O ₂ , NaNi _1-x M ¹ _x O ₂ , NaCo _1-x M ¹ _x O ₂ , NaMn _1-x M ¹ _x O ₂ (wherein M ¹ is one or more elements selected from the group consisting of trivalent metals, and 0 ≦ x <0.5). Among these, complex oxides mainly containing iron and sodium, which are composed of hexagonal crystal structures, can charge and discharge at a high potential. It should be an ion battery.

  Non-patent documents 1 to 3 can be cited as techniques related to the present invention. These will be described later.

Patent No. 3624205 Patent No. 3631202

Journal of Solid State Electrochem DOI 10.1007 / s10008-009-0873-7 Preparation and cycle performance at high temperature for Li [Ni0.5Co0.2Mn0.3] O2 coated with LiFePO4 S.-B. Kim & KJ Lee & W. J. Choi & W.-S. Kim & I. C. Jang & HH Lim & YS LeePublished online; 23 June 2009http: //www.springerlink.com/content/l2pn1526p2233704/fulltext.pdf? Page = 1 Seung-Taek Myung, Kentarou Izumi, Shinichi Komaba, Hitoshi Yashiro, Hyun Joo Bang, Yang-Kook Sun, and Naoaki Kumagai J. Phys. Chem. C 2007, 111, 4061-4067 Seung-Taek Myung ,, Ki-Soo Lee, Chong Seung Yoon, Yang-Kook Sun, Khalil Amine, and Hitoshi Yashiro J. Phys. Chem. C 2010, 114, 4710? 4718

However, a positive electrode active material that charges and discharges at a high potential does not necessarily have a high voltage resistance enough to withstand charge and discharge. For example, Li ₂ CoPO ₄ F starts to discharge from a maximum of 5.15 V, but when such a charge-discharge cycle up to a high voltage is repeated, the discharge voltage and the discharge capacity immediately decrease. In addition, Li ₂ NiPO ₄ F also starts to discharge at a maximum of about 5.3 V, but the discharge voltage and discharge capacity decrease more rapidly than Li ₂ CoPO ₄ F. For this reason, the theoretical energy density with a high angle cannot be fully utilized.

  The present invention has been made in view of the above-described conventional problems, and an object of the present invention is to provide a positive electrode for a secondary battery that can achieve a high discharge voltage and is less likely to have a reduced charge amount even after repeated charge and discharge. And

In order to solve the above-described conventional problems, the present inventors have investigated charge / discharge characteristics at various potentials exceeding 5 V (vs Li / Li ⁺ ) for various positive electrode active materials. As a result, LiFePO ₄ is a substance having extremely excellent withstand voltage characteristics that the charge amount hardly decreases even when charged at an extremely high potential of 6 V (vs Li / Li ⁺ ) and shows stable charge / discharge characteristics. I found out.

However, since the oxidation-reduction potential of LiFePO ₄ is not so high, a secondary battery having a high energy density cannot be obtained even if this is used alone as a positive electrode active material.

For this reason, it was considered that the periphery of the positive electrode active material having a high oxidation-reduction potential was covered with LiFePO ₄ and the Li ion conduction path was secured by LiFePO ₄ . That is, according to Non-Patent Document 1, when Li [Ni _0.5 Co _0.2 Mn _0.3 ], which is known as a positive electrode active material, is charged and discharged at 50 ° C. higher than room temperature, it is discharged as charging and discharging are repeated. Whereas the capacity is reduced, the charge / discharge characteristics of the substance surrounded by LiFePO ₄ around Li [Ni _0.5 Co _0.2 Mn _0.3 ] is reduced even when the discharge capacity is repeated. It shows difficult characteristics. Since LiFePO ₄ is not easily decomposed even at a high potential and can be charged and discharged stably, a positive electrode such as Li ₂ CoPO ₄ F having a higher redox potential than Li [Ni _0.5 Co _0.2 Mn _0.3 ]. The active material was surrounded by LiFePO ₄ , so that a high discharge voltage could be realized, and a positive electrode for a secondary battery in which the charge amount would not easily decrease even after repeated charge and discharge could be realized. As a result, the first invention (positive electrode for lithium ion secondary battery) and the third invention (positive electrode for sodium ion secondary battery) which will be described later have been completed.

That is, the positive electrode for a secondary battery of the first invention is formed so as to cover the first positive electrode active material capable of entering and exiting lithium ions by intercalation and the first positive electrode active material, and Li _{1 -x} FePO ₄ (however, some of the Fe Co, Ni, Mn, Fe , Mg, Cu, Cr, V, Li, Nb, may be substituted by one or more of Ti and Zr, x And a second positive electrode active material comprising a number of 0 or more and less than 1),
The first positive electrode active material is Li ₂ NiPO ₄ F, Li ₂ CoPO ₄ F, LiCoPO ₄ and LiNiPO ₄ (where Ni and Co are Ni, Co, Mn, Fe, Mg, Cu, And may be substituted with one or more of Cr, V, Li, Nb, Ti, and Zr), and spinel-based LiNi _0.5 Mn _1.5 O ₄ , LiCoMnO _4, Li _2-y MnO ₃ (y = 0 to 2) and a solid solution of a layered oxide (where Ni, Co and Mn are Li, Ni, Co, Mn, Fe, Mg, Cu, Cr, V, Li and Nb other than the element) 1 or more selected from the group consisting of Ti and Zr, which may be substituted by one or more of Ti and Zr).

The positive electrode for a secondary battery of the first invention is Li _1-x FePO ₄ having stable charge / discharge characteristics even at a high potential (note that a part of Fe is Co, Ni, Mn, Fe, Mg, Cu, Cr, V, Li, Nb, Ti and Zr may be substituted with one or more of x, x represents a number of 0 or more and less than 1, and lithium ion by intercalation The first positive electrode active material capable of entering and exiting is covered. For this reason, lithium enters and exits between the electrolytic solution and the positive electrode active material through the second positive electrode active material having stable charge / discharge characteristics. For this reason, even if the first positive electrode active material alone is a substance whose charge potential and charge capacity decrease when repeated charge and discharge are performed alone, it can stably charge and discharge at a high potential. , The charge amount is difficult to decrease.
In addition, since the first positive electrode active material is selected from positive electrode active materials having a high redox potential, the energy density is high, a high discharge voltage can be realized, and the charge amount is reduced even after repeated charge and discharge. It is difficult to achieve a positive electrode for a secondary battery.

Next, the process until the completion of the second invention (positive electrode for lithium ion secondary battery) and the fourth invention (positive electrode for sodium ion secondary battery) described later will be described.
According to Non-Patent Document 2, when Li [Li _0.05 Ni _0.4 Co _0.15 Mn _0.4 ] O ₂ is covered with an oxide such as Al ₂ O ₃ or Nb ₂ O ₃ , Al ₂ O ₃ or Nb ₂ O The oxide such as ₃ reacts with hydrogen fluoride generated in the electrolytic solution to form a fluoride film, which prevents corrosion of the positive electrode active material and ensures a high charge capacity. According to Non-Patent Document 3, Li0.35 [Ni1 / 3Co1 / 3Mn1 / 3] O2 coated with AlF ₃ has improved heat resistance as compared with the case where it is not coated.
Based on the above facts, the present inventors considered that the positive electrode active material having a high redox potential was covered with a film made of oxide or fluoride, thereby improving the corrosion resistance of the positive electrode active material. However, even when the corrosion resistance of the positive electrode active material is improved, securing the electron conductivity with the current collecting electrode becomes a problem. This is because carbon black and graphite powder conventionally used to ensure electron conductivity with the collecting electrode are decomposed at a high potential. For this reason, as a result of further earnest studies, the second and fourth inventions described later have been completed.

  In other words, the positive electrode for the secondary battery according to the second invention is formed so as to cover the periphery of the positive electrode active material and the positive electrode active material capable of entering and exiting lithium ions by intercalation, thereby preventing corrosion of the positive electrode active material. And a conductive additive made of at least glassy carbon and / or conductive diamond-like carbon in electrical contact with the corrosion-resistant shell of the positive electrode active material and the corrosion-resistant shell.

The positive electrode for a secondary battery according to the second invention is formed so as to cover the periphery of the positive electrode active material, and the corrosion resistant shell for preventing the corrosion of the positive electrode active material is capable of allowing lithium ions to enter and exit by intercalation. Covers around. For this reason, the positive electrode active material is protected from corrosion by the corrosion-resistant shell, can be stably charged and discharged at a high potential, and the amount of charge is unlikely to decrease.
Further, at least the corrosion-resistant shell of the polar active material and the corrosion-resistant shell is in electrical contact with a conductive assistant composed of glassy carbon and / or conductive diamond-like carbon that has a wide potential window and is stable even at a high potential. ing. For this reason, even if the positive electrode active material has a high charge / discharge potential, the conductive additive is not easily decomposed, electrons can be stably taken in and out at all times, and a positive electrode for a secondary battery having excellent durability can be obtained. be able to.
Therefore, by selecting a material having a high oxidation-reduction potential as the positive electrode active material, a positive electrode for a secondary battery that has a high energy density, can realize a high discharge voltage, and does not easily decrease the charge amount even after repeated charge and discharge. Can be realized.

As the corrosion-resistant shell in the positive electrode for secondary battery of the second invention, Li _1-x FePO ₄ (However, a part of Fe is Co, Ni, Mn, Fe, Mg, Cu, Cr, V, Li, Nb, Ti. And Zr may be substituted with one or more of Zr, and x represents a number of 0 or more and less than 1. In this case, the corrosion-resistant shell made of Li _1-x FePO ₄ has a structure that covers the periphery of the positive electrode active material capable of entering and exiting lithium ions by intercalation. For this reason, lithium enters and exits between the electrolytic solution and the positive electrode active material through Li _1-x FePO ₄ having stable charge / discharge characteristics. For this reason, even if the positive electrode active material alone is a material whose charge potential and charge capacity decrease when repeated charge and discharge are performed alone, it can stably charge and discharge at a high potential, and the amount of charge Becomes difficult to decrease.

Further, as another corrosion-resistant shell material in the secondary battery positive electrode of the second invention, from the group consisting of Al ₂ O ₃ , Nb ₂ O ₃ , Ta ₂ O ₅ , ZrO ₂ , ZnO, AlF ₃ , HfF ₄ , and BiOF. It may be composed of one or more selected. These corrosion-resistant shells can exhibit an excellent anticorrosive effect on the positive electrode active material.

In addition, as the positive electrode active material in the positive electrode for secondary battery of the second invention, Li ₂ NiPO ₄ F, Li ₂ CoPO ₄ F, LiCoPO ₄ , LiNiPO ₄ (where Ni and Co are Ni, Co other than the elements, Ni, Co , Mn, Fe, Mg, Cu, Cr, V, Li, Nb, Ti, and Zr may be substituted with one or more). Also, spinel-based LiNi _0.5 Mn _1.5 O ₄ , LiCoMnO _4, Li _2-y MnO ₃ (y = 0 to 2) and their solid solutions (herein, solid solutions are the above-mentioned oxide-based positive electrode actives) A substance in which metal atoms are mixed in a free ratio. (However, Ni, Co, and Mn are Li, Ni, Co, Mn, Fe, Mg, Cu, Cr, V, and Li other than the element.) , Nb, Ti and Zr, which may be substituted with one or more of them. These positive electrode active materials have a high charge / discharge potential, and can be used as a positive electrode for a lithium ion battery having a high energy density.

A positive electrode for a secondary battery according to a third aspect of the present invention is formed to cover a first positive electrode active material capable of entering and exiting sodium ions by intercalation and the first positive electrode active material, and Na _{1 -x} FePO ₄ (however, some of the Fe Co, Ni, Mn, Fe , Mg, Cu, Cr, V, Li, Nb, may be substituted by one or more of Ti and Zr, x And a second positive electrode active material comprising a number of 0 or more and less than 1),
The first positive electrode active material is Na ₂ NiPO ₄ F, Na ₂ CoPO ₄ F, Na ₂ MnPO ₄ F, Na ₂ FePO ₄ F, NaNiPO ₄ , NaCoPO ₄ , NaMnPO ₄ , NaFeO ₂ , NaNiO ₂ , NaCoO _2. , NaMnO ₂ , NaFe _1-x M ¹ _x O ₂ , NaNi _1-x M ¹ _x O ₂ , NaCo _1-x M ¹ _x O ₂ , NaMn _1-x M ¹ _x O ₂ (where M ¹ is 3 It is one or more elements selected from the group consisting of valent metals, and 0 ≦ x <0.5.), (Li _x Na _(3-x) ) _n Fe ₂ F ₇ (where n is 0 The number is 1 or less, x is a number from 0 to 3, and a part of Fe is one or more of Mg, Ni, Cu, Co, Mn, Al, Cr, Ga, V, and In. Selected from the group consisting of (optionally substituted) Characterized in that the at least one

In the positive electrode for secondary battery of the third invention, Na _1-x FePO ₄ having stable charge / discharge characteristics even at a high potential (however, part of Fe is Co, Ni, Mn, Fe, Mg, Cu, Cr, A second positive electrode active material which may be substituted with one or more of V, Li, Nb, Ti, and Zr, and x represents a number of 0 or more and less than 1, is a sodium ion by intercalation The first positive electrode active material capable of entering and exiting is covered. For this reason, the sodium ion enters and exits between the electrolytic solution and the positive electrode active material via the second positive electrode active material having stable charge / discharge characteristics. For this reason, even if the first positive electrode active material alone is a substance whose charge potential and charge capacity decrease when repeated charge and discharge are performed alone, it can stably charge and discharge at a high potential. It is possible to suppress a decrease in the charge amount.
In addition, since the first positive electrode active material is selected from positive electrode active materials having a high redox potential, the energy density is high, a high discharge voltage can be realized, and the charge amount is reduced even after repeated charge and discharge. It is difficult to achieve a positive electrode for a secondary battery.
Therefore, by selecting a material having a high oxidation-reduction potential as the first positive electrode active material, it is possible to realize a high discharge voltage with a high energy density, and the charge amount is unlikely to decrease even after repeated charge and discharge. A positive electrode for a battery can be realized.

  The positive electrode for a secondary battery according to a fourth aspect of the present invention is formed so as to cover the periphery of the positive electrode active material and a positive electrode active material capable of entering and exiting sodium ions by intercalation to prevent corrosion of the positive electrode active material. And a conductive additive made of at least glassy carbon and / or conductive diamond-like carbon in electrical contact with the corrosion-resistant shell of the positive electrode active material and the corrosion-resistant shell.

A positive electrode for a secondary battery according to a fourth aspect of the present invention is a positive electrode active material formed so as to cover the periphery of the positive electrode active material, and a corrosion-resistant shell that prevents corrosion of the positive electrode active material allows sodium ions to enter and exit by intercalation Covers around. For this reason, the positive electrode active material is protected from corrosion by the corrosion-resistant shell, can be stably charged and discharged at a high potential, and the amount of charge is unlikely to decrease.
Further, at least the corrosion-resistant shell of the polar active material and the corrosion-resistant shell is in electrical contact with a conductive assistant composed of glassy carbon and / or conductive diamond-like carbon that has a wide potential window and is stable even at a high potential. ing. For this reason, even if the positive electrode active material has a high charge / discharge potential, the conductive additive is not easily decomposed, electrons can be stably taken in and out at all times, and a positive electrode for a secondary battery having excellent durability can be obtained. be able to.
Therefore, by selecting a material having a high oxidation-reduction potential as the positive electrode active material, a positive electrode for a secondary battery that has a high energy density, can realize a high discharge voltage, and does not easily decrease the charge amount even after repeated charge and discharge. Can be realized.

As the corrosion-resistant shell in the positive electrode for secondary battery of the fourth invention, Na _1-x FePO ₄ (However, a part of Fe is Co, Ni, Mn, Fe, Mg, Cu, Cr, V, Li, Nb, Ti And Zr may be substituted with one or more of Zr, and x represents a number of 0 or more and less than 1. In this case, the corrosion-resistant shell made of Na _1-x FePO ₄ has a structure that covers the periphery of the positive electrode active material that allows sodium ions to enter and exit by intercalation. For this reason, sodium enters and exits between the electrolytic solution and the positive electrode active material through Na _1-x FePO ₄ having stable charge / discharge characteristics. For this reason, even if the positive electrode active material alone is a material whose charge potential and charge capacity decrease when repeated charge and discharge are performed alone, it can stably charge and discharge at a high potential, and the amount of charge Becomes difficult to decrease.

Further, as another corrosion-resistant shell material in the positive electrode for secondary battery of the fourth invention, a group consisting of Al ₂ O ₃ , Nb ₂ O ₃ , Ta ₂ O ₅ , ZrO ₂ , ZnO, AlF ₃ , HfF ₄ , BiOF is used. It may be composed of at least one selected from the above. These corrosion-resistant shells can exhibit an excellent anticorrosive effect on the positive electrode active material.

The positive electrode active material having a high redox potential in the positive electrode for secondary battery of the fourth invention includes Na ₂ NiPO ₄ F, Na ₂ CoPO ₄ F, Na ₂ MnPO ₄ F, Na ₂ FePO ₄ F, and NaNiPO _4. , NaCoPO ₄ , NaMnPO ₄ , NaFeO ₂ , NaNiO ₂ , NaCoO ₂ , NaMnO ₂ , NaFe _1-x M ¹ _x O ₂ , NaNi _1-x M ¹ _x 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), (Li _x Na _{(3- x)} ) _n Fe ₂ F ₇ (wherein n is a number from 0 to 1, x is a number from 0 to 3, and a part of Fe is Mg, Ni, Cu, Co, Mn, Al) , Cr, Ga, V and In 1 or 2 or more types may be substituted). These positive electrode active materials have a high charge / discharge potential, and can be used as a positive electrode for sodium ion batteries having a high energy density.

In the secondary battery positive electrode according to the first to fourth inventions, it is also preferable that Li _1-x FePO ₄ , Na _1-x FePO _4, and the corrosion-resistant shell are surrounded by glassy carbon and / or conductive diamond-like carbon. . In this case, electrons can enter and leave the second positive electrode active material and the glassy carbon and / or the conductive diamond-like carbon more reliably, and a secondary battery with low internal resistance can be obtained.

2 is a schematic cross-sectional view of a positive electrode for a secondary battery according to Embodiment 1. FIG. 3 is a graph showing the relationship between charge / discharge capacity and voltage in charge / discharge test 1; 4 is a graph showing the relationship between charge / discharge capacity and voltage in charge / discharge test 2. 4 is a schematic cross-sectional view of a two-layer coating particle included in a positive electrode for a secondary battery according to Embodiment 2. FIG. 6 is a schematic cross-sectional view of a positive electrode for a secondary battery according to Embodiment 3. FIG. 6 is a schematic cross-sectional view of two-layer coating particles contained in a positive electrode for a secondary battery according to Embodiment 4. FIG. 6 is a partial schematic cross-sectional view of a positive electrode for a secondary battery according to Embodiment 5. FIG. 7 is a partial schematic cross-sectional view of a positive electrode for a secondary battery according to Embodiment 6. FIG.

Hereinafter, embodiments embodying the present invention will be described.
(Embodiment 1)
As shown in FIG. 1, the positive electrode for a secondary battery of Embodiment 1 has coating particles 11 in which a positive electrode active material 11b surrounds a positive electrode active material 11a for a lithium ion battery. The conductive auxiliary agent 12 is in contact with the particles 11. And the coating particle 11 and the conductive support agent 12 contact the electrical power collector 13, and electronic conductivity is ensured.

As the positive electrode active material 11a, any positive electrode active material capable of entering and exiting lithium ions by intercalation can be used, but in order to increase the energy density and the discharge potential, those having a high redox potential are preferable.
As such a positive electrode active material, for _{example, Li 2 NiPO 4 F, Li} 2 CoPO 4 F, LiCoPO 4, LiNiPO 4, also, _LiNi 0.5 _Mn 1.5 _{O 4} and LiCoMnO ₄ of spinel, Li _{2 -Y} MnO ₃ (y = 0 to 2) and solid solutions thereof (here, the solid solution refers to a material in which metal atoms are mixed in a free ratio in the oxide-based positive electrode active material) (where Ni , Co, and Mn may be substituted with one or more of Li, Ni, Co, Mn, Fe, Mg, Cu, Cr, V, Li, Nb, Ti, and Zr other than the element). These positive electrode active materials may be a single type or a mixture of two or more types.

On the other hand, Li _1-x FePO ₄ (x represents a number of 0 or more and less than 1) can be used as the positive electrode active material 11b. The value of x is a number that varies depending on the state of charge / discharge. As will be described later, this material is stable and does not decompose even when charged at a high potential of 6 V (vs Li / Li ⁺ ). In addition to Li _1-x FePO ₄ , part of Fe is substituted with one or more of Co, Ni, Mn, Fe, Mg, Cu, Cr, V, Li, Nb, Ti, and Zr. Also good.

  Further, the conductive auxiliary agent 12 exists between the coating particle 11 and the current collector 13 and plays a role of ensuring a sufficient electron conduction path between them. The form of the conductive auxiliary agent 12 is not particularly limited as long as an electron conduction path can be formed between the coating particles 11 and the current collector 13. For example, carbon black such as acetylene black, graphite powder, diamond-like carbon, etc. Further, conductive powder (conductive aid) such as glassy 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.

Furthermore, the molding material of the current collector 13 is required to be stable during charging. In particular, when the positive electrode active material 11b uses a phosphate-based or olivine fluoride-based positive electrode active material having an olivine-type crystal structure with a high oxidation-reduction 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 of the electrolytic solution, austenitic stainless steel, Ni, Al, Ti, or the like can be used, but it is appropriately selected in consideration of the operating potential of the positive electrode active material 11b to be used. It is preferable. 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. In addition, when LiTFSI is used as the electrolyte of the electrolytic solution, 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 DLC is coated on a conductive metal material such as Al, the fluoride film is generated only in a very small area of the defective portion of the conductive DLC even if the coating cannot be completely covered and there are holes. . 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 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.

<Manufacturing method>
The positive electrode for secondary batteries of Embodiment 1 can be manufactured as follows.
· LiFePO Synthesis LiCO ₃ of ₄ and _{FeC 2 O} 4 · 2H 2 _O and _(NH 4) and 2HPO ₄ mixing weighed to such that the stoichiometric ratio of LiFePO _4. Then, the mixture is put in a tubular furnace, pre-baked at 400 ° C. for 4 hours in an argon atmosphere, and then baked at 660 ° C. for 2.5 hours. Thus, LiFePO ₄ having an olivine type crystal structure is obtained.

· The powder of the positive electrode active material 11a formed of producing Li ₂ NiPO ₄ F or _Li 2 CoPO ₄ F and LiCoPO _4, etc. of the coating particles 11, the LiFePO ₄ obtained as described above becomes 0.5 to 10 mass% Then, the mixture was mixed and then treated with a hybridization system (NHS-0, Nara Machinery Co., Ltd.) for 3 minutes. Then, the treated product is heated at 500 ° C. for 4 hours to obtain coating particles 11. Then, a predetermined amount of glassy carbon powder or the like is added to the powder composed of the coating particles 11 and mixed, and the positive electrode for a lithium ion battery of Embodiment 1 having a disk shape is obtained by cold pressing.

<Effect>
In the lithium ion battery positive electrode according to Embodiment 1 configured as described above, the positive electrode active material 11b surrounds the positive electrode active material 11a capable of entering and exiting lithium ions by intercalation. For this reason, lithium enters and leaves the electrolyte solution on the positive electrode active material 11b having stable charge / discharge characteristics. For this reason, even if the positive electrode active material 11b alone is a substance whose charge potential and charge capacity decrease when charge and discharge are repeated by itself, charging and discharging can be stably repeated at a high potential. A decrease in the amount can be suppressed. For this reason, by selecting a material having a high redox potential as the positive electrode active material 11a, a secondary battery that has a high energy density, can realize a high discharge voltage, and does not easily decrease the charge amount even after repeated charge and discharge. A positive electrode can be realized.

Charge and discharge characteristics of -Li _1-x FePO ₄ -
The positive electrode for a lithium ion battery according to Embodiment 1 exhibits the above-described effects even if the positive electrode active material 11a is a substance whose charge potential and charge capacity decrease when it is repeatedly charged and discharged by itself. This is because Li _1-x FePO ₄ that is the positive electrode active material 11a can be stably charged and discharged at a high potential. In order to prove this, a lithium ion coin battery was manufactured under the following conditions, and two kinds of charge / discharge tests described later were performed.
-Working electrode LiFePO ₄ / Conductive aid glassy carbon powder / Binder PTFE
・ Counter electrode Li metal ・ Electrolyte Solvent / Sebaonitrile: Ethylene carbonate: Dimethyl carbonate =
50:25:25 (capacity ratio)
Electrolyte / 1.0M LiBF ₄
・ Cell Coin cell SUS316L 2032 type ・ Current collector Substrate SUS316L, Current collector Ti

<Charge / discharge test 1>
Normal charging is performed at a charging rate of 0.01 C (approximately constant at 3.5 V (vs Li / Li ⁺ )), 10 C at the start of charging, and after reaching 6 V (vs Li / Li ⁺ ) Charging / discharging characteristics were measured using two types of charging methods, high potential charging for performing potential charging. As a result, as shown in FIG. 2, the discharge capacity was almost the same for both charging methods of 6V (vs Li / Li ⁺ ) and 3.5 V (vs Li / Li ⁺ ). It was. From this, LiFePO ₄ (lithium ions are released by charging to become Li _1-x FePO ₄ ) can be stably charged and discharged even at a high potential of 6 V (vs Li / Li ⁺ ). I understood.

<Charge / discharge test 2>
Charging up to 175 mAh / g (active material amount) at 5.5 V and constant current discharging at a discharge rate of 0.05 C and a cutoff voltage of 2.5 V were repeated 10 times. As a result, as shown in FIG. 3, the discharge characteristics hardly changed and the discharge capacity was almost constant.

(Embodiment 2)
As shown in FIG. 4, the positive electrode for a secondary battery according to Embodiment 2 is a two-layer coating particle in which a high-voltage resistant film 14 made of diamond-like carbon, glassy carbon, or the like is formed on the surface of the coating particle 11 in Embodiment 1. 15 is included. Such a high voltage resistant film 14 can be easily formed by attaching diamond-like carbon or glassy carbon to the surface of the coating particle 11 by sputtering, ion coating, or plasma CVD. For example, as described in Japanese Patent Application Laid-Open No. 2004-339564, in a vacuum, electrons are accelerated by an electric field to collide with argon gas to ionize argon, and this is accelerated by the electric field to cause solid carbon target collision. And a method of forming diamond-like carbon on the powder of the coating particles 11 (at this time, a negative bias voltage may be applied to the coating particles 11).
The positive electrode for a secondary battery of Embodiment 2 thus obtained maintains a conductive path with the coating particles 11 even under a high voltage by the high voltage resistant film 14, and thus has a more stable charge / discharge characteristic at a high potential. can get.

(Embodiment 3)
As shown in FIG. 5, the positive electrode for secondary battery of Embodiment 3 has a matrix in which a plurality of particles 21 a made of a positive electrode active material for a lithium ion battery are made of a positive electrode active material 21 b made of Li _1-x FePO ₄ . The composite particles 21 are dispersed.
In the positive electrode for secondary battery of Embodiment 3 obtained in this way, the positive electrode active material 21b made of Li _1-x FePO ₄ surrounds the positive electrode active material 21a. For this reason, lithium enters and leaves the electrolyte solution on the positive electrode active material 21b having stable charge / discharge characteristics. For this reason, even if the positive electrode active material 21b is a substance whose charge potential and charge capacity decrease when it is repeatedly charged and discharged by itself, charging and discharging can be stably repeated at a high potential. A decrease in the amount can be suppressed. For this reason, like the positive electrode for secondary batteries of Embodiment 1, by selecting a material having a high redox potential as the positive electrode active material 21a, the energy density is high, a high discharge voltage can be realized, and charging / discharging can be performed. Even if it repeats, the positive electrode for secondary batteries in which a charge amount cannot fall easily is realizable.

(Embodiment 4)
As shown in FIG. 6, the positive electrode for a secondary battery according to the fourth embodiment has a high-voltage resistant film 24 made of diamond-like carbon, glassy carbon, or the like on the surface of the coating particles 21 in the second embodiment. Such a high voltage-resistant film 24 can be easily formed by attaching diamond-like carbon or glassy carbon to the surface of the coating particle 21 by sputtering, ion coating, or plasma CVD as described in the second embodiment. Can be formed.
The positive electrode for a secondary battery of Embodiment 4 obtained in this way maintains a conductive path with the coating particles 21 even under a high voltage by the high voltage resistant film 24, and therefore has a more stable charge / discharge characteristic at a high potential. can get.

(Embodiment 5)
As shown in FIG. 7, the positive electrode for the secondary battery of Embodiment 5 has a structure in which Li _1-x FePO ₄ 31b sparsely covers the peripheral surface of the positive electrode active material 31a for the lithium ion battery. The conductive assistant 32 made of glassy carbon or diamond-like carbon is in contact with the periphery of the substrate.

In the positive electrode for a lithium ion battery according to the fifth embodiment, the peripheral surface of the positive electrode active material 31a is covered with Li _1-x FePO ₄ 31b and sparsely, and further, a conductive additive 32 made of glassy carbon or diamond-like carbon. Since the conductive path of the positive electrode is maintained by Li _1-x FePO ₄ 31b and the conductive additive 32, more stable charge / discharge characteristics can be obtained at a high potential.

(Embodiment 6)
Secondary battery positive electrode according to the sixth embodiment, as shown in FIG. 8, the periphery of the Li _{1-x FePO} 4 _31b is covered with anti-high voltage coating 34 made of glassy carbon and diamond-like carbon. Such a high voltage resistant film 34 can be easily formed by attaching diamond-like carbon or glassy carbon to the surface of the coating particle 21 by sputtering, ion coating, or plasma CVD, as described in the second embodiment. Can be formed.
The positive electrode for a secondary battery of Embodiment 6 obtained in this way is more stable at a high potential because the conductive path is maintained between the Li _1-x FePO ₄ 31b and the Li _1-x FePO ₄ 31b even under high voltage by the high voltage resistant film 34. Charge / discharge characteristics can be obtained.

  The above Embodiments 1 to 6 are all positive electrodes for lithium ion batteries, but by using a positive electrode active material capable of sodium ion in / out by intercalation as a positive electrode active material, a positive electrode for sodium ion batteries It can be. These positive electrodes for sodium ion batteries can exhibit the same effects as described above for the same reason as described in the first to sixth embodiments.

In the _{first to} sixth embodiments, Li _1-x FePO ₄ is used as a material covering the periphery of the positive electrode active material. However, as an alternative to Li _1-x FePO ₄ , Al ₂ O ₃ , Nb ₂ O ₃ , Ta The periphery of the positive electrode active material may be covered with a corrosion-resistant shell made of one or more selected from the group consisting of ₂ O ₅ , ZrO ₂ , ZnO, AlF ₃ , HfF ₄ , and BiOF. In this case, the positive electrode active material is protected from corrosion by the corrosion-resistant shell, can be stably charged and discharged at a high potential, and the amount of charge is unlikely to decrease. Further, at least the corrosion-resistant shell of the polar active material and the corrosion-resistant shell is in electrical contact with a conductive assistant composed of glassy carbon and / or conductive diamond-like carbon that has a wide potential window and is stable even at a high potential. Therefore, even if the positive electrode active material has a high charge / discharge potential, the conductive additive is not easily decomposed, and electrons can be stably taken in and out at all times. can do. For this reason, by selecting a material having a high redox potential as the positive electrode active material, the energy density is high, a high discharge voltage can be realized, and the charge amount is difficult to decrease even after repeated charge and discharge. It becomes the positive electrode.

As a method for covering the positive electrode active material with an oxide or fluoride such as Al ₂ O ₃ , Nb ₂ O ₃ , Ta ₂ O ₅ , ZrO ₂ , ZnO, AlF ₃ , HfF ₄ , BiOF or a mixture thereof, Non-Patent Document 2 Alternatively, the method described in Non-Patent Document 3 can be used. That is, the following method.
・ Method of Non-Patent Document 2
An alkoxide such as Al [OCH (CH ₃ ) ₂ ] ₃ , Nb [OC ₂ H ₅ ] ₅ , Ta [OC ₂ H ₅ ] ₅ , or Zr [OC ₂ H ₅ ] ₅ is dissolved in ethanol at room temperature. (However, in the case of coating with ZnO, Zn (CH ₃ COO) ₂ .2H ₂ O is dissolved in distilled water to adjust the pH to 10.5.) And the ratio of the powder of the positive electrode active material to the coating material is 99: The positive electrode active material powder is slowly put into the solution so as to be 1. Then, after stirring at 80 ° C. for 2 days, the solvent is evaporated at 120 ° C., and further heated at 400 ° C. in the atmosphere for 5 hours. Thus, a positive electrode active material coated with Al ₂ O ₃ , Nb ₂ O ₃ , Ta ₂ O ₅ , ZrO ₂ , ZnO or the like can be obtained. Using the positive electrode active material coated with various metal oxides thus obtained, a positive electrode for a secondary battery can be produced by the same method as in the first embodiment.
-Method of Non-Patent Document 3 An aqueous solution of ammonium fluoride and an aqueous solution of anhydrous aluminum nitrate are prepared. Then, the powder of the positive electrode active material is put in an aluminum nitrate aqueous solution, and further an ammonium fluoride aqueous solution is added until a pH at which AlF ₃ can be precipitated is reached. Then, after stirring at 80 ° C. for 2 days, the solvent is evaporated at 120 ° C., and further heated at 400 ° C. for 5 hours in a nitrogen stream. Thus, coating of AlF ₃ not containing Al ₂ O ₃ can be performed around the positive electrode active material powder.

The present invention is applied to a 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. Especially, when the oxidation-reduction potential of the positive electrode is 4.5 V or more, 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 at least one 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 ester carbonate, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), or the like 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, 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 lone pair of oxygen present in the ether molecule to form a complex, and the electron density of oxygen is reduced. 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 about 0.1 to 3% by weight in the electrolytic solution. 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 0.1 to 3% by weight. When the concentration of the Li salt is less than 0.1% by weight, 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, if it exceeds 3% by weight, the dissolved Li salt precipitates due to environmental changes such as temperature, which is not preferable.

  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”.
Materials that can be used for the first positive electrode active material of the present invention include (1) oxides, (2) phosphates having an olivine crystal structure, (3) olivine fluorides, and (4) metals. Fluoride etc. can be mentioned.

(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) A substance in which metal atoms are mixed in a free ratio in the positive electrode active material. 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. 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 5.2 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 further discharge potential and energy density (Wh / kg) is greater than _{FeF3, (Li x K y Na} 1-x-y) nMF3 ( wherein, x, y and n are 0 or more and 1 or less M is any one of Mn, Co and Ni, and a part of M is substituted with one or more of Mg, Cu, Co, Mn, Al, Cr, Ga, V and In. May be included). 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.

(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 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, prevents a short circuit therebetween, and allows the passage of Li 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 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 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 , 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 about 0.1 to 3 weight% of various additives (for example, vinylene carbonate, fluoroethylene carbonate, ethylene sulfite, propane sultone). 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 the organic solvent, one or more 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 the material that can be used for the first positive electrode active material of the present invention include NaFeO ₂ , NaNiO ₂ , NaCoO ₂ , NaMnO ₂ , and NaFe _1-x M ¹ _x O described in JP-A-2009-129741. ₂ , NaNi _1-x M ¹ _x 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 selected from the group consisting of trivalent metals) Element, 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).
In addition, 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 ) nMF3 (wherein x, y, and n are 0 or more and 1). M is any of Mn, Co, and Ni, and a part of M is substituted with one or more of Mg, Cu, Co, Mn, Al, Cr, Ga, V, and In. May be used). 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 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 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.

  The positive electrode for a secondary battery of the present invention can be suitably used as a positive electrode for a secondary battery that can effectively utilize a positive electrode active material having a high energy density in which a charging reaction is performed at a high potential.

11a, 21a, 31a ... positive electrode active material (first positive electrode active material)
11b, 21b, 31b ... positive electrode active material (second positive electrode active material)
11, 12 ... Positive electrode for secondary battery (11 ... Coating particle, 12 ... Conductive aid)
13 ... current collector

Claims (3)

The first positive electrode active material capable of entering and exiting sodium ions by intercalation and the first positive electrode active material are formed so as to cover the periphery of the first positive electrode active material, and Na _1-x FePO ₄ (however, a part of Fe is Co , Ni, Mn, Fe, Mg, Cu, Cr, V, Li, Nb, Ti, and Zr may be substituted with one or more, and x represents a number from 0 to less than 1. Two positive electrode active materials,
The first positive electrode active material is Na ₂ NiPO ₄ F, Na ₂ CoPO ₄ F, Na ₂ MnPO ₄ F, Na ₂ FePO ₄ F, NaNiPO ₄ , NaCoPO ₄ , NaMnPO ₄ , NaFeO ₂ , NaNiO ₂ , NaCoO _2. , NaMnO ₂ , NaFe _1-x M ¹ _x O ₂ , NaNi _1-x M ¹ _x O ₂ , NaCo _1-x M ¹ _x O ₂ , NaMn _1-x M ¹ _x O ₂ (where M ¹ is 3 It is one or more elements selected from the group consisting of valent metals, and 0 ≦ x <0.5.), (Li _x Na _(3-x) ) _n Fe ₂ F ₇ (where n is 0 The number is 1 or less, x is a number from 0 to 3, and a part of Fe is one or more of Mg, Ni, Cu, Co, Mn, Al, Cr, Ga, V, and In. Selected from the group consisting of (optionally substituted) A positive electrode for a secondary battery, characterized in that the at least one. A positive electrode active material capable of entering and exiting sodium ions by intercalation; a corrosion-resistant shell formed to cover the positive electrode active material and preventing corrosion of the positive electrode active material; the positive electrode active material and the corrosion-resistant shell At least a glassy carbon film and / or a conductive diamond-like carbon film formed on the surface of the corrosion-resistant shell, wherein the corrosion-resistant shell is Na _1-x FePO ₄ (provided that a part of Fe is Co, Ni, Mn, Fe, Mg, Cu, Cr, V, Li, Nb, Ti and Zr may be substituted with one or more, and x represents a number from 0 to less than 1, or The corrosion-resistant shell is a group consisting of Al ₂ O ₃ , Nb ₂ O ₃ , Ta ₂ O ₅ , ZrO ₂ , ZnO, AlF ₃ , HfF ₄ , and BiOF. A positive electrode for a secondary battery, comprising at least one selected from the above. The positive electrode active material is Na ₂ NiPO ₄ F, Na ₂ CoPO ₄ F, Na ₂ MnPO ₄ F, Na ₂ FePO ₄ F, NaNiPO ₄ , NaCoPO ₄ , NaMnPO ₄ , NaFeO ₂ , NaNiO ₂ , NaCoO ₂ , NaMnO _2. NaFe _1-x M ¹ _x O ₂ , NaNi _1-x M ¹ _x O ₂ , NaCo _1-x M ¹ _x O ₂ , NaMn _1-x M ¹ _x O ₂ (where M ¹ is a trivalent metal) 1 or more elements selected from the group consisting of 0 ≦ x <0.5.), (Li _x Na _(3-x) ) _n Fe ₂ F ₇ (wherein n is 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 substituted with one or more of Mg, Ni, Cu, Co, Mn, Al, Cr, Ga, V and In. 1 type selected from the group consisting of A positive electrode for a secondary battery according to claim 2, characterized in that the top.

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