Novel negative and positive electrodes for lithium-ion secondary batteries

Abstract

To provide a novel positive electrode active material and negative electrode active material for a non-aqueous electrolyte secondary battery.SOLUTION: A positive electrode active material for a non-aqueous electrolyte secondary battery includes a compound represented by the formula (1) (R1 and R2 are independently hydrogen, an alkyl group, an alkenyl group, or the like, and n is an integer of 2 or more). The negative electrode active material for the non-aqueous electrolyte secondary batteries is preferably 5-formylsalicylic acid or lithium 5-formylsalicylate.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to novel negative and positive electrodes for lithium-ion secondary batteries. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries are being developed as secondary batteries with high energy density. Lithium-ion secondary batteries (hereinafter simply referred to as lithium secondary batteries) are non-aqueous secondary batteries that offer high voltage and high capacity. Because they are relatively easy to miniaturize and reduce in weight, they are used as power sources for portable electronic devices. In recent years, they have been considered for use as backup power sources in vehicles such as electric bicycles, electric vehicles, and airplanes, as well as mobile phone base stations and hospitals. A typical lithium secondary battery uses a carbonaceous material, such as graphite, capable of absorbing and releasing lithium as the negative electrode active material, and a composite oxide of lithium and a transition metal, such as lithium cobalt oxide (LiCoO), as the positive electrode active material. This combination achieves high voltage and high capacity sufficient for practical use.

[0003] However, the inorganic active materials mentioned above are approaching their limits in terms of discharge capacity and energy density, and negative electrode active materials other than graphite and positive electrode active materials such as lithium cobalt oxide are being explored. Therefore, electrode active materials that are expected to improve capacity are required, and the development of electrode active materials made of not only inorganic materials but also organic materials is being promoted.

[0004] For example, Patent Document 1 discloses a negative electrode active material made of a phenolic resin-based carbon material, and Patent Document 2 discloses a negative electrode active material made of a silicon compound.

[0005] Patent Documents 3 to 5 disclose positive electrode active materials having a quinone structure. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent Publication No. 2010-80123 [Patent Document 2] Patent Publication No. 2017-117656 [Patent Document 3] Patent Publication No. 2013-20760 [Patent Document 4] Patent Publication No. 2016-8227 [Patent Document 5] Patent Publication No. 2020-66681 Summary of the Invention [Problem to be solved by the invention]

[0007] As described above, there is a demand for electrode active materials that are expected to improve capacity.

[0008] The problem to be solved by the present invention is to provide an anode active material, a cathode active material, or both, for a secondary battery made of a specific organic material; an anode having an anode active material, a cathode having a cathode active material, or both; and a secondary battery having an anode, a positive electrode, or both. [Means for solving the problem]

[0009] As a result of extensive research to solve the above problems, the inventors discovered that the capacity of a secondary electrode can be unexpectedly increased by using formyl salicylic acid or an alkali metal salt thereof as the negative electrode active material and polycatechol as the positive electrode active material, and thus completed the present invention.

[0010] The present invention encompasses the embodiments described below.

[0011] Item 1. A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising a compound represented by the following formula (1): [ka] (In the formula, R1, R 2 teetheach independently represents hydrogen, an alkyl group, an alkenyl group, an alkynyl group, an aryl group, an oxyalkylene group, or an electron-withdrawing group, and n is an integer of 2 or greater. Item 2. A positive electrode comprising the positive electrode active material for a nonaqueous electrolyte secondary battery according to Item 1. Item 3. A nonaqueous electrolyte secondary battery having the positive electrode according to Item 2. Item 4. A negative electrode active material for a non-aqueous electrolyte secondary battery, comprising a compound represented by the following formula (2): [ka] (In the formula, A is hydrogen, a metal, or NH3, and R1, R2, and R3 are each independently hydrogen, an alkyl group, or an oxyalkylene group.) Item 5. The negative electrode active material for a non-aqueous electrolyte secondary battery according to Item 4, wherein the compound represented by formula (2) is 5-formylsalicylic acid or lithium 5-formylsalicylate. Item 6. A negative electrode comprising the negative electrode active material for a nonaqueous electrolyte secondary battery according to Item 4 or 5. Item 7. A nonaqueous electrolyte secondary battery having the negative electrode according to Item 6. Item 8. A nonaqueous electrolyte secondary battery comprising the positive electrode according to Item 2 and the negative electrode according to Item 6. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a high-capacity non-aqueous electrolyte secondary battery that uses a specific organic material as an active material. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a schematic exploded side view of a secondary battery according to an embodiment of the present invention; [Figure 2] (A) IR spectra of P(Cat) and catechol P(Cat) at each synthesis temperature, (B) TG graph of the sample synthesized at 4°C. [Figure 3] Graph of XRD measurements of P(Cst) compared to glassy carbon and graphene. [Figure 4](A) SEM image of P(Cat), (B), (C), and (D) TEM images. (C) is an SAED image, and (D) is a high-resolution electron microscope image. [Figure 5] CV measurement results for the positive electrode containing P(Cat). 1st: 1st time, 2nd: 2nd time, 3rd: 3rd time. [Figure 6] (A) Graph of charge / discharge capacity and reaction potential of a positive electrode containing P(Cat), (B) Graph of charge / discharge capacity versus cycles of a positive electrode containing P(Cat). [Figure 7] (A) Graph of charge / discharge capacity and reaction potential of positive electrodes containing different P(Cat). [Figure 8] Plot of the root mean square error (RMSE) of the training data (open circles in Figure 8) versus the literature values ​​of RMSE for six compounds listed in the literature (filled circles in Figure 8). [Figure 9] (A)-(F) Graphs showing the measured capacities of commercially available compounds. [Figure 10] (G)-(L) Graphs showing the measured capacities of commercially available compounds. [Figure 11] (M)-(N) Graph showing the measured capacities of commercially available compounds. [Figure 12] Charge-discharge capacity and reaction potential of electrodes using graphite (dotted line) and a combination of graphite and 5-formylsalicylic acid. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the present disclosure will be described, but the following description does not limit the scope of the claims.

[0015] First embodiment

[0016] <Cathode active material> The positive electrode active material of this embodiment contains a compound represented by the following formula (1). [ka] (In the formula, R1 and R2 each independently represent a hydrogen atom, an alkyl group, an alkenyl group, an alkynyl group, an aryl group, an oxyalkylene group, or an electron-withdrawing group, and n is an integer of 2 or greater.)

[0017] Previous attempts have been made to apply quinone derivatives to cathode materials. For example, Choi et al. proposed a method for reducing solubility by adding anthraquinone to a vinyl group, thereby using it as a cathode material for lithium-air batteries (J. Am. Chem. Soc. 2011, 133, 49, 19839-19843). Nishide et al. also proposed a method for using quinone derivatives as a cathode material for sodium-air batteries by adding two anthraquinones to a single main chain, enabling a four-electron reaction (Macromolecules 2015, 48, 8, 2429-2434). However, these electrode materials have problems such as the lack of electrical conductivity of the main chain and the fact that the main chain is not used for charge / discharge capacity, resulting in limited theoretical capacity.

[0018] When polymerizing quinone derivatives for use as electrode materials, it is necessary to contain a large amount of quinone moieties. This is because increasing the number of quinone moieties increases the theoretical capacity and energy density. Methods for polymerizing quinone derivatives include the side-chain type (T. Kawai, K. Oyaizu, H. Nishide, Macromolecules, 2015, 48, 2429) and the main-chain type (Z. Song, Y. Qian, T. Zhang, M. Otani, H. Zhou, 2015, 2, 1500124.). Each of these methods has its own advantages and challenges.

[0019] First, in the side-chain type, the quinone moieties are closely adjacent, so high rate characteristics can be expected due to electron transfer within the molecule. However, side-chain types have issues such as a lack of conductivity in the main chain portion and a decrease in theoretical capacity because the main chain portion does not contribute to capacity. Therefore, research into main-chain types is being conducted to achieve even higher capacity. While main-chain types are expected to have a higher theoretical capacity than side-chain types, aggregation due to interactions between main chains is considered to be an issue.

[0020] While searching for an electrode material with improved electrochemical properties by incorporating a large amount of quinone moieties into the polymer, the inventors achieved a high reaction potential for use as a cathode material for Li-ion secondary batteries by incorporating an o-quinone structure, which is thought to have a high reaction potential, into the molecule, in contrast to the low reaction potential that was an issue with quinone derivatives. This molecular structure prevents aggregation due to strong stacking between molecules, due to the influence of hydroxyl groups and steric hindrance from neighboring rings. Furthermore, it is believed that the polymer has a high theoretical capacity due to being polymerized with a minimum molecular weight.

[0021] The quinone site refers to a site in a compound where an o-quinone, m-quinone, or p-quinone structure, or a derivative thereof, exists, or its vicinity.

[0022] n is preferably 2 to 100. In a particularly preferred embodiment, n is 3 to 20, and in a more preferred embodiment, n is 5 to 20. It is believed that practically sufficient capacity can be obtained when n is 2 to 100. Furthermore, when n is 3 or more, particularly high capacity can be easily obtained, and when n is 20 or less, synthesis is easy and therefore advantageous in terms of cost.

[0023] The alkyl group is preferably an alkyl group having 1 to 12 carbon atoms, more preferably an alkyl group having 1 to 6 carbon atoms. The fewer the carbon atoms in the alkyl group, the more quinone moieties can be theoretically formed with the minimum molecular weight, making it easier to obtain a high capacity, but if the carbon number is around 6, it does not have a significant effect on the capacity, and if the carbon number is around 12, the effect on the capacity can be reduced.

[0024] Examples of the alkyl group include a linear, branched, or cyclic alkyl group such as a methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, sec-butyl group, tert-butyl group, n-pentyl group, isopentyl group, neopentyl group, tert-pentyl group, n-hexyl group, 1-methylpentyl group, 4-methyl-2-pentyl group, 2-ethylbutyl group, n-heptyl group, 1-methylhexyl group, n-octyl group, 1-methylheptyl group, 2-ethylhexyl group, n-decyl group, n-dodecyl group, cyclopentyl group, and cyclohexyl group, and preferably a methyl group or an ethyl group.

[0025] The alkenyl group preferably has 2 to 12 carbon atoms, and more preferably has 2 to 6 carbon atoms. The fewer the carbon atoms in the alkenyl group, the more quinone moieties can be theoretically formed with the minimum molecular weight, making it easier to obtain a high capacity, but if the carbon number is around 6, it does not have a significant effect on the capacity, and if the carbon number is around 12, the effect on the capacity can be reduced.

[0026] Examples of the alkenyl group include linear, branched, and cyclic alkenyl groups such as vinyl, propenyl, 1-butenyl, isobutenyl, 1-pentenyl, 2-pentenyl, 2-methyl-1-butenyl, 2-cyclopentenyl, and 1-vinylhexyl, with vinyl being preferred.

[0027] The alkynyl group is preferably an alkynyl group having 2 to 12 carbon atoms, more preferably an alkynyl group having 2 to 6 carbon atoms. The fewer the carbon atoms in the alkynyl group, the more quinone moieties can be theoretically formed with the minimum molecular weight, making it easier to obtain a high capacity, but if the carbon number is around 6, it does not have a significant effect on the capacity, and if the carbon number is around 12, the effect on the capacity can be reduced.

[0028] Examples of the alkynyl group include an ethynyl group, a propynyl group, a 1-butynyl group, a 1-pentynyl group, a 2-pentynyl group, a 2-methyl-1-pentynyl group, etc., and preferably an ethynyl group.

[0029] Examples of the aryl group include a phenyl group, a 1-naphthyl group, a 2-naphthyl group, a 2-anthracenyl group, a 1-phenanthryl group, a 2-phenanthryl group, a 3-phenanthryl group, a 1-pyrenyl group, a 2-pyrenyl group, etc., and preferably a phenyl group.

[0030] The oxyalkylene group is preferably an oxyalkylene group having 1 to 8 carbon atoms, and more preferably an oxyethylene group.

[0031] Preferably, the compound represented by the formula (1) is a compound in which R1 and R2 are hydrogen. More preferably, the compound represented by the formula (1) is a compound in which R1 and R2 are hydrogen and n is 5 to 20.

[0032] Examples of the electron-withdrawing group include -CN, -NO2, -CF3, -CCl3, -CHO, -COCH3, -COOC2H5, -COOH, -SO2CH3, -SO3H, etc.

[0033] The positive electrode active material of this embodiment has a high reaction potential as an organic polymer, and the capacity is as high as that of a known positive electrode active material. Therefore, a lithium secondary battery provided with a positive electrode containing such a positive electrode active material has a high capacity and improved high-rate charge-discharge characteristics.

[0034] In the first embodiment, the positive electrode active material may consist only of the compound represented by the formula (1), but as long as it contains the compound represented by the formula (1), conventionally known positive electrode active materials such as LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiNi a Co b M c O2 (wherein M is Mn or Al, and a, b, c satisfy 0 < a < 1, 0 < b < 1, 0 < c < 1, and a + b + c = 1), LiFePO4, etc. may be included.

[0035] When the positive electrode active material contains a compound other than the compound represented by formula (1), the content of the compound represented by formula (1) is not limited, but may be, for example, 5 to 95% by mass, or 10 to 90% by mass. In one embodiment, the amount of the compound represented by formula (1) in the positive electrode active material is 10% by mass or more, 20% by mass or more, 30% by mass or more, 40% by mass or more, 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more. In another embodiment, the amount of the compound represented by formula (1) in the positive electrode active material is 90% by mass or less, 80% by mass or less, 70% by mass or less, 60% by mass or less, 50% by mass or less, 40% by mass or less, 30% by mass or less, 20% by mass or less, or 10% by mass or less. In terms of improving charge / discharge characteristics, a higher content of the compound represented by formula (1) is preferred.

[0036] A method for producing a polycatechol compound represented by formula (1) is described below. First, catechol is dissolved in an aqueous sulfuric acid solution. The sulfuric acid solution may be cooled to 4°C, at room temperature, or heated. A solution of FeCl3 dissolved in sulfuric acid is prepared in a separate container, and this solution is mixed with the aqueous sulfuric acid solution containing catechol and allowed to stand for a certain period of time. The standing temperature may be, for example, 4°C to 40°C, and the standing time may be 30 minutes to 72 hours. Subsequently, unreacted catechol monomer and remaining iron ions are removed using a known purification method such as suction filtration. The residue is washed with a solvent such as hydrochloric acid or acetone to remove oligomers. The resulting purified product is then dried to obtain a black powder. Optionally, this graphite powder may be further reprecipitated. Specifically, 10 mg of the black powder is dissolved in benzyl alcohol and mixed with pure water. The resulting particles are recovered using a known purification method such as suction filtration and washed with an appropriate solvent such as pure water to obtain the product compound represented by formula (1).

[0037] <Negative electrode active material> The negative electrode active material of this embodiment contains a compound represented by the following formula (2). [ka] (In the formula, A is hydrogen, a metal, or NH3, and R1, R2, and R3 are each independently hydrogen, an alkyl group, or an oxyalkylene group.)

[0038] The compound of formula (2) is formed when the functional group -COOA is an ion (COO - A + ) is also included.

[0039] Examples of the metal include metal elements such as alkali metals, alkaline metals, transition metals, etc. A is preferably hydrogen, Li, K, or Na.

[0040] The alkyl group is preferably an alkyl group having 1 to 12 carbon atoms, more preferably an alkyl group having 1 to 6 carbon atoms, and even more preferably a methyl group or an ethyl group. The fewer the carbon atoms in the alkyl group, the more quinone moieties can be theoretically formed with the minimum molecular weight, making it easier to obtain a high capacity, but even if the carbon number is around 12, it does not have a significant effect on the capacity.

[0041] The oxyalkylene group is preferably an oxyalkylene group having 1 to 8 carbon atoms, and more preferably an oxyethylene group.

[0042] Preferably, the compound represented by formula (2) is 5-formylsalicylic acid or lithium 5-formylsalicylate (R1, R2, R3 are hydrogen).

[0043] [ka]

[0044] The negative electrode active material of this embodiment has a low reaction potential for a low molecular weight compound and an extremely high capacity, so that a lithium secondary battery equipped with a negative electrode containing such a negative electrode active material has a high capacity and improved high-rate charge / discharge characteristics.

[0045] In our search for organic negative electrode active materials for lithium secondary batteries, we first performed charge / discharge measurements on 25 known organic compounds (training compounds) and measured their negative electrode capacities (objective variables). Next, based on literature and theoretical values, we prepared a total of 23 explanatory variables, including solubility parameters in the electrolyte, energy levels based on DFT calculations, and molecular weight, which determine capacity. These variables were combined with the measured capacities to create a training dataset. Sparse modeling was performed on this dataset to extract several descriptors correlated with capacity. A capacity prediction model consisting of these descriptors was constructed, and using this model, predicted capacities of commercially available compounds expected to have high negative electrode capacities were calculated. Charge / discharge measurements were then performed on compounds with high predicted values ​​to measure their capacities. Compounds of formula (2) with significantly higher capacities than other compounds were identified.

[0046] In the first embodiment, the negative electrode active material may consist solely of the compound represented by formula (2), but may further contain a conventionally known negative electrode active material, such as a carbon-based negative electrode active material, a silicon-based negative electrode active material, or both, as long as it contains the compound represented by formula (2). Examples of carbon-based negative electrode active materials include graphite, graphene, metal-organic complexes, carbon nanotubes, and carbon nanofibers.

[0047] When the negative electrode active material contains a compound other than the compound represented by formula (2), the amount of the compound represented by formula (2) in the negative electrode active material is not limited, but may be, for example, 5 to 95% by mass, or 10 to 90% by mass. In one embodiment, the amount of the compound represented by formula (2) in the negative electrode active material is 10% by mass or more, 20% by mass or more, 30% by mass or more, 40% by mass or more, 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more. In another embodiment, the amount of the compound represented by formula (2) in the negative electrode active material is 90% by mass or less, 80% by mass or less, 70% by mass or less, 60% by mass or less, 50% by mass or less, 40% by mass or less, 30% by mass or less, 20% by mass or less, or 10% by mass or less. In terms of improving charge / discharge characteristics, a higher content of the compound represented by formula (2) is preferred.

[0048] <Lithium-ion secondary battery> FIG. 1 is a schematic exploded side view of a lithium secondary battery. In the following description, the lithium-ion secondary battery may be abbreviated as "battery".

[0049] The lithium secondary battery 1 as a non-aqueous electrolyte secondary battery includes a positive electrode (also referred to as a positive electrode layer or cathode) 10, a negative electrode (also referred to as a negative electrode layer or anode) 20, an electrolytic solution 40 having lithium-ion conductivity filled between the positive electrode 10 and the negative electrode 20, and a separator 30 disposed between the positive electrode 10 and the negative electrode 20 to insulate between the positive electrode 10 and the negative electrode 20 in the electrolytic solution 40. Both the positive electrode 10 and the negative electrode 20 perform redox reactions and occlude and release lithium ions. The positive electrode 10 includes a positive electrode current collector 12 and a positive electrode mixture layer 14, and the negative electrode 20 includes a negative electrode current collector 22 and a negative electrode mixture layer 24.

[0050] <Positive electrode> The positive electrode 10 of the present embodiment is a sheet-like member. The positive electrode current collector 12 may be, for example, an Al foil, an Al alloy foil, a Ni film, or the like. The positive electrode current collector 12 may have a thickness of, for example, 5 to 50 μm. The positive electrode mixture layer 14 may be formed on the surface of the positive electrode current collector 12. The positive electrode mixture layer 24 may be formed on both the front and back surfaces of the positive electrode current collector 12. The positive electrode mixture layer 14 may have a thickness of, for example, 10 to 200 μm. The positive electrode mixture layer 14 contains the positive electrode active material of the present embodiment. That is, the positive electrode 10 contains at least the positive electrode active material of the present embodiment.

[0051] In addition to the positive electrode active material of the present embodiment, the positive electrode mixture layer 14 may further contain conventionally known positive electrode active materials, such as LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiNi a Co b M c O2 (wherein M is Mn or Al, and a, b, c satisfy 0 < a < 1, 0 < b < 1, 0 < c < 1, and a + b + c = 1), LiFePO4, etc. One kind of positive electrode active material may be used alone, or two or more kinds of positive electrode active material particles may be combined and used.

[0052] The positive electrode mixture layer 14 may contain a conductive material and a binder in addition to the positive electrode active material. The proportions of the positive electrode active material, conductive material, and binder in the positive electrode mixture layer 14 can be appropriately selected to improve capacity. For example, the positive electrode mixture layer 14 may contain 50 to 85 mass % of the positive electrode active material, 10 to 45 mass % of the conductive material, and 5 to 15 mass % of the binder. To achieve high capacity, it is preferable that 50 to 100 mass % of the positive electrode active material particles be the positive electrode active material of this embodiment (compound represented by formula (1)) and it is preferable that 80 to 100 mass % be the positive electrode active material of this embodiment (compound represented by formula (1))).

[0053] The conductive material assists electron conduction within the positive electrode mixture layer 14. The conductive material is not particularly limited. The conductive material may be, for example, carbon black, vapor-grown carbon fiber (VGCF), carbon nanotubes (CNT), graphene, activated carbon, or the like. The carbon black may be, for example, acetylene black (AB), Ketjen Black (registered trademark), or the like. One type of conductive material may be used alone, or two or more types of conductive materials may be used in combination. From the viewpoint of improving charge / discharge capacity, the conductive material preferably includes a combination of vapor-grown carbon fiber (VGCF) and graphene.

[0054] The binder binds the components in the positive electrode mixture layer 14 together. The binder binds the negative electrode mixture layer 24 and the positive electrode current collector 12 together. The binder is not particularly limited. Examples of the binder include polyamide (PA), polyimide (PI), polyamideimide (PAI), polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), vinylidene fluoride-hexafluoropropylene copolymer [poly(VDF-co-HFP)], styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), and polyacrylic acid (PAA). One type of binder may be used alone, or two or more types of binders may be used in combination.

[0055] <Negative electrode> The negative electrode 20 of this embodiment is a sheet-like member. The negative electrode current collector 22 may be, for example, a Cu (copper) foil or a Cu alloy foil. The negative electrode current collector 22 may have a thickness of, for example, 5 to 50 μm. In this specification, the thickness of each component can be measured, for example, with a micrometer.

[0056] The negative electrode mixture layer 24 can be formed on the surface of the negative electrode current collector 22. The negative electrode mixture layer 24 can be formed on both the front and back surfaces of the negative electrode current collector 22. The negative electrode mixture layer 24 may have a thickness of, for example, 10 to 200 μm. The negative electrode mixture layer 24 contains the negative electrode active material of this embodiment. That is, the negative electrode 20 contains at least the negative electrode active material of this embodiment.

[0057] In addition to the negative electrode active material of this embodiment (the compound represented by formula (2)), the negative electrode mixture layer 24 may further contain a conventionally known negative electrode active material, such as a carbon-based negative electrode active material, a silicon-based negative electrode active material, or both. Examples of carbon-based negative electrode active materials include graphite, graphene, metal-organic complexes, carbon nanotubes, and carbon nanofibers. The combination of a silicon-based negative electrode active material and a carbon-based negative electrode active material is expected to improve the balance between cycle characteristics and capacity, for example. The carbon-based negative electrode active material has a graphite structure. One type of carbon-based negative electrode active material may be used alone, or two or more types of carbon-based negative electrode active materials may be used in combination.

[0058] The negative electrode mixture layer 24 may contain a conductive material and a binder in addition to the negative electrode active material of this embodiment. The proportions of the negative electrode active material, conductive material, and binder in the negative electrode mixture layer 24 can be selected as appropriate. For example, the positive electrode mixture layer 24 may contain 50 to 85 mass % of the negative electrode active material, 10 to 45 mass % of the conductive material, and 5 to 15 mass % of the binder. Preferably, 50 to 100 mass % of the negative electrode active material is the negative electrode active material of this embodiment (compound represented by formula (2)), and preferably 80 to 100 mass % is the negative electrode active material of this embodiment (compound represented by formula (2)).

[0059] The conductive material assists electron conduction within the negative electrode mixture layer 24. The conductive material is not particularly limited. The conductive material may be, for example, carbon black, vapor grown carbon fiber (VGCF), carbon nanotubes (CNT), graphene flakes, activated carbon, or the like. The carbon black may be, for example, acetylene black (AB), Ketjen Black (registered trademark), or the like. One type of conductive material may be used alone, or two or more types of conductive materials may be used in combination.

[0060] The binder binds the components in the negative electrode mixture layer 24 together. The binder binds the negative electrode mixture layer 24 and the negative electrode current collector 22 together. The binder is not particularly limited. The binder may be, for example, polyamide (PA), polyimide (PI), polyamideimide (PAI), polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), vinylidene fluoride-hexafluoropropylene copolymer [poly(VDF-co-HFP)], styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), or the like. One type of binder may be used alone, or two or more types of binders may be used in combination.

[0061] <separator> The separator 30 is a sheet-like member. The separator 30 is interposed between the negative electrode 20 and the positive electrode 10. The separator 30 may have a thickness of, for example, 10 to 50 μm. The separator 30 is porous. The separator 30 is made of an electrically insulating material. The separator 30 may be made of, for example, a polyolefin resin such as polyethylene or polypropylene; polyimide; polyvinyl alcohol; a fluororesin such as terminally aminated polyethylene oxide polytetrafluoroethylene; an acrylic resin; nylon; aromatic aramid; inorganic glass; ceramics; or the like. The separator 30 may have, for example, a single-layer structure or a multi-layer structure (for example, a three-layer structure).

[0062] <Electrolytes> The electrolyte 40 is usually a liquid electrolyte, but may be a gel electrolyte or a solid electrolyte. A non-aqueous electrolyte solution in which a lithium salt is dissolved in an organic solvent is preferably used as the electrolyte solution 40, and contains an organic solvent and a lithium salt. The electrolyte solution may contain, for example, 0.5 to 2.0 mol / L of lithium salt. The lithium salt functions as a supporting electrolyte.

[0063] Lithium salts include, for example, inorganic lithium salts such as LiClO4, LiBF4, LiPF6, LiAsF6, LiCl, and LiBr; LiB(C6H5)4, LiN(SO2CF3)2, LiC(SO2CF3)3, LiiOSO2CF3, LiiOSO2C2F5, LiiOSO2C4F9, LiiOSO2C5F 11 , LiiOSO2C6F 13 , and LiiOSO2C7F 15、 The lithium salt may be an organic lithium salt such as Li[N(FSO2)2] or Li[N(CF3SO2)2]. One type of lithium salt may be used alone, or two or more types of lithium salts may be used in combination.

[0064] Examples of organic solvents include lactones, cyclic ethers, chain ethers, cyclic esters, and chain esters. Examples of lactones include γ-butyrolactone (GBL) and δ-valerolactone. Examples of cyclic ethers include tetrahydrofuran (THF), 1,3-dioxolane, and 1,4-dioxane. Examples of cyclic esters include propylene carbonate, butylene carbonate, γ-butyrolactone, vinylene carbonate, 2-methyl-γ-butyrolactone, acetyl-γ-butyrolactone, and γ-valerolactone. Examples of chain esters include dimethyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl butyl carbonate, methyl propyl carbonate, ethyl butyl carbonate, ethyl propyl carbonate, butyl propyl carbonate, alkyl propionate, dialkyl malonate, and alkyl acetate. Examples of cyclic ethers include tetrahydrofuran, alkyl tetrahydrofuran, dialkyl tetrahydrofuran, alkoxy tetrahydrofuran, dialkoxy tetrahydrofuran, 1,3-dioxolane, alkyl-1,3-dioxolane, and 1,4-dioxolane. Examples of chain ethers include 1,2-dimethoxyethane, 1,2-diethoxyethane, diethyl ether, ethylene glycol dialkyl ether, diethylene glycol dialkyl ether, triethylene glycol dialkyl ether, and tetraethylene glycol dialkyl ether.

[0065] In addition to the organic solvent and the lithium salt, the electrolyte solution 40 may further contain various functional additives. The electrolyte solution may contain, for example, 1 to 5 mass % of the functional additive. Examples of the functional additive include a gas generating agent (overcharge additive) and a film forming agent. Examples of the gas generating agent may include cyclohexylbenzene (CHB), biphenyl (BP), etc. Examples of the film forming agent may include vinylene carbonate (VC), vinylethylene carbonate (VEC), Li[B(C2O4)2], LiPO2F2, propane sultone (PS), ethylene sulfite (ES), etc.

[0066] The method for assembling the lithium secondary battery 1 is not particularly limited, and it can be assembled by a known method for assembling a secondary battery. For example, the electrode components consisting of the positive electrode 10, the negative electrode 20, and the separator 30, and an insulator (not shown), are housed in an exterior case (not shown), the exterior case (not shown) is impregnated with an electrolyte solution 40, and the electrolyte 40 is disposed between the positive electrode 10 and the negative electrode 20. Next, the top of the case is sealed, thereby manufacturing the lithium secondary battery 1.

[0067] The exterior packaging material is preferably a metal can, such as a can made of iron, stainless steel, or aluminum. Alternatively, a film-like bag made of extremely thin aluminum laminated with resin may be used. The exterior packaging material may be cylindrical, rectangular, thin, or any other shape. However, since large-sized lithium-ion secondary batteries are often used as assembled batteries, rectangular or thin shapes are preferred.

[0068] <Applications of lithium-ion secondary batteries> The lithium ion secondary battery of this embodiment is expected to have a high capacity. The lithium ion secondary battery of this embodiment is expected to be excellent in at least one of input characteristics and output characteristics. This is because the lithium ion secondary battery of this embodiment has a positive electrode 10 containing the positive electrode active material of this embodiment (compound represented by formula (1)) and a negative electrode 20 containing the negative electrode active material of this embodiment (compound represented by formula (2)).

[0069] The applications of the lithium-ion secondary battery of this embodiment include, but are not limited to, being used as a power source for driving vehicles, a power source for electronic devices, etc., and as a power source for objects that require electrical supply. The lithium-ion secondary battery of this embodiment is applicable to all applications.

[0070] Second Embodiment Only the differences between the lithium-ion secondary battery of the second embodiment and the lithium-ion secondary battery of the first embodiment will be described.

[0071] In the lithium-ion secondary battery of the second embodiment, the negative electrode active material is the same as that of the first embodiment and contains the compound represented by formula (2), but the positive electrode active material does not contain the compound represented by formula (1). Instead, it contains a conventionally known positive electrode active material, such as LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiNi a Co b M c O2 (where in the formula, M is Mn or Al, and a, b, c satisfy 0 < a < 1, 0 < b < 1, 0 < c < 1, and a + b + c = 1), LiFePO4, etc. One kind of positive electrode active material may be used alone, or two or more kinds of positive electrode active material particles may be combined and used.

[0072] Third Embodiment Only the differences between the lithium-ion secondary battery of the third embodiment and the lithium-ion secondary battery of the first embodiment will be described.

[0073] In the lithium-ion secondary battery of the third embodiment, the positive electrode active material is the same as that of the first embodiment and contains a compound represented by formula (1). However, the negative electrode active material does not contain a compound represented by formula (2). Instead, it further contains a conventionally known negative electrode active material, such as a carbon-based negative electrode active material, a silicon-based negative electrode active material, or both. Examples of carbon-based negative electrode active materials include graphite, graphene, metal-organic complexes, carbon nanotubes, and carbon nanofibers. The combination of a silicon-based negative electrode active material and a carbon-based negative electrode active material is expected to improve the balance between cycle characteristics and capacity, for example. The carbon-based negative electrode active material contains a graphite structure. One type of carbon-based negative electrode active material may be used alone, or two or more types of carbon-based negative electrode active materials may be used in combination. [Example]

[0074] The present invention will be specifically described with reference to the following examples, but the present invention is not limited to these examples.

[0075] Example 1: Preparation of polycatechol 1. Synthesis of polycatechol (P(Cat)) 1 mmol of catechol was dissolved in 5 mL of 20 wt% H2SO4 cooled to 4°C. In a separate container, 4 mmol of FeCl3 was added and dissolved in 5 mL of 20 wt% H2SO4 also cooled to 4°C. While further cooling to 4°C, the FeCl3 solution was added to the catechol solution, and the mixture was gently shaken and then allowed to stand in a 4°C thermostatic chamber for 48 hours. Afterwards, the mixture was washed with hydrochloric acid using suction filtration to remove monomers and residual iron ions. Subsequently, the mixture was washed with acetone to remove oligomers. This was then dried in a 60°C thermostatic chamber to obtain a black powder.

[0076] 10 mg of this black powder was dissolved in 10 mL of benzyl alcohol and added to approximately 500 mL of pure water. The resulting particles were collected by suction filtration and washed with pure water to obtain the product. Hereafter, this product will be referred to as polycatechol (P(Cat)). Furthermore, the benzyl alcohol solution added to pure water will be referred to as P(Cat) dispersion.

[0077] 2.Analysis (1) Fourier transform infrared spectroscopy (FT-IR) To confirm the polymerization, a Fourier transform infrared spectrometer (FT / IR-4200 type A, JASCO) was used. The sample was formed into a disk shape using the KBr method, and the measurement range was 4000–400 cm. -1 , resolution 4 cm -1 The measurement was performed with the number of integration times set to 128. (2) Thermogravimetric measurement device (TG-DTA) To confirm the polymerization of the sample, TG-DTA (TG / DTA7200, EXSTER) was used. The temperature range was set to 25°C to 800°C, and the heating rate was 10°C min -1 and measurements were carried out. (3) Scanning electron microscope (SEM) To observe the shape of the P(Cat) samples, a SEM (JSM-7100F, JEOL Ltd.) was used. For observation, the samples were fixed to an aluminum base with conductive tape. After fixing, each sample was coated with approximately 5 nm of osmium using an OSMIUM COATER (HPC-1S, Vacuum Device Co., Ltd.) to ensure conductivity before observation. (4) Transmission electron microscope (TEM) To observe the shape of P(Cat), we used FE-TEM (Tecnai F20, FEI) and TEM-120 (TecnaiSpirit, FEI). The P(Cat) dispersion was dropped onto a Cu grid, and the sample was observed after the solvent evaporated.

[0078] 3. Experimental Results Figures 2(A) and (B) show the changes in the IR spectra of P(Cat) and catechol when the synthesis temperature of P(Cat) was changed to 0, room temperature, 60, and 100°C. Comparison of the spectra in Figure 2(A) revealed that peaks originating from OH stretching vibrations, CH stretching vibrations, C=O stretching vibrations, aromatic ring stretching vibrations, and CO stretching vibrations were assigned. A decrease in the CH stretching vibration peak was also confirmed. Furthermore, the TG graph in Figure 2(B) revealed that the weight loss temperature of P(Cat) was approximately 500°C, which was higher than the weight loss temperature of the monomer catechol, approximately 200°C. This indicates that P(Cat) has a higher molecular weight.

[0079] Furthermore, the results of XRD measurement of P(Cst) are shown in Figure 3. From these results, the XRD peak of P(Cat) is very small compared to glassy carbon and graphene, suggesting low crystallinity. In addition, the broad peak indicates an amorphous structure. Furthermore, the results of elemental analysis and the structure of P(Cat) estimated from it are shown in Table 1.

[0080] [Table 1]

[0081] It was suggested that most of P(Cat) has a linear structure, and that some of it has a peroxidized partial structure.

[0082] Next, we observed the morphology. First, the SEM image in Figure 4(A) shows that the P(Cat) particles are irregular particles of approximately several hundred nanometers in size. Furthermore, the particles appear to be aggregated. The results of TEM observation of these particles are shown below. In the TEM observations in Figures 4(B)-(D), similar irregular particles of several hundred nanometers in size were observed. Further observations using high-resolution electron microscopy showed that no lattice fringes were observed, indicating that the particles were amorphous.

[0083] Example 2: Preparation of cathode material and evaluation of charge / discharge characteristics 1. Production of cathode materials The P(Cat) produced in Example 1 was mixed with a polycatechol sample, a co-electrode material (acetylene black), and a binder (polyvinylidene fluoride) in a ratio of 6:3:1, and the mixture was made into a slurry using NMP. The slurry was then applied to a SUS mesh (Nilaco) and dried to produce a positive electrode.

[0084] 2.Analysis (1) Cyclic voltammetry (CV) To investigate the electrochemical activity of the electrode containing P(Cat), cyclic voltammetry (HSV-110, Hokuto Denko) was used. The working electrode was the positive electrode, the reference electrode was lithium metal, the counter electrode was lithium metal, and the electrolyte was 1 mol cm. -3 LiPF6 EC / DMC (1:2 v / v%) was used. The scan range was 2.5 to 4.5 V (vs. Li / Li+) at a scan rate of 1 mV s -1 was set and measurements were carried out. (2) Charge / discharge measurement To investigate the electrochemical activity of the electrode containing P(Cat), a charge-discharge apparatus (HJ1001SD8, Hokuto Denko Corporation) was used. The working electrode was the positive electrode, the reference electrode was lithium metal, the counter electrode was lithium metal, and the electrolyte was 1 mol cm3 LiPF6 EC / DMC (1:2 v / v%). The scan range was 2.5 to 4.5 V (vs. Li / Li). + The current density was set to 50 mA g-1 to 5000 mA g-1. -1 Measurements were carried out as follows.

[0085] 3. Experimental Results The electrochemical properties of the electrode containing P(Cat) were evaluated. The CV results are shown in Figure 5. From these results, in LiPF6EC / DMC (1:2 v / v%), a peak due to the oxidation of the quinone moiety was observed between 3.5 and 4.3 V, and a peak due to the reduction of the quinone moiety was observed between 3.5 and 2.8 V. Furthermore, the capacities were 119.7, 99.8, and 99.7 mA h g-1, respectively, from the first cycle. This confirmed that the polymerization had a certain effect of suppressing dissolution.

[0086] The results of the charge-discharge measurements are shown in Figures 6(A) and 6(B). From these graphs, a charge plateau was observed between 3.5 and 4.5 V, and a discharge plateau was observed between 3.5 and 3.0 V. The capacity was 129 mA h g at 50 mA g. The capacity also decreased with increasing current density.

[0087] The electrochemical properties of these P(Cat)s were compared with those of previous studies on cathode materials for Li-ion secondary batteries using organic materials (Sci. Rep, 2014, 4,3591; Jpn. J. Appl. Phys,1984, 23, L892; Electrochem. Acta, 2006, 51, 2589; J. Appl. Electrochem, 1992, 22, 307; Adv. Sci, 2015, 2, 1500124; J. Mater. Chem,2012, 22, 4032; Angew. Chem, 2015, 127, 14153; Macromolecules, 2010, 43, 10382; Macromol. Rapid Commun, 2007, 28, 1929; Energ. Compared with the prior art (Environ. Sci. 2017, 10, 2334; Chem. Phys. Lett. 2002, 359, 351), the organic sulfur compounds in the prior art had capacities of 200–400 mA h g-1 at a potential of approximately 2.5 V. Furthermore, radical compounds had capacities of less than 100 mA h g-1 at a potential of approximately 3.5 V. Materials using quinones had capacities of 200–300 mA h g-1 at a potential of approximately 2–3 V. The discharge potential of P(Cat) was 3.5 V, which was significantly higher than the 2–3 V of materials using quinone derivatives. Furthermore, the capacity of 129 mA h g-1 was higher than that of radical compounds with similar reaction potentials.

[0088] Example 3 Further investigation of positive electrode materials A positive electrode material was produced according to the method described in Example 2, using a mixture of vapor grown carbon fiber (VGCF) and graphene as the conductive material instead of acetylene black.

[0089] The results of charge-discharge measurements of the electrode containing P(Cat) are shown in Figure 7. Significant improvements in discharge capacity at 50 mA g-1 and 100 mA g-1 were observed.

[0090] Example 4 Selection of negative electrode active material In our search for organic negative electrode active materials for lithium secondary batteries, we first performed charge-discharge measurements on 25 known organic compounds (training compounds) shown as compounds (1)-(25) in Scheme 1 below, and measured their negative electrode capacity (objective variable). The training compounds selected were compounds with a carbazole skeleton (dibenzothiophene / dibenzofuran), compounds with a thiophene skeleton, compounds with a conjugated skeleton, and these compounds with a carboxyl group.

[0091] Next, based on literature and theoretical values, we prepared a total of 23 explanatory variables, including solubility parameters in the electrolyte that determine capacity, energy levels based on DFT calculations, and molecular weight (Table 2), and combined these with the measured capacity to create a training dataset (Table 3). Each explanatory variable is publicly known. Sparse modeling was performed on this dataset (R version 3.4.0, data analysis by MCP, packages ncverg and ggplot2) to identify six descriptors, x4, X6, and X, that correlate with capacity. 10 , X 11 , X 13 , X 22 The capacity prediction model consisting of these descriptors is: y=ax4-b6+cX 10 + dX 11 + eX 13 + fX 22 + g (a, b, c, d, e, f, g are constants) (1) was constructed.

[0092] Here, the root mean square error (RMSE) of the training data (open circles in Figure 8) and the literature values ​​of RMSE for a total of six compounds (three compounds described in X. Han, G. Qing, J. Sun, T. Sun, Angew. Chem. Int. Ed., 2012, 51, 5147 and three compounds described in HH Lee, Y. Park, KH. Shin, KT Lee, SY Houng, ACS Appl. Mater. Interfaces. 2014, 6, 19118) (black circles in Figure 8) were 213 and 418 mAhg, respectively. -1 It was confirmed that the prediction accuracy was high compared to the literature values.

[0093] Using this model (1), we calculated the predicted capacities of commercially available compounds (26)-(127) in Scheme 2 below, which are expected to have high negative electrode capacities. The commercially available compounds selected were cinnamic acid and its derivatives, acetylsalicylic acid and its derivatives, and compounds with a conjugated backbone and a carboxyl group.

[0094] Of these, charge / discharge measurements were performed on 14 compounds with high predicted values ​​(Scheme 3, Table 4), and the capacity was measured three times for each compound (Figures 9-11). Note that compounds ranked 11 and above were selected taking into consideration the structural similarity of the compounds. As a result, it was confirmed that the measured capacity of compound (66), lithium 5-formyl salicylate, was significantly higher than that of the other compounds.

[0095] As shown in Figure 12, when graphite was used as the negative electrode active material, the discharge capacity of the electrode was 372 mA hg at 0.01 V. -1 The discharge capacity of the electrode using a combination of graphite and lithium 5-formylsalicylate was 1262 mA hg at 0.01 V. -1 It was.

[0096] [ka]

[0097] [Table 2]

[0098] [Table 3]

[0099] [ka]

[0100] [ka]

[0101] [Table 4] [Explanation of symbols]

[0102] 10...Positive electrode, 12...Positive electrode active material, 20...Negative electrode, 22...Negative electrode active material.

Claims

1. A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising a compound represented by the following formula (1): 【Chemistry 1】 (In the formula, R 1 , R 2 are each independently a hydrogen atom, an alkyl group, an alkenyl group, an alkynyl group, an aryl group, an oxyalkylene group, or an electron-withdrawing group, and n is an integer of 2 or more.

2. A positive electrode comprising the positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1.

3. A non-aqueous electrolyte secondary battery comprising the positive electrode according to claim 2.

4. A negative electrode active material for a non-aqueous electrolyte secondary battery, comprising a compound represented by the following formula (2): 【Chemistry 2】 (wherein A is hydrogen, a metal, or NH 3 and R 1 , R 2 , R 3 are each independently a hydrogen atom, an alkyl group, or an oxyalkylene group.

5. 5. The negative electrode active material for a non-aqueous electrolyte secondary battery according to claim 4, wherein the compound represented by formula (2) is 5-formylsalicylic acid or lithium 5-formylsalicylate.

6. A negative electrode comprising the negative electrode active material for a non-aqueous electrolyte secondary battery according to claim 4 or 5.

7. A non-aqueous electrolyte secondary battery comprising the negative electrode according to claim 6.

8. A non-aqueous electrolyte secondary battery comprising the positive electrode according to claim 2 and the negative electrode according to claim 6.

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