JP5350845B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery.

  Conventionally, as a lithium ion secondary battery, an active material containing tungsten dioxide doped with a metal dopant that improves charge / discharge capacity per unit weight of an active material such as aluminum or manganese and a polymer binder such as polyvinylidene fluoride are used. There has been proposed an electrode composition having an electrode composition (see, for example, Patent Document 1). In the lithium secondary battery of Patent Document 1, the charge / discharge capacity per unit weight of the active material is improved by the metal dopant, and the decrease in battery capacity due to the charge / discharge cycle is suppressed.

Japanese translation of PCT publication No. 2003-509829

  By the way, although tungsten oxide is a material that is thermally stable and exhibits a high theoretical capacity density, the electrolyte solution solvent in the vicinity of 0.5 to 1.2 V where charge / discharge efficiency decreases or charge / discharge progresses. In some cases, the internal resistance may increase due to a high-resistance film formed on the negative electrode by decomposition. Further, for example, in the battery described in Patent Document 1, a decrease in battery capacity due to a charge / discharge cycle is suppressed, but it is not yet sufficient, and it has been desired to further suppress a decrease in battery capacity.

  This invention is made in view of such a subject, In what provided the negative electrode active material containing a group 6 element oxide, the fall of the initial charge / discharge efficiency was suppressed more, and the battery capacity by a charge / discharge cycle It is a main object to provide a lithium ion secondary battery that can further suppress the decrease in the internal resistance and the increase in the internal resistance.

  As a result of intensive research to achieve the above-described object, the present inventors have represented a cation and a general formula (1) in a lithium ion secondary battery using a negative electrode active material containing a Group 6 element oxide. A non-aqueous ion conducting medium containing an electrolyte compound containing an anionic compound and a supporting salt so that a ratio of the electrolyte compound to the whole of the electrolyte compound and the supporting salt is in a range of 2 mol% to 80 mol% is used. Thus, the inventors have found that the first reduction in charge / discharge efficiency can be further suppressed, and that the decrease in battery capacity and the increase in internal resistance before and after the charge / discharge cycle can be further suppressed, and the present invention has been completed.

That is, the lithium ion secondary battery of the present invention is
A positive electrode having a positive electrode active material comprising lithium and a transition metal;
A negative electrode having a negative electrode active material containing a Group 6 element oxide;
An electrolyte compound containing a cation and an anion compound represented by the general formula (1) and a supporting salt interposed between the positive electrode and the negative electrode to conduct lithium ions, and the electrolyte compound and the supporting salt And a non-aqueous ion conductive medium in which the ratio of the electrolyte compound to the whole is in the range of 2 mol% or more and 80 mol% or less.

（但し、Ｍは、遷移元素、周期表の１３族、１４族又は１５族元素を表す；ｂは１〜３の整数、ｍは１〜４の整数、ｎは０〜８の整数、ｑは０又は１を表す；Ｒ¹は、炭素数１〜１０のアルキレン、炭素数１〜１０のハロゲン化アルキレン、炭素数６〜２０のアリーレン又は炭素数６〜２０のハロゲン化アリーレン（これらのアルキレン及びアリーレンはその構造中に置換基、ヘテロ原子を持っていてもよく、またｑが１でｍが２〜４のときにはｍ個のＲ¹はそれぞれが結合していてもよい）を表す；Ｒ²は、ハロゲン、炭素数１〜１０のアルキル、炭素数１〜１０のハロゲン化アルキル、炭素数６〜２０のアリール、炭素数６〜２０のハロゲン化アリール（これらのアルキル及びアリールはその構造中に置換基、ヘテロ原子を持っていてもよく、またｎが２〜８のときにはｎ個のＲ²はそれぞれが結合して環を形成してもよい）又は−Ｘ³Ｒ³を表す；Ｘ¹，Ｘ²及びＸ³は、それぞれが独立でＯ，Ｓ又はＮＲ⁴を表す；Ｒ³及びＲ⁴は、それぞれが独立で水素、炭素数１〜１０のアルキル、炭素数１〜１０のハロゲン化アルキル、炭素数６〜２０のアリール、炭素数６〜２０のハロゲン化アリール（これらのアルキル及びアリールはその構造中に置換基、ヘテロ原子を持っていてもよく、Ｒ³又はＲ⁴は複数個存在する場合にはそれぞれが結合して環を形成してもよい）を表す）

(However, M represents a transition element, a group 13, 14 or 15 element of the periodic table; b is an integer of 1 to 3, m is an integer of 1 to 4, n is an integer of 0 to 8, q is R ¹ represents alkylene having 1 to 10 carbon atoms, halogenated alkylene having 1 to 10 carbon atoms, arylene having 6 to 20 carbon atoms or halogenated arylene having 6 to 20 carbon atoms (these alkylene and Arylene may have a substituent or a hetero atom in the structure, and when q is 1 and m is 2 to 4, m R ^{1 s} may be bonded to each other); R ² Is a halogen, an alkyl having 1 to 10 carbon atoms, an alkyl halide having 1 to 10 carbon atoms, an aryl having 6 to 20 carbon atoms, an aryl halide having 6 to 20 carbon atoms (these alkyl and aryl are in the structure) May have substituents, heteroatoms And n represents the n number of R ² may be each bonded to form a ring) or -X ³ R ³ at the time of 2~8; X ^1, X ² and X ³ are each independently in represents O, S or NR ^4; R ³ and R ⁴ are each hydrogen independently alkyl having 1 to 10 carbon atoms, a halogenated alkyl having 1 to 10 carbon atoms, 6 to 20 carbon atoms aryl, carbon A number 6 to 20 aryl halide (These alkyls and aryls may have a substituent or a heteroatom in the structure, and when a plurality of R ³ or R ⁴ are present, they are bonded to form a ring. May represent))

In this lithium ion secondary battery, it is possible to further suppress the decrease in the initial charge / discharge efficiency, and to further suppress the decrease in battery capacity and the increase in internal resistance before and after the charge / discharge cycle. The reason why such an effect is obtained is not clear, but when the anionic compound represented by the above compound formula (1) is added to the ion conductive medium, it decomposes on the negative electrode active material during the first charge and is stable SEI ( (Solid Electrolyte Interface) film is considered to be formed. In particular, when a Group 6 element oxide is used as the negative electrode active material, a better form is obtained at a relatively low potential in the vicinity of 0.5 to 1.2 V (where the reference electrode is Li ⁺ / Li) where charge and discharge proceed. The SEI film is considered to be formed. And this SEI film is considered to have an effect of suppressing the formation of a high resistance film by suppressing the decomposition of the ion conductive medium on the negative electrode active material due to repeated charge and discharge. Thereby, it is thought that the fall of battery capacity can be suppressed.

1 is a schematic diagram showing an example of a lithium ion secondary battery 10.

  The lithium ion secondary battery of the present invention is interposed between a positive electrode having a positive electrode active material containing lithium and a transition metal, a negative electrode having a negative electrode active material containing a Group 6 element oxide, and the positive electrode and the negative electrode. An electrolyte compound that conducts lithium ions and includes a cation and an anion compound having a specific structure and a supporting salt, and a ratio of the electrolyte compound to the whole of the electrolyte compound and the supporting salt is in a range of 2 mol% to 80 mol%. A non-aqueous ion conductive medium.

In the lithium ion secondary battery of the present invention, the positive electrode has a positive electrode active material containing lithium and a transition metal. For the positive electrode, for example, a positive electrode active material, a conductive material, and a binder are mixed, and an appropriate solvent is added to form a paste-like positive electrode material, which is applied to the surface of the current collector and dried. It may be formed by compression to increase the density. Any positive electrode active material may be used as long as it contains lithium and a transition metal and occludes / releases lithium ions, and may be an oxide or the like. Specifically, Li _(1-x) MnO ₂ (0 <x <1, etc., the same shall apply hereinafter), lithium manganese composite oxides such as Li _(1-x) Mn ₂ O ₄ , Li _(1-x) CoO and lithium-cobalt composite oxides such as _2, Li _(1-x) lithium nickel composite oxide such as NiO _2, a lithium vanadium composite oxide such as LiV ₂ O _3, and transition metal oxides such as V ₂ O ₅ is It is done. Of these, lithium transition metal composite oxides such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , and LiV ₂ O ₃ are preferable. A lithium nickel composite oxide such as Li _(1-x) NiO ₂ to which Co, Al, Mg or the like is added is also suitable. The conductive material is not particularly limited as long as it is an electron conductive material that does not adversely affect the battery performance of the positive electrode. For example, graphite such as natural graphite (scale-like graphite, scale-like graphite) or artificial graphite, acetylene black, carbon black, Examples thereof include ketjen black, carbon whisker, needle coke, carbon fiber, and a mixture of one or more of metals (copper, nickel, aluminum, silver, gold, etc.). Among these, as the conductive material, carbon black and acetylene black are preferable from the viewpoints of electron conductivity and coatability. The binder serves to bind the active material particles and the conductive material particles. For example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), a fluorine-containing resin such as fluororubber, polypropylene, Thermoplastic resins such as polyethylene, ethylene-propylene-dienemer (EPDM), sulfonated EPDM, natural butyl rubber (NBR) and the like can be used alone or as a mixture of two or more. Examples of the solvent for dispersing the positive electrode active material, the conductive material, and the binder include N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, methylethylketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, N, N- Examples thereof include organic solvents such as dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran. Further, a slurry obtained by adding a dispersant, a thickener, or the like to water and slurrying the active material with a latex such as SBR may be used. As the thickener, for example, polysaccharides such as carboxymethyl cellulose and methyl cellulose can be used alone or as a mixture of two or more. Current collectors include aluminum, titanium, stainless steel, nickel, iron, calcined carbon, conductive polymer, conductive glass, etc., as well as surfaces such as aluminum for the purpose of improving adhesion, conductivity and oxidation resistance. Are treated with carbon, nickel, titanium, silver or the like. For these, the surface may be oxidized. Examples of the shape of the current collector include foil, film, sheet, net, punched or expanded, lath, porous, foam, and formed fiber group. Examples of the thickness of the current collector include 1 to 500 μm.

In the lithium ion secondary battery of the present invention, the negative electrode has a negative electrode active material containing a Group 6 element oxide. As the Group 6 element oxide, for example, at least one of tungsten oxide (for example, WO ₂ ), molybdenum oxide (for example, MoO ₂ ), chromium oxide (for example, CrO ₂ ), and the like can be used. The Group 6 element oxide is not particularly limited, but preferably has a rutile structure. In these oxides, lithium can be inserted into and extracted from a tunnel-like site surrounded by a rutile chain, and thus can function as an electrode of a lithium secondary battery. The group 6 element oxide is preferably tungsten oxide. Tungsten oxide undergoes lithium insertion / extraction at a lower potential than molybdenum oxide or chromium oxide. For this reason, when tungsten oxide is used as the negative electrode, the battery voltage determined by the potential difference between the positive electrode and the negative electrode can be increased. Further, it is thermally stable and exhibits a high theoretical capacity density. The negative electrode for a lithium secondary battery of the present invention is, for example, a mixture of a negative electrode active material, a negative electrode binder, and a conductive material, which is added to a paste-like negative electrode material by adding a suitable solvent, and is applied to the surface of the current collector and dried. And it may be formed by compressing to increase the electrode density as necessary. The conductive material is not particularly limited as long as it is an electron conductive material that does not adversely affect the battery performance of the positive electrode. For example, graphite such as natural graphite (scale-like graphite, scale-like graphite) or artificial graphite, acetylene black, carbon black, What mixed 1 type (s) or 2 or more types, such as ketjen black, carbon whisker, needle coke, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.) can be used. Among these, as the conductive material, carbon black and acetylene black are preferable from the viewpoints of electron conductivity and coatability. Examples of the solvent for dispersing the negative electrode active material, the negative electrode binder, and the negative electrode conductive material include N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, and N, N-dimethylaminopropyl. Organic solvents such as amine, ethylene oxide, and tetrahydrofuran are listed. Further, a slurry obtained by adding a dispersant, a thickener, or the like to water and slurrying the active material with a latex such as SBR may be used. As the thickener, for example, polysaccharides such as carboxymethyl cellulose and methyl cellulose can be used alone or as a mixture of two or more. Examples of the application method include roller coating such as applicator roll, screen coating, doctor blade method, spin coating, bar coater, and the like, and any of these can be used to obtain an arbitrary thickness and shape.

  In the lithium ion secondary battery of the present invention, the non-aqueous ion conductive medium is an electrolyte compound including a cation and an anion compound represented by the general formula (1) that conducts lithium ions interposed between the positive electrode and the negative electrode. And the supporting salt, and the ratio of the electrolyte compound to the whole of the electrolyte compound and the supporting salt is in the range of 2 mol% or more and 80 mol% or less. In the general formula (1), M is a transition element, a group 13, 14 or 15 element of the periodic table, among which Al, B, V, Ti, Si, Zr, Ge, Sn, Cu, Y, Zn, Ga, Nb, Ta, Bi, P, As, Sc, Hf or Sb are preferable, and Al, B or P is more preferable. In the case where M is Al, B or P, the synthesis of the anionic compound becomes relatively easy, and the production cost can be suppressed. The valence b of the anion is 1 to 3, of which 1 is preferable. When the valence b is larger than 3, it is not preferable because the salt of the anion compound tends to be hardly dissolved in the mixed organic solvent. The constants m and n are values related to the number of ligands and are determined by the type of M. m is an integer of 1 to 4, and n is an integer of 0 to 8. The constant q is 0 or 1. When q is 0, the chelating ring is a five-membered ring, and when q is 1, the chelating ring is a six-membered ring.

R ¹ represents alkylene having 1 to 10 carbons, halogenated alkylene having 1 to 10 carbons, arylene having 6 to 20 carbons, or halogenated arylene having 6 to 20 carbons. These alkylene and arylene may have a substituent or a hetero atom in the structure. Specifically, instead of hydrogen on alkylene and arylene, halogen, chain or cyclic alkyl group, aryl group, alkenyl group, alkoxy group, aryloxy group, sulfonyl group, amino group, cyano group, carbonyl group, acyl It may have a group, an amide group or a hydroxyl group as a substituent, or may have a structure in which nitrogen, sulfur or oxygen is introduced instead of carbon on alkylene and arylene. When q is 1 and m is 2 to 4, m R ^{1 s} may be bonded to each other. Examples thereof include a ligand such as ethylenediaminetetraacetic acid.

R ² is halogen, alkyl having 1 to 10 carbon atoms, alkyl halide having 1 to 10 carbon atoms, aryl having 6 to 20 carbon atoms, aryl halide having 6 to 20 carbon atoms, or —X ³ R ³ (X ³ , R ³ will be described later. Alkyl and aryl here may also have a substituent or a hetero atom in the structure in the same manner as R ^1, and when n is 2 to 8, n R ² are bonded to each other to form a ring. May be formed. R ² is preferably an electron-withdrawing group, particularly preferably a fluorine atom. This is because in the case of a fluorine atom, the solubility and dissociation degree of the salt of the anion compound are improved, and the ionic conductivity is improved accordingly. Moreover, it is because oxidation resistance improves and generation | occurrence | production of a side reaction can be suppressed by this.

X ¹ , X ² and X ³ each independently represent O, S or NR ⁴ . That is, the ligand is bonded to M through these heteroatoms.

R ³ and R ⁴ each independently represent hydrogen, an alkyl having 1 to 10 carbon atoms, an alkyl halide having 1 to 10 carbon atoms, an aryl having 6 to 20 carbon atoms, or an aryl halide having 6 to 20 carbon atoms. . Similarly to R ¹ , these alkyls and aryls may have a substituent or a hetero atom in the structure. Further, when a plurality of R ³ or R ⁴ are present, they may be bonded to each other to form a ring.

  Examples of the cation paired with the anionic compound include lithium, sodium, potassium, magnesium, calcium, barium, cesium, rubidium, silver, zinc, copper, cobalt, iron, nickel, manganese, titanium, lead, chromium, vanadium, and ruthenium. In addition to cations such as yttrium, lanthanoid, and actinoid, tetraalkylammonium (alkyl is methyl, ethyl, butyl, etc.), ammonium cation such as triethylammonium, pyridinium, imidazolium, protons, and the like. Among these, a lithium cation, a sodium cation, or a potassium cation is preferable.

  In such an anion compound, by charging the lithium ion secondary battery at least once, all or a part of the anion compound is decomposed, and the surface of both the positive electrode and the negative electrode or one of the positive electrode active material and the negative electrode active material. It is thought that a film is formed by coating both or one surface. This coating can be detected by, for example, X-ray photoelectron spectroscopy (XPS) or IR analysis. Such an anionic compound is preferably at least one of BFO, PTFO, PFO, and PO represented by formulas (2) to (5). The reason is that the solubility and dissociation degree of the salt of the anion compound are improved, so that the ionic conductivity of the non-aqueous ion conductive medium is improved and the oxidation resistance is improved. In particular, when the lithium-containing transition metal nitride containing Co is used as the positive electrode active material and the additive compound contains PFO as an anion, a higher effect of maintaining the discharge capacity can be obtained even if overdischarge occurs. . In addition, it is guessed from the Example (especially Table 1, 2) of Unexamined-Japanese-Patent No. 2007-18945 that BFO, PTFO, PFO, and PO show the same effect in a lithium ion secondary battery, for example.

As a method for synthesizing such an anionic compound, for example, in the case of BFO, LiBF ₄ and 2-fold moles of lithium alkoxide are reacted in a non-aqueous solvent, and then oxalic acid is added to bind to boron. There is a method of substituting alkoxide with oxalic acid. In the case of PFO, a method of reacting LiPF ₆ with 4 times moles of lithium alkoxide in a non-aqueous solvent and then adding oxalic acid to replace the alkoxide bonded to phosphorus with oxalic acid. Etc. In these cases, a lithium salt of an anionic compound can be obtained.

  In the lithium ion secondary battery of the present invention, the non-aqueous ion conductive medium may be a non-aqueous electrolyte solution in which an electrolyte compound containing a cation and an anion compound represented by the general formula (1) and a supporting salt are dissolved. . Examples of the solvent for the nonaqueous electrolytic solution include carbonates, esters, ethers, nitriles, furans, sulfolanes and dioxolanes, and these can be used alone or in combination. Specifically, as carbonates, cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t-butyl carbonate, di- chain carbonates such as i-propyl carbonate and t-butyl-i-propyl carbonate, cyclic esters such as γ-butyllactone and γ-valerolactone, chain esters such as methyl formate and ethyl acetate, dimethoxyethane, Ethers such as ethoxymethoxyethane, nitriles such as acetonitrile and benzonitrile, furans such as tetrahydrofuran and methyltetrahydrofuran, sulfolanes such as sulfolane and tetramethylsulfolane Examples include holanes, dioxolanes such as 1,3-dioxolane and methyldioxolane. Among these, the combination of cyclic carbonates and chain carbonates is preferable. According to this combination, not only the cycle characteristics representing the battery characteristics in repeated charge and discharge are excellent, but also the viscosity of the electrolyte, the electric capacity of the obtained battery, the battery output, etc. should be balanced. it can. The cyclic carbonates are considered to have a relatively high relative dielectric constant and increase the dielectric constant of the electrolytic solution, and the chain carbonates are considered to suppress the viscosity of the electrolytic solution. Also, instead of a liquid non-aqueous electrolyte, a solid ion conductive polymer, an inorganic solid electrolyte, a mixed material of an organic polymer electrolyte and an inorganic solid electrolyte, or an inorganic solid powder bound by an organic binder is used. be able to.

Examples of the supporting salt include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiSbF ₆ , LiSiF ₆ , LiAlF ₄ , LiSCN, Examples include LiClO ₄ , LiCl, LiF, LiBr, LiI, and LiAlCl ₄ . Among these, from the group consisting of inorganic salts such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ , and organic salts such as LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO _{2) 3.} It is preferable from the viewpoint of electrical characteristics to use a combination of one or two or more selected salts. Of these, LiPF ₆ is preferred. The supporting salt does not include an electrolyte compound containing the anion compound represented by the general formula (1). The concentration of the electrolyte compound containing the anion compound represented by the general formula (1) and the supporting salt in the entire non-aqueous ion conductive medium is preferably 0.1 mol / L or more and 5 mol / L or less, and 0.5 mol More preferably, it is / L or more and 2 mol / L or less. If it is 0.1 mol / L or more, a sufficient current density can be obtained, and if it is 5 mol / L or less, the electrolytic solution can be made more stable. Moreover, you may add flame retardants, such as a phosphorus type and a halogen type, to this non-aqueous ion conduction medium.

  The electrolyte compound containing the anion compound represented by the general formula (1) is included in the nonaqueous ion conductive medium in a range where the ratio of the electrolyte compound is 2 mol% or more and 80 mol% or less with respect to the entire electrolyte compound and the supporting salt. What is necessary is just to be a thing, It is preferable that it is contained in the range of 3 mol% or more and 50 mol% or less, and it is more preferable that it is contained in the range of 5 mol% or more and 20 mol% or less. If it is 2 mol% or more, a sufficient addition effect can be obtained, and if it is 80 mol% or less, the electrolyte compound containing an anionic compound and its decomposition film are not excessively suppressed from becoming a resistance component. It is considered possible.

  The lithium ion secondary battery of the present invention may include a separator between the positive electrode and the negative electrode. The separator is not particularly limited as long as the composition can withstand the use range of the lithium ion secondary battery. For example, a polymer nonwoven fabric such as a polypropylene nonwoven fabric or a polyphenylene sulfide nonwoven fabric, or a thin olefin resin such as polyethylene or polypropylene is used. A microporous membrane is mentioned. These may be used alone or in combination.

  The shape of the lithium ion secondary battery of the present invention is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, and a square type. Moreover, you may apply to the large sized thing etc. which are used for an electric vehicle etc. FIG. 1 is a schematic view showing an example of a lithium ion secondary battery 10 of the present invention. The lithium ion secondary battery 10 includes a positive electrode sheet 13 in which a positive electrode active material 12 is formed on a current collector 11, a negative electrode sheet 18 in which a negative electrode active material 17 is formed on the surface of the current collector 14, a positive electrode sheet 13 and a negative electrode A separator 19 provided between the sheet 18 and a non-aqueous ion conductive medium 20 filling between the positive electrode sheet 13 and the negative electrode sheet 18 is provided. In this lithium ion secondary battery 10, a separator 19 is sandwiched between a positive electrode sheet 13 and a negative electrode sheet 18, and these are wound and inserted into a cylindrical case 22, and a positive electrode terminal 24 and a negative electrode sheet connected to the positive electrode sheet 13. And a negative electrode terminal 26 connected to each other.

  It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

[Example 1]
(Production of battery)
LiNi _0.75 Co _0.15 Al _0.05 Mg _0.05 O ₂ was synthesized as a positive electrode active material by a known coprecipitation method using nitrate of each metal as a raw material. 85% by weight of this positive electrode active material, 10% by weight of carbon black as a conductive material, 5% by weight of polyvinylidene fluoride as a binder, and an appropriate amount of N-methyl-2-pyrrolidone as a dispersing agent is added and dispersed. Thus, a slurry composite was obtained. These slurry composites were uniformly applied to both surfaces of a 20 μm thick aluminum foil current collector and dried by heating to prepare a coated sheet. Thereafter, the coated sheet was passed through a roll press to be densified and cut into 52 mm × 450 mm to obtain a positive electrode sheet (positive electrode). Next, tungsten dioxide (WO ₂ manufactured by High Purity Chemical) was used as the negative electrode active material, 85% by weight of the negative electrode active material, 10% by weight of carbon black as the conductive material, and polyvinylidene fluoride as the binder. 5% by weight was mixed, and an appropriate amount of N-methyl-2-pyrrolidone was added and dispersed as a dispersing material to obtain a slurry-like composite material. These slurry composites were uniformly applied on both sides of a 10 μm thick copper foil current collector and dried by heating to prepare a coated sheet. Thereafter, the coated sheet was passed through a roll press to be densified and cut into 54 mm × 500 mm to obtain a negative electrode sheet (negative electrode). The positive electrode sheet and the negative electrode sheet thus prepared were wound around a 25 μm thick polyethylene separator to prepare a roll electrode body and inserted into a 18650 type cylindrical case. Next, a nonaqueous electrolytic solution was prepared as follows. First, in general formula (1), b = 1, n = 2, q = 0, R ² is F, X ¹ and X ² are O, and M is P (PFO, see formula (4)) LPFO and LiPF ₆ were mixed so that LPFO was 5 mol% with respect to the whole of LPFO and LiPF ₆ , which are compounds composed of Li ⁺ . This was dissolved in an organic solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 30:70 so as to have a concentration of 1 mol / L to prepare a nonaqueous electrolytic solution. The prepared non-aqueous electrolyte was impregnated in the above-mentioned 18650 type cylindrical case and sealed to produce a cylindrical battery. Thus, the battery of Example 1 was obtained.

(Evaluation of initial charge / discharge efficiency)
Using the battery obtained as described above, charging was performed up to the upper limit voltage (3.7 V) at a constant current of a current density of 0.2 mA / cm ² to measure the initial charge capacity (mAh / g). Next, the battery was discharged to a lower limit voltage (2.0 V) at a constant current of a current density of 0.2 mA / cm ² and the initial discharge capacity (mAh / g) was measured. Then, the initial charge / discharge efficiency (%) was calculated using an equation of (initial discharge capacity / initial charge capacity) × 100. In addition, since the upper limit voltage and the lower limit voltage differ depending on the type of electrode used, etc., values considered to be optimal in each battery configuration were used.

(Charge / discharge cycle test)
Subsequently, was charged at a constant current of current density of 2 mA / cm ² to an upper limit voltage (3.7V), then the charge to discharge at a constant current of current density of 2 mA / cm ² until the lower limit voltage (2.0 V) discharge Was repeated for 500 cycles under a temperature condition of 60 ° C.

(Evaluation of capacity maintenance rate)
Before and after the charge / discharge cycle test described above, after charging at constant current and constant voltage over 7 hours up to the upper limit voltage (3.7 V) at a current density of 0.2 mA / cm ² under a temperature condition of 20 ° C., 0.1 mA The discharge capacity (mAh / g) when discharging to a lower limit voltage (2.0 V) with a constant current of / cm ² was measured. And discharge capacity maintenance factor (%) was computed using the formula of (discharge capacity after cycle test / discharge capacity before cycle test) × 100. Based on the discharge capacity obtained by this measurement, SOC adjustment in the following evaluation of DC resistance was performed.

(Evaluation of DC resistance increase rate)
After adjusting to 50% of the battery capacity (SOC = 50%), current of 0.12A, 0.4A, 1.2A, 1.8A, 2.4A, 4.8A was passed under the temperature condition of 20 ° C. The battery voltage after 10 seconds was measured. The applied current and voltage were linearly approximated, and the direct current resistance of the battery was calculated from the slope of the straight line. Then, the DC resistance increase rate (%) was calculated using the formula {(DC resistance after cycle test−DC resistance before cycle test) / DC resistance before cycle test} × 100.

[Examples 2 to 6]
Example similar to Example 1 except that an anion compound of formula (3) (PTFO) and a compound composed of Li ⁺ (LiPF ₄ (C ₂ O ₄ ), hereinafter referred to as LPTFO) was used instead of LPFO. A battery of 2 was produced. Further, in the same manner as in Example 1 except that a compound composed of an anion compound (PO) of formula (5) and Li ⁺ (LiP (C ₂ O ₄ ) ₃ , hereinafter referred to as LPO) was used instead of LPFO. A battery of Example 3 was produced. Further, except for using compound anion compound (PFO) consists of K ⁺ in the formula (4) instead of _{LPFO (KPF 2 (C 2 O} 4) 2, hereinafter referred to as KPFO) is as in Example 1 A battery of Example 4 was produced. Further, LPFO was manufactured in the same manner as the battery of Example 5 as in Example 1 except that a mixture of a LPFO and LiPF ₆ so that 2 mol% relative to the total of LPFO and LiPF _6. Further, LPFO was manufactured in the same manner as the battery of Example 6 as in Example 1 except that a mixture of a LPFO and LiPF ₆ so that 80 mol% relative to the total of LPFO and LiPF _6. Using the battery thus obtained, the initial charge / discharge efficiency, capacity retention rate, and DC resistance increase rate were evaluated in the same manner as in Example 1.

[Comparative Examples 1-3]
A battery of Comparative Example 1 was prepared in the same manner as Example 1 except that 1 mol / L LiPF ₆ was used without using LPFO. Further, LPFO was prepared in the same manner as in Comparative Example 2 cell as in Example 1 except that a mixture of a LPFO and LiPF ₆ so that 1 mol% relative to the total of LPFO and LiPF _6. Further, LPFO was prepared in the same manner as in Comparative Example 3 batteries of Example 1 except that a mixture of a LPFO and LiPF ₆ so that 90 mol% relative to the total of LPFO and LiPF _6. Using the battery thus obtained, the initial charge / discharge efficiency, capacity retention rate, and DC resistance increase rate were evaluated in the same manner as in Example 1.

[Comparative Example 4]
A battery of Comparative Example 4 was produced in the same manner as Comparative Example 1 except that graphite was used instead of WO ₂ as the negative electrode active material. Using this, the initial charge / discharge efficiency, capacity retention rate, and DC resistance increase rate were evaluated in the same manner as in Example 1 except that the upper limit voltage was 4.1 V and the lower limit voltage was 3.0 V.

[Comparative Example 5]
A battery of Comparative Example 5 was produced in the same manner as Example 1 except that graphite was used instead of WO ₂ as the negative electrode active material. Using this, as in Comparative Example 4, the initial charge / discharge efficiency, capacity retention rate, and DC resistance increase rate were evaluated.

[Experimental result]
Table 1 shows the initial charge / discharge efficiency, capacity retention rate, and DC resistance increase rate of Examples 1 to 6 and Comparative Examples 1 to 5. As shown in Table 1, when any negative electrode active material is used, the electrolyte compound such as LPFO or its related compounds such as LPTFO, LPO, and KPFO is 5 mol% based on the total amount of the electrolyte compound and the supporting salt. By using in this manner, the decrease in discharge capacity in the charge / discharge cycle at 60 ° C. was suppressed, the capacity retention rate was improved, and the resistance increase rate was reduced. In particular, it was found that when WO ₂ was used as the negative electrode active material, the capacity maintenance rate was further improved and the resistance increase rate was further reduced as compared with the case where graphite was used. In Examples 1 to 3 using LPFO, LPTFO, and LPO, the capacity retention rate and the resistance increase rate were good, and therefore the anion compound is represented by the general formula (1) even if it is not PFO. It was inferred that the capacity retention rate improved and the resistance increase rate decreased. In Example 4 using KPFO, since the capacity retention rate and the resistance increase rate were good, the cation paired with the anion compound is not limited to lithium, and may be sodium or potassium. Inferred. Moreover, when the ratio of LPFO to the whole of LPFO and LiPF ₆ was changed, the initial charge / discharge efficiency, capacity retention rate, and resistance increase rate were all good when LPFO was in the range of 2 mol% to 80 mol%. On the other hand, in Comparative Example 2 in which LPFO was 1 mol%, the initial charge / discharge efficiency was reduced, and in Comparative Example 3 in which LPFO was 90 mol%, the capacity retention rate was reduced and the resistance increase rate was increased. From this, it was found that the ratio of LPFO to the whole of LPFO and LiPF ₆ is preferably in the range of 2 mol% to 80 mol%. Moreover, since the initial charge / discharge efficiency, capacity retention rate, and resistance increase rate when LPTFO, LPO, and KPFO were 5 mol%, all showed values similar to those when LPFO was 5 mol%, the general formula (1) It was speculated that the electrolyte compound containing an anion compound represented by the formula is preferably in the range of 2 mol% or more and 80 mol% or less, even if other than LPFO. From the above, the lithium ion secondary battery includes a negative electrode having a negative electrode active material containing a Group 6 element oxide, an electrolyte compound containing a cation and an anion compound represented by the general formula (1), and a supporting salt. By using a non-aqueous ion conductive medium, the ratio of the electrolyte compound to the whole of the electrolyte compound and the supporting salt is in the range of 2 mol% or more and 80 mol% or less. As a result, it was found that the effect of decreasing the resistance increase rate was obtained. Further, in this example, since the capacity retention rate and the resistance increase rate were good even by repeated cycles at 60 ° C., it was presumed that the capacity maintenance rate and the resistance increase rate were also good in the repeated cycles at room temperature. .

  DESCRIPTION OF SYMBOLS 10 Lithium ion secondary battery, 11 Current collector, 12 Positive electrode active material, 13 Positive electrode sheet, 14 Current collector, 17 Negative electrode active material, 18 Negative electrode sheet, 19 Separator, 20 Nonaqueous ion conduction medium, 22 Cylindrical case, 24 Positive terminal, 26 Negative terminal.

Claims (1)

A positive electrode having a positive electrode active material comprising lithium and a transition metal;
A negative electrode having a negative electrode active material made of tungsten oxide ;
Lithium ions are conducted between the positive electrode and the negative electrode, and are at least one selected from the group consisting of cations and BFO, PTFO, PFO and PO represented by the general formulas (1) to (4). A non-aqueous ion conducting medium comprising an electrolyte compound containing an anionic compound and a supporting salt, wherein the ratio of the electrolyte compound to the whole of the electrolyte compound and the supporting salt is in the range of 2 mol% to 80 mol%,
Lithium ion secondary battery equipped with.

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