Negative electrode for non-aqueous electrolyte secondary battery, separator for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery

Abstract

According to the present invention, a negative electrode or separator of a nonaqueous electrolyte secondary battery is configured to contain an iron cyano complex. The iron cyano complex is, for example, at least one compound that is selected from the group consisting of ferrocyanides and ferricyanides. A ferrocyanide is composed of, for example, at least one compound that is selected from the group consisting of potassium ferrocyanide, sodium ferrocyanide, calcium ferrocyanide and ammonium ferrocyanide; and a ferricyanide is composed of at least one compound that is selected from the group consisting of potassium ferricyanide, sodium ferricyanide, calcium ferricyanide and ammonium ferricyanide.

Description

[Technical Field]

[0001] The present invention relates to a negative electrode for a non-aqueous electrolyte secondary battery, a separator for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries are used as power sources for small devices such as smartphones, in-vehicle power sources, etc. Non-aqueous electrolyte secondary batteries include a positive electrode, a negative electrode, and a non-aqueous electrolyte. Examples of non-aqueous electrolyte secondary batteries include lithium ion secondary batteries and lithium (metal) secondary batteries. The negative electrode of a lithium ion secondary battery absorbs lithium ions during charging and releases lithium ions during discharging. In lithium (metal) secondary batteries, lithium metal is deposited on the negative electrode during charging and dissolves during discharging.

[0003] Patent Document 1 proposes an electrolyte solution for a lithium secondary battery that contains a chelating agent that forms a complex with transition metal ions in the battery, a non-aqueous solvent, and an electrolyte salt. The chelating agent used is EDTA (ethylenediaminetetraacetic acid), NTA (nitrilotriacetic acid), or the like. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5507083 specification Summary of the Invention [Problem to be solved by the invention]

[0005] When charging a secondary battery, side reactions other than the battery reaction (release of lithium ions at the positive electrode and absorption of lithium ions at the negative electrode) may occur. Specifically, some of the metals (excluding lithium, mainly including transition metals such as copper and iron) present in the positive electrode may be oxidized, causing metal ions to leach into the non-aqueous electrolyte. The leached metal ions may then be reduced at the negative electrode, resulting in metal precipitation. Metal precipitation may reduce the charge / discharge efficiency of the secondary battery.

[0006] The chelating agent added to the electrolyte solution described in Patent Document 1 can suppress metal deposition at the negative electrode to some extent by forming a complex with the eluted metal ions. However, it is difficult to add a large amount of the chelating agent from the standpoint of solubility in the electrolyte solution, and the effect is limited. In addition, when the amount of eluted metal ions is large, it is difficult to suppress metal deposition at the negative electrode. [Means for solving the problem]

[0007] One aspect of the present invention relates to a negative electrode for a non-aqueous electrolyte secondary battery, which contains an iron cyano complex.

[0008] Another aspect of the present invention relates to a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode is the above-mentioned negative electrode for a non-aqueous electrolyte secondary battery.

[0009] Yet another aspect of the present invention relates to a separator for a non-aqueous electrolyte secondary battery, which contains an iron cyano complex.

[0010] Yet another aspect of the present invention relates to a nonaqueous electrolyte secondary battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte, wherein the separator is the above-described separator for nonaqueous electrolyte secondary batteries. [Effects of the Invention]

[0011] According to the present invention, the charge / discharge efficiency of a non-aqueous electrolyte secondary battery can be improved. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a partially cutaway perspective view of a nonaqueous electrolyte secondary battery according to one embodiment of the present invention. [Figure 2] FIG. 2 is a diagram schematically illustrating the configuration of the electrode group in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0013] A negative electrode for a non-aqueous electrolyte secondary battery according to one embodiment of the present invention contains an iron-cyano complex. A separator for a non-aqueous electrolyte secondary battery according to one embodiment of the present invention contains an iron-cyano complex. In the non-aqueous electrolyte secondary battery, at least one of the negative electrode and the separator (hereinafter also referred to as the negative electrode, etc.) may contain the iron-cyano complex.

[0014] During charging of a secondary battery, some of the metals (excluding lithium; hereafter referred to as metal A) present in the positive electrode are oxidized and eluted into the non-aqueous electrolyte. Metal A primarily contains transition metals such as iron and copper, and originates from impurities introduced into the positive electrode material (mainly the positive electrode active material) and / or during the battery manufacturing process (such as the production of the positive electrode). The eluted metal A ions can form complex salts with iron cyano complexes that are less soluble in the non-aqueous electrolyte. This suppresses the deposition of metal A at the negative electrode, thereby suppressing the decrease in battery voltage and the accompanying decrease in charge / discharge efficiency that would otherwise result from the deposition of metal A at the negative electrode.

[0015] The addition of the iron cyano complex to the negative electrode can be achieved by adhering the iron cyano complex to the surface of the negative electrode mixture layer and / or the negative electrode current collector sheet (hereinafter also referred to as the negative electrode surface), or by adding the iron cyano complex to the negative electrode mixture layer. The addition of the iron cyano complex to the separator can be achieved by adhering the iron cyano complex to the surface of the substrate sheet (hereinafter also referred to as the separator surface), or by adding the iron cyano complex to the substrate sheet. Even when the amount of metal A is high (e.g., when the amount of transition metal derived from the positive electrode active material is high), the amount of iron cyano complex added can be appropriately adjusted depending on the amount of metal A within a range that does not impair the performance of the negative electrode, etc.

[0016] Iron cyanide complexes consist of an iron ion (central metal ion) and a cyanide ion (CN - ) (ligand). Iron cyano complexes are added to the negative electrode, etc., as salts of alkali metal elements (excluding lithium), Group 2 elements, ammonium, etc. Iron cyano complexes added to the negative electrode, etc., are difficult to dissolve in non-aqueous electrolytes and tend to remain in the negative electrode, etc., even after the battery is constructed.

[0017] The iron cyano complex added to the negative electrode or the like is formed of a cation and an anion (complex ion). Examples of the cation include cations of alkali metal elements, cations of Group 2 elements, and ammonium cations (NH4 + ) is preferred. Examples of cations of alkali metal elements include potassium (K) cation and sodium (Na) cation. Examples of cations of Group 2 elements include calcium (Ca) cation. Among these, potassium cation is more preferred. Examples of complex ions include ferrocyanide ion: [Fe(CN)6] 4- , ferricyanide ion: [Fe(CN)6] 3- Examples include:

[0018] When an iron cyano complex is contained in an anode or the like as a salt of an alkali metal element, a salt of a Group 2 element, or an ammonium salt, from the viewpoint of chemical stability, the iron cyano complex is likely to form a stable complex salt with metal A (mainly a transition metal).

[0019] The iron cyano complex is preferably at least one selected from the group consisting of ferrocyanide and ferricyanide. The ferrocyanide is preferably at least one selected from the group consisting of potassium ferrocyanide, sodium ferrocyanide, calcium ferrocyanide, and ammonium ferrocyanide. The ferricyanide is preferably at least one selected from the group consisting of potassium ferricyanide, sodium ferricyanide, calcium ferricyanide, and ammonium ferricyanide. Among these, potassium ferrocyanide and potassium ferricyanide are more preferred from the viewpoint of easily forming a salt insoluble in the non-aqueous electrolyte with ions of metal A.

[0020] The shape of the iron cyano complex is not particularly limited, but is preferably particulate. The iron cyano complex is difficult to dissolve in a nonaqueous electrolyte, tends to remain in a solid state even after the battery is constructed, and tends to remain in the negative electrode, etc. The average particle size of the iron cyano complex particles may be 0.1 μm or more and 50 μm or less, or 1 μm or more and 50 μm or less. When the average particle size of the iron cyano complex particles is 0.1 μm or more (or 1 μm or less), precipitation of metal A at the negative electrode is likely to be suppressed, and charge / discharge efficiency is likely to be improved. Furthermore, when the average particle size of the iron cyano complex particles is 50 μm or less, the influence of the addition of the iron cyano complex on the negative electrode, etc. is likely to be reduced, and high capacity is likely to be obtained. Here, the term "particle" is a concept that includes primary particles, secondary particles, and aggregates formed by aggregation of these particles, and includes concepts such as aggregation and agglomeration in addition to general particles or particles. The particle size (or diameter of an aggregate) may be considered as the diameter of an equivalent circle having the same area as the area enclosed by the outline of the particle (or aggregate) when observing a cross-sectional sample as described below.

[0021] When the negative electrode contains an iron cyano complex, the average particle size of the iron cyano complex particles can be determined by the following method. A battery (either a battery before the first charge or a battery in an initial fully discharged state) is disassembled, the negative electrode is removed, and an image of the negative electrode surface (or negative electrode cross section) is obtained using a scanning electron microscope (SEM). The fully discharged state described above refers to a state in which the secondary battery has been discharged to a depth of discharge (DOD) of 90% or more, e.g., a state in which the secondary battery has been discharged to the lower limit voltage within a specified voltage range in the field of equipment in which the secondary battery is used. Alternatively, an SEM image of the negative electrode surface (or negative electrode cross section) immediately after fabrication may be obtained.

[0022] Using the SEM image, the area of ​​any 20 to 30 particles of the iron cyano complex is measured, the diameter of a perfect circle (circle equivalent diameter) corresponding to the area is calculated, and the average of these is calculated as the average particle size. The iron cyano complex contained in the negative electrode can be confirmed by SEM-EDX (energy dispersive X-ray spectroscopy) analysis. When iron cyano complex particles are attached to the negative electrode surface, the average particle size can be determined using an SEM image of the negative electrode surface. When iron cyano complex particles are contained in the negative electrode mixture layer, the average particle size can be determined using an SEM image of the negative electrode cross section. When the separator contains an iron cyano complex, the average particle size can be determined in the same manner as above.

[0023] The content of the iron cyano complex in the negative electrode may be 0.01 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the negative electrode active material (e.g., a carbonaceous material such as graphite). When the negative electrode surface is covered with an iron cyano complex, the content of the iron cyano complex in the negative electrode refers to the amount of the iron cyano complex attached to the negative electrode surface. In this case, deposition of metal A on the negative electrode can be easily suppressed while maintaining negative electrode characteristics (e.g., low negative electrode resistance). The content of the iron cyano complex in the negative electrode can be determined by analysis such as inductively coupled plasma mass spectrometry (ICP-MS).

[0024] The iron cyano complex is preferably attached to the surface of the negative electrode (the surface of the negative electrode mixture layer and / or the negative electrode current collector sheet). The iron cyano complex is preferably attached to at least the surface of the negative electrode facing the positive electrode. In this case, deposition of metal A on the negative electrode and the resulting decrease in charge / discharge efficiency can be efficiently suppressed.

[0025] When an iron cyano complex is attached to the surface of the negative electrode, the iron derived from the iron cyano complex is absorbed within 1 cm of the negative electrode surface. 2 The amount of iron attached per unit area may be, for example, 1 nmol or more and 30 μmol or less. The amount of iron attached per unit area derived from the iron cyano complex can be determined, for example, by analyzing the negative electrode of the secondary battery before the first charge (or the negative electrode immediately after production) using X-ray photoelectron spectroscopy (XPS) or the like.

[0026] The negative electrode may include a negative electrode mixture layer containing a negative electrode active material, a negative electrode current collector sheet supporting the negative electrode mixture layer, and an iron cyano complex attached to the surface of the negative electrode mixture layer opposite to the side supported by the negative electrode current collector sheet. When the negative electrode current collector sheet has a region that does not support the negative electrode mixture layer, the iron cyano complex may be attached to the surface of that region of the negative electrode current collector sheet. The negative electrode mixture layer may be formed on one surface of the negative electrode current collector sheet, or may be formed on both surfaces of the negative electrode current collector sheet.

[0027] In the case of a lithium (metal) secondary battery in which lithium metal is deposited on the negative electrode during charging and the lithium metal dissolves during discharging, the negative electrode may include a negative electrode current collector sheet and an iron cyano complex attached to the surface of the negative electrode current collector sheet.

[0028] The separator may include a substrate sheet and an iron cyano complex attached to a surface of the substrate sheet. The iron cyano complex may be attached to one surface of the substrate sheet or to both surfaces of the substrate sheet.

[0029] The coverage of the negative electrode (or separator) surface with the iron cyano complex is preferably 5% or more and 95% or less, and more preferably 10% or more and 90% or less. When the coverage is 5% or more (or 10% or more), deposition of metal A on the negative electrode is likely to be suppressed, and charge / discharge efficiency is likely to be improved. When the coverage is 95% or less (or 90% or more), the influence of the addition of the iron cyano complex on the negative electrode, etc. is likely to be reduced (for example, low negative electrode resistance is likely to be maintained), and high capacity is likely to be obtained.

[0030] The coverage of the negative electrode (or separator) surface with the iron-cyano complex refers to the ratio of the area of ​​the region on the negative electrode (or separator) surface to the total area of ​​the surface facing the positive electrode where the iron-cyano complex is attached. For example, when the negative electrode is in the form of a sheet having a first surface and a second surface, and both the first surface and the second surface face the positive electrode, the coverage of both the first surface and the second surface is preferably within the above range.

[0031] The coverage of the negative electrode surface with the iron cyano complex can be determined by the following method. The battery (before the first charge or in an initial fully discharged state) is disassembled, the negative electrode is removed, and an SEM image of the negative electrode surface (the surface facing the positive electrode) is obtained. An SEM image of the negative electrode surface immediately after fabrication may also be obtained. Using the SEM image, the area S0 of the entire negative electrode surface (for example, the area of ​​the field of view: 0.04 mm) is calculated. 2 ~1.00mm 2 ) and the area S1 of the negative electrode surface where the iron cyano complex is attached are calculated, and the ratio (percentage) of S1 to S0 is calculated as the coverage rate. The coverage rates are calculated for 5 to 10 arbitrary locations on the negative electrode surface, and the average value is calculated. The iron cyano complex on the negative electrode surface can be confirmed by SEM-EDX analysis. The coverage rate of the separator surface with the iron cyano complex can also be calculated in the same manner as above.

[0032] The iron cyano complex can be attached to the negative electrode surface (or separator surface) by, for example, applying a treatment liquid containing the iron cyano complex and water to the negative electrode surface (or separator surface) and drying it. The iron cyano complex is easily soluble in water, and using an aqueous solution of the iron cyano complex as the treatment liquid makes it easy to uniformly attach the iron cyano complex to the negative electrode surface (or separator surface). The amount (coverage) of the iron cyano complex attached to the negative electrode surface (or separator surface) may be adjusted by changing the concentration of the iron cyano complex in the treatment liquid. The treatment liquid may further contain components other than the iron cyano complex (e.g., a binder). Examples of binders include fluororesins and acrylic resins, which will be described later. The method for applying the treatment liquid is not particularly limited, and methods such as coating using various coaters, dipping, and spraying may be used.

[0033] A nonaqueous electrolyte secondary battery according to one embodiment of the present invention includes a positive electrode, a negative electrode, and a nonaqueous electrolyte, where the negative electrode is the above-described negative electrode for a nonaqueous electrolyte secondary battery. Also, a nonaqueous electrolyte secondary battery according to another embodiment of the present invention includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte, where the separator is the above-described separator for a nonaqueous electrolyte secondary battery. In the nonaqueous electrolyte secondary battery according to another embodiment of the present invention, the negative electrode may be the above-described negative electrode for a nonaqueous electrolyte secondary battery. Each component of the nonaqueous electrolyte secondary battery will be specifically described below.

[0034] (positive electrode) The positive electrode includes a positive electrode active material capable of absorbing and releasing lithium ions. The positive electrode may include a positive electrode mixture layer including the positive electrode active material and a positive electrode current collector sheet supporting the positive electrode mixture layer. The positive electrode mixture layer can be formed by applying a positive electrode slurry, in which the positive electrode mixture is dispersed in a dispersion medium, to the surface of the positive electrode current collector sheet and drying the applied layer. The dried coating may be rolled as necessary. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode current collector sheet.

[0035] The positive electrode mixture contains a positive electrode active material as an essential component, and may contain a binder, a thickener, a conductive agent, and the like as optional components.

[0036] As the positive electrode active material, for example, a lithium transition metal composite oxide having a layered rock salt structure is used. Among them, a lithium transition metal composite oxide containing Ni, Co, and at least one of Al and Mn (hereinafter also referred to as composite oxide NC) is promising because it exhibits high capacity and high voltage. Here, if the Ni content of the composite oxide NC can be increased, it is advantageous in terms of cost and can ensure a higher capacity.

[0037] The composition of the composite oxide NC is, for example, Li α Ni (1-x1-x2-x3-y) Co x1 Mn x2 Al x3 M y O 2+β (0.95 ≦ α ≦ 1.05, 0.5 ≦ 1 - x1 - x2 - x3 - y ≦ 0.95, 0 < x1 ≦ 0.04, 0 ≦ x2 ≦ 0.1, 0 ≦ x3 ≦ 0.1, 0 < x2 + x3 ≦ 0.2, 0 ≦ y ≦ 0.1, -0.05 ≦ β ≦ 0.05). However, M is at least one selected from the group consisting of Ti, Zr, Nb, Mo, W, Fe, Zn, B, Si, Mg, Ca, Sr, and Y.

[0038] (1 - x1 - x2 - x3 - y), which indicates the ratio (atomic ratio) of Ni, is preferably 0.8 ≦ 1 - x1 - x2 - x3 - y ≦ 0.95 from the viewpoint of increasing the capacity, and more preferably 0.9 ≦ 1 - x1 - x2 - x3 - y ≦ 0.95.

[0039] x1, which indicates the ratio (atomic ratio) of Co, is greater than 0 and 0.04 or less, preferably 0.02 or less, and more preferably 0.015 or less.

[0040] x2, which indicates the ratio (atomic ratio) of Mn, is 0 ≦ x2 ≦ 0.1, and preferably 0 < x2 ≦ 0.1. The composite oxide NC containing Mn is relatively inexpensive and has a high capacity.

[0041] x3, which indicates the ratio (atomic ratio) of Al, satisfies 0≦x3≦0.1, preferably 0.03≦x3≦0.1, and may be 0.05≦x3≦0.1. When the composite oxide NC contains Al, the crystal structure is stabilized, making it easier to ensure high cycle characteristics.

[0042] The contents of the elements constituting the composite oxide NC can be measured by an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron probe microanalyzer (EPMA), or an EDX.

[0043] Furthermore, as the positive electrode active material, a lithium transition metal composite oxide having an olivine structure (LiFePO4, etc.) or a lithium transition metal composite oxide having a spinel structure (LiMn2O4, etc.) may be used.

[0044] As the binder, for example, a resin material is used. Examples of the binder include fluororesins (e.g., polyvinylidene fluoride (PVDF), polyhexafluoropropylene, polytetrafluoroethylene), polyolefin resins (e.g., polyethylene, polypropylene), polyamide resins (e.g., aramid resin), polyimide resins (e.g., polyimide, polyamideimide), acrylic resins (e.g., polyacrylic acid, polymethacrylic acid, acrylic acid-methacrylic acid copolymer, ethylene-acrylic acid copolymer, or salts thereof), vinyl resins (e.g., polyvinyl acetate), and rubber-like materials (e.g., styrene-butadiene copolymer rubber (SBR)). One type of binder may be used alone, or two or more types may be used in combination.

[0045] Examples of thickeners include cellulose derivatives such as cellulose ether. Examples of cellulose derivatives include carboxymethyl cellulose (CMC) and its modified forms, methyl cellulose, etc. Modified forms of CMC also include salts of CMC. Examples of salts include alkali metal salts (e.g., sodium salts) and ammonium salts. One type of thickener may be used alone, or two or more types may be used in combination.

[0046] Examples of conductive agents include conductive fibers and conductive particles. Examples of conductive fibers include carbon fibers, carbon nanotubes, and metal fibers. Examples of conductive particles include conductive carbon and metal powder. Examples of conductive carbon include carbon black such as acetylene black (AB), and graphite. One type of conductive agent may be used alone, or two or more types may be used in combination.

[0047] Examples of the dispersion medium used in the positive electrode slurry include alcohol (eg, ethanol), ether (eg, tetrahydrofuran), amide (eg, dimethylformamide), N-methyl-2-pyrrolidone (NMP), and mixed solvents thereof.

[0048] The positive electrode current collector sheet is selected depending on the type of nonaqueous electrolyte secondary battery. A non-perforated or perforated (mesh-like, etc.) metal sheet (metal foil, etc.) can be used as the positive electrode current collector sheet. Examples of materials for the positive electrode current collector sheet include stainless steel, aluminum, aluminum alloy, and titanium.

[0049] (Negative electrode) The negative electrode may include a negative electrode mixture layer containing a negative electrode active material and a negative electrode current collector sheet supporting the negative electrode mixture layer. The negative electrode mixture layer can be formed by applying a negative electrode slurry, in which the negative electrode mixture is dispersed in a dispersion medium, to the surface of the negative electrode current collector sheet and drying the applied slurry. Water can be used as the dispersion medium. The dried coating film may be rolled as necessary. The negative electrode mixture layer may be formed on one surface or both surfaces of the negative electrode current collector sheet. In the case of a lithium secondary battery, the negative electrode may include a negative electrode current collector sheet.

[0050] The negative electrode mixture contains a negative electrode active material as an essential component, and may contain a binder, a thickener, a conductive agent, and the like as optional components.

[0051] The iron cyano complex is preferably attached to the negative electrode surface (the surface of the negative electrode mixture layer and / or the negative electrode current collector sheet), but may also be contained in the negative electrode mixture (layer). That is, the negative electrode may include a negative electrode mixture layer containing a negative electrode active material and an iron cyano complex, and a negative electrode current collector sheet supporting the negative electrode mixture layer. In this case, the negative electrode may be prepared by adding the iron cyano complex to the above-mentioned negative electrode slurry.

[0052] As the negative electrode active material, metallic lithium, lithium alloys, etc. may be used, but materials capable of electrochemically absorbing and desorbing lithium ions are preferably used. Examples of such materials include carbonaceous materials, Si-containing materials, Sn-containing materials, etc. The negative electrode may contain one type of negative electrode active material or a combination of two or more types. As the negative electrode active material, carbonaceous materials and Si-containing materials are preferred. A carbonaceous material and a Si-containing material may be used in combination.

[0053] Examples of carbonaceous materials include graphite, easily graphitizable carbon (soft carbon), and non-graphitizable carbon (hard carbon). One type of carbonaceous material may be used alone, or two or more types may be used in combination.

[0054] Graphite is preferred as the carbonaceous material because of its excellent charge / discharge stability and low irreversible capacity. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles. The graphite particles may partially contain amorphous carbon, graphitizable carbon, or non-graphitizable carbon.

[0055] Examples of the Si-containing material include simple Si, silicon alloys, and silicon compounds (silicon oxides, silicates, etc.). Examples of silicon oxides include SiOx particles. x is, for example, 0.5≦x<2, and may be 0.8≦x≦1.6. One type of Si-containing material may be used alone, or two or more types may be used in combination.

[0056] The binder may be a resin material exemplified for the positive electrode. The conductive agent may be selected from those exemplified for the positive electrode. The negative electrode current collector sheet may be a non-porous or porous (mesh-like, etc.) metal sheet (metal foil, etc.). Examples of materials for the negative electrode current collector sheet include stainless steel, nickel, nickel alloy, copper, and copper alloy.

[0057] (separator) The separator has high ion permeability and adequate mechanical strength and insulation. The substrate sheet constituting the separator can be, for example, a microporous thin film, a woven fabric, a nonwoven fabric, or a laminate of at least two selected from these. The material of the substrate sheet is preferably polyolefin (e.g., polypropylene, polyethylene).

[0058] The iron cyano complex is preferably attached to the surface of the separator (the surface of the substrate sheet), but may also be contained in the substrate sheet. For example, the iron cyano complex may be contained in the pores of a microporous thin film, or a woven or nonwoven fabric may be obtained using a fiber material to which the iron cyano complex is attached.

[0059] (Non-aqueous electrolyte) The non-aqueous electrolyte solution contains, for example, a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the non-aqueous electrolyte solution is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0060] Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate (MA), ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. The non-aqueous electrolyte may contain one type of non-aqueous solvent or a combination of two or more types.

[0061] Examples of lithium salts include LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, and LiB 10 Cl 10 Examples of the lithium salts include lithium bis(1,2-benzenediolate(2-)-O,O')borate, lithium bis(2,3-naphthalenediolate(2-)-O,O')borate, lithium bis(2,2'-biphenyldiolate(2-)-O,O')borate, and lithium bis(5-fluoro-2-oleate-1-benzenesulfonic acid-O,O')borate. Examples of the imide salts include lithium bisfluorosulfonylimide (LiN(FSO)), lithium bistrifluoromethanesulfonyllimide (LiN(CFSO)), lithium trifluoromethanesulfonyltri ... The non-aqueous electrolyte may contain one type of lithium salt or a combination of two or more types of lithium salts.

[0062] An example of the structure of a nonaqueous electrolyte secondary battery is a structure in which an electrode group in which a positive electrode and a negative electrode are wound with a separator interposed therebetween and a nonaqueous electrolyte is housed in an exterior body. Alternatively, instead of a wound electrode group, an electrode group of another form may be used, such as a stacked electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween. The nonaqueous electrolyte secondary battery may be in any form, such as a cylindrical type, a prismatic type, a coin type, a button type, a laminate type, or the like.

[0063] The structure of a prismatic nonaqueous electrolyte secondary battery as an example of a nonaqueous electrolyte secondary battery according to one embodiment of the present invention will be described below with reference to Fig. 1. Fig. 1 is a schematic perspective view of a nonaqueous electrolyte secondary battery according to one embodiment of the present invention, with a portion cut away. Fig. 2 is a diagram schematically illustrating the configuration of the electrode group of Fig. 1.

[0064] As shown in Fig. 1, the battery includes a bottomed prismatic battery case 4, and an electrode group 1 and a nonaqueous electrolyte (not shown) housed in the battery case 4. The electrode group 1 is composed of a long strip-shaped positive electrode 9, a long strip-shaped negative electrode 10, and a separator 11 interposed therebetween. The electrode group 1 is formed by winding the positive electrode 9, the negative electrode 10, and the separator 11 around a flat plate-shaped winding core and then removing the winding core.

[0065] The negative electrode 10 has a negative electrode body 12 and a coating layer 15 of an iron cyano complex covering both sides of the negative electrode body 12. More specifically, the negative electrode body 12 includes a negative electrode current collector sheet 13 and a negative electrode mixture layer 14 supported on both sides of the negative electrode current collector sheet 13. The coating layer 15 of the iron cyano complex is formed on the surface of the negative electrode mixture layer 14 (the surface opposite to the side supported on the negative electrode current collector sheet 13). When the coverage of the negative electrode surface with the iron cyano complex is high (when the amount of iron cyano complex attached to the negative electrode surface is large), the iron cyano complex can be formed in a layer, as shown by the coating layer 15 in FIG. 2 .

[0066] 2 shows the iron cyano complex (coating layer 15) formed in a layer, but when the coverage of the negative electrode surface with the iron cyano complex is low (when the amount of iron cyano complex attached to the negative electrode surface is small), the iron cyano complex may be dispersed in particulate form on the negative electrode surface. Also, while FIG. 2 shows the coating layer 15 formed on the surface of the negative electrode body 12, the coating layer may also be formed on the surface of the separator 11.

[0067] The negative electrode current collector sheet of the negative electrode is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector sheet of the positive electrode is connected to the back surface of the sealing plate 5 via a positive electrode lead 2. In other words, the positive electrode lead 2 is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The periphery of the sealing plate 5 is fitted into the open edge of the battery case 4, and the fitting portion is laser welded. The electrolyte injection hole provided in the sealing plate 5 is closed with a seal 8.

[0068] [Example] The present invention will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0069] Examples 1 to 5 A non-aqueous electrolyte secondary battery (evaluation cell) was fabricated and evaluated according to the following procedure.

[0070] (Preparation of positive electrode) 100 parts by mass of the positive electrode active material, 1 part by mass of AB, 1 part by mass of PVDF, and an appropriate amount of NMP were mixed to obtain a positive electrode slurry. Next, the positive electrode slurry was applied to one side of an aluminum foil, the coating was dried, and then rolled to form a positive electrode mixture layer (thickness 95 μm, density 3.6 g / cm) on one side of the aluminum foil. 3 The positive electrode active material was LiNi 0.88 Co 0.09 Al 0.03 O2 was used.

[0071] (Preparation of negative electrode) A negative electrode slurry was prepared by mixing 98 parts by weight of the negative electrode active material (graphite), 1 part by weight of sodium salt of CMC, 1 part by weight of SBR, and an appropriate amount of water. The negative electrode slurry was then applied to one side of a copper foil serving as a negative electrode current collector sheet. The coating was dried and then rolled to form a negative electrode mixture layer on one side of the copper foil.

[0072] (Coating of the negative electrode surface with an iron cyano complex) An aqueous solution of the iron cyano complex was applied to the surface of the negative electrode mixture layer (the surface facing the positive electrode) and dried. In this way, the surface of the negative electrode mixture layer was coated with the iron cyano complex. Potassium ferrocyanide: K4 [Fe(CN)6] was used as the iron cyano complex. The concentration of the iron cyano complex in the aqueous solution was changed so that the coverage of the negative electrode surface (the surface of the negative electrode mixture layer) with the iron cyano complex would be the value shown in Table 1.

[0073] In Examples 1 to 5, the amount of the iron cyano complex attached to the negative electrode surface was in the range of 0.08 parts by mass to 1.6 parts by mass per 100 parts by mass of the negative electrode active material. In Examples 1 to 5, the average particle size of the iron cyano complex particles attached to the negative electrode surface, determined by the method described above, was in the range of 10 μm to 30 μm.

[0074] (Preparation of non-aqueous electrolyte) A non-aqueous electrolyte solution was prepared by dissolving LiPF6 in a mixed solvent of EC and EMC (EC:EMC = 3:7 (volume ratio)). The concentration of LiPF6 in the non-aqueous electrolyte solution was 1.0 mol / L.

[0075] (Fabrication of non-aqueous electrolyte secondary battery) First, the positive electrode was cut into a predetermined shape to obtain a positive electrode for evaluation. The positive electrode had a 20 mm × 20 mm area to function as a positive electrode and a 5 mm × 5 mm area to connect to the tab lead. The positive electrode mixture layer formed on the connection area was then scraped off to expose the positive electrode current collector sheet. The exposed portion of the positive electrode current collector sheet was then connected to the positive electrode tab lead, and a predetermined area around the periphery of the positive electrode tab lead was covered with an insulating tab film. Copper powder with a diameter of approximately 100 μm was then intentionally embedded near the center of the positive electrode mixture layer.

[0076] The negative electrode was cut into the same shape as the positive electrode to obtain a negative electrode for evaluation. The negative electrode mixture layer formed on the connection area formed in the same manner as the positive electrode was peeled off to expose the negative electrode current collector sheet. The exposed portion of the negative electrode current collector sheet was then connected to a negative electrode tab lead in the same manner as the positive electrode, and a predetermined area around the periphery of the negative electrode tab lead was covered with an insulating tab film.

[0077] A cell was fabricated using a positive electrode and a negative electrode for evaluation. First, the positive electrode and the negative electrode were placed opposite each other with a separator between them so that the positive electrode mixture layer and the negative electrode mixture layer overlapped, to obtain an electrode assembly. A 12 μm thick polyethylene separator was used as the separator. Next, an Al laminate film (100 μm thick) cut into a 60 × 90 mm rectangle was folded in half, and the end of the 60 mm long side was heat-sealed at 230 °C to form a 60 × 45 mm cylindrical shape. The electrode assembly was then placed into the cylinder, and the end faces of the Al laminate film were aligned with the insulating tab films of each tab lead, and heat-sealed at 230 °C. Next, 0.3 cm of nonaqueous electrolyte was poured into the short side of the Al laminate film that was not heat-sealed. 3 After the injection, the cells were left standing for 5 minutes under a reduced pressure of 0.06 MPa to allow the nonaqueous electrolyte to penetrate into each composite layer. Finally, the end face of the Al laminate film on the injected side was heat-sealed at 230°C to prepare an evaluation cell. The evaluation cell was prepared in a dry environment with a dew point of -50°C or less.

[0078] Examples 6 to 10 Evaluation cells of Examples 6 to 10 were fabricated in the same manner as in Examples 1 to 5, except that potassium ferricyanide: K3[Fe(CN)6] was used as the iron cyano complex instead of potassium ferrocyanide.

[0079] Comparative Example 1 An evaluation cell of Comparative Example 1 was produced in the same manner as in Example 1, except that the surface of the negative electrode was not coated with an iron cyano complex.

[0080] [evaluation] The evaluation cell was clamped between a pair of 80 × 80 cm stainless steel clamps (2 mm thick) and pressurized to 0.2 MPa. Then, the cell was charged and discharged in a thermostatic chamber at 25°C under the following conditions, and the initial charge-discharge efficiency (%) was determined.

[0081] The following first and second charging procedures were carried out. (1st charge) In a 25°C environment, the battery was charged at a constant current of 0.5C (1C is the current value that discharges the design capacity in 1 hour) until the battery voltage reached 3.6V. The current value and charging time at that time were measured, and the charge capacity C1 (mAh) was calculated. The evaluation cell was then left in a 25°C environment for 24 hours.

[0082] (2nd charge) In an environment of 25°C, the battery was charged at a constant current of 0.3 C until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current value fell below 0.05 C. The current value and charging time at that time were measured, and the charge capacity C2 (mAh) was calculated.

[0083] (discharge) After the second charge, the evaluation cell was left in an environment of 25°C for 20 minutes. Thereafter, a constant current discharge was performed at 0.3 C until the voltage reached 2.5 V, and then a constant voltage discharge was performed at 2.5 V until the current value became less than 0.05 C. The current value and discharge time at this time were measured, and the discharge capacity D (mAh) was calculated.

[0084] Using the charge capacities C1 and C2 and the discharge capacity D obtained above, the initial charge / discharge efficiency (%) was calculated according to the following formula. Initial charge / discharge efficiency = {D / (C1+C2)} x 100

[0085] The evaluation results are shown in Table 1. In Table 1, the "coverage" indicates the coverage of the negative electrode surface (the surface of the negative electrode mixture layer) with the iron cyano complex, as determined by the method described above. The "discharge capacity ratio" (%) indicates the ratio (percentage) of the discharge capacity D of each evaluation cell to the discharge capacity D of the evaluation cell of Comparative Example 1.

[0086] [Table 1]

[0087] In the evaluation cells of Examples 1 to 5, the potassium ferrocyanide covering the negative electrode surface captured copper ions eluted from the positive electrode, suppressing copper deposition on the negative electrode and improving the initial charge-discharge efficiency. In particular, the evaluation cells of Examples 2 to 4, which had a coverage rate of 10% to 90% with potassium ferrocyanide, achieved high initial charge-discharge efficiency while maintaining a discharge capacity ratio similar to that of the evaluation cell of Comparative Example 1.

[0088] In the evaluation cells of Examples 6 to 10, the potassium ferricyanide covering the negative electrode surface captured copper ions eluted from the positive electrode, suppressing copper deposition on the negative electrode and improving the initial charge-discharge efficiency. In particular, the evaluation cells of Examples 7 to 9, which had a potassium ferricyanide coverage of 10% to 90%, achieved high initial charge-discharge efficiency while maintaining a discharge capacity ratio similar to that of the evaluation cell of Comparative Example 1.

[0089] In the evaluation cell of Comparative Example 1, in which the surface of the negative electrode was not coated with an iron cyano complex, the copper ions eluted from the positive electrode were not captured by the iron cyano complex, resulting in copper deposition on the negative electrode and a decrease in the initial charge-discharge efficiency. [Industrial Applicability]

[0090] The nonaqueous electrolyte secondary battery according to the present invention is used, for example, as a power source for small devices such as smartphones, an on-board power source, and the like. While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention. [Explanation of symbols]

[0091] 1: electrode group, 2: positive electrode lead, 3: negative electrode lead, 4: battery case, 5: sealing plate, 6: negative electrode terminal, 7: gasket, 8: sealing plug, 9: positive electrode, 10: negative electrode, 11: separator, 12: negative electrode body, 13: negative electrode current collecting sheet, 14: negative electrode mixture layer, 15: coating layer

Claims

1. a negative electrode active material and an iron cyano complex; The content of the iron cyano complex is 0.01 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the negative electrode active material.

2. 2. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the iron-cyano complex is at least one selected from the group consisting of ferrocyanides and ferricyanides.

3. 3. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the iron-cyano complex is at least one selected from the group consisting of potassium ferrocyanide, sodium ferrocyanide, calcium ferrocyanide, ammonium ferrocyanide, potassium ferricyanide, sodium ferricyanide, calcium ferricyanide, and ammonium ferricyanide.

4. 4. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the iron-cyano complex particles have an average particle size of 0.1 μm or more and 50 μm or less.

5. A negative electrode for a non-aqueous electrolyte secondary battery as described in any one of claims 1 to 4, comprising: a negative electrode mixture layer containing the negative electrode active material; a negative electrode current collecting sheet supporting the negative electrode mixture layer; and the iron cyano complex attached to the surface of the negative electrode mixture layer opposite to the side supported by the negative electrode current collecting sheet.

6. 6. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 5, wherein a coverage of the surface of the negative electrode mixture layer with the iron-cyano complex is 5% or more and 95% or less.

7. The battery comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte solution, The negative electrode is the negative electrode for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 6.

8. containing an iron cyano complex, The iron-cyano complex particles have an average particle size of 0.1 μm or more and 50 μm or less.

9. 9. The separator for a non-aqueous electrolyte secondary battery according to claim 8, wherein the iron-cyano complex is at least one selected from the group consisting of ferrocyanides and ferricyanides.

10. 10. The separator for a non-aqueous electrolyte secondary battery according to claim 8, wherein the iron-cyano complex is at least one selected from the group consisting of potassium ferrocyanide, sodium ferrocyanide, calcium ferrocyanide, ammonium ferrocyanide, potassium ferricyanide, sodium ferricyanide, calcium ferricyanide, and ammonium ferricyanide.

11. 11. The separator for a non-aqueous electrolyte secondary battery according to claim 8, comprising: a substrate sheet; and the iron-cyano complex attached to a surface of the substrate sheet.

12. 12. The separator for a non-aqueous electrolyte secondary battery according to claim 11, wherein a coverage of the surface of the substrate sheet with the iron-cyano complex is 5% or more and 95% or less.

13. a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte; A non-aqueous electrolyte secondary battery, wherein the separator is the separator for a non-aqueous electrolyte secondary battery according to any one of claims 8 to 12.

14. A negative electrode comprising a negative electrode active material and an iron cyano complex, The iron-cyano complex particles have an average particle size of 1.0 μm or more and 50 μm or less.

15. 15. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 14, wherein the iron-cyano complex is at least one selected from the group consisting of ferrocyanides and ferricyanides.

16. 16. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 14 or 15, wherein the iron-cyano complex is at least one selected from the group consisting of potassium ferrocyanide, sodium ferrocyanide, calcium ferrocyanide, ammonium ferrocyanide, potassium ferricyanide, sodium ferricyanide, calcium ferricyanide, and ammonium ferricyanide.

17. A negative electrode for a non-aqueous electrolyte secondary battery as described in any one of claims 14 to 16, comprising: a negative electrode mixture layer containing the negative electrode active material; a negative electrode current collecting sheet supporting the negative electrode mixture layer; and the iron cyano complex adhered to the surface of the negative electrode mixture layer opposite to the side supported by the negative electrode current collecting sheet.

18. 18. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 17, wherein a coverage of the surface of the negative electrode mixture layer with the iron-cyano complex is 5% or more and 95% or less.

19. The battery comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte solution, The negative electrode is the negative electrode for a nonaqueous electrolyte secondary battery according to any one of claims 14 to 18.

20. A substrate sheet and an iron cyano complex attached to the surface of the substrate sheet, A separator for a non-aqueous electrolyte secondary battery, wherein a coverage rate of the iron cyano complex on the surface of the substrate sheet is 5% or more and 95% or less.

21. 21. The separator for a non-aqueous electrolyte secondary battery according to claim 20, wherein the iron-cyano complex is at least one selected from the group consisting of ferrocyanide and ferricyanide.

22. 22. The separator for a non-aqueous electrolyte secondary battery according to claim 20, wherein the iron-cyano complex is at least one selected from the group consisting of potassium ferrocyanide, sodium ferrocyanide, calcium ferrocyanide, ammonium ferrocyanide, potassium ferricyanide, sodium ferricyanide, calcium ferricyanide, and ammonium ferricyanide.

23. a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte; A non-aqueous electrolyte secondary battery, wherein the separator is the separator for a non-aqueous electrolyte secondary battery according to any one of claims 20 to 22.

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