Lithium-ion secondary battery

Abstract

To provide a lithium ion secondary battery with excellent cycle characteristics.SOLUTION: A lithium ion secondary battery includes a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte, and the negative electrode contains silicon or a silicon compound, a binder, and a conductive aid, the binder contains polyimide, and the negative electrode after discharge is observed by nuclear magnetic resonance (NMR) analysis using a single pulse magic angle spinning method (SP-MAS method), and when the peaks are separated with Gaussian function, Lorentzian function, or Voigt function, the NMR spectrum of the solid 7 Li nucleus has a first peak, and the first peak has a peak top in a chemical shift range of 0.5 ppm or more and 1.5 ppm or less when Li in LiCoO2 is set to -0.5 ppm.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a lithium ion secondary battery. [Background technology]

[0002] Lithium-ion secondary batteries are also widely used as a power source for mobile devices such as mobile phones and laptop computers, as well as hybrid cars.

[0003] The capacity of a lithium-ion secondary battery depends mainly on the active material of the electrode. Graphite is generally used as the negative electrode active material, but there is a demand for a negative electrode active material with higher capacity. Therefore, silicon (Si), which has a theoretical capacity much larger than that of graphite (372 mAh / g), has attracted attention.

[0004] Negative electrode active materials containing Si undergo significant volume expansion during charging. This volume expansion of the negative electrode active material causes a decrease in the cycle characteristics of the battery. When the negative electrode active material expands in volume, for example, the conductive paths between the negative electrode active materials are severed, peeling occurs at the interface between the negative electrode active material layer and the current collector, cracks occur in the SEI (Solid Electrolyte Interphase) coating, and electrolyte decomposition occurs. These all reduce the cycle characteristics of the battery.

[0005] In order to improve the cycle characteristics of the battery, a binder is used in the negative electrode active material layer. For example, Patent Document 1 discloses a lithium ion secondary battery that uses polyimide as a binder. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-242405 Summary of the Invention [Problem to be solved by the invention]

[0007] Further improvement in cycle characteristics is required.

[0008] The present disclosure has been made in view of the above problems, and has an object to provide a lithium ion secondary battery with excellent cycle characteristics. [Means for solving the problem]

[0009] In order to solve the above problems, the following means are provided.

[0010] (1) A lithium ion secondary battery according to a first aspect includes a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte. The negative electrode contains silicon or a silicon compound and a binder. The binder contains polyimide. When the negative electrode after discharge is observed by nuclear magnetic resonance (NMR) analysis using a single pulse magic angle spinning method (SP-MAS method) and peaks are separated using a Gaussian function, a Lorentz function, or a Voigt function, a solid 7 The NMR spectrum of the Li nucleus has a first peak, and the first peak has a peak top in the chemical shift range of 0.5 ppm to 1.5 ppm when the Li of LiCoO2 is set to -0.5 ppm.

[0011] (2) In the lithium ion secondary battery according to the above aspect, the solid 7 The MAS NMR spectrum of the Li nucleus may further have a second peak, which has a peak top at a chemical shift of 2.4 ppm when the Li of LiCoO2 is set to -0.5 ppm.

[0012] (3) In the lithium ion secondary battery according to the above aspect, when the negative electrode is observed after charging and discharging by nuclear magnetic resonance (NMR) analysis using a cross polarization magic angle spinning (CP-MAS) method, it is found that the solid 13The MAS NMR spectrum of the C nucleus may have a third peak, which has a chemical shift in the range of 120 ppm to 170 ppm when the upfield peak of the NMR spectrum of hexamethylbenzene is set to 16.81 ppm.

[0013] (4) In the lithium ion secondary battery according to the above aspect, the polyimide may be an aromatic polyimide. [Effects of the Invention]

[0014] The lithium ion secondary battery according to the above embodiment has excellent cycle characteristics. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment. [Figure 2] 1 shows the results of nuclear magnetic resonance (NMR) analysis of the negative electrode according to the present embodiment using the single pulse magic angle spinning method (SP-MAS method). [Figure 3] 1 is a diagram showing an enlarged view of the MAS NMR spectrum of the solid 7Li nucleus of the negative electrode according to the present embodiment in a predetermined range (horizontal axis: −40 ppm to 50 ppm). [Figure 4] 1 shows the results of a nuclear magnetic resonance (NMR) analysis of the negative electrode according to the present embodiment using the cross polarization magic angle spinning method (CP-MAS method). [Figure 5] 1 shows the results of a nuclear magnetic resonance (NMR) analysis of the negative electrode according to the present embodiment using the cross polarization magic angle spinning method (CP-MAS method). DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, the embodiments will be described in detail with reference to the drawings as appropriate. The drawings used in the following description may show characteristic portions enlarged for convenience in order to make the features easier to understand, and the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto. Appropriate changes can be made within the scope of the present invention.

[0017] "Lithium-ion secondary battery" FIG. 1 is a schematic diagram of a lithium-ion secondary battery according to a first embodiment. The lithium-ion secondary battery 100 shown in FIG. 1 includes a power generating element 40, an exterior body 50, and a non-aqueous electrolyte (not shown). The exterior body 50 covers the periphery of the power generating element 40. The power generating element 40 is connected to the outside via a pair of connected terminals 60, 62. The non-aqueous electrolyte is accommodated in the exterior body 50. Although FIG. 1 illustrates a case where one power generating element 40 is provided within the exterior body 50, multiple power generating elements 40 may be stacked.

[0018] (power generating element) The power generating element 40 includes a separator 10, a positive electrode 20, and a negative electrode 30. The power generating element 40 may be a laminate in which these are stacked, or a wound body in which a structure in which these are stacked is wound.

[0019] <Positive electrode> The positive electrode 20 includes, for example, a positive electrode current collector 22 and a positive electrode active material layer 24. The positive electrode active material layer 24 is in contact with at least one surface of the positive electrode current collector 22.

[0020] [Positive electrode current collector] The positive electrode current collector 22 is, for example, a conductive plate material. The positive electrode current collector 22 is, for example, a thin metal plate made of aluminum, copper, nickel, titanium, stainless steel, or the like. Aluminum, which is light in weight, is preferably used for the positive electrode current collector 22. The average thickness of the positive electrode current collector 22 is, for example, 10 μm or more and 30 μm or less.

[0021] [Cathode active material layer] The positive electrode active material layer 24 contains, for example, a positive electrode active material. The positive electrode active material layer 24 may contain a conductive assistant and a binder as required.

[0022] The positive electrode active material includes an electrode active material capable of reversibly proceeding with the occlusion and release of lithium ions, the desorption and intercalation (intercalation) of lithium ions, or the doping and dedoping of lithium ions and counter anions.

[0023] The positive electrode active material is, for example, a composite metal oxide. The composite metal oxide is, for example, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganate (LiMnO2), lithium manganese spinel (LiMn2O4), and the general formula: LiNi x Co y Mn z M a O2 compound (in the general formula, x + y + z + a = 1, 0 ≦ x < 1, 0 ≦ y < 1, 0 ≦ z < 1, 0 ≦ a < 1, M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, Cr), lithium vanadium compound (LiV2O5), olivine type LiMPO4 (where M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr or VO), lithium titanate (Li4Ti5O 12 )、LiNi x Co y Al z O2 (0.9 < x + y + z < 1.1). The positive electrode active material may be an organic substance. For example, the positive electrode active material may be polyacetylene, polyaniline, polypyrrole, polythiophene, or polyacene.

[0024] The positive electrode active material may be a lithium-free material. Examples of the lithium-free material include FeF3, conjugated polymers containing organic conductive materials, Chevrel phase compounds, transition metal chalcogenides, vanadium oxides, and niobium oxides. The lithium-free material may be any one of these materials alone or in combination. When the positive electrode active material is a lithium-free material, for example, discharge is first performed. Lithium is inserted into the positive electrode active material by discharging. Alternatively, lithium may be pre-doped chemically or electrochemically into the lithium-free positive electrode active material.

[0025] The conductive additive enhances the electronic conductivity between the positive electrode active materials. Examples of the conductive additive include carbon powder, carbon nanotubes, carbon materials, metal powder, a mixture of carbon materials and metal powder, and conductive oxides. Examples of the carbon powder include carbon black, acetylene black, and ketjen black. Examples of the metal powder include powders of copper, nickel, stainless steel, and iron.

[0026] There are no particular limitations on the content of the conductive additive in the positive electrode active material layer 24. For example, the content of the conductive additive relative to the total mass of the positive electrode active material, conductive additive, and binder is 0.5 mass% or more and 20 mass% or less, and preferably 1 mass% or more and 5 mass% or less.

[0027] The binder in the positive electrode active material layer 24 binds the positive electrode active material together. Known binders can be used. The binder is preferably one that is insoluble in the electrolyte, has oxidation resistance, and has adhesive properties. The binder is, for example, a fluororesin. Examples of the binder include polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), polyamide (PA), polyimide (PI), polyamideimide (PAI), polybenzimidazole (PBI), polyethersulfone (PES), polyacrylic acid and its copolymers, metal ion crosslinked polyacrylic acid and its copolymers, maleic anhydride-grafted polypropylene (PP) or polyethylene (PE), and mixtures thereof. PVDF is particularly preferred as the binder used in the positive electrode active material layer.

[0028] The binder content in the positive electrode active material layer 24 is not particularly limited. For example, the binder content relative to the total mass of the positive electrode active material, conductive additive, and binder is 1% by mass or more and 15% by mass or less, and preferably 1.5% by mass or more and 5% by mass or less. If the binder content is low, the adhesive strength of the positive electrode 20 will be weakened. If the binder content is high, the binder will be electrochemically inactive and will not contribute to the discharge capacity, resulting in a low energy density of the lithium-ion secondary battery 100.

[0029] <Negative electrode> The negative electrode 30 includes, for example, a negative electrode current collector 32 and a negative electrode active material layer 34. The negative electrode active material layer 34 is formed on at least one surface of the negative electrode current collector 32.

[0030] [Negative electrode current collector] The negative electrode current collector 32 is, for example, a conductive plate material. The negative electrode current collector 32 may be the same as the positive electrode current collector 22.

[0031] [Negative electrode active material layer] The negative electrode active material layer 34 contains a negative electrode active material and a binder. The negative electrode active material layer may contain a conductive additive, a dispersion stabilizer, and the like, as necessary.

[0032] The negative electrode active material contains silicon or a silicon compound. The silicon or silicon compound preferably accounts for 50% by mass or more, and more preferably 70% by mass or more, of the total amount of the negative electrode active material. The silicon compound may be, for example, a silicon alloy or silicon oxide. For example, the silicon or silicon compound may be crystalline or amorphous. Amorphous silicon or silicon compound can be produced by a melt-spun method, a gas atomization method, or the like.

[0033] Silicon alloys are n It is represented by Si. X is a cation. For example, X is Ba, Mg, Al, Zn, Sn, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge, Y, Zr, Nb, Mo, W, Au, Ti, Na, K, etc. n satisfies 0≦n≦0.5. Silicon oxide is SiO x where x satisfies, for example, 0.8≦x≦2. Silicon oxide may consist of only SiO2, may consist of only SiO, or may be a mixture of SiO and SiO2. Silicon oxide may also have some oxygen deficiencies.

[0034] The negative electrode active material may be a composite of silicon or a silicon compound. The composite is a silicon or silicon compound particle in which at least a portion of the surface is coated with a conductive material. The conductive material may be, for example, a carbon material, Al, Ti, Fe, Ni, Cu, Zn, Ag, Sn, or the like. For example, a silicon-carbon composite material (Si-C) is an example of the composite. The amount of the conductive material coated on the silicon or silicon compound particle is, for example, 0.01% by mass or more and 30% by mass or less, and preferably 0.1% by mass or more and 20% by mass or less, based on the total mass of the composite. The composite can be produced, for example, by mechanical alloying, chemical vapor deposition, a wet process, or a method in which a polymer is coated and then thermally decomposed to carbonize the polymer.

[0035] The specific surface area of ​​the negative electrode active material determined by the BET method is, for example, 0.5 m 2 / g or more 100m 2 / g or less, preferably 1.0m 2 / g or more 20m2 / g or less. If the specific surface area is small, it becomes difficult for Li ions to be inserted and removed between the negative electrode active material. If the specific surface area is large, a large amount of binder is required to form the electrode, resulting in a small capacity per unit volume.

[0036] The binder in the negative electrode active material layer 34 includes polyimide. In addition to polyimide, the binder may include, for example, any of the binders that can be used in the positive electrode active material layer 24, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), etc.

[0037] Polyimide is, for example, a polymer containing an imide structure in its repeating unit structure. Polyimide is a polymer of an acid anhydride and a diamine compound, and it is preferable that both the acid anhydride and the diamine compound have an aromatic ring. Polyimide may, for example, contain a cyclic imide structure and an aromatic compound in its repeating unit structure. The cyclic imide structure includes, for example, a five-membered ring imide.

[0038] The binder content in the negative electrode active material layer 34 is not particularly limited. For example, the binder content relative to the total mass of the negative electrode active material, conductive additive, and binder is 1% by mass or more and 20% by mass or less, and preferably 3% by mass or more and 15% by mass or less. If the binder content is low, the adhesive strength of the negative electrode 30 will be weakened. If the binder content is high, the binder will be electrochemically inactive and will not contribute to the discharge capacity, resulting in a low energy density of the lithium-ion secondary battery 100.

[0039] The conductive additive in the negative electrode active material layer 34 enhances the electronic conductivity between the negative electrode active materials. The conductive additive may be the same as that in the positive electrode active material layer 24.

[0040] The content of the conductive additive in the negative electrode active material layer 34 is not particularly limited. For example, the content of the conductive additive relative to the total mass of the negative electrode active material, the conductive additive, and the binder is 5% by mass or more and 20% by mass or less, and preferably 1% by mass or more and 12% by mass or less. The BET specific surface area of ​​the conductive additive is, for example, 100 m 2 / g or more 200m 2 / g or less.

[0041] The dispersion stabilizer in the negative electrode active material layer 34 is, for example, polyvinylpyrrolidone (PVP). The dispersion stabilizer prevents aggregation of the negative electrode active material during preparation of the slurry.

[0042] 2 shows the results of nuclear magnetic resonance (NMR) analysis of the negative electrode according to this embodiment using the single pulse magic angle spinning method (SP-MAS method). 7 This is the NMR spectrum of the Li nucleus. The top graph in Figure 2 shows the results of measuring the negative electrode after charging, and the bottom graph shows the results of measuring the negative electrode after discharging. The horizontal axis is the chemical shift (ppm), expressed as (frequency of absorbed electromagnetic waves - absorption frequency of reference material) / magnetic field strength. As a reference, Li in LiCoO2 was set to -0.5 ppm. The strength of the applied magnetic field was 6.4 T (272 MHz).

[0043] The magic angle spinning method is a method for sharpening the signal of solid-state NMR by rotating the sample at high speed around an axis tilted relative to the direction of the applied magnetic field. The sample rotation speed was set to 15 kHz.

[0044] FIG. 3 shows the solid-state negative electrode according to this embodiment. 7 This is a diagram showing an enlarged view of the NMR spectrum of Li nucleus in a predetermined range (horizontal axis: -40 ppm to 50 ppm). The spectrum shown in Fig. 3 shows the result of peak separation of overlapping peaks using a Voigt function. Peak separation of overlapping peaks may also be performed using a Lorentzian function or a Gaussian function.

[0045] As shown in Figure 3, the negative electrode solid 7 The NMR spectrum of Li nuclei has, for example, a first peak p1 and a second peak p2. The terms "first" and "second" are simply ordinal numbers and do not indicate the magnitude of the peaks. The first peak p1 shown in FIG. 3 has its peak top at a chemical shift of 0.95 ppm. The second peak p2 shown in FIG. 3 has its peak top at a chemical shift of 2.4 ppm.

[0046] The first peak p1 is a peak derived from the reaction product of polyimide and lithium (polyimide absorbing lithium). The first peak p1 is only observed after the polyimide and lithium have sufficiently reacted. In other words, even if polyimide and lithium are simply contained simultaneously as constituent materials for preparing the negative electrode active material layer, this peak will not be observed. A method for sufficiently reacting polyimide and lithium will be described later. The peak top position of the first peak p1 varies depending on the state of the polyimide (type, polymerization state, and degree of polymerization). The first peak p1 has a chemical shift in the range of 0.5 ppm to 1.5 ppm.

[0047] 4 and 5 show the results of nuclear magnetic resonance (NMR) analysis of the negative electrode according to this embodiment using the cross polarization magic angle spinning (CP-MAS) method. 13 The C NMR spectrum results are shown in Figure 5. 13 This is the NMR spectrum result for the C nucleus. The horizontal axis is the chemical shift (ppm), expressed as (frequency of electromagnetic wave where absorption occurred - absorption frequency of reference material) / magnetic field strength. As a reference, the peak on the high magnetic field side of the NMR spectrum of hexamethylbenzene was set to 16.81 ppm. The applied magnetic field strength was 4.1 T (176 MHz). The sample rotation speed was 15 kHz.

[0048] As shown in Figures 4 and 5, the negative electrode solid 13 The NMR spectrum of C nucleus, for example, has a broad third peak. The "third" is simply an ordinal number and does not indicate the magnitude of the peak. The third peak shown in Figures 4 and 5 has its peak top at a chemical shift of 130 ppm.

[0049] The third peak is a peak derived from the reaction product of polyimide and lithium (polyimide absorbing lithium). Nuclear magnetic resonance analysis of polyimide before incorporation into the anode reveals several peaks based on the bonding groups. When polyimide absorbs lithium during charge / discharge reactions, the bonding state changes, and these peaks broaden. The third peak is one of the broad peaks. The third peak is only observed after sufficient reaction between polyimide and lithium. In other words, even if polyimide and lithium are simply included as constituent materials for preparing the anode active material layer, this peak will not be observed. A method for sufficiently reacting polyimide with lithium will be described later. The peak top position of the third peak varies depending on the state of the polyimide (type, polymerization state, and degree of polymerization). The third peak has a chemical shift in the range of 120 ppm to 170 ppm.

[0050] <separator> The separator 10 is sandwiched between the positive electrode 20 and the negative electrode 30. The separator 10 separates the positive electrode 20 from the negative electrode 30 and prevents short-circuiting between the positive electrode 20 and the negative electrode 30. The separator 10 extends in-plane along the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

[0051] The separator 10 has, for example, an electrically insulating porous structure. The separator 10 is, for example, a monolayer or laminate of a polyolefin film. The separator 10 may be a stretched membrane of a mixture of polyethylene, polypropylene, or the like. The separator 10 may be a fibrous nonwoven fabric made of at least one material selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. The separator 10 may be, for example, a solid electrolyte. Examples of the solid electrolyte include a polymer solid electrolyte, an oxide-based solid electrolyte, and a sulfide-based solid electrolyte. The separator 10 may also be an inorganic-coated separator. The inorganic-coated separator is formed by coating the surface of the above-mentioned film with a mixture of a resin such as PVDF or CMC and an inorganic material such as alumina or silica. The inorganic-coated separator has excellent heat resistance and suppresses the deposition of transition metals eluted from the positive electrode onto the negative electrode surface.

[0052] <Electrolyte> The electrolytic solution is sealed in the exterior body 50 and impregnates the power generating element 40. The non-aqueous electrolytic solution includes, for example, a non-aqueous solvent and an electrolytic salt. The electrolytic salt is dissolved in the non-aqueous solvent.

[0053] The solvent is not particularly limited as long as it is a solvent generally used in lithium ion secondary batteries. The solvent includes, for example, any of a cyclic carbonate compound, a chain carbonate compound, a cyclic ester compound, and a chain ester compound. The solvent may include a mixture of these compounds in any ratio. Examples of the cyclic carbonate compound include ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate, and vinylene carbonate. Examples of the chain carbonate compound include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and the like. Examples of the cyclic ester compound include γ-butyrolactone, and the like. Examples of the chain ester compound include propyl propionate, ethyl propionate, and ethyl acetate, and the like.

[0054] The electrolytic salt is, for example, a lithium salt. Examples of the electrolyte include LiPF6, LiClO4, LiBF4, LiCF3SO3, LiCF3CF2SO3, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(CF3CF2CO)2, LiBOB, and LiN(FSO2)2. One lithium salt may be used alone, or two or more may be used in combination. From the viewpoint of the degree of ionization, it is preferable that the electrolyte contains LiPF6. The degree of dissociation of the electrolytic salt in a carbonate solvent at room temperature is preferably 10% or more.

[0055] The electrolyte is preferably, for example, LiPF6 dissolved in a carbonate solvent. The concentration of LiPF6 is, for example, 1 mol / L. When the polyimide resin contains a large amount of aromatics, the polyimide resin may exhibit charging behavior similar to that of soft carbon. When the electrolyte is a carbonate electrolyte solvent containing a cyclic carbonate, lithium can be reacted uniformly with the polyimide. In this case, the cyclic carbonate is preferably ethylene carbonate, fluoroethylene carbonate, or vinylene carbonate.

[0056] <Exterior body> The exterior body 50 seals the power generating element 40 and the non-aqueous electrolyte solution inside. The exterior body 50 prevents the non-aqueous electrolyte solution from leaking to the outside and prevents moisture and the like from entering the lithium-ion secondary battery 100 from the outside.

[0057] 1, the exterior body 50 has a metal foil 52 and a resin layer 54 laminated on each side of the metal foil 52. The exterior body 50 is a metal laminate film in which the metal foil 52 is coated on both sides with a polymer film (resin layer 54).

[0058] For example, aluminum foil can be used as the metal foil 52. A polymer film such as polypropylene can be used as the resin layer 54. The materials constituting the inner and outer resin layers 54 may be different. For example, the outer material may be a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide (PA), and the inner polymer film may be made of polyethylene (PE), polypropylene (PP), or the like.

[0059] <Terminal> Terminals 60 and 62 are connected to the positive electrode 20 and the negative electrode 30, respectively. The terminal 60 connected to the positive electrode 20 is a positive electrode terminal, and the terminal 62 connected to the negative electrode 30 is a negative electrode terminal. The terminals 60 and 62 are responsible for electrical connection to the outside. The terminals 60 and 62 are made of a conductive material such as aluminum, nickel, or copper. The connection method may be welding or screw fastening. It is preferable to protect the terminals 60 and 62 with insulating tape to prevent short circuits.

[0060] "Method of manufacturing lithium-ion secondary batteries" The lithium ion secondary battery 100 is fabricated by preparing and assembling the negative electrode 30, the positive electrode 20, the separator 10, the electrolyte, and the exterior body 50. An example of a method for manufacturing the lithium ion secondary battery 100 will be described below.

[0061] The negative electrode 30 is produced, for example, by sequentially carrying out a slurry production step, an electrode application step, a drying step, and a rolling step.

[0062] The slurry preparation process involves mixing a negative electrode active material (silicon or a silicon compound), a binder, a conductive additive, and a solvent to prepare a slurry. The binder is as described above. Adding a dispersion stabilizer to the slurry can suppress aggregation of the negative electrode active material.

[0063] Specifically, the slurry is prepared as follows: First, polyamic acid, which serves as the binder, is diluted with N-methyl-2-pyrrolidone (NMP) solution in a resin container. The resin container should have a low contact angle with the main solvent, and a fluororesin container, for example, is preferable. Using a resin container can prevent the container from reacting with the slurry.

[0064] The solution is then stirred at 1000 rpm or higher to ensure uniform dispersion of the polyamic acid. The conductive additive is added to the solution in multiple portions, with stirring at 1000 rpm or higher for several minutes after each addition. The dispersion stabilizer is added, for example, simultaneously with the conductive additive.

[0065] Next, the negative electrode active material is added to the slurry containing the polyamic acid and the conductive additive in several batches, and the mixture is stirred at 1000 rpm or higher for several minutes after each addition of the negative electrode active material. The optimal stirring speed for preparing the slurry varies depending on the apparatus used, so any method can be used as long as it can produce a uniform dispersion.

[0066] If the binder is water-soluble, the desired slurry can be produced by replacing the diluted solution with water and carrying out the same process. When using water as the main solvent, it is preferable to use a resin container such as polypropylene rather than fluororesin.

[0067] By increasing the dispersibility of the slurry, lithium ions during charging and discharging can react with the negative electrode active material and the negative electrode binder.

[0068] The electrode coating step is a step of coating the surface of the negative electrode current collector 32 with a slurry. The method of coating the slurry is not particularly limited. For example, a slit die coating method or a doctor blade method can be used as the method of coating the slurry. The slurry is coated at room temperature, for example.

[0069] The drying step is a step of removing the solvent from the slurry. For example, the negative electrode current collector 32 coated with the slurry is dried in an atmosphere at 80°C to 350°C. The drying step may also cause a ring-closing reaction of the precursor polyamic acid. In this case, the drying step is preferably performed at a temperature of 200°C to 350°C. By drying the slurry or causing the ring-closing reaction to proceed, the negative electrode active material layer 34 is formed on the negative electrode current collector 32.

[0070] The rolling step is performed as necessary. The rolling step is a step of applying pressure to the negative electrode active material layer 34 to adjust the density of the negative electrode active material layer 34. The rolling step is performed, for example, using a roll press device. After the rolling step, it can be confirmed that each component is uniformly dispersed in the negative electrode active material layer by measuring the resistance of the negative electrode with an electrical resistance meter or observing the cross section with a scanning electron microscope.

[0071] The positive electrode 20 can be produced by the same procedure as that for the negative electrode 30. The separator 10 and the outer casing 50 can be commercially available products.

[0072] Next, the prepared positive electrode 20 and negative electrode 30 are stacked so that the separator 10 is positioned between them to prepare the power generating element 40. When the power generating element 40 is a wound body, the positive electrode 20, the negative electrode 30, and one end side of the separator 10 are wound around the axis.

[0073] Finally, the power generation element 40 is sealed in the exterior body 50. A non-aqueous electrolyte solution is poured into the exterior body 50. After the non-aqueous electrolyte solution is poured, the pressure is reduced, heating, etc. is performed, and the non-aqueous electrolyte solution is impregnated into the power generation element 40. The lithium-ion secondary battery 100 is obtained by sealing the exterior body 50 by applying heat, etc. Note that the power generation element 40 may be impregnated with the electrolyte solution instead of pouring the electrolyte solution into the exterior body 50. After the electrolyte solution is poured into the power generation element, it is preferable to leave it to stand for 24 hours.

[0074] In the lithium-ion secondary battery 100 according to the first embodiment, the binder is a polyimide, which has higher strength than polyacrylic acid (PAA) and the like, and can follow the expansion and contraction of the negative electrode active material. Furthermore, since the polyimide is dispersed sufficiently uniformly within the negative electrode active material layer, the polyimide absorbs lithium uniformly, improving the cycle characteristics. If the lithium absorption in the polyimide is non-uniform, a predetermined peak does not appear in NMR. If the lithium absorption in the polyimide is non-uniform, the electrode reaction also becomes non-uniform, and the lithium-ion secondary battery cannot exhibit sufficient cycle characteristics.

[0075] The above describes the embodiments of the present invention in detail with reference to the drawings. However, each configuration and combination thereof in each embodiment is an example, and additions, omissions, substitutions, and other modifications of the configurations are possible within the scope that does not deviate from the spirit of the present invention. [Example]

[0076] "Example 1" One surface of a 15 μm thick aluminum foil was coated with a positive electrode slurry prepared by mixing a positive electrode active material, a conductive additive, a binder, and a solvent.

[0077] The positive electrode active material is Li x CoO2 was used. Acetylene black was used as the conductive additive. Polyvinylidene fluoride (PVDF) was used as the binder. N-methyl-2-pyrrolidone was used as the solvent. 97 parts by mass of the positive electrode active material, 1 part by mass of the conductive additive, 2 parts by mass of the binder, and 70 parts by mass of the solvent were mixed to prepare a positive electrode slurry. The amount of the positive electrode active material supported in the positive electrode active material layer after drying was 25 mg / cm. 2 The solvent was removed from the positive electrode slurry in a drying furnace to prepare a positive electrode active material layer, which was then pressed with a roll press to prepare a positive electrode.

[0078] Next, a negative electrode slurry was prepared. The negative electrode active material was silicon with an average particle size of 3 μm. Carbon black was used as the conductive additive. The binder was a polymerized polyamic acid of the following chemical formula (1). The weight-average molecular weight of the binder was 35,000.

[0079] [ka]

[0080] First, the above compound was diluted with N-methyl-2-pyrrolidone (NMP) solution in a resin container. The solution was then stirred at 1000 rpm for 1 minute to ensure uniform dispersion of the above compound. Next, the conductive additive and dispersion stabilizer were added to the solution in multiple batches, with stirring at 1000 rpm for several minutes after each addition of the conductive additive. The dispersion stabilizer was polyvinylpyrrolidone. Next, silicon was added to the slurry in multiple batches, with stirring at 1000 rpm for several minutes after each silicon addition.

[0081] The negative electrode slurry was applied to one side of a copper foil having a thickness of 10 μm and then dried. After drying, the amount of the negative electrode active material carried in the negative electrode active material layer was 2.5 mg / cm. 2 The negative electrode active material layer was pressed with a roll press and then thermally baked at 300° C. or higher for 5 hours in a nitrogen atmosphere.

[0082] Next, an electrolyte solution was prepared. The solvent for the electrolyte solution was fluoroethylene carbonate (FEC):ethylene carbonate (EC):diethyl carbonate (DEC) = 10% by volume:20% by volume:70% by volume. The electrolyte solution was also supplemented with additives for improving output, gas suppression, cycle characteristics improvement, and safety performance improvement. LiPF6 was used as the electrolyte salt. The concentration of LiPF6 was 1 mol / L.

[0083] (Fabrication of lithium-ion secondary batteries for evaluation) The prepared negative electrode and positive electrode were laminated with a separator (porous polyethylene sheet) interposed between them so that the positive electrode active material layer and the negative electrode active material layer faced each other, to obtain a laminate. This laminate was inserted into an exterior body made of aluminum laminate film and heat-sealed except for one peripheral location to form a closed opening. Finally, the above-mentioned electrolyte solution was injected into the exterior body, and the remaining location was heat-sealed while reducing the pressure using a vacuum sealer, to produce a lithium-ion secondary battery. The fabricated lithium-ion secondary battery was left to stand for 24 hours. Two lithium-ion secondary batteries were fabricated under the same conditions, one for evaluating cycle characteristics and the other for NMR measurement.

[0084] (Measurement of capacity retention rate after 300 cycles) The cycle characteristics of the lithium ion secondary battery were measured using a secondary battery charge / discharge tester (manufactured by Hokuto Denko Corporation).

[0085] The battery was charged at a constant current charge rate of 1 C (the current value at which charging is completed in 1 hour when constant current charging is performed at 25°C) until the battery voltage reached 4.2 V, and then discharged at a constant current discharge rate of 1.0 C until the battery voltage reached 2.5 V. The discharge capacity after charging and discharging was detected, and the battery capacity Q1 before the cycle test was calculated.

[0086] The battery whose battery capacity Q1 was determined above was again charged using a secondary battery charge / discharge tester at a constant current charge rate of 1C until the battery voltage reached 4.2V, and then discharged at a constant current discharge rate of 1C until the battery voltage reached 2.5V. This charge / discharge cycle was counted as one cycle, and 300 charge / discharge cycles were performed. The discharge capacity after 300 charge / discharge cycles was then measured, and the battery capacity Q2 after 300 cycles was calculated. The capacity retention rate E after 300 cycles was calculated from the capacities Q1 and Q2 determined above. The capacity retention rate E was calculated by E = Q2 / Q1 × 100. The capacity retention rate of Example 1 was 82.4%.

[0087] (NMR measurement) The fabricated lithium-ion secondary battery was charged to a fully charged state over 5 hours and left to stand in the fully charged state for 12 hours. The fully charged lithium-ion secondary battery was then discharged over 5 hours. The discharged sample was then disassembled in a glove box under an argon gas atmosphere, and the negative electrode was removed. The negative electrode was washed with dimethyl carbonate (DMC). The negative electrode active material layer was then peeled off from the copper foil (negative electrode current collector) using a Teflon (registered trademark) spatula and dried.

[0088] The dried negative electrode active material layer was placed in a sample tube made of zirconia and subjected to solid-state NMR measurement at 700 MHz. 7 Li nuclei were measured by SP-MAS method, 13 The C nucleus was measured by CP-MAS. Figures 2 to 5 show the evaluation results of Example 1. As shown in Figures 2 to 5, the first to third peaks were all confirmed. The peak top of the first peak was at a chemical shift of 0.95 ppm. The peak top of the second peak was at a chemical shift of 2.4 ppm. The peak top of the third peak was at a chemical shift of 130 ppm.

[0089] "Comparative Example 1" Comparative Example 1 differs from Example 1 in that, when preparing the slurry, polyamic acid, silicon, and a conductive additive were added to an N-methyl-2-pyrrolidone (NMP) solution all at once, without adding a dispersion stabilizer. The slurry was prepared by adding polyamic acid, silicon, and a conductive additive to an N-methyl-2-pyrrolidone (NMP) solution all at once, and then stirring at less than 1000 rpm for several minutes.

[0090] The capacity retention rate of Comparative Example 1 was 54.3%. Furthermore, the first peak and the third peak were not observed in the negative electrode active material layer of Comparative Example 1. The second peak was also observed in the negative electrode active material layer of Comparative Example 1.

[0091] "Example 2" Example 2 differs from Example 1 in that the polyimide material was changed. Example 2 used a polymerized polyamic acid represented by the following chemical formula (2) as a binder. Chemical formula (2) differs in that the imide rings are bonded via phenyl groups. Other conditions in Example 2 were the same as those in Example 1.

[0092] [ka]

[0093] The capacity retention rate of Example 2 was 81.2%. Furthermore, when the negative electrode active material layer of Example 2 was subjected to NMR analysis, all of the first to third peaks were confirmed. The peak top of the first peak was at a chemical shift of 0.5 ppm. The peak top of the second peak was at a chemical shift of 2.4 ppm. The peak top of the third peak was at a chemical shift of 140 ppm.

[0094] "Example 3" Example 3 differs from Example 1 in that an aqueous aromatic polyimide binder was used. The other conditions were the same as those of Example 1.

[0095] The capacity retention rate of Example 3 was 80.4%. Furthermore, when the negative electrode active material layer of Example 2 was subjected to NMR analysis, all of the first to third peaks were confirmed. The peak top of the first peak was at a chemical shift of 1.5 ppm. The peak top of the second peak was at a chemical shift of 2.4 ppm. The peak top of the third peak was at a chemical shift of 130 ppm.

[0096] "Comparative Example 2" Comparative Example 2 differs from Example 1 in that a non-aromatic cyclic imide polymer represented by the following chemical formula (3) was used as the binder.

[0097] [ka]

[0098] The capacity retention rate of Comparative Example 2 was 63.2%. A weak first peak at 0.15 ppm was detected in the negative electrode active material layer of Comparative Example 1. A second peak was also observed in the negative electrode active material layer of Comparative Example 1.

[0099] The results of the examples and comparative examples are summarized below.

[0100] [Table 1]

[0101] In Table 1, br means a broad peak, and sh means a sharp peak. <50sh means that there is a sharp peak below 50 ppm. The first and second peaks are 7 This is a Li nucleus NMR peak, and is the result of peak separation using the Voigt function, so there is no distinction between broad and sharp. The third peak is 13 The observed peaks of C nuclear NMR are listed, 1 Following the conventions for notating H nuclear NMR, broad and sharp spectra were used. The conventions were based on the Spectroscopic Identification of Organic Compounds, 6th Edition. [Explanation of symbols]

[0102] 10 Separator 20 positive electrode 22 Positive electrode current collector 24 Cathode active material layer 30 negative electrode 32 Negative electrode current collector 34 Negative electrode active material layer 40 Power generating element 50 Exterior body 52 Metal foil 54 Resin layer 60, 62 terminals 100 Lithium-ion secondary battery

Claims

1. a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte; the negative electrode comprises silicon or a silicon compound and a binder; the binder comprises an aromatic polyimide; The negative electrode after discharge is observed by nuclear magnetic resonance (NMR) analysis using a single pulse magic angle spinning (SP-MAS) method, and peak separation using a Gaussian function, a Lorentz function, or a Voigt function reveals that the solid ７ The NMR spectrum of the Li nucleus has a first peak, The first peak is LiCoO ２ The lithium ion secondary battery has a chemical shift peak top in the range of 0.5 ppm to 1.5 ppm when Li is set to -0.5 ppm.

2. The solid ７ The NMR spectrum of the Li nucleus further has a second peak, The second peak is LiCoO ２ The lithium ion secondary battery according to claim 1, wherein the chemical shift when Li is set to -0.5 ppm has a peak top at a position of 1.6 ppm or more and 3.2 ppm or less.

3. When the negative electrode was observed after charging and discharging by nuclear magnetic resonance (NMR) analysis using the cross polarization magic angle spinning (CP-MAS) method, a solid １３ The MAS NMR spectrum of the C nucleus has a third peak, 2. The lithium ion secondary battery according to claim 1, wherein the third peak has a peak top in a chemical shift range of 100 ppm to 170 ppm when the upfield peak of the NMR spectrum of hexamethylbenzene is set to 16.81 ppm.

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