JP7730289B2 - Lithium-ion secondary battery - Google Patents

Description

The present invention relates to a lithium-ion secondary battery.

Lithium-ion secondary batteries are also widely used as power sources for mobile devices such as mobile phones and laptops, as well as hybrid cars.

The capacity of a lithium-ion secondary battery depends primarily on the active material of the electrodes. Graphite is generally used as the negative electrode active material, but there is a demand for negative electrode active materials with higher capacities. For this reason, silicon (Si), which has a theoretical capacity far greater than that of graphite (372 mAh/g), is attracting attention.

Negative electrode active materials containing Si undergo significant volume expansion during charging. This volume expansion of the negative electrode active material can cause a decrease in the battery's cycle characteristics. When the negative electrode active material expands in volume, for example, the conductive paths between the negative electrode active material can be severed, peeling can occur at the interface between the negative electrode active material layer and the current collector, cracks can form in the SEI (Solid Electrolyte Interphase) coating, and electrolyte decomposition can occur. These factors reduce the battery's cycle characteristics.

To improve the battery's cycle characteristics, 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.

Japanese Patent Application Laid-Open No. 2007-242405

Further improvements in cycle characteristics are required.

This disclosure has been made in consideration of the above problems, and aims to provide a lithium-ion secondary battery with excellent cycle characteristics.

To solve the above problems, we provide the following solutions.

(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 (SP-MAS) method and peak separation is performed using a Gaussian function, a Lorentz function, or a Voigt function, the NMR spectrum of a solid ^7Li 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 in _LiCoO2 is set to -0.5 ppm.

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

(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 cross polarization magic angle spinning (CP-MAS), the solid-state ^13C MAS NMR spectrum may have a third peak, which has a peak top in a chemical shift range of 120 ppm to 170 ppm when the upfield peak of the NMR spectrum of hexamethylbenzene is set to 16.81 ppm.

(4) In the lithium ion secondary battery according to the above aspect, the polyimide may be an aromatic polyimide.

The lithium-ion secondary battery according to the above aspect has excellent cycle characteristics.

1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment. 1 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). 1 is a diagram showing an enlarged MAS NMR spectrum of the solid ⁷ Li nucleus of the negative electrode according to this embodiment in a predetermined range (horizontal axis: −40 ppm to 50 ppm). 1 shows the results of a nuclear magnetic resonance (NMR) analysis of the negative electrode according to this embodiment using the cross polarization magic angle spinning method (CP-MAS method). 1 shows the results of a nuclear magnetic resonance (NMR) analysis of the negative electrode according to this embodiment using the cross polarization magic angle spinning method (CP-MAS method).

Embodiments will be described in detail below, with appropriate reference to the drawings. The drawings used in the following description may show characteristic parts enlarged for the sake of clarity, and the dimensional proportions 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 to them. Appropriate changes can be made within the scope of the present invention.

"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 contained within the exterior body 50. While FIG. 1 illustrates a case where one power generating element 40 is contained within the exterior body 50, multiple power generating elements 40 may be stacked.

(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 elements are stacked, or a wound body in which a structure in which these elements are stacked is wound.

<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.

[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 lightweight, 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.

[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 additive and a binder as necessary.

The positive electrode active material includes an electrode active material that can reversibly absorb and release lithium ions, desorb and insert lithium ions (intercalation), or dope and dedope lithium ions and counter anions.

The positive electrode active material is, for example, a composite metal oxide. Examples of the composite metal oxide include lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiMnO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and compounds of the general formula: LiNi _x Co _y Mn _z M _a O ₂ (wherein x + y + z + a = 1, 0≦x<1, 0≦y<1, 0≦z<1, 0≦a<1, and M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr), lithium vanadium compounds (LiV ₂ O ₅ ), olivine-type LiMPO ₄ (wherein M is one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr, or VO), and lithium titanate (Li _{4 Ti5O12} ), _{LiNixCoyAlzO2 (} 0.9< _x +y+ _z <1.1). The positive _electrode active material may be an organic material. For example, the positive electrode active material may be polyacetylene, polyaniline, polypyrrole, polythiophene, or polyacene.

The positive electrode active material may be a lithium-free material. Examples of the lithium-free material include FeF ₃ , 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. In addition, lithium may be pre-doped chemically or electrochemically into a lithium-free positive electrode active material.

Conductive additives increase the electronic conductivity between the positive electrode active materials. Examples of conductive additives include carbon powder, carbon nanotubes, carbon materials, metal powder, a mixture of carbon materials and metal powder, and conductive oxides. Examples of carbon powder include carbon black, acetylene black, and ketjen black. Examples of metal powder include powders of copper, nickel, stainless steel, and iron.

The content of the conductive additive in the positive electrode active material layer 24 is not particularly limited. 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% by mass or more and 20% by mass or less, and preferably 1% by mass or more and 5% by mass or less.

The binder in the positive electrode active material layer 24 binds the positive electrode active material together. Known binders can be used. Preferably, the binder is insoluble in the electrolyte, oxidation-resistant, and adhesive. Examples of binders include fluororesin. Examples of binders 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.

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.

<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.

[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.

[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.

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. Examples of silicon compounds include silicon alloys and silicon oxide. For example, the silicon or silicon compound may be crystalline or amorphous. Amorphous silicon or silicon compounds can be produced by melt-spun methods, gas atomization methods, etc.

Silicon alloys are represented by _XnSi . X is a cation. X is, for example, 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 represented by _SiOx . x satisfies, for example, 0.8≦x≦2. Silicon oxide may be composed of only _SiO2 , may be composed of only SiO, or may be a mixture of SiO and _SiO2 . Silicon oxide may also have a partial oxygen deficiency.

The negative electrode active material may be a composite of silicon or a silicon compound. The composite is composed of silicon or silicon compound particles with at least a portion of their surfaces coated with a conductive material. Examples of conductive materials include carbon materials, Al, Ti, Fe, Ni, Cu, Zn, Ag, and Sn. One example of a composite is silicon-carbon composite material (Si-C). The amount of conductive material coated on the silicon or silicon compound particles is, for example, 0.01% by mass to 30% by mass, and preferably 0.1% by mass to 20% by mass, relative to the total mass of the composite. The composite can be produced by, for example, mechanical alloying, chemical vapor deposition, wet processing, or a method in which a polymer is coated and then thermally decomposed to carbonize the polymer.

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 and 100 ^{m 2} /g or less, preferably 1.0 ^{m 2} /g or more and 20 m ² /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 an electrode, resulting in a small capacity per unit volume.

The binder in the negative electrode active material layer 34 contains polyimide. In addition to polyimide, the binder may also contain, for example, any of the binders that can be used in the positive electrode active material layer 24, such as carboxymethyl cellulose (CMC) or styrene-butadiene rubber (SBR).

Polyimide is, for example, a polymer containing an imide structure in its repeating unit structure. Polyimide is formed by polymerizing 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.

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.

The conductive additive in the negative electrode active material layer 34 enhances electronic conductivity between the negative electrode active materials. The conductive additive can be the same as that used in the positive electrode active material layer 24.

There are no particular limitations on the content of the conductive additive in the negative electrode active material layer 34. 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 mass% or more and 20 mass% or less, and preferably 1 mass% or more and 12 mass% or less. The BET specific surface area of the conductive additive is, for example, 100 ^m2 /g or more and 200 ^m2 /g or less.

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 when preparing the slurry.

FIG. 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 (SP-MAS) method. FIG. 2 shows the NMR spectrum of the solid ⁷ Li nucleus obtained by NMR analysis of the negative electrode. The upper graph in FIG. 2 shows the results of measuring the negative electrode after charging, and the lower graph shows the results of measuring the negative electrode after discharging. The horizontal axis represents the chemical shift (ppm), expressed as ("frequency of absorbed electromagnetic wave" - "absorption frequency of reference material") / "magnetic field strength." As a reference, the Li in LiCoO ₂ was set to -0.5 ppm. The strength of the applied magnetic field was 6.4 T (272 MHz).

The magic angle spinning method involves rotating a sample at high speed around an axis tilted relative to the direction of the applied magnetic field, sharpening the solid-state NMR signal. The sample rotation speed was 15 kHz.

3 is a diagram showing an NMR spectrum of the solid ⁷ Li nucleus of the negative electrode according to this embodiment, enlarged over 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.

As shown in Figure 3, the NMR spectrum of the solid ^7Li nucleus of the negative electrode has, for example, a first peak p1 and a second peak p2. "First,""second," etc. are simply ordinal numbers and do not indicate the magnitude of the peak. The first peak p1 shown in Figure 3 has a peak top at a chemical shift of 0.95 ppm. The second peak p2 shown in Figure 3 has a peak top at a chemical shift of 2.4 ppm.

The first peak p1 is a peak resulting from the reaction product of polyimide and lithium (polyimide absorbing lithium). The first peak p1 is only observed after sufficient reaction between polyimide and lithium. In other words, this peak would not be observed even if polyimide and lithium were simply contained simultaneously as constituent materials for forming the negative electrode active material layer. Methods 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.

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. FIG. 4 shows the NMR spectrum of the solid ^13C nucleus of the negative electrode after charging, and FIG. 5 shows the NMR spectrum of the solid ^13C nucleus of the negative electrode after discharging. The horizontal axis represents the chemical shift (ppm), expressed as ("frequency of absorbed electromagnetic wave" - "absorption frequency of reference material") / "magnetic field strength." As a reference, the high magnetic field peak 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.

As shown in Figures 4 and 5, the NMR spectrum of the ^solid C nucleus of the negative electrode has, for example, 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.

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 negative electrode 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 forming the negative electrode active material layer, this peak will not be observed. Methods for sufficient reaction between polyimide and lithium are described below. 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.

<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.

The separator 10 has, for example, an electrically insulating porous structure. It is, for example, a monolayer or laminate of polyolefin film. It may also be a stretched membrane of a mixture of polyethylene, polypropylene, or the like. It may also be a fibrous nonwoven fabric made of at least one constituent material selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. The separator 10 may also be, for example, a solid electrolyte. Examples of solid electrolytes include polymer solid electrolytes, oxide-based solid electrolytes, and sulfide-based solid electrolytes. The separator 10 may also be an inorganic-coated separator. An 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. Inorganic-coated separators have excellent heat resistance and suppress the deposition of transition metals eluted from the positive electrode onto the negative electrode surface.

<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.

The solvent is not particularly limited as long as it is a solvent commonly used in lithium-ion secondary batteries. Examples of the solvent include cyclic carbonate compounds, chain carbonate compounds, cyclic ester compounds, and chain ester compounds. The solvent may contain a mixture of these compounds in any ratio. Examples of cyclic carbonate compounds include ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate, and vinylene carbonate. Examples of chain carbonate compounds include diethyl carbonate (DEC) and ethyl methyl carbonate (EMC). Examples of cyclic ester compounds include gamma-butyrolactone. Examples of chain ester compounds include propyl propionate, ethyl propionate, and ethyl acetate.

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 type _of lithium salt may be used alone, or _two or more types may be used in combination. From the viewpoint of the degree of ionization _, it is preferable that the electrolyte contains _LiPF6 . The dissociation rate of the electrolytic salt in a carbonate solvent at room temperature is preferably 10% or more.

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.

<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.

As shown in Figure 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).

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

<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.

"Method of manufacturing lithium-ion secondary batteries"
The lithium ion secondary battery 100 is fabricated by 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.

The negative electrode 30 is produced, for example, by sequentially performing a slurry preparation process, an electrode application process, a drying process, and a rolling process.

The slurry preparation process involves mixing the negative electrode active material (silicon or silicon compound), binder, conductive additive, and solvent to create a slurry. The binder used is the one described above. Adding a dispersion stabilizer to the slurry can prevent the negative electrode active material from agglomerating.

Specifically, the slurry is prepared using the following procedure. First, polyamic acid, which will serve as the binder, is diluted with an 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.

Then, stir the solution at 1000 rpm or more to ensure that the polyamic acid is uniformly dispersed within the solution. The conductive additive is added to the solution in multiple batches, stirring at 1000 rpm or more for several minutes after each addition. The dispersion stabilizer is added, for example, at the same time as the conductive additive.

Next, the negative electrode active material is added in multiple batches to the slurry containing the polyamic acid and conductive additive, 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 equipment used, so any method that can produce a uniform dispersion is acceptable.

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 a fluororesin container.

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

The electrode coating process is a process of coating the surface of the negative electrode current collector 32 with a slurry. There are no particular limitations on the method for coating the slurry. For example, slit die coating or doctor blade coating can be used as a method for coating the slurry. The slurry is applied, for example, at room temperature.

The drying process is a process 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 process may also promote a ring-closing reaction of the precursor polyamic acid. In this case, the drying process is preferably carried out at a temperature of 200°C or higher and 350°C or lower. By drying the slurry or promoting the ring-closing reaction, a negative electrode active material layer 34 is formed on the negative electrode current collector 32.

The rolling process is performed as needed. The rolling process applies pressure to the negative electrode active material layer 34 to adjust the density of the negative electrode active material layer 34. The rolling process is performed, for example, using a roll press device. After the rolling process, the resistance of the negative electrode can be measured with an electrical resistance meter or the cross section can be observed with a scanning electron microscope to confirm that each component is uniformly dispersed within the negative electrode active material layer.

The positive electrode 20 can be produced using the same procedure as the negative electrode 30. The separator 10 and outer casing 50 can be commercially available.

Next, the prepared positive electrode 20 and negative electrode 30 are stacked with the separator 10 positioned between them to prepare the power generating element 40. If the power generating element 40 is a wound body, the positive electrode 20, negative electrode 30, and separator 10 are wound around one end of the electrode as the axis.

Finally, the power generation element 40 is sealed in the exterior body 50. The non-aqueous electrolyte is poured into the exterior body 50. After pouring the non-aqueous electrolyte, the pressure is reduced, heating, etc. is performed, and the non-aqueous electrolyte 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 instead of pouring the electrolyte into the exterior body 50, the power generation element 40 may be impregnated with the electrolyte. After pouring the electrolyte into the power generation element, it is preferable to leave it to stand for 24 hours.

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 accommodate 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 cycle characteristics. If the lithium absorption in the polyimide is uneven, the specified peak will not appear in NMR. If the lithium absorption in the polyimide is uneven, the electrode reaction will also be uneven, and the lithium-ion secondary battery will not be able to exhibit sufficient cycle characteristics.

The above describes embodiments of the present invention in detail with reference to the drawings. However, the configurations and combinations thereof in each embodiment are merely examples, and additions, omissions, substitutions, and other modifications to the configurations are possible without departing from the spirit of the present invention.

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

Li _x CoO ₂ was used as the positive electrode active material. 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 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. The positive electrode active material layer was pressed with a roll press to prepare a positive electrode.

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.

First, the above compound was diluted with an N-methyl-2-pyrrolidone (NMP) solution in a resin container. The solution was then stirred at 1000 rpm for 1 minute to ensure the above compound was uniformly dispersed within the solution. 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.

The negative electrode slurry was then applied to one side of a 10 μm-thick copper foil and dried. The amount of negative electrode active material supported in the negative electrode active material layer after drying was 2.5 mg/ ^cm2 . 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.

Next, an electrolyte solution was prepared. The solvent of the electrolyte solution was fluoroethylene carbonate (FEC): ethylene carbonate (EC): diethyl carbonate (DEC) = 10 vol %: 20 vol %: 70 vol %. In addition, additives for improving output, gas suppression additives, cycle characteristic improvement additives, safety performance improvement additives, etc. were added to the electrolyte solution. LiPF ₆ was used as the electrolyte salt. The concentration of LiPF ₆ was 1 mol/L.

(Preparation of Lithium-ion Secondary Battery for Evaluation)
The prepared negative electrode and positive electrode were laminated with a separator (porous polyethylene sheet) 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 aluminum laminate film exterior and heat-sealed except for one peripheral location to form a closed opening. Finally, the above-mentioned electrolyte solution was injected into the exterior, 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 was used for evaluating cycle characteristics, and the other was used for NMR measurement.

(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).

The battery was charged at a constant current charge rate of 1 C (a 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 to determine the battery capacity _Q1 before the cycle test.

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 detected, and the battery capacity _Q2 after 300 cycles was determined. 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%.

(NMR measurement)
The fabricated lithium-ion secondary battery was charged to a fully charged state over 5 hours and then 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.

The dried negative electrode active material layer was placed in a zirconia sample tube and subjected to solid-state NMR measurement at 700 MHz. ⁷ Li nuclei were measured using the SP-MAS method, ^{and 13} C nuclei were measured using the CP-MAS method. Figures 2 to 5 show the evaluation results for Example 1. As shown in Figures 2 to 5, all of the first to third peaks were 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.

"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 all at once to an N-methyl-2-pyrrolidone (NMP) solution, and no dispersion stabilizer was added. The slurry was prepared by adding polyamic acid, silicon, and a conductive additive all at once to an N-methyl-2-pyrrolidone (NMP) solution, and then stirring the mixture at less than 1000 rpm for several minutes.

The capacity retention rate of Comparative Example 1 was 54.3%. Furthermore, the first and third peaks 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.

"Example 2"
Example 2 differs from Example 1 in that the polyimide material was changed. In Example 2, a polymerized polyamic acid represented by the following chemical formula (2) was used as the 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.

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.

"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.

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.

"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. The other conditions were the same as those of Example 1.

The capacity retention rate of Comparative Example 2 was 63.2%. Furthermore, 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.

The results of the examples and comparative examples are summarized below.

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 ^7Li NMR peaks, and are the result of peak separation using the Voigt function, so there is no distinction between broad and sharp. The third peak is an observed peak in ^13C NMR, so it is distinguished as broad or sharp in accordance with the convention for ^1H NMR. The convention used was based on Spectroscopic Identification of Organic Compounds, 6th Edition.

REFERENCE SIGNS LIST 10 Separator 20 Positive electrode 22 Positive electrode current collector 24 Positive electrode 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 layers 60, 62 Terminal 100 Lithium ion secondary battery

Claims (3)

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 method (SP-MAS method), and peak separation is performed using a Gaussian function, a Lorentz function, or a Voigt function. The NMR spectrum of the solid ⁷ Li nucleus has a first peak,
The first peak has a peak top in a chemical shift range of 0.5 ppm to 1.5 ppm when the Li of LiCoO ₂ is set to −0.5 ppm. The NMR spectrum of the solid ⁷ Li nucleus further has a second peak;
The lithium ion secondary battery according to claim 1, wherein the second peak has a peak top at a chemical shift position of 1.6 ppm or more and 3.2 ppm or less when the Li of LiCoO ₂ is set to −0.5 ppm. When the negative electrode was observed after charging and discharging by nuclear magnetic resonance (NMR) analysis using a cross polarization magic angle spinning (CP-MAS) method, the MAS NMR spectrum of the solid ^13C nucleus had 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.