JP2013218855A - Negative electrode carbon material, negative electrode active material, negative electrode and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery excellent in cycle characteristics and capacity retention during high-rate charge/discharge.SOLUTION: In a half cell, an electrode is formed by using a negative electrode carbon material of the present invention in a predetermined condition, lithium metal is used as a counter electrode, an electrolyte obtained by adding LiPFat a ratio of 1 mol/dmto a mixed solvent including an ethylene carbonate and a diethyl carbonate mixed at a volume ratio of 1:1 is used. When, in the half cell, a relationship between a potential difference of both electrodes and charge capacity in charging at 25°C with a constant current of 25 mA/g as a current value per unit weight of the negative electrode carbon material is represented in a graph having a vertical axis (Y axis) that represents the potential difference [V] of both electrodes and a horizontal axis (X axis) that represents charge capacity [mAh/g] per unit weight of the negative electrode carbon material, a relationship represented by the following expression (1) is satisfied in a region where X in the graph is 50 mAh/g or more and 100 mAh/g or less. -0.007X+0.8<Y<-0.007X+1.3...(1)

Description

  The present invention relates to a negative electrode carbon material, a negative electrode active material, a negative electrode, and a lithium ion secondary battery.

  Conventionally, a carbon material has been used for a negative electrode of a lithium ion secondary battery (see, for example, Patent Document 1). This is because even if the charge / discharge cycle proceeds, dendritic lithium is hardly deposited on the negative electrode using the carbon material, and safety is guaranteed.

  However, when a conventional carbon material is used, the cycle characteristics (retention rate of charge / discharge capacity when charging / discharging is repeated) and the capacity during rapid charge / discharge of the lithium ion secondary battery manufactured using the carbon material There were problems such as poor retention.

JP-A-5-74457

  An object of the present invention is to provide a lithium ion secondary battery excellent in cycle characteristics (retention rate of charge / discharge capacity when repeated charge / discharge is repeated) and capacity retention during rapid charge / discharge, and cycle characteristics (charge / discharge capacity). Providing a negative electrode that can be suitably applied to a lithium ion secondary battery excellent in capacity retention at the time of rapid charge and discharge, and cycle characteristics ( A carbon material for a negative electrode and a negative electrode active material that can be suitably used for the production of a lithium ion secondary battery excellent in capacity retention during rapid charge / discharge) It is to provide.

Such an object is achieved by the present invention described in the following (1) to (9).
(1) A carbon material for a negative electrode used in a lithium ion secondary battery,
An electrode is formed using a composition in which a carbon material for a negative electrode, carboxymethyl cellulose, styrene-butadiene rubber, and acetylene black are mixed at a weight ratio of 100: 1.5: 1.5: 2, and lithium metal is used as a counter electrode. The unit weight of the carbon material for the negative electrode is a half cell using a mixed solvent in which ethylene carbonate and diethyl carbonate are mixed at a volume ratio of 1: 1 as an electrolytic solution and LiPF ₆ is added at a ratio of 1 mol / dm ^3. The relationship between the potential difference between both electrodes when charged at 25 ° C. with a constant current value of 25 mA / g and the charge capacity, the vertical axis (Y axis) is the potential difference [V] of both electrodes, and the horizontal axis (X axis) ) In the graph of the charge capacity [mAh / g] of the negative electrode carbon material per unit weight, X in the graph is in the region of 50 mAh / g or more and 100 mAh / g or less. The negative electrode carbon material which satisfies the relationship of the following formula (1).
−0.007X + 0.8 <Y <−0.007X + 1.3 (1)

  (2) The carbon material for a negative electrode according to the above (1), wherein the carbon material for a negative electrode is composed of a material containing amorphous carbon.

(3) Under the following conditions (A) to (E), the positron lifetime of the amorphous carbon measured by the positron annihilation method is 370 picoseconds or more and 480 picoseconds or less,
(A) Positron beam source: A positron is generated from an electron / positron pair using an electron accelerator (B) Gamma ray detector: BaF ₂ scintillator and photomultiplier tube (C) Measurement temperature and atmosphere: 25 ° C. in vacuum (D ) Count of annihilation γ-ray: 3 × 10 ⁶ or more (E) Positron beam energy: 10 keV

  And the carbon material for negative electrodes as described in said (2) whose half value width of the peak recognized by 285 eV vicinity measured by X-ray Photoelectron Spectroscopy (XPS method) is 0.8 eV or more and 1.8 eV or less.

  (4) In the half cell, after charging to 0 V at a constant current of 25 mA / g, charging is terminated when the current decays to 2.5 mA / g by holding 0 V, and the cutoff potential of the discharge condition is 1 The carbon material for a negative electrode according to any one of the above (1) to (3), wherein the charge / discharge efficiency obtained under the condition of 0.5 V is 75% or more.

  (5) A negative electrode active material comprising the carbon material for a negative electrode according to any one of (1) to (4) above.

  (6) The negative electrode active material according to claim 5, wherein the negative electrode active material is composed of a material containing graphite in addition to the carbon material for a negative electrode according to any one of (1) to (4) above. material.

  (7) The negative electrode active material according to (6), wherein the graphite content in the negative electrode active material is 50 wt% or more and 96.2 wt% or less.

  (8) A negative electrode manufactured using the negative electrode active material according to any one of (5) to (7) above.

  (9) A lithium ion secondary battery manufactured using the negative electrode active material according to any one of (5) to (7) above.

  According to the present invention, it is possible to provide a lithium ion secondary battery excellent in cycle characteristics (retention rate of charge / discharge capacity when charge / discharge is repeated) and capacity retention during rapid charge / discharge, and cycle characteristics. (Retention rate of charge / discharge capacity when charge / discharge is repeated) and carbon material for negative electrode and negative electrode active material that can be suitably used for the production of a lithium ion secondary battery excellent in capacity retention during rapid charge / discharge Can be provided.

It is a figure which shows the relationship between the count number of annihilation gamma rays, and positron annihilation time. It is a schematic diagram of a lithium ion secondary battery.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
<Carbon material for negative electrode>
First, the carbon material for negative electrodes of the present invention (hereinafter sometimes simply referred to as a carbon material) will be described.

  By the way, conventionally, a carbon material has been used for a negative electrode of a lithium ion secondary battery. This is because even if the charge / discharge cycle proceeds, dendritic lithium is hardly deposited on the negative electrode using the carbon material, and safety is guaranteed.

  However, when a conventional carbon material is used, the cycle characteristics (retention rate of charge / discharge capacity when charging / discharging is repeated) and the capacity during rapid charge / discharge of the lithium ion secondary battery manufactured using the carbon material There were problems such as poor retention.

Therefore, the present inventor has conducted intensive research for the purpose of solving the above problems, and has reached the present invention. That is, the negative electrode carbon material of the present invention uses a composition in which the negative electrode carbon material, carboxymethyl cellulose, styrene-butadiene rubber, and acetylene black are mixed at a weight ratio of 100: 1.5: 1.5: 2. The electrode is formed, lithium metal is used as the counter electrode, and a mixed solvent in which ethylene carbonate and diethyl carbonate are mixed at a volume ratio of 1: 1 is used as an electrolytic solution to which LiPF ₆ is added at a ratio of 1 mol / dm ³ . The relationship between the potential difference between the two electrodes and the charging capacity when the current value per unit weight of the carbon material for the negative electrode was charged at 25 ° C. at a constant current of 25 mA / g, and the vertical axis (Y axis) is the bipolar electrode Potential difference [V], and the horizontal axis (X axis) is a graph of the charge capacity [mAh / g] of the carbon material for negative electrode per unit weight, X in the graph is 50 mAh / g or more In the following area 00mAh / g, characterized by satisfying the following relationship formula (1).
−0.007X + 0.8 <Y <−0.007X + 1.3 (1)

  With such a configuration, the above problems can be solved. On the other hand, when the above conditions are not satisfied, the excellent effects as described above cannot be obtained. That is, in the region where X is 50 mAh / g or more and 100 mAh / g or less, when Y is −0.007X + 0.8 or less, the resistance is large and the capacity reduction during rapid charge / discharge is large. Further, in the region where X is 50 mAh / g or more and 100 mAh / g or less, when Y is −0.007X + 1.3 or more, in the lithium ion secondary battery, it is consumed for reaction with the electrolytic solution on the active material surface. The amount of Li is large, the irreversible capacity increases, and the cycle characteristics deteriorate.

  In addition, “charging” for the half cell means moving lithium ions from an electrode made of lithium metal to an electrode made of a material containing a carbon material by applying a voltage, and “discharging” means This refers to a phenomenon in which lithium ions move from an electrode made of a material containing a carbon material to an electrode made of lithium metal.

  As described above, in the present invention, in the region where X in the graph is 50 mAh / g or more and 100 mAh / g or less, the relationship of −0.007X + 0.8 <Y <−0.007X + 1.3 may be satisfied. , −0.007X + 0.83 <Y <−0.007X + 1.25 is preferably satisfied, and more preferably −0.007X + 0.85 <Y <−0.007X + 1.2. Thereby, the effects as described above can be made more remarkable.

  When measuring the half cell, an electrode using the carbon material for a negative electrode of the present invention can be suitably produced as follows. That is, a predetermined amount of the carbon material for negative electrode of the present invention, carboxymethyl cellulose, styrene-butadiene rubber, acetylene black and water are mixed, stirred with a mixer to obtain a slurry, and then the slurry is a current collector The electrode can be suitably manufactured by coating on a copper foil, pre-drying at 60 ° C. for 2 hours, then vacuum-drying at 120 ° C. for 15 hours, and then cutting out to a predetermined size. .

In the half cell, a negative electrode (an electrode manufactured using the negative electrode carbon material of the present invention) composed of a negative electrode material layer and a negative electrode current collector has a disc shape with a diameter of 13 mm, and the negative electrode material layer has a thickness of The counter electrode (electrode made of Li) can be a disk having a diameter of 12 mm and a thickness of 1 mm.
The half cell may have a 2032 coin cell shape.

  Further, in the half cell, after charging to 0 V at a constant current of 25 mA / g, charging was terminated when the current was attenuated to 2.5 mA / g by holding 0 V, and the cutoff potential of the discharge condition was 1. The charge / discharge efficiency obtained under the condition of 5 V is preferably 75% or more, more preferably 77% or more, and further preferably 80% or more. Thereby, while being excellent in a rapid charge / discharge characteristic, when it is set as a battery, a battery with high energy density can be obtained.

  In the present invention, the ratio of the sum of the total volume of the mesopores and the total volume of the macropores to the total pore volume of the carbon material for the negative electrode is 80% or more and 98% or less, and the unit of the carbon material for the negative electrode The volume of the macropores per weight is preferably 0.005 ml / g or more and 0.030 ml / g or less. Thereby, the charge / discharge capacity of the lithium ion secondary battery manufactured using the carbon material for negative electrodes of the present invention and the current density during charge / discharge can be made particularly excellent.

  As described above, the ratio of the sum of the total volume of the mesopores and the total volume of the macropores to the total pore volume of the carbon material for the negative electrode is preferably 80% or more and 98% or less, but 85% or more. It is more preferably 95% or less, and further preferably 85% or more and 92% or less. Thereby, the effects as described above can be exhibited more remarkably.

  In addition, as described above, the volume of the macropores per unit weight of the carbon material for negative electrode is preferably 0.005 ml / g or more and 0.030 ml / g or less, but 0.007 ml / g or more and 0.025 ml. / G or less is more preferable. Thereby, the effects as described above can be exhibited more remarkably.

  The volume of mesopores per unit weight of the carbon material for negative electrode of the present invention is preferably 0.001 ml / g or more and 0.025 ml / g or less, and 0.003 ml / g or more and 0.020 ml / g or less. It is more preferable that

  The ratio of the total volume of mesopores to the total pore volume of the negative electrode carbon material of the present invention is preferably 20% or more and 50% or less, and more preferably 30% or more and 45% or less.

  Further, the ratio of the total volume of macropores to the total pore volume of the negative electrode carbon material of the present invention is preferably 30% or more and 70% or less, and more preferably 40% or more and 60% or less.

  The negative electrode carbon material of the present invention is preferably composed of a material containing amorphous carbon. Thereby, the discharge characteristic in low temperature (for example, -20 degreeC) can be made especially excellent.

Amorphous carbon, unlike graphite, is an amorphous (amorphous) carbon material.
Amorphous carbon can be suitably obtained by carbonizing a resin or resin composition.

  Examples of amorphous carbon include hard carbon and soft carbon, but the carbon material for negative electrode of the present invention preferably contains hard carbon. Thereby, in the lithium ion secondary battery manufactured using the carbon material for negative electrodes of the present invention, the cycle characteristics can be made particularly excellent, and the input / output characteristics of a large current can be made particularly excellent. .

Amorphous carbon preferably has an average interplanar spacing d ₀₀₂ of (002) planes of 3.4 to 3.9 mm calculated from the wide-angle X-ray diffraction method using the Bragg equation. When the average interplanar distance d ₀₀₂ is 3.4 mm or more, particularly 3.6 mm or more, the interlayer contraction / expansion due to the occlusion of lithium ions is less likely to occur, so that the reduction in charge / discharge cycle performance can be suppressed.

On the other hand, when the average interplanar distance d ₀₀₂ is 3.9 mm or less, particularly 3.8 mm or less, lithium ions are smoothly occluded / desorbed, and a decrease in charge / discharge efficiency can be suppressed.

  Further, the amorphous carbon preferably has a crystallite size Lc in the c-axis direction (the (002) plane orthogonal direction) of 8 to 50 mm.

  By setting Lc to 8 mm or more, particularly 9 mm or more, there is an effect that a carbon interlayer space capable of inserting and extracting lithium ions is formed, and a sufficient charge / discharge capacity can be obtained. By doing so, it is possible to suppress the collapse of the carbon laminate structure due to the insertion / desorption of lithium ions and the reductive decomposition of the electrolytic solution, and to suppress the decrease in charge / discharge efficiency and charge / discharge cycle performance.

Lc is calculated as follows.
It was determined from the half width of the 002 plane peak and the diffraction angle in the spectrum obtained from the X-ray diffraction measurement using the following Scherrer equation.

Lc = 0.94λ / (βcosθ) (Scherrer equation)
Lc: Crystallite size λ: Characteristic X-ray Kα1 wavelength output from the cathode β: Half width of peak (radian)
θ: Reflection angle of spectrum

  The X-ray diffraction spectrum in the amorphous carbon is measured by an X-ray diffraction apparatus “XRD-7000” manufactured by Shimadzu Corporation. The method for measuring the average interplanar spacing in amorphous carbon is as follows.

  From the spectrum obtained from the X-ray diffraction measurement of amorphous carbon, the average interplanar spacing d was calculated from the following Bragg equation.

λ = 2d _hkl sin θ (Bragg equation) (d _hkl = d ₀₀₂ )
λ: wavelength of characteristic X-ray Kα1 output from the cathode θ: reflection angle of spectrum

  The particle size (average particle size) at 50% accumulation in the volume-based cumulative distribution of amorphous carbon is preferably 1 μm or more and 50 μm or less, and more preferably 2 μm or more and 30 μm or less. Thereby, a high-density electrode can be produced.

  The resin used as the raw material for amorphous carbon or the resin contained in the resin composition is not particularly limited. For example, a thermosetting resin, a thermoplastic resin, or petroleum-based tar produced as a by-product during production of ethylene and Petroleum, coal-based tar or pitch, such as pitch, coal tar produced during coal carbonization, heavy components and pitch obtained by distilling off low boiling components of coal tar, tar and pitch obtained by coal liquefaction, and Those obtained by crosslinking the tar, pitch and the like can be contained, and one or more of these can be used in combination.

  Further, as will be described later, the resin composition contains the above resin as a main component, and can contain a curing agent, an additive and the like, and further, a crosslinking treatment by oxidation or the like can be appropriately performed. it can.

  The thermosetting resin is not particularly limited. For example, phenol resins such as novolac type phenol resin and resol type phenol resin, epoxy resins such as bisphenol type epoxy resin and novolac type epoxy resin, melamine resin, urea resin, and aniline resin. , Cyanate resin, furan resin, ketone resin, unsaturated polyester resin, urethane resin and the like. In addition, modified products obtained by modifying these with various components can also be used.

  The thermoplastic resin is not particularly limited, and examples thereof include polyethylene, polystyrene, polyacrylonitrile, acrylonitrile-styrene (AS) resin, acrylonitrile-butadiene-styrene (ABS) resin, polypropylene, vinyl chloride, methacrylic resin, polyethylene terephthalate. , Polyamide, polycarbonate, polyacetal, polyphenylene ether, polybutylene terephthalate, polyphenylene sulfide, polysulfone, polyethersulfone, polyetheretherketone, polyetherimide, polyamideimide, polyimide, polyphthalamide and the like.

  In particular, a thermosetting resin is preferable as a resin as a main component used in the production of amorphous carbon. Thereby, the effect that a quick charge / discharge characteristic is excellent is acquired.

  In particular, among thermosetting resins, those selected from novolac type phenol resins, resol type phenol resins, melamine resins, furan resins, aniline resins, and modified products thereof are preferable. Thereby, the charge / discharge efficiency can be made particularly excellent. In addition, the degree of freedom in designing the carbon material is widened and can be manufactured at a low price. Further, the stability during cycling and the input / output characteristics of a large current can be further improved.

Moreover, when using a thermosetting resin, the hardening | curing agent can be used together.
The curing agent to be used is not particularly limited. For example, in the case of a novolac type phenol resin, hexamethylenetetramine, resol type phenol resin, polyacetal, paraform and the like can be used. In the case of epoxy resin, known as epoxy resins such as polyamine compounds such as aliphatic polyamines and aromatic polyamines, acid anhydrides, imidazole compounds, dicyandiamide, novolac type phenol resins, bisphenol type phenol resins, resol type phenol resins, etc. Can be used.

  In addition, even if it is a thermosetting resin that normally uses a predetermined amount of a curing agent, the resin composition used in the present embodiment is used in a smaller amount than usual or without using a curing agent. You can also.

  Moreover, in the resin composition as a raw material of amorphous carbon, an additive can be blended in addition to the above components.

  Although it does not specifically limit as an additive used here, For example, the carbon material precursor carbonized at 200 to 800 degreeC, an organic acid, an inorganic acid, a nitrogen-containing compound, an oxygen-containing compound, an aromatic compound, and And non-ferrous metal elements. These additives can be used alone or in combination of two or more depending on the type and properties of the resin used.

  As a resin used as a raw material for amorphous carbon, nitrogen-containing resins described later may be included as a main component resin. Moreover, when the nitrogen-containing resin is not contained in the main component resin, at least one kind of nitrogen-containing compound may be included as a component other than the main component resin, or the nitrogen-containing resin is included as the main component resin. In addition, a nitrogen-containing compound may be included as a component other than the main component resin. By carbonizing such a resin, amorphous carbon containing nitrogen can be obtained. When nitrogen is contained in amorphous carbon, suitable electrical characteristics can be imparted to amorphous carbon (a negative electrode carbon material) depending on the electronegativity of nitrogen. Thereby, occlusion and discharge | release of lithium ion can be accelerated | stimulated and a high charging / discharging characteristic can be provided.

Here, the following can be illustrated as nitrogen-containing resin.
Examples of thermosetting resins include melamine resins, urea resins, aniline resins, cyanate resins, urethane resins, phenol resins modified with nitrogen-containing components such as amines, and epoxy resins.

  Examples of the thermoplastic resin include polyacrylonitrile, acrylonitrile-styrene (AS) resin, acrylonitrile-butadiene-styrene (ABS) resin, polyamide, polyetherimide, polyamideimide, polyimide, polyphthalamide, and the like.

Moreover, the following can be illustrated as resin other than nitrogen-containing resin.
Examples of the thermosetting resin include a phenol resin, an epoxy resin, a furan resin, and an unsaturated polyester resin.

  Examples of the thermoplastic resin include polyethylene, polystyrene, polypropylene, vinyl chloride, methacrylic resin, polyethylene terephthalate, polycarbonate, polyacetal, polyphenylene ether, polybutylene terephthalate, polyphenylene sulfide, polysulfone, polyethersulfone, and polyetheretherketone. .

In addition, when a nitrogen-containing compound is used as a component other than the main component resin, the type thereof is not particularly limited. For example, hexamethylenetetramine, which is a curing agent for a novolac type phenol resin, and aliphatic polyamine, which is a curing agent for an epoxy resin. Besides aromatic polyamines, dicyandiamide and the like, in addition to the curing agent component, amine compounds that do not function as curing agents, ammonium salts, nitrates, nitro compounds, and other nitrogen-containing compounds can be used.
As the nitrogen-containing compound, one type may be used or two or more types may be used in combination, whether or not the main component resin contains nitrogen-containing resins.

  The resin composition used as the raw material for amorphous carbon or the nitrogen content in the resin is not particularly limited, but is preferably 5% by weight or more and 65% by weight or less, preferably 10% by weight or more and 20% by weight or less. More preferably.

  The carbon atom content in the amorphous carbon obtained by carbonizing the resin composition or resin is preferably 95% by weight or more, and the nitrogen atom content is preferably 0.5% by weight. The content is preferably 5% by weight or less.

  By containing nitrogen atoms in an amount of 0.5% by weight or more, particularly 0.8% by weight or more in this way, it is possible to impart electrical characteristics suitable for amorphous carbon depending on the electronegativity of nitrogen. Thereby, occlusion and discharge | release of lithium ion can be accelerated | stimulated and a high charging / discharging characteristic can be provided.

  Further, by setting the nitrogen atom to 5% by weight or less, particularly 3% by weight or less, the electrical characteristics imparted to the amorphous carbon are suppressed from becoming excessively strong, and the occluded lithium ions are nitrogenated. Prevents electroadsorption with atoms. Thereby, an increase in irreversible capacity can be suppressed and high charge / discharge characteristics can be obtained.

  Nitrogen content in amorphous carbon is the above-mentioned resin composition or nitrogen content in the resin, as well as conditions for carbonizing the resin composition or resin, and when curing or pre-carbonization is performed before carbonization. In addition, these conditions can also be adjusted by appropriately setting.

  For example, as a method for obtaining the carbon material having the nitrogen content as described above, the nitrogen content in the resin composition or the resin is set as a predetermined value, and conditions for carbonizing the resin composition, in particular, the final temperature is adjusted. There are methods.

  The method for preparing a resin composition used as a raw material for amorphous carbon is not particularly limited, for example, a method of blending the above main component resin and other components at a predetermined ratio, and melt-mixing them, It can be prepared by a method in which these components are dissolved in a solvent and mixed, or a method in which these components are pulverized and mixed.

In the present specification, the nitrogen content is measured by a thermal conductivity method.
In this method, a measurement sample is converted into a simple gas (CO ₂ , H ₂ O, and N ₂ ) using a combustion method, and then the gasified sample is homogenized and then passed through a column. . Thereby, these gas is isolate | separated in steps and content of carbon, hydrogen, and nitrogen can be measured from each thermal conductivity.

  In the present invention, an element analysis measuring device “PE2400” manufactured by PerkinElmer Co., Ltd. was used.

  Further, the amorphous carbon constituting the carbon material for a negative electrode of the present invention preferably has a positron lifetime measured by a positron annihilation method of 370 picoseconds or more and 480 picoseconds or less, and 380 picoseconds or more and 460 picoseconds. The following are more preferable.

  When the positron lifetime measured by the positron annihilation method is 370 picoseconds or more and 480 picoseconds or less, it can be said that a void having a size in which lithium easily enters and exits is formed in amorphous carbon as described later. The charge capacity and discharge capacity of the carbon material can be further increased.

Note that the measurement of the positron lifetime by the positron annihilation method is performed under the following conditions.
(A) Positron beam source: A positron is generated from an electron / positron pair using an electron accelerator (B) Gamma ray detector: BaF ₂ scintillator and photomultiplier tube (C) Measurement temperature and atmosphere: 25 ° C. in vacuum (D ) Count of annihilation γ-ray: 3 × 10 ⁶ or more (E) Positron beam energy: 10 keV

Here, the relationship between the positron lifetime and the void size will be described.
The positron lifetime method is a method for measuring the size of the void by measuring the time from when the positron (e ⁺ ) enters the sample until it disappears.

A positron is an antimatter of electrons and has the same static mass as an electron, but its charge is positive.
It is known that when a positron enters a substance, it becomes a pair (positron-electron pair (positronium)) with an electron and then disappears. When a positron is injected into the carbon material, the positron (e ⁺ ) is combined with one of the electrons struck out in the polymer to form positronium. Positronium is trapped in a portion having a low electron density in the polymer material, that is, a local void in the polymer, and overlaps with the electron cloud emitted from the void wall and disappears. When positronium is present in voids in the polymer, the size of the voids and the annihilation lifetime of positronium are inversely related. That is, when the void is small, the overlap between the positronium and the surrounding electrons is increased, and the positron annihilation lifetime is shortened. On the other hand, when the void is large, the probability that the positronium overlaps with other electrons that have exuded from the void wall and disappears becomes low, and the annihilation lifetime of the positronium becomes long. Therefore, the size of the voids in the carbon material can be evaluated by measuring the annihilation lifetime of positronium.

As described above, the positron incident on the carbon material loses energy, and then forms positronium and disappears together with the electron. At this time, γ rays are emitted from the carbon material.
Accordingly, the emitted γ rays serve as a measurement end signal.

For the measurement of positron annihilation lifetime, often used radioactive isotopes ^{22 Na} as that of an electron accelerator and generic as positron source. ²² Na emits positrons and 1.28 MeV gamma rays simultaneously when β ⁺ decays to ²² Ne. The positron incident on the carbon material emits 511 keV gamma rays through an annihilation process. Therefore, the annihilation lifetime of the positron can be obtained by measuring the time difference between the 1.28 MeV γ ray and the 511 kev γ ray as the end signal. Specifically, a positron lifetime spectrum as shown in FIG. 1 is obtained. The slope A of the positron lifetime spectrum indicates the positron lifetime, and the positron lifetime of the carbon material can be grasped from the positron lifetime spectrum.

When an electron accelerator is used as the positron source, electron / positron pair generation is caused by the braking X-rays generated by irradiating the target made of tantalum or tungsten with an electron beam to generate positrons. In the case of an electron accelerator, the time when the positron beam is incident on the sample is set as a measurement start point (corresponding to the start signal in the ²² Na), and the end signal is measured on the same principle as in the case of ²² Na.

  When the positron lifetime measured by the positron annihilation method is less than 370 picoseconds, the pore size is too small and it becomes difficult to occlude and release lithium ions. In addition, when the positron lifetime measured by the positron annihilation method exceeds 480 picoseconds, the amount of occlusion of lithium is increased, but it is assumed that lithium is less likely to be released due to an increase in electrostatic capacity due to intrusion of other substances such as an electrolytic solution. Is done.

  In the case of amorphous carbon, the half width of the peak observed around 285 eV measured by XPS method is preferably 0.8 eV or more and 1.8 eV or less, more preferably 0.9 eV or more and 1.6 eV or less. preferable. When the half-value width of the peak observed around 285 eV measured by XPS method is 1.8 eV or less, most of the elements present on the amorphous carbon surface are due to inactive C—C bonds, etc. A functional group or impurity that reacts with an active substance related to ion conduction such as ions is substantially absent. Moreover, when the half width of the peak recognized in the vicinity of 285 eV is 0.8 eV or more, problems such as excessive crystallization do not occur. Therefore, when the full width at half maximum of the peak observed in the vicinity of 285 eV measured by XPS method is 0.8 eV or more and 1.8 eV or less like the carbon material according to the present invention, the charge / discharge efficiency is reduced due to the irreversible capacity. Is suppressed.

Next, the relationship between the XPS measurement and the surface state will be described.
The XPS measurement method irradiates the surface of a solid sample with X-rays and measures the kinetic energy of photoelectrons emitted from the excited atoms. As a result, the binding energy of electrons in the atoms (a value specific to each atom is determined). It is a method for identifying constituent elements present on the surface.

  The surface state can also be analyzed by the FT-IR method, which identifies chemical bonds existing about 1 μm from the surface, whereas the XPS measurement method identifies elements present several kilometers from the surface. be able to. From this, it is preferable to use the XPS measurement method to identify the functional group closer to the surface.

Further, the amorphous carbon preferably has a specific surface area of 15 m ² / g or less and 1 m ² / g or more according to the BET three-point method in nitrogen adsorption.

Reaction with a carbon material and electrolyte solution can be suppressed because the specific surface area by BET 3 point method in nitrogen adsorption is 15 m < ² > / g or less.

Moreover, there exists an effect that appropriate permeability to the carbon material of electrolyte solution is acquired by making the specific surface area by BET 3 point method in nitrogen adsorption into 1 m < ² > / g or more.

The calculation method of the specific surface area is as follows.
The monomolecular adsorption amount Wm was calculated from the following formula (1), the total surface area Total was calculated from the following formula (2), and the specific surface area S was calculated from the following formula (3).
1 / [W (Po / P-1) = (C-1) / WmC (P / Po) / WmC (1)

In the formula (1), P: pressure of the adsorbate gas in the adsorption equilibrium, Po: saturated vapor pressure of the adsorbate at the adsorption temperature, W: adsorption amount at the adsorption equilibrium pressure P, Wm: monomolecular layer adsorption amount, C : Constant on the magnitude of the interaction between the solid surface and the adsorbate (C = exp {(E1-E2) RT}) [E1: heat of adsorption of the first layer (kJ / mol), E2: measurement temperature of the adsorbate Liquefaction heat (kJ / mol)]

Total = (WmNAcs) M (2)
In formula (2), N: Avogadro number, M: molecular weight, Acs: adsorption cross section

S = Total / w (3)
In formula (3), w: sample weight (g)

  The amorphous carbon as described above can be produced as follows in a typical example of a resin or a resin composition.

First, a resin or resin composition to be carbonized is manufactured.
The apparatus for preparing the resin composition is not particularly limited. For example, when melt mixing is performed, a kneading apparatus such as a kneading roll, a single screw or a twin screw kneader can be used. Moreover, when performing melt | dissolution mixing, mixing apparatuses, such as a Henschel mixer and a disperser, can be used. And when performing pulverization mixing, apparatuses, such as a hammer mill and a jet mill, can be used, for example.

  The resin composition thus obtained may be one obtained by physically mixing a plurality of types of components, or is applied during mixing (stirring, kneading, etc.) during preparation of the resin composition. A part of the material may be chemically reacted with mechanical energy and thermal energy converted from the mechanical energy. Specifically, a mechanochemical reaction using mechanical energy or a chemical reaction using thermal energy may be performed.

Amorphous carbon is obtained by carbonizing the above resin composition or resin.
Here, the conditions for the carbonization treatment are not particularly limited. For example, the temperature is raised from room temperature to 1 ° C./hour to 200 ° C./hour, and from 800 ° C. to 3000 ° C., 0.1 to 50 hours, preferably Can be carried out by holding for 0.5 hours or more and 10 hours or less. The atmosphere during the carbonization treatment is an inert atmosphere such as nitrogen or helium gas, or a substantially inert atmosphere where a trace amount of oxygen is present in the inert gas, or a reducing gas atmosphere. Is preferred. By doing in this way, thermal decomposition (oxidative decomposition) of resin can be suppressed and a desired carbon material can be obtained.

  Conditions such as temperature and time during carbonization can be adjusted as appropriate in order to optimize the characteristics of amorphous carbon.

  In addition, in order to obtain a carbon material having a peak half-value width measured in the vicinity of 285 eV measured by the XPS method of 0.8 eV or more and 1.8 eV or less, conditions may be appropriately determined according to the resin or the like. The temperature during the carbonization treatment may be set to 1000 ° C. or higher, and the temperature increase rate may be set to less than 200 ° C./hour.

  By doing so, the surface of the amorphous carbon is caused by an inactive functional group or the like, and the half width of the peak observed near 285 eV measured by XPS method is 0.8 eV or more and 1.8 eV or less. It is estimated that crystalline carbon can be obtained.

In addition, before performing the said carbonization process, a pre carbonization process can be performed.
Although it does not specifically limit as conditions for pre carbonization here, For example, it can carry out at 200 degreeC or more and 600 degrees C or less for 1 hour or more and 10 hours or less. Thus, even if the resin composition or resin is infusibilized by performing the pre-carbonization treatment before the carbonization treatment, and the resin composition or the resin is pulverized before the carbonization treatment step, The resin composition or the resin can be prevented from being re-fused during carbonization, and a desired carbon material can be obtained efficiently.

  At this time, as an example of a method for obtaining amorphous carbon having a positron lifetime measured by the positron annihilation method of 370 picoseconds or more and 480 picoseconds or less, in the absence of a reducing gas or an inert gas, Carburizing treatment is performed.

  Further, when a thermosetting resin or a polymerizable polymer compound is used as the resin for producing amorphous carbon, the resin composition or the resin can be cured before the pre-carbonization treatment. .

  Although it does not specifically limit as a hardening processing method, For example, it can carry out by the method of giving the heat quantity which can perform hardening reaction to a resin composition, the method of thermosetting, or the method of using resin and a hardening | curing agent together. As a result, the pre-carbonization treatment can be performed substantially in the solid phase, so that the carbonization treatment or the pre-carbonization treatment can be performed while maintaining the resin structure to some extent, and the structure and characteristics of the amorphous carbon can be controlled. become able to.

  In addition, when performing the carbonization treatment or the pre-carbonization treatment, a metal, a pigment, a lubricant, an antistatic agent, an antioxidant, or the like is added to the resin composition to impart desired characteristics to the carbon material. You can also.

  When the curing treatment and / or the pre-carbonization treatment are performed, the processed product may be pulverized before the carbonization treatment. In such a case, variation in thermal history during carbonization can be reduced, and the uniformity of the surface state of amorphous carbon can be improved. And the handleability of a processed material can be made favorable.

  Further, in order to obtain amorphous carbon having a positron lifetime measured by the positron annihilation method of 370 picoseconds or more and 480 picoseconds or less, for example, in the presence of a reducing gas or an inert gas after carbonization treatment as necessary. Then, it may be naturally cooled to 500 ° C. or higher and 800 ° C. or lower, and then cooled at 100 ° C./hour until it becomes 100 ° C. or lower.

  By doing so, cracks of amorphous carbon due to rapid cooling are suppressed, and the formed voids can be maintained, so that the positron lifetime measured by the positron annihilation method is 370 picoseconds or more and 480 picoseconds or less. It is speculated that some amorphous carbon can be obtained.

  The negative electrode carbon material of the present invention may be composed of a material containing graphite in addition to the amorphous carbon. As a result, the lithium ion secondary battery manufactured using the carbon material for a negative electrode of the present invention has a high charge capacity and discharge capacity, and a particularly excellent balance of charge capacity, discharge capacity and charge / discharge efficiency. Can do.

  Graphite is one of the allotropes of carbon, and is a hexagonal hexagonal plate crystal material that forms a layered lattice made of layers of six-carbon rings.

  The particle size (average particle size) at the time of 50% accumulation in the volume-based cumulative distribution of graphite is preferably 2 μm or more and 50 μm or less, and more preferably 5 μm or more and 30 μm or less. This makes it possible to produce a high-density electrode while maintaining high charge / discharge efficiency.

<Negative electrode active material>
Next, the negative electrode active material of the present invention (hereinafter sometimes simply referred to as an active material) will be described.

  The negative electrode active material refers to a material capable of taking in and out Li ions in a lithium ion secondary battery.

  The negative electrode active material of the present invention includes the above-described negative electrode carbon material of the present invention. Thereby, the negative electrode active material which can be suitably used for the manufacture of a lithium ion secondary battery excellent in cycle characteristics (retention rate of charge / discharge capacity upon repeated charge / discharge) and capacity retention during rapid charge / discharge Can provide.

  Although the negative electrode active material of this invention should just contain the carbon material for negative electrodes of this invention mentioned above, in addition to that, it may contain another component. Examples of such components include silicon, silicon monoxide, and graphite.

  Especially, it is preferable that the negative electrode active material of this invention contains graphite in addition to the carbon material for negative electrodes of this invention mentioned above. As a result, the lithium ion secondary battery manufactured using the carbon material for a negative electrode of the present invention has a high charge capacity and discharge capacity, and a particularly excellent balance of charge capacity, discharge capacity and charge / discharge efficiency. Can do.

  The particle size (average particle size) at the time of 50% accumulation in the volume-based cumulative distribution of graphite is preferably 2 μm or more and 50 μm or less, and more preferably 5 μm or more and 30 μm or less. This makes it possible to produce a high-density electrode while maintaining high charge / discharge efficiency.

  The content of graphite in the negative electrode active material of the present invention is preferably 50% by weight or more and 96.2% by weight or less, more preferably 55% by weight or 90% by weight or less, and 60% by weight or 85% by weight. More preferably, it is as follows. When the graphite content is within the above range, high energy density and high current input / output characteristics can be achieved at a high level while the charge / discharge efficiency is high.

When the carbon material for a negative electrode of the present invention contains amorphous carbon and the negative electrode active material of the present invention contains graphite, the content of the amorphous carbon in the negative electrode active material is expressed as X _H [Mass%] When the content of the graphite in the negative electrode active material is X _G [mass%], it is preferable to satisfy the relationship of 1 ≦ X _G / X _H ≦ 20, and 1.2 ≦ X _It is more preferable to satisfy the relationship of _{G 1} / X _H ≦ 9, and it is even more preferable to satisfy the relationship of 1.5 ≦ X _G / X _H ≦ 5.7. By satisfying such a relationship, the stability during cycling and the input / output characteristics of a large current can be further effectively improved without impairing the excellent charge / discharge efficiency.

<Negative electrode / Lithium ion secondary battery>
Next, the negative electrode and lithium ion secondary battery (hereinafter, also simply referred to as “secondary battery”) of the present invention will be described.

  The negative electrode of the present invention is produced using the negative electrode active material of the present invention as described above. This provides a negative electrode that can be suitably applied to a lithium ion secondary battery excellent in cycle characteristics (retention rate of charge / discharge capacity when charge / discharge is repeated) and capacity retention during rapid charge / discharge. be able to. The lithium ion secondary battery of the present invention is manufactured using the negative electrode active material of the present invention. Thereby, it is possible to provide a lithium ion secondary battery excellent in cycle characteristics (retention rate of charge / discharge capacity when charge / discharge is repeated) and capacity retention during rapid charge / discharge.

Hereinafter, preferred embodiments of the negative electrode and the lithium ion secondary battery of the present invention will be described.
FIG. 2 is a schematic diagram of a lithium ion secondary battery.

  As illustrated in FIG. 2, the secondary battery 10 includes a negative electrode 13, a positive electrode 21, an electrolytic solution 16, and a separator 18.

As shown in FIG. 2, the negative electrode 13 includes a negative electrode material 12 and a negative electrode current collector 14.
The negative electrode current collector 14 is made of, for example, copper foil or nickel foil.

The negative electrode material 12 is composed of the negative electrode active material of the present invention as described above.
The negative electrode 13 can be manufactured as follows, for example.

  1 part by weight or more and 30 parts by weight or less of organic polymer binder (fluorine polymer including polyethylene, polypropylene, etc., rubbery polymer such as butyl rubber, butadiene rubber, etc.) with respect to 100 parts by weight of the negative electrode active material, And an appropriate amount of a viscosity adjusting solvent (N-methyl-2-pyrrolidone, dimethylformamide, etc.) or water and kneaded to form a paste-like mixture into sheets, pellets, etc. by compression molding, roll molding, etc. The negative electrode material 12 can be obtained by molding. And the negative electrode 13 can be obtained by laminating | stacking the negative electrode material 12 and the negative electrode collector 14 which were obtained in this way.

  In addition, 1 part by weight or more and 30 parts by weight of an organic polymer binder (fluorine polymer including polyethylene, polypropylene, etc., rubbery polymer such as butyl rubber, butadiene rubber, etc.) with respect to 100 parts by weight of the negative electrode active material. The following mixture is used as the negative electrode material 12 by adding a kneaded mixture of an appropriate amount of a viscosity adjusting solvent (N-methyl-2-pyrrolidone, dimethylformamide, etc.) or water, and using this mixture as the negative electrode material 12. The negative electrode 13 can also be manufactured by coating and molding on the material 14.

  In addition, the negative electrode 13 may satisfy the same conditions as the electrodes included in the half cell described above, or may have different conditions.

  The electrolyte solution 16 fills the space between the positive electrode 21 and the negative electrode 13 and is a layer in which lithium ions move by charge / discharge.

  As the electrolytic solution 16, a solution obtained by dissolving a lithium salt serving as an electrolyte in a non-aqueous solvent is used.

  As this non-aqueous solvent, it is possible to use a mixture of cyclic esters such as propylene carbonate, ethylene carbonate and γ-butyrolactone, chain esters such as dimethyl carbonate and diethyl carbonate, chain ethers such as dimethoxyethane, and the like. it can.

As the electrolyte, lithium metal salts such as LiClO ₄ and LiPF ₆ , tetraalkylammonium salts, and the like can be used. Further, the above salts can be mixed with polyethylene oxide, polyacrylonitrile or the like and used as a solid electrolyte.

  It does not specifically limit as the separator 18, For example, porous films, such as polyethylene and a polypropylene, a nonwoven fabric, etc. can be used.

As shown in FIG. 2, the positive electrode 21 includes a positive electrode material 20 and a positive electrode current collector 22.
The positive electrode material 20 is not particularly limited. For example, a composite oxide such as lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiMn ₂ O ₄ ), polyaniline, Conductive polymers such as polypyrrole can be used.

As the positive electrode current collector 22, for example, an aluminum foil can be used.
And the positive electrode 21 in this embodiment can be manufactured with the manufacturing method of the known positive electrode.

  As mentioned above, although this invention was demonstrated based on suitable embodiment, this invention is not limited to these.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail based on an Example and a comparative example, this invention is not limited to this. In the examples and comparative examples, “part” indicates “part by weight” and “%” indicates “% by weight”.

[1] Measuring Method First, measuring methods in the following examples and comparative examples will be described.

1. Particle size distribution Using a laser diffraction particle size distribution measuring device LA-920 manufactured by Horiba, Ltd., by laser diffraction, the particle size distribution of the carbon material, the particle size distribution of graphite constituting the carbon material, and the amorphous carbon constituting the carbon material The particle size distribution was measured. From the measurement results, for the carbon material, the particle size (D50, average particle size) at the time of 50% accumulation in the volume-based cumulative distribution is determined, and the graphite constituting the carbon material and the amorphous carbon constituting the carbon material , 50% cumulative particle size (D50, average particle size) was determined.

2. Measurement method of positron lifetime by positron lifetime method Using positron / positronium lifetime measurement / nano-hole measuring device (manufactured by National Institute of Advanced Industrial Science and Technology), measure electromagnetic wave (annihilation γ-ray) generated when positron disappears, The positron lifetime was measured.

Specifically, it is as follows.
(A) Positron beam source: A positron is generated from electron-positron pair generation using the electron accelerator of the National Institute of Advanced Industrial Science and Technology (Measurement Frontier Research Department). The electron accelerator irradiates a target (tantalum) with an electron beam, Causes generation of electron-positron pairs, generating positrons)

(B) Gamma ray detector: BaF ₂ scintillator and photomultiplier tube (C) Measurement temperature and atmosphere: 25 ° C. in vacuum (1 × 10 ⁻⁵ Pa (1 × 10 ⁻⁷ Torr))

(D) Count of annihilation γ-rays: 3 × 10 ⁶ or more (E) Positron beam energy: 10 keV
(F) Sample size: Powder is applied to a sample holder (aluminum plate) with a thickness of 0.1 mm.

3. Analysis of surface condition by XPS measurement Using Escalab-220iXL (Thermo Fisher Scientific), measurement was performed under the following conditions, and the half width of the peak observed near 285 eV was obtained by the following calculation method. Calculated.

(Measurement condition)
X-ray source: Mg-Kα
Output: 12kV-10mA

(Method of calculation)
Based on the obtained spectrum, the peak intensity and the peak half-value width are obtained as follows.
In order to obtain the peak intensity, a baseline is drawn from both ends of the target peak, and the intensity from the baseline to the peak apex is defined as the peak intensity. This is because the base line of the spectrum usually obtained changes depending on the environment at the time of measurement, the difference in the sample, and the like. When a plurality of peaks overlap in the obtained spectrum, a baseline is drawn from both ends of the overlapping peaks. The half width of the peak is obtained by drawing a line parallel to the base line from a point having an intensity of ½ of the peak intensity obtained from the peak apex, and reading the energy at the intersection with both ends of the peak.

4). Specific surface area It measured by the BET 3 point | piece method in nitrogen adsorption | suction using the Nova-1200 apparatus made from Yuasa. A specific calculation method is as described in the above embodiment.

5. Carbon content, nitrogen content Measured using an elemental analysis measuring device “PE2400” manufactured by PerkinElmer. The measurement sample is converted into CO ₂ , H ₂ O, and N ₂ using a combustion method, and then the gasified sample is homogenized and then passed through the column. Thereby, these gases were separated stepwise, and the contents of carbon, hydrogen, and nitrogen were measured from the respective thermal conductivities.

A) Carbon content The obtained amorphous carbon was dried at 110 ° C./vacuum for 3 hours, and then the carbon composition ratio was measured using an elemental analyzer.

B) Nitrogen content The obtained amorphous carbon was dried at 110 ° C./vacuum for 3 hours, and then the nitrogen composition ratio was measured using an elemental analyzer.

6). Charging curve, charge / discharge efficiency (1) Production of bipolar half-cell For 100 parts of carbon material for negative electrode obtained in each example and each comparative example, carboxymethyl cellulose (manufactured by Daicel Finechem, CMC Daicel 2200) 1.5 Add parts by weight, 1.5 parts by weight of styrene-butadiene rubber (manufactured by JSR, TRD-2001), 2 parts by weight of acetylene black (manufactured by Denki Kagaku Kogyo, Denka Black), and 100 parts by weight of distilled water. The mixture was stirred and mixed with a slurry to prepare a slurry-like negative electrode mixture.

  The prepared negative electrode slurry mixture was applied to one side of a copper foil having a thickness of 14 μm (Furukawa Electric, NC-WS), followed by preliminary drying in air at 60 ° C. for 2 hours, and then at 120 ° C. for 15 Vacuum dried for hours. After vacuum drying, the electrode was pressure-formed by a roll press. This was cut into a disk shape having a diameter of 13 mm to produce a negative electrode. The thickness of the negative electrode material layer was 50 μm.

  Lithium metal was formed in a disk shape with a diameter of 12 mm and a thickness of 1 mm to produce a positive electrode. In addition, a polyolefin porous film (manufactured by Celgard, trade name: Celgard 2400) was used as a separator.

Using the negative electrode, the positive electrode, and the separator described above, a mixture of ethylene carbonate and diethyl carbonate mixed at a volume ratio of 1: 1 as an electrolyte and LiPF ₆ added at a ratio of 1 mol / dm ³ , argon was used. A 2032 type coin cell-shaped bipolar half cell was manufactured in a glove box under an atmosphere, and the evaluation described below was performed on the half cell.

(2) Charging curve of bipolar half-cell Charging condition is that the current value per unit weight of the carbon material for negative electrode at 25 ° C. is charged to 0 V at a constant current of 25 mA / g, and then the current value is 2 at 0 V holding. The charging was terminated when it attenuated to 0.5 mA / g.

  Further, based on the value of the charging capacity obtained under the above conditions, the charging capacity X [mAh / g] per unit weight of the carbon material for the negative electrode is obtained, and further, the relationship between the potential difference during charging and the charging capacity is obtained. A graph was prepared in which the vertical axis (Y axis) is the potential difference [V] between the two electrodes, and the horizontal axis (X axis) is the charge capacity [mAh / g] of the carbon material for the negative electrode per unit weight.

(3) Charging / Discharging Efficiency The half cell charged under the condition (2) was discharged at a constant current of 25 mA / g per unit weight of the carbon material for negative electrode up to a cutoff voltage of 1.5 V at 25 ° C. The charge / discharge efficiency was calculated by the following formula.
Charge / discharge efficiency (%) = [discharge capacity / charge capacity] × 100

7). Battery Performance Evaluation (1) Production of Lithium Ion Secondary Battery With respect to 100 parts of the negative electrode active material obtained in each Example and each Comparative Example, 1.5 parts by weight of carboxymethyl cellulose (manufactured by Daicel Finechem, CMC Daicel 2200), Add 1.5 parts by weight of styrene-butadiene rubber (manufactured by JSR, TRD-2001), 2 parts by weight of acetylene black (Denka Black, manufactured by Denki Kagaku Kogyo), and 100 parts by weight of distilled water, and stir with a rotating / revolving mixer. By mixing, a slurry-like negative electrode mixture was prepared.

  The prepared negative electrode slurry mixture was applied to one side of a copper foil having a thickness of 14 μm (Furukawa Electric, NC-WS), followed by preliminary drying in air at 60 ° C. for 2 hours, and then at 120 ° C. for 15 Vacuum dried for hours. After vacuum drying, the electrode was pressure-formed by a roll press. This was cut into a disk shape having a diameter of 13 mm to produce a negative electrode. The thickness of the negative electrode material layer was 50 μm.

A single-layer sheet (Pionix Co., Ltd., trade name: Pioxel C-100) using LiCoO ₂ as an active material and an aluminum foil as a current collector was formed into a disk shape having a diameter of 12 mm.

  As the separator, a polyolefin porous film (manufactured by Celgard, trade name: Celgard 2400) was used.

Argon atmosphere using the above negative electrode, positive electrode, separator, and a mixture of ethylene carbonate and diethyl carbonate mixed at a volume ratio of 3: 7 with LiPF6 added at a ratio of 1 mol / dm ³ as an electrolytic solution. In the lower glove box, a 2032 type coin cell-shaped lithium ion secondary battery was manufactured, and the evaluation described below was performed on the lithium ion secondary battery.

(2) Battery performance evaluation (A) Aging treatment At 25 ° C., after charging to 4.2 V at a constant current of 25 mA / g current value per unit weight of the negative electrode active material, the current value was 4.2 V constant voltage. The battery was charged until it decayed to 2.5 mA / g. Next, it was discharged to 2.5 V at a constant current of 25 mA / g. Thereafter, charging and discharging were performed under the same conditions, and a total of five cycles of charging and discharging were performed.

(B) Charging / discharging efficiency The charging / discharging efficiency of the 1st cycle in the said aging process was computed by the following formula.
Charge / discharge efficiency (%) = [discharge capacity / charge capacity] × 100

(C) Cycle characteristics After the aging treatment, the battery was charged to 4.2 V with a constant current of 25C and 1 C, and then discharged to 2.5 V with a constant current of 1 C. The discharge capacity retention rate after repeating 200 cycles of charging and discharging under the above conditions was calculated by the following formula.

200 cycle capacity retention rate (%) = [200th cycle discharge capacity in 1C charge / discharge / first cycle discharge capacity in 1C charge / discharge] × 100

  1C was set to a current value (mA) sufficient to discharge the discharge capacity at the fifth cycle in (A) in one hour.

(D) Rapid discharge characteristics at 25 ° C. After charging to 4.2 V at a constant current of 25 ° C. and 0.2 C, and charging until the current value decays to 0.02 C at a constant voltage of 4.2 V, a constant current of 1 C After charging to 4.2 V with a discharge capacity of 2.5 V and a constant current of 25 ° C. and 0.2 C, and charging until the current value decays to 0.02 C with a constant voltage of 4.2 V The ratio of discharge capacity [5C discharge capacity / 1C discharge capacity] when discharged to 2.5 V at a constant current of 5 C was used as an index of the 25 ° C. rapid discharge characteristics.

(E) -20 ° C. capacity maintenance rate At 25 ° C., the battery was charged to 4.2 V with a constant current of 0.2 C, charged until the current value was attenuated to 0.02 C with a constant voltage of 4.2 V, and then 25 ° C. The discharge capacity when discharged to 2.5V with a constant current of 1C and at 25 ° C, the battery is charged to 4.2V with a constant current of 0.2C, and the current value becomes 0.02C with a constant voltage of 4.2V. After charging until decay, the ratio [−20 ° C. discharge capacity / 25 ° C. discharge capacity × 100] with the discharge capacity when discharged to 2.5V at a constant current of 1 C at −20 ° C. is the −20 ° C. capacity maintenance rate It was.

[2] Production of carbon material for negative electrode (Example 1)
As a resin composition, phenol resin PR-217 (manufactured by Sumitomo Bakelite Co., Ltd.) was processed in the order of the following steps (a) to (f) to obtain a carbon material 1 for a negative electrode.

  (A) Without any of reducing gas replacement, inert gas replacement, reducing gas flow, and inert gas flow, the temperature is raised from room temperature to 500 ° C. at 100 ° C./hour.

  (B) Without any reducing gas replacement, inert gas replacement, reducing gas flow, or inert gas flow, cooling after degreasing treatment at 500 ° C. for 2 hours

(C) Fine grinding with a vibrating ball mill (d) Temperature rise from room temperature to 1200 ° C at 100 ° C / hour under inert gas (nitrogen) substitution and circulation

(E) Carbonization treatment at 1200 ° C. for 8 hours under an inert gas (nitrogen) flow (f) After natural cooling to 600 ° C. under an inert gas (nitrogen) flow, from 600 ° C. to 100 ° C. or less, 100 ° C. / Cooling in time

(Example 2)
The same resin composition as in Example 1 was used.

  Further, in the treatment of the resin composition, the negative electrode carbon material 2 was obtained in the same manner as in Example 5 except that the steps (d) and (e) of Example 1 were as follows.

(D) Under inert gas (nitrogen) substitution and flow, temperature rise from room temperature to 1100 ° C. at 100 ° C./hour (e) Carbonization treatment at 1100 ° C. for 8 hours under flow of inert gas (nitrogen)

(Example 3)
In Example 1, an aniline resin (synthesized by the following method) was used instead of the phenol resin.

  100 parts of aniline, 697 parts of 37% formaldehyde aqueous solution, and 2 parts of oxalic acid were placed in a three-necked flask equipped with a stirrer and a condenser, reacted at 100 ° C. for 3 hours, and dehydrated to obtain 110 parts of aniline resin. The obtained aniline resin had a weight average molecular weight of about 800.

  The resin composition obtained by pulverizing and mixing 100 parts of the aniline resin and 10 parts of hexamethylenetetramine obtained as described above was treated in the same process as in Example 1 to obtain a carbon material 3 for negative electrode. .

Example 4
A resin composition obtained by mixing 100 parts by weight of the aniline resin similar to Example 3 and 5 parts by weight of graphite (mesophase carbon microbeads) with a mixer is treated in the same manner as in claim 1, and the carbon for negative electrode is obtained. Material 4 was obtained.

(Comparative Example 1)
Graphite (mesophase carbon microbeads) was prepared and used as the carbon material 5 for the negative electrode.

(Comparative Example 2)
The same resin composition as in Example 1 was used.

  Further, in the treatment of the resin composition, the negative electrode carbon material 6 was obtained in the same manner as in Example 1 except that the steps (d) and (e) of Example 1 were as follows.

  (D) Temperature increase from room temperature to 600 ° C. at 100 ° C./hour under inert gas (nitrogen) substitution and circulation

(E) Carbonization treatment at 600 ° C. for 8 hours under inert gas (nitrogen) flow The obtained amorphous carbon was used as it was without mixing with graphite.

[3] Negative electrode active material (Examples 5 to 11 and Comparative Examples 3 to 5)
The carbon materials for negative electrodes obtained in Examples 1 to 4 and Comparative Examples 1 and 2 were mixed at the component ratios shown in Table 2 to obtain negative electrode active materials 1 to 10.

[4] Production of Negative Electrode and Lithium Ion Secondary Battery Using the negative electrode active materials 1 to 10, a negative electrode and a lithium ion secondary battery were produced according to the above-described [1] measurement method.

[5] Results Regarding the carbon materials for negative electrodes obtained in Examples 1 to 4, the relationship between X and Y in the region where X in the graph created by the above-described [1] measurement method is 50 mAh / g or more and 100 mAh / g or less. , D50, positron lifetime, XPS, specific surface area, carbon content, nitrogen content are shown in Table 1.
As for the relationship between X and Y, in Table 1, in the region where X in the graph is 50 mAh / g or more and 100 mAh / g or less, the relationship of −0.007X + 0.85 <Y <−0.007X + 1.2 is satisfied. “A” indicates that it is satisfied, and “B” indicates that that satisfies the relationship of −0.007X + 0.83 <Y <−0.007X + 1.25 (excluding those that satisfy the condition of “A”), − 0.007X + 0.8 <Y <−0.007X + 1.3 satisfying the relationship (excluding those satisfying the condition of “A” and those satisfying the condition of “B”) “C”, Those not satisfying the relationship of −0.007X + 0.8 <Y <−0.007X + 1.3 are indicated by “D”.

  In addition, using the negative electrode active materials obtained in Examples 5 to 11 and Comparative Examples 3 to 5, the initial charge and discharge efficiency of the lithium ion secondary battery prepared and measured according to the above-described [1] measurement method, Table 2 shows the cycle characteristics, the 25 ° C. rapid discharge characteristics, and the −20 ° C. capacity retention rate.

  The relationship between X and Y in the region where X in the graph created by the above-described [1] measurement method is 50 mAh / g or more and 100 mAh / g or less is the relationship of −0.007X + 0.8 <Y <−0.007X + 1.3. Lithium ion secondary batteries (Examples 5 and 6) produced using a negative electrode active material composed only of a satisfactory negative electrode carbon material have a 200 cycle capacity retention rate, a 25 ° C. rapid discharge characteristic, and a −20 ° C. capacity retention rate. Became extremely high. Further, the carbon material for negative electrode 3 in which the relationship between X and Y in the region where X in the graph is 50 mAh / g or more and 100 mAh / g or less satisfies the relationship of −0.007X + 0.85 <Y <−0.007X + 1.2. A lithium ion secondary battery (Example 6) produced using a negative electrode active material composed only of lithium is excellent in the balance between charge / discharge efficiency, 200 cycle capacity retention, 25 ° C. rapid discharge characteristics, and −20 ° C. capacity retention. It was.

  Furthermore, the lithium ion secondary battery (Examples 8-11) produced using the negative electrode active material comprised from the carbon material for negative electrodes of this invention and graphite, all have high charging / discharging efficiency and 200 cycle capacity. The maintenance rate, 25 ° C. rapid discharge characteristics, and −20 ° C. capacity maintenance rate were highly compatible.

  Lithium ion secondary battery manufactured using a negative electrode active material composed only of a carbon material for a negative electrode where X in the graph does not satisfy the relationship of −0.007X + 0.8 <Y <−0.007X + 1.3 (comparative example) As for 3-5), 200 cycle capacity maintenance rate, 25 degreeC rapid discharge characteristic, and -20 degreeC capacity maintenance rate all became low.

DESCRIPTION OF SYMBOLS 10 Secondary battery 12 Negative electrode material 14 Negative electrode collector 13 Negative electrode 20 Positive electrode material 22 Positive electrode collector 21 Positive electrode 16 Electrolytic solution 18 Separator

Claims (9)

A carbon material for a negative electrode used in a lithium ion secondary battery,
An electrode is formed using a composition in which a carbon material for a negative electrode, carboxymethyl cellulose, styrene-butadiene rubber, and acetylene black are mixed at a weight ratio of 100: 1.5: 1.5: 2, and lithium metal is used as a counter electrode. The unit weight of the carbon material for the negative electrode is a half cell using a mixed solvent in which ethylene carbonate and diethyl carbonate are mixed at a volume ratio of 1: 1 as an electrolytic solution and LiPF ₆ is added at a ratio of 1 mol / dm ^3. The relationship between the potential difference between both electrodes when charged at 25 ° C. with a constant current value of 25 mA / g and the charge capacity, the vertical axis (Y axis) is the potential difference [V] of both electrodes, and the horizontal axis (X axis) ) In the graph of the charge capacity [mAh / g] of the negative electrode carbon material per unit weight, X in the graph is in the region of 50 mAh / g or more and 100 mAh / g or less. The negative electrode carbon material which satisfies the relationship of the following formula (1).
−0.007X + 0.8 <Y <−0.007X + 1.3 (1)   The carbon material for a negative electrode according to claim 1, wherein the carbon material for a negative electrode is composed of a material containing amorphous carbon. Under the following conditions (A) to (E), the positron lifetime of the amorphous carbon measured by the positron annihilation method is 370 picoseconds or more and 480 picoseconds or less,
(A) Positron beam source: A positron is generated from an electron / positron pair using an electron accelerator (B) Gamma ray detector: BaF ₂ scintillator and photomultiplier tube (C) Measurement temperature and atmosphere: 25 ° C. in vacuum (D ) Count of annihilation γ-ray: 3 × 10 ⁶ or more (E) Positron beam energy: 10 keV
The carbon material for a negative electrode according to claim 2, wherein the half-value width of a peak observed in the vicinity of 285 eV measured by X-ray Photoelectron Spectroscopy (XPS method) is 0.8 eV or more and 1.8 eV or less.   In the half cell, after charging to 0 V at a constant current of 25 mA / g, charging is terminated when the current decays to 2.5 mA / g by holding 0 V, and the cutoff potential of the discharge condition is 1.5 V The carbon material for a negative electrode according to any one of claims 1 to 3, wherein the charge / discharge efficiency obtained under the conditions for the negative electrode is 75% or more.   A negative electrode active material comprising the carbon material for a negative electrode according to any one of claims 1 to 4.   The negative electrode active material according to claim 5, wherein the negative electrode active material is composed of a material containing graphite in addition to the carbon material for a negative electrode according to any one of claims 1 to 4.   The negative electrode active material according to claim 6, wherein a content of the graphite in the negative electrode active material is 50 wt% or more and 96.2 wt% or less.   A negative electrode manufactured using the negative electrode active material according to any one of claims 5 to 7.   A lithium ion secondary battery manufactured using the negative electrode active material according to claim 5.

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