JP7599196B2 - Negative electrode active material for lithium-ion secondary batteries - Google Patents

Description

The present invention relates to a negative electrode active material for lithium ion secondary batteries. More specifically, the present invention relates to a negative electrode active material for lithium ion secondary batteries, nitrogen-doped carbon particles that can be suitably used for the negative electrode active material for lithium ion secondary batteries and a method for producing the same, and a lithium ion secondary battery having a negative electrode containing the negative electrode active material for lithium ion secondary batteries.

Nitrogen-doped carbon is used as an electrochemical catalyst in reactions such as redox reactions and oxygen evolution reactions, and as a material for supercapacitors. It is generally believed that the higher the nitrogen content in nitrogen-doped carbon, the higher the redox activity of the nitrogen-doped carbon (see, for example, paragraph [0002] of Patent Document 1).

As a method for producing nitrogen-doped carbon with a high nitrogen content, a method for producing nitrogen-containing carbon has been proposed that includes the steps of using a quartz firing vessel to fire carbon in an atmosphere containing carbon dioxide gas, lowering the internal temperature of the firing vessel to 100°C or lower, and replacing the internal atmosphere of the firing vessel with an ammonia-containing gas atmosphere and firing the carbon (see, for example, claim 1 of Patent Document 1).

However, since the nitrogen content in the nitrogen-containing carbon obtained by the method for producing nitrogen-containing carbon is about 1.5% by weight (see, for example, paragraph [0013] and Figure 2 of Patent Document 1), the redox activity of the nitrogen-containing carbon is not particularly high, and an improvement in the discharge capacity is desired.

In recent years, there has been a demand for the development of nitrogen-doped carbon that has a high discharge capacity, excellent durability against charging and discharging, and excellent high-speed charging properties that allow charging in a short time.

JP 2015-67487 A

The present invention has been made in consideration of the above-mentioned conventional technology, and aims to provide nitrogen-doped carbon that has a high discharge capacity, excellent durability against charging and discharging, and excellent high-speed charging that allows charging in a short time, a method for producing the same, a negative electrode active material for lithium ion secondary batteries that contains the nitrogen-doped carbon, and a lithium ion secondary battery having a negative electrode that contains the negative electrode active material for lithium ion secondary batteries.

The present invention relates to
(1) Nitrogen-doped carbon particles having a nitrogen atom content of 15% by mass or more;
(2) A method for producing the nitrogen-doped carbon particles according to (1) above, comprising calcining polybenzimidazole at a temperature of 550 to 1500° C. in a nitrogen gas-containing atmosphere.
(3) A negative electrode active material for a lithium ion secondary battery, comprising the nitrogen-doped carbon particles according to (1) above; and (4) a lithium ion secondary battery having a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, the negative electrode comprising the negative electrode active material for a lithium ion secondary battery according to (3) above.

In the present invention, nitrogen-doped carbon refers to carbon that is doped (contains) nitrogen.

The present invention provides nitrogen-doped carbon that has a high discharge capacity, excellent durability against charging and discharging, and excellent high-speed charging ability that allows charging in a short time, a method for producing the same, a negative electrode active material for lithium-ion secondary batteries that contains the nitrogen-doped carbon, and a lithium-ion secondary battery having a negative electrode that contains the negative electrode active material for lithium-ion secondary batteries.

4(a) to (c) are powder X-ray diffraction patterns of the nitrogen-doped carbon particles obtained in Examples 1 to 3, respectively. 1 is a scanning electron microscope photograph showing the results of energy dispersive X-ray analysis of the nitrogen-doped carbon particles obtained in Example 3. 1A is a scanning electron microscope photograph showing the results of energy-dispersive X-ray analysis of the carbon contained in the nitrogen-doped carbon particles obtained in Example 3, FIG. 1B is a scanning electron microscope photograph showing the results of energy-dispersive X-ray analysis of the nitrogen contained in the nitrogen-doped carbon particles, and FIG. 1C is a scanning electron microscope photograph showing the results of energy-dispersive X-ray analysis of the oxygen contained in the nitrogen-doped carbon particles. 4(a) and (b) are transmission electron microscope photographs of the nitrogen-doped carbon particles obtained in Example 3. 1 is a graph showing the results of energy dispersive X-ray analysis of the nitrogen-doped carbon particles obtained in Example 3. 6A is a graph showing the results of X-ray photoelectron spectroscopy of the nitrogen-doped carbon particles obtained in Example 3, and FIG. 6B is a graph showing an enlarged view of the peak based on N1s in the graph showing the results of X-ray photoelectron spectroscopy of the nitrogen-doped carbon particles shown in FIG. 6A. 1A is a graph showing the relationship between the charge transfer resistance at the interface between the electrolyte and the electrode at frequencies of 1 MHz to 100 mHz in the nitrogen-doped carbon particles obtained in Example 3, and FIG. 1B is a graph showing the results of cyclic voltammetry measurements. 1A is a graph showing the measurement results of current values when cyclic voltammetry of a negative electrode half-cell using the nitrogen-doped carbon particles obtained in Example 3 was examined at voltages of 0.1 V/s, 0.3 V/s, 0.5 V/s, 0.7 V/s, or 1.0 V/s; and FIG. 1B is a graph showing the relationship between the scan rate and peak current in cyclic voltammetry of the negative electrode half-cell. Graph A shows the relationship between the number of charge/discharge cycles of a negative electrode half-cell and the discharge capacity, and graph B shows the relationship between the number of charge/discharge cycles of a negative electrode half-cell and the charge/discharge efficiency (Coulomb efficiency).

As described above, the nitrogen-doped carbon particles of the present invention have a nitrogen atom content of 15% by mass or more. The nitrogen atom content in the nitrogen-doped carbon particles is a value measured based on the method described in the following examples. The nitrogen-doped carbon particles of the present invention have a nitrogen atom content of 15% by mass or more, preferably 15 to 25% by mass, and more preferably 18 to 25% by mass, and therefore have a high discharge capacity, excellent durability against charging and discharging, and excellent high-speed charging properties that allow charging in a short time.

The nitrogen-doped carbon particles of the present invention can be produced, for example, by sintering polybenzimidazole at a temperature of 550 to 1500°C in a nitrogen gas-containing atmosphere.

[Polybenzimidazole]
Polybenzimidazoles have the formula:

A polymer having a repeating unit represented by the formula: In addition to the repeating unit, polybenzimidazole may contain repeating units other than the repeating unit, as long as the object of the present invention is not hindered. Polybenzimidazole is readily available commercially, but can be prepared as follows:

As a raw material for polybenzimidazole, for example, 3,4-diaminobenzoic acid dihydrochloride can be used. 3,4-Diaminobenzoic acid dihydrochloride can be easily prepared by reacting 3,4-diaminobenzoic acid with hydrochloric acid.

Polybenzimidazole can be prepared, for example, by heating 3,4-diaminobenzoic acid dihydrochloride in the presence of polyphosphoric acid in an inert gas atmosphere at a temperature of about 80 to 150°C until polybenzimidazole is produced. Examples of inert gases include nitrogen gas and argon gas, but the present invention is not limited to these examples. The produced polybenzimidazole may be washed, dried, and pulverized to a desired particle size, if necessary.

From the viewpoint of obtaining nitrogen-doped carbon with high discharge capacity, excellent durability against charging and discharging, and excellent high-speed charging ability that can be charged in a short time, the number average molecular weight of polybenzimidazole is preferably 2,000 to 100,000, more preferably 3,000 to 100,000 if it does not have a crosslinked structure, and is preferably 2,000 to 1,000,000, more preferably 3,000 to 1,000,000 if it has a crosslinked structure.

The number average molecular weight of polybenzimidazole is the value measured by gel permeation chromatography (GPC) under the following measurement conditions:

[Measurement conditions]
Apparatus: Showa Denko K.K., product name: Shodex-101
Concentration at time of injection: 0.01% by mass
Injection volume: 100 μL
・Flow rate: 1mL/min
Solvent: N,N-dimethylformamide Column: Showa Denko K.K., trade name: Shodex KD-803 and trade name: Shodex KD-804
Column temperature: 40°C
・Standard: Polymethyl methacrylate

[Nitrogen-doped carbon particles]
Next, the polybenzimidazole is baked to obtain nitrogen-doped carbon particles. It is preferable to pulverize the polybenzimidazole before baking in order to improve the baking efficiency of the polybenzimidazole. The particle size of the pulverized polybenzimidazole is not particularly limited, but is preferably 1 mm or less, more preferably 500 μm or less, and even more preferably 300 μm or less, from the viewpoint of improving the baking efficiency of the polybenzimidazole.

When sintering polybenzimidazole, for example, an electric furnace or the like can be used. The atmosphere when sintering polybenzimidazole is a nitrogen gas-containing atmosphere from the viewpoint of efficiently producing nitrogen-doped carbon particles. The nitrogen gas atmosphere is ideally a nitrogen gas atmosphere, but the nitrogen gas atmosphere may contain, for example, an inert gas such as argon gas, air, oxygen gas, carbon dioxide gas, etc., within a range that does not impede the object of the present invention.

There is no particular limitation on the rate of temperature rise during baking of polybenzimidazole. The rate of temperature rise during baking of polybenzimidazole may be changed stepwise or may be maintained constant. From the viewpoint of efficiently producing nitrogen-doped carbon that has excellent durability against charging and discharging and has excellent high-speed charging ability that can be charged in a short time, the baking temperature of polybenzimidazole is preferably 550 to 1500°C, more preferably 600 to 1300°C, and even more preferably 600 to 1100°C.

The baking time of polybenzimidazole cannot be determined in general because it varies depending on the baking temperature of polybenzimidazole, but is usually about 0.5 to 6 hours. After baking the polybenzimidazole, the baked polybenzimidazole is allowed to cool, preferably to a temperature of 40°C or less, more preferably to room temperature. The baking temperature of polybenzimidazole can be measured using a thermocouple.

In this manner, nitrogen-doped carbon particles can be obtained. If the obtained nitrogen-doped carbon particles are aggregated, the nitrogen-doped carbon particles may be disintegrated using a pin mill, hammer mill, or the like, if necessary.

The nitrogen-doped carbon particles of the present invention show a (002) plane peak by X-ray diffraction, but because the (002) plane peak is broad, they are amorphous carbon.

In addition, when the nitrogen-doped carbon particles are observed with a transmission electron microscope, it is found that the nitrogen-doped carbon particles contain macropores with a pore size exceeding 50 nm and mesopores with a pore size in the range of 2 to 50 nm. Therefore, since the nitrogen-doped carbon particles have macropores and mesopores, they are considered to have excellent lithium ion diffusibility.

X-ray photoelectron spectroscopy (XPS) of the nitrogen-doped carbon particles revealed that the nitrogen-doped carbon particles contained a compound represented by the formula (Ia):

A compound having graphitic nitrogen (graphite-type nitrogen) represented by formula (Ib):

Pyridine represented by formula (Ic):

Pyrrole represented by formula (Id):

It can be seen that the nitrogen-doped carbon particles contain oxygen-containing pyridine compounds represented by the formulae (Ia) to (Id). From this, it is believed that the nitrogen-doped carbon particles contain the compounds represented by the formulae (Ia) to (Id) that are generated when the polybenzimidazole used as the raw material for the nitrogen-doped carbon particles is decomposed by baking.

The spacing between the crystal planes of the nitrogen-doped carbon particles of the present invention is wider than the spacing between the crystal planes of graphite, being 0.35 to 0.37 nm, as confirmed in the following examples.

The average particle diameter of nitrogen-doped carbon particles when observed with a transmission electron microscope is usually within the range of 50 nm to 10 μm. The average particle diameter of nitrogen-doped carbon particles is calculated by randomly selecting 100 nitrogen-doped carbon particles from an image taken with a scanning electron microscope, calculating the average values on the vertical and horizontal axes of each nitrogen-doped carbon particle, summing up the average values for the 100 nitrogen-doped carbon particles, and dividing the total by 100.

[Negative electrode active material for lithium ion secondary batteries]
The negative electrode active material for lithium ion secondary batteries of the present invention contains the nitrogen-doped carbon particles. The negative electrode active material for lithium ion secondary batteries of the present invention may be composed of only the nitrogen-doped carbon particles, or may contain other negative electrode active materials within a range that does not impede the object of the present invention. Examples of other negative electrode active materials include powders of carbon materials such as graphite, graphite, and coke, but the present invention is not limited to only these examples.

[Lithium-ion secondary battery]
The lithium ion secondary battery of the present invention has the same structure as commonly used lithium secondary ion batteries.

The lithium ion secondary battery of the present invention typically has a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. Examples of the shape of the lithium ion secondary battery include a cylindrical type and a laminated type, but the present invention is not limited to these examples.

When the lithium ion secondary battery of the present invention is, for example, a CR2025 lithium ion secondary battery, the negative electrode, separator, and non-aqueous electrolyte are housed in a case. Below, an embodiment in which the lithium ion secondary battery is a CR2025 lithium ion secondary battery is described, but the present invention is not limited to only this embodiment.

The case used in the CR2025 lithium-ion secondary battery is hollow, has an opening, and doubles as a positive electrode container. A lid is provided on the opening of the case, and the lid also doubles as a negative electrode lid. A gasket is provided between the case and the lid to maintain an insulating and sealed state between the case and the lid. The space between the case and the lid contains an electrode and a non-aqueous electrolyte.

(1) Electrodes The electrodes include a positive electrode, a separator, and a negative electrode, which are arranged in this order. The positive electrode is in contact with the inner surface of the case, and the negative electrode is in contact with the inner surface of the lid.

The positive electrode may be the same as the positive electrode used in commonly used lithium secondary ion batteries, and the present invention is not limited by the composition and structure of the positive electrode.

The positive electrode can be produced, for example, by mixing the positive electrode active material, conductive material, and binder in a predetermined ratio, adding to the resulting mixture an appropriate amount of a solvent, and if necessary, activated carbon, a viscosity adjusting additive, etc., kneading the resulting mixture to prepare a positive electrode composite paste, which is then applied to the surface of a current collector, press-molded if necessary, and dried.

Representative examples of positive electrode active materials include lithium and lithium-manganese composite oxides, but the present invention is not limited to these examples. Examples of conductive materials include carbon black such as acetylene black, natural graphite, artificial graphite, and expanded graphite, but the present invention is not limited to these examples. Examples of binders include polyvinylidene fluoride (PVDF), 1-methyl-2-pyrrolidone (NMP), polytetrafluoroethylene (PTFE), fluororubber, and ethylene propylene diene rubber, but the present invention is not limited to these examples. Examples of solvents include N-methyl-2-pyrrolidone, but the present invention is not limited to these examples.

The negative electrode uses the negative electrode active material of the present invention. The negative electrode can be produced, for example, by mixing the negative electrode active material of the present invention with a binder, adding a solvent to the resulting mixture to prepare a paste-like negative electrode mixture, applying the negative electrode mixture to the surface of a metal foil collector such as copper, drying it, and, if necessary, press-molding and drying it.

The negative electrode active material used is the negative electrode active material of the present invention, but powders such as graphite and carbon black may be included as necessary. Examples of binders include polyvinylidene fluoride (PVDF), 1-methyl-2-pyrrolidone (NMP), polytetrafluoroethylene (PTFE), fluororubber, and ethylene propylene diene rubber, but the present invention is not limited to these examples. Examples of solvents include N-methyl-2-pyrrolidone, but the present invention is not limited to these examples.

The separator is used between the positive electrode and the negative electrode. The separator separates the positive electrode and the negative electrode, holds the electrolyte, and forms a path for the movement of lithium ions between the positive electrode and the negative electrode. The separator can be a foil membrane of a polyolefin resin such as polyethylene or polypropylene, and the thin film has many micropores formed in it.

(2) Nonaqueous Electrolyte Examples of the nonaqueous electrolyte include cyclic carbonates such as ethylene carbonate, diethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate; and ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxyethane; however, the present invention is not limited to these examples.

The lithium ion secondary battery of the present invention has a high discharge capacity because it uses a negative electrode containing the negative electrode active material for lithium ion secondary batteries of the present invention as the negative electrode active material, and has excellent durability against charging and discharging because the decrease in discharge capacity is small even after 1200 charge and discharge cycles, and has excellent high-speed charging properties because it can be charged in a short time.

Next, the present invention will be described in detail based on examples, but the present invention is not limited to these examples.

The physical properties measured in the following examples were measured according to the following methods.
[Powder X-ray Diffraction]
Powder X-ray diffraction was performed using a powder/thin film X-ray diffractometer (manufactured by Rigaku Corporation, product name: SmartLab) with CuKα radiation having a wavelength (λ) of 0.154 nm in the measurement range (2θ) of 10 to 80°.

[Energy Dispersive X-ray Analysis (EDS)]
The measurement was carried out using a tabletop microscope (manufactured by Hitachi High-Technologies Corporation, product name: Miniscope (registered trademark) TM3030).

[Observation by Transmission Electron Microscope (TEM)]
Observation was carried out using a transmission electron microscope (Hitachi, Ltd., product number: H-7100, accelerating voltage: 100 kV).

[X-ray photoelectron spectroscopy (XPS)]
X-ray photoelectron spectroscopy was performed using an X-ray photoelectron spectroscopy analyzer (manufactured by Fison Instruments, product name: S-PROBE™ 2803).

[Cyclic Voltammetry]
Cyclic voltammetry was measured at room temperature using a potentiostat (Biological Co., Ltd.) at a frequency of 1 MHz to 100 mHz and an output of 10 mV. The potential range during cyclic voltammetry was 10 to 2100 mV, and the scan rate was adjusted to 0.1 mV/s.

[Charge/Discharge]
The negative electrode half cell was charged and discharged using a compact charge/discharge device (manufactured by EC Frontier Co., Ltd., product number: ECAD-1000) while adjusting the potential difference with respect to Li/Li ⁺⁺ to 10 to 2100 mV.

Production Example 1 [Preparation of polybenzimidazole]
(1) Preparation of 3,4-diaminobenzoic acid dihydrochloride 3 g (19.72 mmol) of 3,4-diaminobenzoic acid was placed in a beaker at room temperature, and 20 mL of methanol was added to the beaker, and the contents of the beaker were then stirred for about 1 hour.

Next, 3 mL of 10% hydrochloric acid was added dropwise to the beaker while stirring the contents of the beaker, and the contents of the beaker were then stirred for approximately 4 hours. The contents of the beaker were filtered through a Büchner funnel to remove unreacted precipitates and recover the filtrate.

The solvent (methanol) was removed from the filtrate obtained above using a rotary evaporator to recover the solids. The solids were dried under reduced pressure for 5 to 6 hours to obtain 3,4-diaminobenzoic acid dihydrochloride.

(2) Preparation of Polybenzimidazole While introducing nitrogen gas into a 100 mL three-neck flask, 30 g of polyphosphoric acid was added to the flask under stirring, and the polyphosphoric acid was heated at 120° C. for 1 hour to remove moisture from the polyphosphoric acid.

Next, 1.32 g of the 3,4-diaminobenzoic acid dihydrochloride obtained above was added to the flask and stirred at 120°C for 1 hour, after which the temperature of the contents of the flask was raised to 200°C and the contents of the flask were left at that temperature for approximately 14 hours with stirring to obtain polybenzimidazole.

(3) Purification of polybenzimidazole The polybenzimidazole obtained above was added to distilled water in a beaker at room temperature and left for about 24 hours to dissolve the remaining unreacted 3,4-diaminobenzoic acid dihydrochloride in water, thereby purifying the polybenzimidazole. The number average molecular weight of the polybenzimidazole obtained above was 36,000.

Next, the polybenzimidazole was removed from the beaker, placed in a dryer, and dried at 180°C under reduced pressure for about 6 to 7 hours. The dried polybenzimidazole was pulverized in a pulverizer (Japan Analytical Industry Co., Ltd., product number: JF-300) at a rotation speed of 1430 rpm for 20 minutes to obtain a polybenzimidazole powder.

The polybenzimidazole powder obtained above was added to a 10% aqueous potassium hydroxide solution at room temperature and stirred for about 12 hours, after which the polybenzimidazole powder was thoroughly washed with deionized water. The polybenzimidazole powder was then filtered through a fluororesin (polytetrafluoroethylene) filter membrane with a pore size of 0.1 μm, and the resulting residue was placed in a dryer and dried at a temperature of 80° C. for about 12 hours to obtain purified polybenzimidazole.

Example 1
After the atmosphere inside a programmable tubular electric furnace (manufactured by AS ONE Corporation, product number: TMF-300N) was replaced with nitrogen gas, a silica-alumina combustion boat (manufactured by AS ONE Corporation) containing 100 mg of the polybenzimidazole obtained above was placed in the electric furnace, and the polybenzimidazole was baked while continuously introducing nitrogen gas into the electric furnace.

The polybenzimidazole was sintered by heating the polybenzimidazole from room temperature to 750°C at a heating rate of 5°C/min, and when the temperature reached 750°C, heating it to 800°C at a heating rate of 1°C/min, maintaining the temperature at 800°C for 1 hour, and then cooling it to room temperature. The yield of the sintered product obtained was 72 mg, confirming that the yield was 72% by mass.

The fired product obtained above was ground in an agate mortar and pestle, then added to 30 mL of 10% hydrochloric acid, and ultrasonic waves were applied to the resulting mixture for 10 to 15 minutes to obtain a dispersion of fired product powder. The dispersion obtained above was filtered through a fluororesin (polytetrafluoroethylene) filter membrane with a pore size of 0.1 μm, placed in a dryer, and dried at a temperature of 80°C for approximately 12 hours to obtain nitrogen-doped carbon particles.

Example 2
After the atmosphere inside a programmable tubular electric furnace (manufactured by AS ONE Corporation, product number: TMF-300N) was replaced with nitrogen gas, a silica-alumina combustion boat (manufactured by AS ONE Corporation) containing 100 mg of the polybenzimidazole obtained above was placed in the electric furnace, and the polybenzimidazole was baked while continuously introducing nitrogen gas into the electric furnace.

The polybenzimidazole was sintered by heating the polybenzimidazole from room temperature to 950°C at a heating rate of 5°C/min, and when the temperature reached 950°C, heating it to 1000°C at a heating rate of 1°C/min, maintaining the temperature at 1000°C for 1 hour, and then cooling it to room temperature. The yield of the sintered product obtained was 71 mg, confirming that the yield was 71% by mass.

The fired product obtained above was ground in an agate mortar and pestle, then added to 30 mL of 10% hydrochloric acid, and ultrasonic waves were applied to the resulting mixture for 10 to 15 minutes to obtain a dispersion of fired product powder. The dispersion obtained above was filtered through a fluororesin (polytetrafluoroethylene) filter membrane with a pore size of 0.1 μm, placed in a dryer, and dried at a temperature of 80°C for approximately 12 hours to obtain nitrogen-doped carbon particles.

Example 3
After the atmosphere inside a programmable tubular electric furnace (manufactured by AS ONE Corporation, product number: TMF-300N) was replaced with nitrogen gas, a silica-alumina combustion boat (manufactured by AS ONE Corporation) containing 100 mg of the polybenzimidazole obtained above was placed in the electric furnace, and the polybenzimidazole was baked while continuously introducing nitrogen gas into the electric furnace.

The polybenzimidazole was sintered by heating the polybenzimidazole from room temperature to 550°C at a heating rate of 5°C/min, and when the temperature reached 550°C, heating it to 600°C at a heating rate of 1°C/min, maintaining the temperature at 600°C for 1 hour, and then cooling it to room temperature. The yield of the sintered product obtained was 73 mg, confirming that the yield was 73% by mass.

The fired product obtained above was ground in an agate mortar and pestle, then added to 30 mL of 10% hydrochloric acid, and ultrasonic waves were applied to the resulting mixture for 10 to 15 minutes to obtain a dispersion of fired product powder. The dispersion obtained above was filtered through a fluororesin (polytetrafluoroethylene) filter membrane with a pore size of 0.1 μm, placed in a dryer, and dried at a temperature of 80°C for approximately 12 hours to obtain nitrogen-doped carbon particles.

Next, the powder X-ray diffraction patterns of the nitrogen-doped carbon particles obtained in Examples 1 to 3 are shown in Figure 1. From the results shown in Figure 1, the Miller indices of the nitrogen-doped carbon particles are (002), and the broad peak indicates that amorphous carbon is produced. In addition, the Bragg equation:
nλ=2d sin θ
(In the formula, n is a positive integer, λ is the wavelength of the X-ray, d is the spacing between the crystal planes, and θ is the angle between the crystal planes and the X-ray.)
The spacing between the crystal planes of the nitrogen-doped carbon particles calculated based on the above was 0.37 nm in Example 1, 0.36 nm in Example 2, and 0.35 nm in Example 3, all of which are larger than the spacing between the crystal planes of the graphite powder (0.33 nm).

Next, the nitrogen-doped carbon particles obtained in Example 3 were used to examine the following physical properties.
[Energy Dispersive X-ray Analysis (EDS)]
The results of energy dispersive X-ray analysis of the nitrogen-doped carbon particles are shown in Fig. 2. Fig. 2 is a graph showing the results of energy dispersive X-ray analysis of the nitrogen-doped carbon particles obtained in Example 3.

Energy dispersive X-ray analysis was also performed on the carbon, nitrogen and oxygen contained in the nitrogen-doped carbon particles. The results are shown in Figure 3. In Figure 3, (a) is a graph showing the results of energy dispersive X-ray analysis of the carbon contained in the nitrogen-doped carbon particles, (b) is a graph showing the results of energy dispersive X-ray analysis of the nitrogen contained in the nitrogen-doped carbon particles, and (c) is a graph showing the results of energy dispersive X-ray analysis of the oxygen contained in the nitrogen-doped carbon particles.

The results shown in Figures 2 and 3 show that carbon, nitrogen and oxygen are uniformly dispersed in the nitrogen-doped carbon particles.

In addition, the results shown in Figure 2 confirmed that the particle diameter of the nitrogen-doped carbon particles was in the range of 50 nm to 10 μm.

[Observation by Transmission Electron Microscope (TEM)]
A transmission electron microscope photograph of the nitrogen-doped carbon particles is shown in Figure 4. From the transmission electron microscope photograph of the nitrogen-doped carbon particles shown in Figure 4(a), it can be seen that the nitrogen-doped carbon particles have macropores with a pore size exceeding 50 nm and mesopores with a pore size in the range of 2 to 50 nm. It is believed that the macropores and mesopores of the nitrogen-doped carbon particles contribute to the diffusion and storage of lithium ions.

The transmission electron microscope photograph of the nitrogen-doped carbon particles shown in Figure 4(b) shows that the nitrogen-doped carbon particles have a layered structure, a structure in which the carbon is interconnected, and are porous.

[Energy Dispersive X-ray Analysis (EDS)]
A graph showing the results of energy dispersive X-ray analysis of the nitrogen-doped carbon particles is shown in Fig. 5. The contents of elements contained in the nitrogen-doped carbon particles obtained from the results of energy dispersive X-ray analysis of the nitrogen-doped carbon particles are shown in Table 1.

As shown in Table 1, the nitrogen content in the nitrogen-doped carbon particles is 19.51% by mass, which is significantly higher than the nitrogen content of the nitrogen-containing carbon described in Patent Document 1, which is about 1.5% by weight (see Figure 2 in Reference 1), and is therefore recognized to have excellent redox activity. In addition, when the nitrogen contents of the nitrogen-doped carbon particles obtained in Examples 1 and 2 were also examined, they were found to be 19.32% by mass and 20.25% by mass, respectively, indicating that these nitrogen-doped carbon particles have excellent redox activity, similar to the nitrogen-doped carbon particles obtained in Example 3.

[X-ray photoelectron spectroscopy (XPS)]
The results of the X-ray photoelectron spectroscopy of the nitrogen-doped carbon particles are shown in Fig. 6. In Fig. 6, (a) is a graph showing the results of the X-ray photoelectron spectroscopy of the nitrogen-doped carbon particles, and (b) is a graph showing the results of the X-ray photoelectron spectroscopy of the nitrogen-doped carbon particles. 6(b) is a graph showing the results of X-ray photoelectron spectroscopy of the nitrogen-doped carbon particles shown in FIG. 6(a), in which the peak due to N1s is enlarged. In FIG. 6(b), A is represented by formula (Ia) A is a graph of a compound having graphitic nitrogen (graphite-type nitrogen), B is a graph of pyridine represented by formula (Ib), C is a graph of pyrrole represented by formula (Ic), D is a graph of oxygen represented by formula (Id), Graph of pyridine atom-containing compounds.

From the results shown in Figure 6 (b), it was confirmed that the nitrogen-doped carbon particles contained 10.3 mass% of a compound having graphitic nitrogen (graphite-type nitrogen) represented by formula (Ia), 5.9 mass% of pyridine represented by formula (Ib), 0.6 mass% of pyrrole represented by formula (Ic), and 0.7 mass% of an oxygen-atom-containing pyridine compound represented by formula (Id). From this, it is considered that the polybenzimidazole used as the raw material for the nitrogen-doped carbon particles was decomposed by baking, and the compounds represented by formulas (Ia) to (Id) were generated.

[Cyclic Voltammetry]
80 g of nitrogen-doped carbon particles, 10 g of acetylene black, 10 g of polyvinylidene fluoride as a binder, and 400 g of N-methyl-2-pyrrolidone were mixed to a uniform composition, and the resulting slurry was applied to a copper foil (length: 20 mm, width: 100 mm, thickness: 20 μm) with a doctor blade, dried at a temperature of 80° C. under reduced pressure for about 12 hours, and pressed with a roll to produce an electrode with a coating thickness of 0.06 mm after drying.

In order to incorporate the electrode obtained above into a CR2025 lithium ion secondary battery, a disk having a diameter of 15 mm was punched out, and the disk obtained above was used as the negative electrode, lithium foil was used as the counter electrode, a polypropylene separator (manufactured by Asahi Kasei Corporation, product name: Celgard 2500, thickness: 25 μm) was used as the separator, and an electrolyte obtained by dissolving LiPF ₆ in a solvent mixed with ethylene carbonate and diethylene carbonate in a mass ratio of 1:1 to a concentration of 1 M was used to assemble a negative electrode half cell in an argon gas atmosphere. The obtained negative electrode half cell was left at room temperature for 12 hours for stabilization.

The cyclic voltammetry of the negative electrode half-cell obtained above was examined. The results are shown in Figures 7 and 8.

Figure 7(a) is a graph showing the relationship of charge transfer resistance at the interface between the electrolyte and the electrode at frequencies from 1 MHz to 100 mHz. In Figure 7(a), A is a graph showing the relationship of charge transfer resistance at the interface between the electrolyte and the electrode before cyclic voltammetry is performed, and B is a graph showing the relationship of charge transfer resistance at the interface between the electrolyte and the electrode after cyclic voltammetry is performed.

The results shown in Figure 7(a) show that when cyclic voltammetry is performed, the charge transfer resistance at the interface between the electrolyte and the electrode decreases, indicating that a favorable interface is formed between the electrolyte and the electrode.

Figure 7(b) is a graph showing the measurement results of cyclic voltammetry. In Figure 7(b), the symbols 1 to 5 are graphs showing the measurement results of cyclic voltammetry for cycles 1 to 5, respectively.

The results shown in Figure 7(b) show that the current value increases significantly toward the negative side when the potential is between 0 and 0.1 V, indicating that the electrode is undergoing lithiation, and that the current value becomes positive as the potential increases, indicating that delithiation is progressing. This shows that the nitrogen-doped carbon particles used in the negative electrode half cell have excellent charge/discharge characteristics.

Figure 8 (a) is a graph showing the current measurement results when cyclic voltammetry of the negative electrode half cell obtained above was examined at voltages of 0.1 V/s, 0.3 V/s, 0.5 V/s, 0.7 V/s, and 1.0 V/s.

In Figure 8(a), A to E are graphs showing the measurement results of the current value when voltages of 0.1 V/s, 0.3 V/s, 0.5 V/s, 0.7 V/s, and 1.0 V/s were applied, respectively. Figure 8(b) is a graph showing the relationship between the scan rate and peak current in the cyclic voltammetry of the negative electrode half cell.

The peak current is calculated by the formula:
Ip=2.69×10 ⁵ ×n ^3/2 ×A×D _Li ^1/2 ×C _Li ×V ^1/2
(In the formula, Ip is the peak current (A), n is the lithium ion equivalent/mol, A is the electrode area (cm ² ), D _Li is the diffusion coefficient (cm ² /s), C _Li is the lithium ion concentration (mol/cm ³ ), and V is the scan rate (V/s).)
It is calculated by the Randles-Sevcik formula shown below.

The lithium ion diffusion coefficient was examined based on the Randles-Sevcik equation and found to be 1.11×10 ⁻⁸ cm ² /s, indicating that the negative electrode half cell obtained above has excellent lithium ion diffusibility (mobility).

[Charge and discharge of negative electrode half-cell]
The negative electrode half cell obtained above was repeatedly charged and discharged. The results are shown in Fig. 9. In Fig. 9, A is a graph showing the relationship between the number of charge/discharge cycles of the negative electrode half cell and the discharge capacity, and B is a graph showing the relationship between the number of charge/discharge cycles of the negative electrode half cell and the charge/discharge efficiency (Coulomb efficiency).

As shown in the graph of Figure 9, which shows the relationship between the number of charge/discharge cycles and the discharge capacity of the negative electrode half cell, the discharge capacity of the negative electrode half cell obtained above is about 180 mA·h/g or more even after 1,300 cycles. This shows that the durability against charge/discharge (cycle characteristics) is remarkably superior, at least about twice as much, compared to the durability against charge/discharge (cycle characteristics) of a conventional negative electrode that uses carbon as the electrolyte, which is at most 500 cycles.

In addition, the discharge capacity of the negative electrode half cell obtained above was constant at about 100% coulomb efficiency even after 1,300 charge/discharge cycles, indicating that the quality is stable and the battery has excellent durability.

[Fast charging]
The negative electrode half cell obtained above was used to examine the time required for charging by changing the charging current. The results are shown in Table 2.

The results shown in Table 2 show that when the charging current is about 2.86 mA/g, the charging capacity of the negative electrode half-cell obtained above is maintained at 115 mA·h/g or more even after 1000 charge/discharge cycles, and the negative electrode half-cell can be charged in a short time of about 10 minutes.

The lithium-ion secondary battery of the present invention therefore has a high discharge capacity, excellent durability against charging and discharging, and excellent fast charging capability, and is therefore believed to contribute to the rapid charging of EV vehicles in places such as highway service areas in a short period of time.

The nitrogen-doped carbon of the present invention, the negative electrode active material for a lithium ion secondary battery containing the nitrogen-doped carbon, and the lithium ion secondary battery having a negative electrode containing the negative electrode active material for a lithium ion secondary battery have a high discharge capacity, excellent durability against charging and discharging, and excellent high-speed charging ability that allows charging in a short time, and are therefore expected to be used in, for example, smartphones, tablet personal computers, hybrid automobiles, electric automobiles, and the like.

Claims (4)

A negative electrode active material for a lithium ion secondary battery comprising nitrogen-doped carbon particles having a nitrogen atom content of 15% by mass or more, the nitrogen-doped carbon particles being made of a compound represented by the formula:

and a number average molecular weight of 2,000 to 100,000 is used, and the polybenzimidazole is calcined to contain nitrogen-doped carbon particles composed of carbon atoms, nitrogen atoms, and oxygen atoms . The nitrogen doped carbon particles are coated with a compound of formula (Ia):

A compound represented by formula (Ib):

pyridine represented by formula (Ic):

and pyrrole represented by formula (Id):

2. The negative electrode active material for a lithium ion secondary battery according to claim 1, comprising an oxygen atom-containing pyridine compound represented by the formula: 3. The negative electrode active material for a lithium ion secondary battery according to claim 1, wherein the spacing between crystal planes of the nitrogen-doped carbon particles is 0.35 to 0.37 nm. A lithium ion secondary battery having a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein the negative electrode comprises the negative electrode active material for lithium ion secondary batteries according to claim 3 .

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