Anode carbon material of lithium-ion battery

Abstract

To reduce reaction resistance in a Li+ insertion / extraction reaction regarding the anode carbon material of a lithium-ion battery.SOLUTION: The anode carbon material of a lithium-ion battery comprises a structure in which nitrogen atoms are doped to a carbon material. With the anode carbon material, the bind form of carbon and nitrogen atoms that the carbon material contains is such that the ratio of (A) pyridine type of nitrogen atoms to the total number of (A) pyridine type, (B) amine type, (C) pyrrole type, and (D) quaternary type of nitrogen atoms is 17% to 78% inclusive. Moreover, the ratio of sum of (C) pyrrole type and (D) quaternary type of nitrogen atoms to the total number is 5% to 60% inclusive.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a negative electrode carbon material for a lithium ion battery.

Background Art

[0002] In recent years, lithium ion batteries have been required to have further high performance such as rapid charge and discharge. In order to meet such requirements, new negative electrode carbon materials for lithium ion batteries have been proposed. For example, Patent Documents 1 and 2 disclose nitrogen-doped carbon materials doped with nitrogen atoms.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] For improving the performance of a lithium ion battery, a negative electrode carbon material for a lithium ion battery with reduced reaction resistance in the Li + insertion / desorption reaction is required.

Means for Solving the Problems

[0005] Each aspect of the negative electrode carbon material for a lithium ion battery that solves the above problems will be described. Aspect 1 has a structure in which nitrogen atoms are doped into a carbon material, and the bonding form between the carbon atoms and the nitrogen atoms contained in the carbon material is such that the ratio of the (A) pyridine-type nitrogen atoms to the total number of (A) pyridine-type, (B) amine-type, (C) pyrrole-type, and (D) quaternary-type nitrogen atoms is 17% or more and 78% or less, and the total ratio of the (C) pyrrole-type nitrogen atoms and the (D) quaternary-type nitrogen atoms to the total number is 5% or more and 60% or less.

[0006] In the above configuration, the value of the Boltzmann factor is larger compared to the case where at least one of the ratio of the (A) pyridine-type nitrogen atoms to the total number and the total ratio of the (C) pyrrole-type nitrogen atoms and the (D) quaternary-type nitrogen atoms to the total number is outside the above range. As a result, in the lithium-ion battery adopting the above configuration, Li + The reaction resistance in the insertion and desorption reaction becomes smaller.

Advantages of the Invention

[0007] According to the present invention, Li + The reaction resistance in the insertion and desorption reaction can be reduced.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, an embodiment of the negative electrode carbon material of a lithium ion battery will be described with reference to FIGS. 1 to 5. The negative electrode carbon material of the lithium ion battery of this embodiment has a structure in which nitrogen atoms are doped into the carbon material. Hereinafter, the negative electrode carbon material of the lithium ion battery may also be simply referred to as the negative electrode carbon material.

[0010] <Skeleton of Negative Electrode Carbon Material> The carbon material that is the skeleton of the negative electrode carbon material is not particularly limited. For example, carbon black, graphite, graphene nanoplatelet, graphene, carbon fiber, carbon nanotube, carbon nanofiber, carbon nanohorn, carbon nanobrush, activated carbon, porous carbon, nanoporous carbon, etc. can be mentioned.

[0011] Examples of carbon black include acetylene black, ketjen black, furnace black, medium thermal carbon black, graphitized carbon black, etc.

[0012] <Nitrogen Doping> 〔Examples of Nitrogen Doping Types〕 The compound represented by the following formula (IX) is an example of a nitrogen-doped carbon material having a structure in which nitrogen atoms are doped into a carbon material. With reference to the compound represented by formula (IX), the bonding form between carbon atoms and nitrogen atoms will be described. The bonding form between the carbon atoms and nitrogen atoms contained in the carbon material is sometimes referred to as a nitrogen-doped type.

[0013]

Chemical formula

[0014] The bonding form between the carbon atom and the nitrogen atom shown as (B) in formula (IX) is referred to as (B) amine type. In formula (IX), the bonding form between the carbon atom and the nitrogen atom shown as (C) is referred to as (C) pyrrole type.

[0015] In formula (IX), the bonding form between the carbon atom and the nitrogen atom shown as (D) is referred to as (D) quaternary type. 〔Ratio of nitrogen-doped type in the negative electrode carbon material〕 The negative electrode carbon material contains a nitrogen-doped type so as to satisfy the following aspects. The position where nitrogen atoms are doped in the negative electrode carbon material is not particularly limited.

[0016] The negative electrode carbon material has (A) pyridine type. Further, the negative electrode carbon material has either one of (C) pyrrole type and (D) quaternary type, or both (C) pyrrole type and (D) quaternary type. It is preferable that the negative electrode carbon material has at least (D) quaternary type among (C) pyrrole type and (D) quaternary type. The negative electrode carbon material may have (B) amine type. The ratio of each nitrogen-doped type in the negative electrode carbon material is as follows.

[0017] In the negative electrode carbon material, the bonding form between the carbon atoms and nitrogen atoms contained in the carbon material is such that the ratio of the (A) pyridine-type nitrogen atoms to the total number of (A) pyridine-type, (B) amine-type, (C) pyrrole-type, and (D) quaternary-type nitrogen atoms is 17% or more and 78% or less.

[0018] The ratio of the (A) pyridine-type nitrogen atoms to the total number is preferably 17% or more and 60% or less, more preferably 20% or more and 60% or less, and still more preferably 33% or more and 60% or less.

[0019] In the negative electrode carbon material, the total ratio of the (C) pyrrole-type nitrogen atoms and (D) quaternary-type nitrogen atoms to the total number is 5% or more and 60% or less. The total ratio of the (C) pyrrole-type nitrogen atoms and (D) quaternary-type nitrogen atoms to the total number is preferably 20% or more and 60% or less, more preferably 30% or more and 60% or less, and still more preferably 40% or more and 60% or less.

[0020] Hereinafter, the ratio of the (A) pyridine-type nitrogen atoms to the total number may also be referred to as the (A) type ratio. The total ratio of the (C) pyrrole-type nitrogen atoms and (D) quaternary-type nitrogen atoms to the total number may also be referred to as the (C)+(D) type ratio.

[0021] [Method for Measuring the Ratio of Nitrogen Doping Types] The ratio of each nitrogen doping type in the negative electrode carbon material can be measured by a known method. For example, the ratio of each nitrogen doping type in the negative electrode carbon material can be measured by X-ray photoelectron spectroscopy (XPS). More specifically, based on the peak area corresponding to each nitrogen doping type obtained by X-ray photoelectron spectroscopy (XPS), the ratio of each nitrogen doping type can be calculated.

[0022] [Ratio of the Content of Nitrogen Atoms to the Content of Carbon Atoms] The ratio (molar ratio) of the nitrogen atom content to the carbon atom content on the surface of the negative electrode carbon material is not particularly limited. Hereinafter, this ratio may also be referred to as the N / C ratio.

[0023] The N / C ratio on the surface of the negative electrode carbon material is, for example, 3% or more and 15% or less, preferably 4.7% or more and 12.1% or less. The N / C ratio on the surface of the negative electrode carbon material can be calculated, for example, using X-ray photoelectron spectroscopy (XPS).

[0024] <Manufacturing method of negative electrode carbon material> The negative electrode carbon material can be manufactured by a known method. The negative electrode carbon material can be manufactured using a carbon material as a base material and a nitrogen source as raw materials. The negative electrode carbon material is manufactured so as to satisfy the above-described nitrogen doping ratio. By changing the type of nitrogen source and the blending ratio of the nitrogen source, the nitrogen doping type and the nitrogen doping amount can be arbitrarily adjusted.

[0025] An example of the manufacturing method of the negative electrode carbon material will be described. First, the carbon material as a base material and the nitrogen source are kneaded. Examples of the carbon material include acetylene black, ketjen black, expanded graphite, carbon nanotube (CNT), polyacetylene, and the like.

[0026] Examples of the nitrogen source include nitrogen-containing cyclic compounds such as polyacrylonitrile, N-methylpyrrolidone, pyridine, melamine, and pyrrole; compounds having an amino group such as amino acid, urea, and aniline; compounds having an amide bond such as formamide, acetamide, and acetanilide; and compounds having a cyano group such as acetonitrile.

[0027] The kneading may be performed by known means such as a high-pressure homogenizer, a rotary homogenizer, an ultrasonic homogenizer bead mill, a ball mill, a three-roll mill, a planetary mixer, a disperser mixer, a Henschel mixer, and a kneader.

[0028] The amount of the nitrogen source may be, for example, 8 parts by mass or more and 12 parts by mass or less with respect to 100 parts by mass of the carbon material. Next, the kneaded carbon material and the nitrogen source are heated in an inert atmosphere.

[0029] The heating temperature is not particularly limited, but it is preferably a predetermined temperature higher than the thermal decomposition temperature of the nitrogen source. The predetermined temperature is, for example, 50°C or more and 100°C or less. When the nitrogen source is polyacrylonitrile, the heating temperature is, for example, 750°C or more and 850°C or less.

[0030] The heating time is not particularly limited, but it is preferably sufficient for the nitrogen source to thermally decompose. When the nitrogen source is polyacrylonitrile, the heating time is, for example, 1 hour or more and 2 hours or less. The time sufficient for thermal decomposition may be estimated by kinetic analysis of the thermal decomposition reaction using TG-MS.

[0031] The nitrogen source liquefied or vaporized at high temperature comes into contact with the highly reactive sites of the carbon material. As a result, the carbon atoms at the highly reactive sites of the carbon material are replaced by nitrogen elements, or nitrogen elements are added onto the carbon atoms. Thus, the negative electrode carbon material is obtained. Also, the remaining nitrogen source is removed from the kneaded product by being thermally decomposed.

[0032] <Performance Evaluation of Negative Electrode Carbon Material> An example of a method for evaluating the performance of the negative electrode carbon material will be described. Li in a lithium-ion battery + The reaction resistance in the insertion / desorption reaction of lithium ions is proportional to the reciprocal of the Boltzmann factor. That is, when the value of the Boltzmann factor increases, the reaction tends to be faster. On the other hand, when the value of the Boltzmann factor decreases, the reaction tends to be slower. Therefore, the reaction resistance can be evaluated based on the value of the Boltzmann factor in the negative electrode carbon material.

[0033] An example of a method for evaluating the performance of a negative electrode carbon material is based on electronic state calculations using density functional theory. An example of a method for evaluating the performance of a negative electrode carbon material includes a step of obtaining a correlation between an activation energy and a Mulliken charge using a bonding form model, and a step of calculating a Boltzmann factor of the negative electrode carbon material using the correlation.

[0034] The steps involving various calculations shown below can be performed using software such as Gaussian (manufactured by Gaussian, Inc.), GAMESS, and Spartan (manufactured by Wavefunction, Inc.). It is preferable to use software with good calculation accuracy. For example, as the functional / basis function, conditions using B3LYP / 6-31G(d,p) or B3LYP / 6-31G(d) are preferable. It is preferable to incorporate the solvent effect using the polarized dielectric model method (PCM method) for the calculation. At that time, an arbitrary value can be used for the dielectric constant of the solvent. For example, the dielectric constant of ethylene carbonate (EC) can be set to 90.

[0035] Note that the steps involving various calculations are not limited to those described above. For example, for reducing the calculation load, B3LYP / 3-21G can be used as the functional / basis function, and the polarized dielectric model method (PCM method) may not be used.

[0036] 〔Correlation between activation energy and Mulliken charge in the bonding form model〕 FIG. 1 shows the flow of the step of obtaining the correlation between the activation energy and the Mulliken charge using the bonding form model.

[0037] First, in step S101, a solvation model M + in which solvent molecules are coordinated to Li S is created. As the solvent, for example, carbonate solvents such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) can be used. Next, the step of step S102 is performed.

[0038] In step S102, the solvation model M S is subjected to structure optimization. Next, the process of step S103 is performed. In step S103, the total energy E S of the structure-optimized solvation model M S is calculated. Next, the process of step S104 is performed.

[0039] In step S104, a binding form model M C is created. The binding form model M C may be a minimum unit including a nitrogen-doped site or may be one with an extended conjugated system by a carbon skeleton. The following shows, as an example of the binding form model M C , a pyridine-type model M C (A), an amine-type model M C (B), a pyrrole-type model M C (C), a quaternary-type model M C (D) and a non-nitrogen-doped model M C (0).

[0040]

Chemical formula

[0041] In step S105, the binding form model M C is subjected to structure optimization. Next, the process of step S106 is performed. In step S106, the total energy E C of the structure-optimized binding form model M C is calculated. Next, the process of step S107 is performed.

[0042] In step S107, the solvation model M S and the binding form model M C are arranged. More specifically, Li S in the solvation model M + and the binding form model M CArrange them so that the nitrogen atom in [the relevant context] faces the other. The arranged solvation model M S and the binding form model M C are structurally optimized. Then, separate the distance between Li + and the nitrogen atom by the maximum distance Dmax. The distance between Li + and the nitrogen atom is also referred to as distance D.

[0043] Figure 2(a) shows the solvation model M S and the binding form model M C arranged with a separation of the maximum distance Dmax. The maximum distance Dmax can be set, for example, as the distance at which the solvation model M S and the binding form model M C do not interact. An example of the maximum distance Dmax is 15 Å. Regarding the non-nitrogen-doped type model M C in the binding form model M C (0), arrange the Li S in the solvation model M + and the carbon atom in the non-nitrogen-doped type model M C (0) with a separation of the maximum distance Dmax. When the solvation model M S and the binding form model M C are arranged with a separation of the maximum distance Dmax, the process of step S108 is then performed.

[0044] In step S108, the total energy E S of the solvation model M C and the binding form model M SC (1) arranged in the above step S107 is calculated. Also, set the variable i to "1". Next, the process of step S109 is performed.

[0045] In step S109, increase the variable i by "1". Then, the process of step S110 is performed. In step S110, the Li S in the solvation model M + is [related to the following operation in the binding form model M CMove it by a moving distance d toward the nitrogen atom. The moving distance d can be set, for example, as a predetermined ratio to the maximum distance Dmax. An example of the moving distance d is 1 Å. Next, perform the step of step S111.

[0046] In step S111, the total energy E S of the solvation model M C and the binding form model M SC (i) is calculated. When the value of the total energy E SC (i) is memorized, then proceed to the step of step S112.

[0047] In step S112, it is determined whether the distance D between Li + and the nitrogen atom in the binding form model M C is smaller than the threshold value Dth. An example of the threshold value Dth is 2 Å. Li + and the distance D between Li C and the nitrogen atom in the binding form model M + becomes smaller every time the step of step S110 is performed. In other words, every time the step of step S110 is performed, Li + approaches the nitrogen atom. As a result, the positional relationship between Li C and the binding form model M + gradually changes from the state shown in Fig. 2(a) to the state shown in Fig. 2(b).

[0048] In step S112, when the distance D between Li C and the nitrogen atom in the binding form model M + is smaller than the threshold value Dth (S112: YES), proceed to the step of step S113. On the other hand, when the distance D between Li C and the nitrogen atom in the binding form model M + is greater than or equal to the threshold value Dth (S112: NO), proceed to the step of step S109. That is, until the distance D between Li C and the nitrogen atom in the binding form model M becomes smaller than the threshold value Dth, repeat the steps of step S109 to step S112.

[0049] In step S113, the total energy E SC (1), E SC (2), …E SC (i) among them, the maximum value is obtained as the maximum value E max Next, the process of step S114 is performed. In step S114, the activation energy E a is calculated.

[0050] Figure 3 shows an example of the relationship between the integrated value Σd of the moving distance d and the total energy E SC by repeating the processes of step S109 to step S112. As shown in Figure 3, the activation energy E S is calculated as the value obtained by subtracting the sum of the total energy E C and the total energy E max from the maximum value E a . Note that the number of plots shown in Figure 3 is an example and is not limited thereto. After calculating the activation energy E a , next, the process of step S115 is performed.

[0051] In step S115, the Mulliken charge q of the bonding form model M C is calculated. Specifically, for the above pyridine-type model M C (A), amine-type model M C (B), pyrrole-type model M C (C), quaternary-type model M C (D) or non-nitrogen-doped model M C (0), the Mulliken charge q of the part enclosed by the dashed line is calculated. Next, the process of step S116 is performed.

[0052] In step S116, the activation energy E a calculated in step S114 and the Mulliken charge q calculated in step S115 are associated. Thereby, the activation energy E C in the bonding form model M aObtain the correlation with the Mulliken charge q. Then, end this step.

[0053] The activation energy E described above a Obtain the correlation with the Mulliken charge q for each bond form model M C That is, for the pyridine-type model M C (A), the amine-type model M C (B), the pyrrole-type model M C (C), the quaternary-type model M C (D) and the non-nitrogen-doped model M C (0), perform the steps of S101 to S116.

[0054] 〔An example of the correlation〕 FIG. 4 is an example of a graph plotting the correlation between the activation energy E C calculated according to the flow described with reference to FIG. 1 and the Mulliken charge q for each bond form model M a FIG. 4 shows the regression line α for the pyridine-type model M C (A), the amine-type model M C (B), the pyrrole-type model M C (C) and the quaternary-type model M C (D).

[0055] Referring to the regression line α, for the bond form model M C containing a nitrogen atom, the relationship between the Mulliken charge q and the activation energy E a can be understood. Specifically, in the bond form model M C containing a nitrogen atom, it can be seen that the larger the Mulliken charge q, the larger the activation energy E a .

[0056] As shown in FIG. 4, the activation energy E C of the non-nitrogen-doped model M a (0) is the activation energy E indicated by the regression line α aIt can be seen that the tendency is lower compared to. The non-nitrogen-doped model M is substituted into the equation representing the regression line α C with the value "0" which is the Mulliken charge q of M(0), and the activation energy E of the non-nitrogen-doped model M C (0) is subtracted from the value, and the resulting value is defined as the energy difference ΔE a a .

[0057] [Calculation of the Boltzmann factor in the negative electrode carbon material] FIG. 5 shows the flow of the process for calculating the Boltzmann factor of the negative electrode carbon material. In this process, instead of directly outputting and handling the energy values themselves, since the molecules are also large, for reducing the computational load, B3LYP / 3-21G may be used as the functional / basis function.

[0058] First, in step S201, a negative electrode carbon material model is created. Next, the process of step S202 is performed. In step S202, the structure of the negative electrode carbon material model is optimized. Next, the process of step S203 is performed.

[0059] In step S203, for the structure-optimized negative electrode carbon material model, the Mulliken charges q(1), q(2),... q(j) of the nitrogen-doped part or non-nitrogen-doped part on the surface side are calculated. Here, the variable j corresponds to the total number of the nitrogen-doped part and non-nitrogen-doped part on the surface side. Next, the process of step S204 is performed.

[0060] In step S204, based on the relationship between the Mulliken charge q and the activation energy E as illustrated in FIG. 4 a the Mulliken charges q(1), q(2),... q(j) are converted into the activation energies E a (1), E a (2),... E a (j).

[0061] A specific example of the conversion method will be described. ​Regarding the nitrogen-doped part in the negative electrode carbon material model, the activation energy E can be calculated based on the Mulliken charge q of the nitrogen-doped part and the regression straight line α. a can be calculated.

[0062] Regarding the non-nitrogen-doped part in the negative electrode carbon material model, the activation energy E is calculated as the value obtained by subtracting the energy difference ΔE from the value calculated based on the Mulliken charge q and the regression straight line α of the non-nitrogen-doped part. a Subtracting the value, a can be calculated.

[0063] In step S204, when the Mulliken charges q(1), q(2),... q(j) are converted into the activation energies E a (1), E a (2),... E a (j), the process of step S205 is then performed.

[0064] In step S205, based on the formula (F) in the figure, the Boltzmann factor integrated value ΣF of the negative electrode carbon material model B is calculated. The Boltzmann factor F B (k) in the nitrogen-doped part or the non-nitrogen-doped part on the surface side of the negative electrode carbon material model can be calculated based on the relational expression between the Boltzmann factor F B (k) and the activation energy E a (k) (k = 1 to j). In the formula (F), R is the gas constant and T is the absolute temperature.

[0065] As shown in the formula (F), the Boltzmann factor integrated value ΣF of the negative electrode carbon material model B can be calculated as the integrated value of the Boltzmann factor F B (k) in the nitrogen-doped part or the non-nitrogen-doped part on the surface side of the negative electrode carbon material model.

[0066] By treating the Boltzmann factor integrated value ΣF B as the Boltzmann factor of the negative electrode carbon material, the Boltzmann factor integrated value ΣF BBased on this, the performance of the negative electrode carbon material can be evaluated. For example, based on the relative magnitude of the Boltzmann factor integrated value ΣF B the magnitude of the reaction resistance in the Li + insertion / desorption reaction can be evaluated.

[0067] <Actions and Effects of this Embodiment> The actions and effects of this embodiment will be described. (1) Regarding the nitrogen-doped type contained in the negative electrode carbon material of this embodiment, for the (A) type ratio, it is 17% or more and 78% or less. Further, the (C)+(D) type ratio is 5% or more and 60% or less.

[0068] When the (A) type ratio or the (C)+(D) type ratio of the negative electrode carbon material of this embodiment is outside the above-mentioned predetermined numerical range, and when both the (A) type ratio and the (C)+(D) type ratio are outside the above-mentioned predetermined numerical range, the value of the Boltzmann factor is large. As a result, in the lithium-ion battery employing the negative electrode carbon material of this embodiment, the reaction resistance in the Li + insertion / desorption reaction becomes small.

[0069] (2) On the negative electrode of the lithium-ion battery, a side reaction occurs in which a film is formed by the reduction of components of the electrolyte solution such as a solvent and an additive. Due to this side reaction, a decrease in battery capacity and an increase in battery resistance occur. Considering the likelihood of the side reaction from the magnitude relationship between the Highest Occupied Molecular Orbital (HOMO) on the negative electrode surface and the Lowest Unoccupied Moleculat Orbital (LUMO) of the electrolyte solution component, the following can be said.

[0070] Consider the case where electrons move from the nitrogen-doped portion on the negative electrode surface to the electrolyte components. That is, the electrons move from the HOMO at the nitrogen-doped portion on the negative electrode surface. At this time, if the HOMO on the negative electrode surface is higher than the LUMO of the electrolyte components, the electrons are more likely to move. In a lithium-ion battery, the cell voltage increases after charging compared to the state before charging. Therefore, the negative electrode potential decreases after charging. This corresponds to an increase in the energy level of the HOMO. The smaller the HOMO on the negative electrode surface before charging, the greater the energy required for the HOMO on the negative electrode surface to become higher than the LUMO of the electrolyte components.

[0071] On the other hand, consider the relationship between the energy level of the HOMO in the nitrogen-doped portion and the energy level of the HOMO in the non-nitrogen-doped portion. When the HOMO of the nitrogen-doped negative electrode surface becomes smaller and the magnitude relationship with the HOMO of the non-nitrogen-doped negative electrode is such that the HOMO of the nitrogen-doped negative electrode is lower than that of the non-nitrogen-doped negative electrode, electrons are more likely to move from the non-nitrogen-doped portion to the nitrogen-doped portion. That is, electrons are more likely to move to the surface portion in contact with the electrolyte components. In other words, when the HOMO of the nitrogen-doped negative electrode is higher than the HOMO of the non-nitrogen-doped negative electrode, it becomes more difficult for electrons to move to the surface portion in contact with the electrolyte components. From this, it can be said that when the HOMO of the nitrogen-doped negative electrode is higher than the HOMO of the non-nitrogen-doped negative electrode, the reduction reaction of the electrolyte components is less likely to occur.

[0072] According to the negative electrode carbon material of the present embodiment, the energy level of the HOMO can be increased compared to the non-nitrogen-doped carbon material. Therefore, in a lithium-ion battery employing the negative electrode carbon material of the present embodiment, the reduction reaction of the electrolyte components is less likely to occur.

[0073] (Modified example) The present embodiment can be implemented with the following modifications. The present embodiment and the following modified examples can be implemented in combination with each other within a technically non-conflicting range.

[0074] · Activation energy E in the binding form model M C ina The relationship with the Mulliken charge q may be calculated each time when calculating the integrated value ΣF of the Boltzmann factor of the negative electrode carbon material model, or the previously calculated value may be used. For the bonding form model M B the activation energy E C in may be calculated each time when calculating the integrated value ΣF of the Boltzmann factor of the negative electrode carbon material model, or the previously calculated value may be used. For the bonding form model M a if the correlation between the activation energy E B and the Mulliken charge q is stored in advance in the storage unit of the device or the like, by performing each step according to FIG. 5 using the correlation, the integrated value ΣF of the Boltzmann factor of the negative electrode carbon material model

[0075] · The method for evaluating the reaction resistance of the negative electrode carbon material is not limited to the method described in the above embodiment. · The negative electrode carbon material may have a bonding form other than (A) pyridine type, (B) amine type, (C) pyrrole type, and (D) quaternary type with respect to the bonding form between carbon atoms and nitrogen atoms. The negative electrode carbon material can contain other bonding forms as long as the effects of the negative electrode carbon material are not impaired.

Example

[0076] The negative electrode carbon material of the lithium ion battery will be described in more detail based on the following examples. Note that the negative electrode carbon material of the lithium ion battery is not limited to the configuration described in the example column.

[0077] Negative electrode carbon material models of Examples 1 to 17 and Comparative Examples 1 to 18 shown in Tables 1 to 3 were created. For each model, evaluation was performed according to the following test examples. Gaussian16W (manufactured by Gaussian) was used for the calculation. For the process in FIG. 1, the functional was set to B3LYP, the basis function was set to 6-31G(d,p), and the continuous dielectric model (PCM) with a dielectric constant of 90 was applied. Also, for the process in FIG. 5, in order to reduce the calculation load, the functional B3LYP and the basis function 3-21G were used.

[0078]

Table 1

[0079]

Table 2

[0080] (Comparative Examples 1 to 6) A graphene model of 9 rows and 2 columns was created. According to the nitrogen-doped type ratio shown in Table 1 and the ratio of the content of nitrogen atoms to the content of carbon atoms (N / C ratio), a negative electrode carbon material model in which nitrogen atoms were doped into the graphene model was obtained. The nitrogen doping positions were randomly determined by random numbers.

[0081] (Examples 2 to 17 and Comparative Examples 7 to 18) A negative electrode carbon material model was obtained in the same manner as in Example 1, except that the nitrogen-doped type ratio was changed according to the nitrogen-doped type ratio shown in Tables 2 to 3.

[0082] Note that Comparative Example 8 has a different N / C ratio from Comparative Example 7. Comparative Examples 17 and 18 described later are different in that Comparative Example 17 contains one (A) pyridine type, while Comparative Example 18 contains two (A) pyridine types.

[0083] <Boltzmann factor integrated value ΣF B Calculation of According to the flow described with reference to FIG. 1 in the above embodiment, the correlation between the activation energy and the Mulliken charge in the bonding form model was obtained.

[0084] For each model of Examples 1 to 17 and Comparative Examples 1 to 16, according to the flow described with reference to FIG. 5 in the above embodiment, the Boltzmann factor integrated value ΣF B was calculated using the above correlation. The results are shown in Tables 1 to 2.

[0085] 〔Setting of evaluation index〕 Figure 6 is a graph plotting, for each of Comparative Examples 1 to 6, the ratio of the (D) quartanary type on the horizontal axis and the Boltzmann factor integrated value ΣF B on the vertical axis. In Figure 6, the regression line β based on each plot is shown as a solid line. Referring to the regression line β, it can be seen that the smaller the ratio of the (D) quartanary type, the larger the tendency of the Boltzmann factor integrated value ΣF B . Considering the variation of the regression line β, the line considering the variation is taken as the regression line β + 3σ, and the Boltzmann factor integrated value ΣF B when 10% is substituted as the ratio of the (D) quartanary type into the expression representing the regression line β + 3σ is used as the evaluation index. The evaluation index is 1.03×10 -12 .

[0086] 〔Evaluation of the Boltzmann factor integrated value ΣF B 〕 For Examples 1 to 17 and Comparative Examples 7 to 16, the values of the Boltzmann factor integrated value ΣF B were evaluated. The evaluation criteria are as follows. The results are shown in Table 2.

[0087] ·Evaluation criteria for the Boltzmann factor integrated value ΣF B ○ (Good): The Boltzmann factor integrated value ΣF B is 1.03×10 -12 or more. × (Not acceptable): The Boltzmann factor integrated value ΣF B is less than 1.03×10 -12 .

[0088] In Examples 1 to 17, it was confirmed that the Boltzmann factor integrated value ΣF B was 1.03×10 -12 or more. From this result, it can be seen that in Examples 1 to 17, the reaction resistance in the Li + insertion / desorption reaction is small.

[0089] ​Examples 1 to 17 have a structure in which nitrogen atoms are doped into a carbon material and satisfy both of the following conditions [1] and [2]. [1] The bonding form of carbon atoms and nitrogen atoms contained in the carbon material is such that the ratio of (A) pyridine-type nitrogen atoms to the total number of (A) pyridine-type, (B) amine-type, (C) pyrrole-type, and (D) quaternary-type nitrogen atoms is 17% or more and 78% or less. [2] The bonding form of carbon atoms and nitrogen atoms contained in the carbon material is such that the total ratio of (C) pyrrole-type nitrogen atoms and (D) quaternary-type nitrogen atoms to the total number is 5% or more and 60% or less.

[0090] On the other hand, Comparative Examples 7 to 16 do not satisfy either one of the above conditions [1] and [2], or do not satisfy both of the above conditions [1] and [2]. More specifically, in Comparative Examples 7, 8, 9, 10, 11, and 15, the ratio of (A) pyridine-type nitrogen atoms is less than 17%, and the total ratio of (C) pyrrole-type nitrogen atoms and (D) quaternary-type nitrogen atoms is greater than 60%. Further, in Comparative Examples 12, 13, and 14, the total ratio of (C) pyrrole-type nitrogen atoms and (D) quaternary-type nitrogen atoms is 60%, but the ratio of (A) pyridine-type nitrogen atoms is less than 17%. Further, in Comparative Example 16, the ratio of (A) pyridine-type nitrogen atoms is 17% or more, but the total ratio of (C) pyrrole-type nitrogen atoms and (D) quaternary-type nitrogen atoms is greater than 60%. All of these Comparative Examples 7 to 16 have a Boltzmann factor integrated value ΣF B less than 1.03×10 -12 From this result, it can be seen that in Comparative Examples 7 to 16, the reaction resistance in the Li + insertion / desorption reaction is large.

[0091] From the above results, according to the negative electrode carbon material of a lithium ion battery having a structure in which nitrogen atoms are doped into a carbon material and satisfying both of the above conditions [1] and [2], it can be seen that the reaction resistance in the Li + insertion / desorption reaction is small.

[0092] <Calculation of the energy level of HOMO on the negative electrode surface> For Examples 1 to 17 and Comparative Examples 7 to 18, the energy levels of the HOMO were calculated. The results are shown in Table 3.

[0093]

Table 3

[0094] 〔Evaluation of the Energy Level of HOMO on the Negative Electrode Surface〕 It was confirmed that the energy levels of the HOMO in Examples 1 to 17 were all higher than the energy level of the non-nitrogen-doped model, which is the evaluation criterion. From this result, it can be seen that in Examples 1 to 17, the reduction reaction of the electrolyte components is less likely to occur.

[0095] Incidentally, Fig. 7 is a graph plotted for Examples 1 to 17 and Comparative Examples 7 to 18 with the proportion of (A) type on the horizontal axis and the energy level of the HOMO on the vertical axis. When a regression line γ1 is created based on each plot in Fig. 7, it can be seen that the lower the proportion of (A) pyridine-type nitrogen atoms, the higher the energy level of the HOMO. According to the equation representing the regression line γ1, in the range where the proportion of (A) pyridine-type nitrogen atoms is 78% or less, the energy level of the HOMO becomes higher than -4.04 eV. From this, it can be expected that in the negative electrode carbon material where the proportion of (A) pyridine-type nitrogen atoms is 78% or less, the energy level of the HOMO is higher than that of the non-nitrogen-doped carbon material. Therefore, it can be expected that in a lithium-ion battery employing a negative electrode carbon material where the proportion of (A) pyridine-type nitrogen atoms is 78% or less, the reduction reaction of the electrolyte components is less likely to occur.

[0096] Further, FIG. 8 is a graph plotting the energy level of HOMO with the horizontal axis representing the ratio of (C)+(D) type and the vertical axis representing the energy level of HOMO for Examples 1 to 17 and Comparative Examples 7 to 18. When a regression line γ2 is created based on each plot in FIG. 8, it can be seen that the higher the total ratio of the (C) pyrrole-type nitrogen atom and the (D) quaternary-type nitrogen atom, the higher the energy level of HOMO. According to the equation representing the regression line γ2, in the range where the total ratio of the (C) pyrrole-type nitrogen atom and the (D) quaternary-type nitrogen atom is 5% or more, the energy level of HOMO becomes higher than -4.04 eV. From this, it can be expected that in a negative electrode carbon material where the total ratio of the (C) pyrrole-type nitrogen atom and the (D) quaternary-type nitrogen atom is 5% or more, the energy level of HOMO is higher than that of a non-nitrogen-doped carbon material. Therefore, according to a lithium-ion battery employing a negative electrode carbon material where the total ratio of the (C) pyrrole-type nitrogen atom and the (D) quaternary-type nitrogen atom is 5% or more, it can be expected that the reduction reaction of the electrolyte component will be less likely to occur.

[0097] Among Examples 1 to 17, even those with a relatively high energy level of HOMO are at the same level as the energy level of HOMO in a conventional negative electrode carbon material (-3.69 eV or more and -3.17 eV or less). Therefore, the energy level of HOMO in Examples 1 to 17 is not excessively high.

[0098] From the above results, it can be seen that according to a lithium-ion battery employing a negative electrode carbon material of a lithium-ion battery having a structure in which a carbon material is doped with nitrogen atoms and satisfying both the above conditions [1] and [2], the following can be understood. According to a lithium-ion battery employing the negative electrode carbon material, the reaction resistance in the Li + insertion / desorption reaction can be reduced, and it can be expected that the reduction reaction of the electrolyte component will be less likely to occur.

Claims

【Claim 1】 Comprising a structure in which nitrogen atoms are doped into a carbon material, The bonding form between the carbon atoms and the nitrogen atoms contained in the carbon material is such that the ratio of the (A) pyridine-type nitrogen atoms to the total number of (A) pyridine-type, (B) amine-type, (C) pyrrole-type, and (D) quaternary-type nitrogen atoms is 17% or more and 78% or less, and the total ratio of the (C) pyrrole-type nitrogen atoms and the (D) quaternary-type nitrogen atoms to the total number is 5% or more and 60% or less The negative electrode carbon material of a lithium ion battery.

Images