Anode carbon material for lithium-ion secondary batteries and lithium-ion secondary batteries

Abstract

By lowering the activation energy of the desolvation process at the interface between the negative electrode and the electrolyte, the internal resistance of the lithium-ion secondary battery is reduced. [Solution] The negative electrode carbon material of the lithium-ion secondary battery has a structure in which sulfur atoms are doped into the carbon material, and when the bonding forms of carbon atoms and sulfur atoms contained in the carbon material are (A) sulfide type, (B) sulfoxide type, and (C) sulfonyl type sulfur atoms, the proportion A of (A) sulfide type is 0% of the total number of sulfur atoms of all bonding forms.

Description

Technical Field

[0001] The present disclosure relates to a negative electrode carbon material for a lithium-ion secondary battery and a lithium-ion secondary battery.

Background Art

[0002] As a power source for electric vehicles or hybrid vehicles, lithium-ion secondary batteries are used. One of the problems of lithium-ion secondary batteries is the reduction of internal resistance.

[0003] In order to reduce the internal resistance, it is considered to smoothly progress elementary processes such as charge transfer occurring at the interface between the active material and the electrolyte, and the movement of lithium ions in the active material. Lithium ions are stabilized in the electrolyte solution in a structure coordinated with a plurality of solvent molecules. When the solvated lithium ions are inserted into the carbon material which is the negative electrode active material, they are in a desolvated state. The activation barrier of this desolvation process of lithium ions is known to be large and becomes the rate-determining step of the battery reaction.

[0004] On the other hand, it has been proposed to support an appropriate amount of hydrophilic carbon black on the surface of graphite which is the negative electrode active material (see, for example, Patent Document 1). By supporting hydrophilic carbon black on the graphite surface in this way, it is estimated that the activation barrier of the desolvation process of lithium ions is reduced.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] The desolvation and process strongly depend on the surface structure of the negative electrode active material and the substances adsorbed on the surface. Although it is estimated that the reaction resistance of the negative electrode is reduced according to the above method, a further reduction in the reaction resistance is required.

Means for Solving the Problems

[0007] The present disclosure provides a negative electrode carbon material for a lithium ion secondary battery. The negative electrode carbon material of the lithium ion secondary battery has a structure in which sulfur atoms are doped into the carbon material. When the bonding forms of the carbon atoms contained in the carbon material and the sulfur atoms are the sulfur atoms of (A) sulfide type, (B) sulfoxide type, and (C) sulfonyl type, with respect to the total number of the sulfur atoms of all the bonding forms, the ratio A of the sulfur atoms of the (A) sulfide type bonding form is 0% < A ≤ 28.6%, with respect to the total number of the sulfur atoms of the bonding forms, the ratio B of the sulfur atoms of the (B) sulfoxide type bonding form is 50.0% ≤ B ≤ 79.0%, the ratio S / C of the number of sulfur atoms to the number of carbon atoms of the carbon material is 5.66% ≤ S / C ≤ 8.70%, and when the ratio of the sulfur atoms of the (C) sulfonyl type bonding form to the total number of the sulfur atoms of all the bonding forms is defined as ratio C, the sum of the ratios A to C is 100%.

[0008] The carbon material may have a structure in which a polycyclic aromatic hydrocarbon skeleton is provided and carbon atoms at the ends of the skeleton are replaced with sulfur atoms to form the bonding form. The present disclosure provides a lithium ion secondary battery. The lithium ion secondary battery includes a negative electrode containing the above negative electrode carbon material, a positive electrode containing a lithium composite metal oxide, and an electrolytic solution.

Advantages of the Invention

[0009] According to the present disclosure, the internal resistance of the lithium ion secondary battery can be reduced by reducing the activation energy of the desolvation process at the interface of the negative electrode and the electrolytic solution.

Brief Description of the Drawings

[0010] [Figure 1] This is a schematic diagram of the electrode body of a lithium-ion secondary battery according to one embodiment. [Figure 2] This flowchart shows the evaluation procedure for a negative electrode carbon material in one embodiment. [Figure 3] This figure shows a model of the negative electrode carbon material in one embodiment. [Figure 4] This graph shows the correlation between partial charge and activation energy in one embodiment. [Figure 5] This figure illustrates a barrier in a sulfur-doped carbon structure according to one embodiment. [Figure 6] This flowchart shows the procedure for obtaining the correlation between partial charge and activation energy. [Figure 7] (a) is a diagram of the solvation model, and (b) is a diagram of the bond morphology model and the solvation model. [Figure 8] This graph shows the relationship between activation energy and the distances in the solvation model and the bond morphology model. [Figure 9] This table shows the estimated energy levels of the LUMO for the examples and comparative examples. [Figure 10] This graph shows the distribution of good and poor energy levels in the examples and comparative examples, expressed as the proportion of each coupling configuration. [Modes for carrying out the invention]

[0011] Below, one embodiment of the negative electrode carbon material for a lithium-ion secondary battery will be described with reference to Figures 1 to 6. [Lithium-ion rechargeable battery] Figure 1 shows the electrode body 20 of the lithium-ion secondary battery 10. The electrode body 20 is housed together with a non-aqueous electrolyte (or non-aqueous electrolyte) in a case (not shown). Hereafter, the non-aqueous electrolyte will simply be referred to as the electrolyte.

[0012] The positive electrode plate 21 comprises a positive electrode substrate 22 and a positive electrode composite layer 23. For example, the positive electrode substrate 22 is made of metal foil. The positive electrode composite layer 23 contains a positive electrode active material. For example, the positive electrode active material is a lithium composite metal oxide such as a layered system or spinel system. The lithium composite metal oxide contains lithium, one or more metal elements, and oxygen atoms. For example, LiNiO2, LiCoO2, LiFeO2, LiMn2O4, LiNi 0.5 Mn 1.5 O4, LiCrMnO4, LiFePO4, etc. can be used.

[0013] The negative electrode plate 24 comprises a negative electrode base material 25 and a negative electrode composite layer 26. For example, the negative electrode base material 25 is a metal foil made of copper or an alloy mainly composed of copper. The negative electrode composite layer 26 contains a carbon material as the negative electrode active material. The negative electrode carbon material in this embodiment has a structure in which sulfur atoms are doped into the carbon material.

[0014] As the carbon material that forms the backbone of the negative electrode carbon material, graphene can be used as a material having a polycyclic aromatic hydrocarbon skeleton. The polycyclic aromatic hydrocarbon skeleton is a skeleton in which aromatic rings are fused together. Other carbon materials include carbon black, graphite, graphene nanoplatelets, carbon fibers, carbon nanotubes, carbon nanofibers, carbon nanohorns, carbon nanobrushes, activated carbon, porous carbon, and nanoporous carbon. Examples of carbon black include acetylene black, Ketjen black, furnace black, medium thermal carbon black, and graphitized carbon black. In this embodiment, graphene is used.

[0015] [Sulfur-doped carbon structure] When the bonding configurations between carbon atoms and sulfur atoms contained in the negative electrode carbon material are (A) sulfide type (CSC), (B) sulfoxide type (C-SO-C), and (C) sulfonyl type (C-SO2-C), the negative electrode carbon material satisfies the following conditions 1 to 3.

[0016] (Condition 1) The proportion A of sulfide-type sulfur atoms relative to the total number of sulfur atoms in all bond forms is 0% <A≦28.6%である。 (Condition 2) The proportion B of sulfoxide-type sulfur atoms relative to the total number of sulfur atoms in bonded forms is 50.0% ≤ B ≤ 79.0%.

[0017] (Condition 3) The ratio of sulfur atoms to carbon atoms in the carbon material (S / C ratio) is 5.66% ≤ S / C ≤ 8.70%. Furthermore, let C be the ratio of sulfur atoms in the (C) sulfonyl type (C-SO2-C) bond to the total number of sulfur atoms in all bond forms. The sum of ratios A to C ("ratio A + ratio B + ratio C") is "100%". The (A) sulfide type, (B) sulfoxide type, and (C) sulfonyl type are represented by the following equations (1) to (3).

[0018] [ka]

[0019] [ka]

[0020] [ka] Factors affecting the internal resistance of the lithium-ion secondary battery 10 include the reaction resistance of the desolvation process of solvated lithium ions and the resistance due to the thickness of the film formed on the negative electrode carbon material. Conditions 1 to 3 were set with a focus on reducing the internal resistance of the lithium-ion secondary battery 10.

[0021] Regarding condition 1, if the proportion A of the sulfide type exceeds the upper limit of 28.6%, the desolvation process of the solvated lithium ions becomes less likely to proceed, and the reaction resistance increases. Regarding condition 2, if the proportion of sulfoxide type (B) falls below the lower limit of 50.0%, the reaction resistance of the desolvation process increases. Also, if the proportion of sulfoxide type (B) exceeds the upper limit of 79.0%, the electrical resistance due to the film formed by the decomposition of the electrolyte increases.

[0022] Regarding condition 3, if the sulfur atom ratio (S / C) falls below the lower limit of 5.66%, the reaction resistance of the desolvation process increases. Conversely, if the S / C ratio exceeds the upper limit of 8.70%, the electrical resistance due to the coating increases.

[0023] [Method for manufacturing negative electrode carbon material] The negative electrode carbon material can be manufactured by known methods. The negative electrode carbon material can be manufactured using a base carbon material and a sulfur source as raw materials. The negative electrode carbon material is manufactured to satisfy the sulfur-doped ratio described above.

[0024] For example, as described in "Japanese Patent Publication No. 6691782," methods for producing sulfur-doped negative electrode carbon materials include a method of plasma treatment in liquid with a slurry containing graphene and a sulfur source containing sulfur atoms, and a method of heating a mixture of the sulfur source and carbon material in an inert atmosphere. By changing the type of sulfur source and the mixing ratio of the sulfur source, the sulfur-doped type and the amount of sulfur doping can be arbitrarily adjusted.

[0025] The heating temperature is preferably higher than the thermal decomposition temperature of the sulfur source. The sulfur source, liquefied or vaporized by the high temperature, comes into contact with the highly reactive sites of the carbon material. As a result, carbon atoms in the highly reactive sites of the carbon material are replaced by sulfur atoms, or sulfur atoms are added to carbon atoms. Furthermore, any remaining sulfur source is removed from the mixture by thermal decomposition.

[0026] [Method for measuring sulfur-doped proportion and S / C ratio] The proportion of each sulfur-doped bonding configuration in the negative electrode carbon material can be measured by known methods. For example, the proportion of each sulfur-doped bonding configuration can be measured by X-ray photoelectron spectroscopy (XPS). By assigning the S2p peaks originating from the 2p orbitals of the sulfur atoms to each bonding configuration and separating the peaks assigned to each configuration, the area of ​​each peak is calculated and quantified. Then, by dividing the area of ​​the peak for which the proportion is to be determined by the total area of ​​the S2p peaks, the proportions A to C of sulfur atoms in each bonding configuration relative to the total amount of sulfur atoms can be calculated. The S / C ratio can also be measured similarly using the XPS method.

[0027] [Performance evaluation of negative electrode carbon materials] In the following, a negative electrode carbon material doped with sulfur atoms is referred to as a sulfur-doped carbon material. The degree to which sulfur-doped carbon materials influence the desolvation process can be evaluated by quantum chemical calculations. Furthermore, the resistance to electrolyte decomposition caused by sulfur-doped carbon materials can be evaluated by estimating the energy levels of the LUMO of the bonding configuration.

[0028] (1. 1st performance index) The first performance index will now be explained. The first performance index is an index that indicates the performance of sulfur-doped carbon materials in reducing the reaction resistance in the desolvation process.

[0029] The activation energy required for solvated lithium ions to undergo the desolvation process depends on the surface structure of the negative electrode carbon material. Therefore, by designing the surface structure of the negative electrode carbon material to reduce its activation energy, the reaction resistance of the desolvation process can be reduced. The inventors, through quantum chemical calculations and other methods, discovered that doping the negative electrode carbon material with sulfur atoms lowers the activation energy. Furthermore, they found that the sulfoxide type "C-SO-C" and the sulfonyl type "C-SO2-C" can lower the barrier as bonding forms for sulfur-doped carbon materials. They also found that the proportion of each bonding form affects the magnitude of the activation energy.

[0030] Reaction resistance R in the desolvation process of the solvent and lithium ions ct The reciprocal of ct is proportional to the Boltzmann factor F B The Boltzmann factor F B can be expressed by the following formula (4).

[0031]

Equation

[0032] The activation energy Ea becomes smaller as the desolvation process barrier of the doped carbon material becomes lower. Under the condition of constant temperature, when the activation energy Ea becomes smaller, the value of the Boltzmann factor F B becomes larger. The increase in the value of the Boltzmann factor F B means that the proportion of molecules that can cross the barrier increases, so the reaction resistance of the desolvation resistance becomes smaller and the reaction tends to be faster. Conversely, the decrease in the value of the Boltzmann factor F B means that the proportion of molecules that can cross the barrier decreases, so the reaction resistance R ct of the desolvation resistance increases and the reaction tends to be slower. Therefore, by using the value of the Boltzmann factor F B in the doped carbon material, the reaction resistance R ct of the desolvation process can be evaluated.

[0033] Referring to FIGS. 2 and 3, the procedure of the performance evaluation method using the Boltzmann factor F B will be described. This calculation is performed by executing a program stored in the memory by one or more processors such as a CPU, MPU, NPU, etc. The memory includes any storage medium such as volatile memory and non-volatile memory.

[0034] As shown in Figure 2, first a sulfur-doped carbon structure is created (step S1). A sulfur-doped carbon structure is a carbon structure doped with sulfur atoms. For example, as illustrated in the upper model of Figure 3, an 18x2 graphene skeleton G is created. Alternatively, the graphene skeleton G may be 15x2 or 21x2. Then, the number of (A) sulfide type, (B) sulfoxide type, and (C) sulfonyl type bond forms at each level is set, and sulfur-doped carbon structures G1 are created accordingly. At this time, which of the multiple six-membered rings constituting the surface of the graphene skeleton G will be used for each bond form is randomly determined by a random number generator. In the example of the lower model in Figure 3, the hydrogen atoms at the end of the six-membered rings in the 2nd, 5th, 7th, and 10th columns of the 2nd row and the carbon atoms bonded to them are replaced with the (B) sulfoxide type bond form DB, which consists of a sulfur atom S and an oxygen atom O. The carbon and hydrogen atoms in the 13th column's six-membered ring are substituted with (A) sulfide-type DA, and the carbon and hydrogen atoms in the 16th column are substituted with (C) sulfonyl-type DC.

[0035] Next, the sulfur-doped carbon structure is optimized (step S2). This determines the molecularly stable structure, i.e., the structure with the minimum energy. Next, energy calculations are performed on the structurally optimized sulfur-doped carbon structure (Step S3). Here, the total energy of the molecule is determined based on electron density using density functional theory.

[0036] Next, the partial charges qk(q1,q2…qn) of the sulfur-doped or unsulfur-doped portions of the sulfur-doped carbon structure are identified (Step S4). In the example in Figure 3, partial charges q1 to q7 are determined for the six sulfur-doped and unsulfur-doped portions of the sulfur-doped carbon structure. For the unsulfur-doped portions, the partial charge of the hydrogen atom is determined. (A) For the sulfide-type "CSC" bond configuration, the partial charge of the sulfur atom is determined, and (B) for the sulfoxide-type "C-SO-C" bond configuration, the partial charge of the oxygen atom is determined. Furthermore, for the sulfonyl-type "C-SO2-C" bond configuration (C), the one with the larger absolute value of the partial charge of the two oxygen atoms is used.

[0037] Furthermore, the LUMO levels of the sulfur-doped and unsulfur-doped portions are identified (step S5). These LUMO levels will be used in the processing described later. Furthermore, the activation energy Ea(Ea1,Ea2…Ean) of the desolvation process is obtained from the partial charge qk value obtained in step S3 (step S6).

[0038] Figure 4 is a graph for converting partial charge qk to activation energy Ea. The graph shows the correlation between partial charge qk and activation energy. In the example in Figure 4, the regression lines are shown plotting the activation energies of sulfoxide type, sulfonyl type, unsulfur-doped structure, and sulfide type in order from smallest partial charge qk. The process for obtaining the correlation between partial charge qk and activation energy Ea will be described later.

[0039] Then, using equation (4), the Boltzmann factor F for each binding configuration is obtained from the activation energy Ea. B (F B 1, F B 2,...F B Find n) (Step S7). Furthermore, the Boltzmann factor F B The performance index normalized by molecular weight (first performance index) is calculated (Step S8). Specifically, the Boltzmann factor F B The cumulative value of the Boltzmann factor ΣF B The first performance index is calculated and normalized by dividing it by the molecular weight on the surface (the molecular weight in the second row in the example in Figure 3), and the normalized value is taken as the first performance index (first performance index = ΣF B (Surface molecular weight M). By normalizing in this way, performance per unit mass can be compared. The larger this first performance index, the lower the reaction resistance in the desolvation process.

[0040] (2.Second performance index) Next, we consider the electrical resistance caused by the film formed on the negative electrode surface. When electrons move from the surface of the negative electrode carbon material to the electrolyte, the electrolyte undergoes reductive decomposition. The decomposed electrolyte forms an SEI film on the surface of the negative electrode carbon material. Although the SEI film has the effect of suppressing further decomposition of the electrolyte, if it becomes excessively thick, the electrical resistance increases. Considering the ease with which this decomposition reaction occurs based on the magnitude of the LUMO (Lowest Unoccupied Molecular Orbital) of the sulfur-doped negative electrode surface, the following can be said.

[0041] When electrons move from a sulfur-doped negative electrode surface to the electrolyte, the sulfur-doped portion is thought to act as a barrier to electron conduction. The ease with which electron transfer occurs can be evaluated using the probability of a tunneling effect occurring through the barrier (tunneling probability). The tunneling probability decreases as the barrier size increases.

[0042] As shown in Figure 5, the maximum barrier height ΔE is considered to correspond to the LUMO of the sulfur-doped portion. In other words, the higher the LUMO energy level, the less likely electrons are to move into the electrolyte. In the following, the LUMO energy level will be used as the second performance indicator.

[0043] The procedure for evaluation using the second performance indicator will be explained. First, a multiple regression analysis is performed with the LUMO energy levels as the dependent variable and the proportions A to C and the S / C ratio as the independent variables. The following equation (5) is used for the multiple regression analysis.

[0044] Y=a1·X1+a2·X2+a3·X3+a4·X4+b …(5) "Y" is the dependent variable. "X1" to "X4" are independent variables, corresponding to proportions A to C and the S / C ratio, respectively. "a1" to "a4" are coefficients, indicating the sensitivity of independent variables "X1" to "X4," respectively. "b" is the intercept.

[0045] This multiple regression analysis allows us to determine the coefficients a1-a4 and intercept b for each proportion A-C and the S / C ratio (explanatory variable D). The coefficients a1-a4 represent the sensitivity of the influence that the explanatory variables, each proportion A-C and the S / C ratio, have on the energy levels of the dependent variable, the LUMO.

[0046] Next, we estimate the energy levels of the LUMO, which corresponds to the barrier to charge transfer. Specifically, we prepare 200 patterns of levels by changing the ratios A to C and the S / C ratio. Then, we set the coefficients and intercept calculated in step S10 to equation (5) above, and substitute the ratios A to C and the S / C ratio for each level to calculate the energy levels of the LUMO.

[0047] A barrier level higher than the reference energy level is considered good, while an energy level below the reference level is judged as poor. (3. Relationship between partial charge and activation energy Ea) Next, we will explain how to obtain the relationship between partial charge and activation energy Ea used in "1. First Performance Index".

[0048] The total energy of a solvated lithium ion increases as it approaches a sulfur-doped carbon structure from a position farther away. However, when the relative distance between the solvated lithium ion and the sulfur-doped carbon structure falls below a certain distance, the slightly negatively charged sulfur-doped portion neutralizes the positive charge of the lithium ion, stabilizing it. Therefore, the total energy is highest when the relative distance between the solvated lithium ion and the sulfur-doped carbon structure is constant, and decreases as the solvated lithium approaches the sulfur-doped carbon structure and the relative distance falls below a certain distance. The activation energy corresponds to the maximum total energy minus the sum of the total energy of the solvated lithium ion and the total energy of the sulfur-doped portion.

[0049] Refer to Figure 6 to explain the procedure for obtaining the relationship between partial charge and activation energy Ea. Also, refer to Figures 7(a) and (b) for the coupling configuration model. In step S101, the solvent molecule SL is converted into lithium ions (L+ ) Solvation model M coordinated to S Prepare the following. As a solvent, carbonate-based solvents such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) can be used.

[0050] In step S102, the solvation model M S Optimize its structure. In step S103, the coupling morphology model M C Create. As shown in Figure 7(b), the coupling morphology model M C This may be the smallest unit containing a sulfur-doped site, or it may be an extended conjugated system with a carbon skeleton. Figure 7(b) shows (B) sulfoxide type bond morphology model M C This indicates.

[0051] In step S104, the coupling morphology model M C Optimize its structure. In step S105, the solvation model M S and combined form model M C Place and at a distance D.

[0052] As shown in Figure 7(b), the solvation model M S Lithium ions and binding morphology model M C The sulfur atoms S are positioned opposite each other. Then the solvation model M is formed. S and combined form model M C The structure is optimized. Then, the lithium ion and sulfur atom are separated by a distance D.

[0053] The maximum value Dmax of distance D is, for example, in the solvation model M. S and combined form model M C This can be set as the distance at which they do not interact. An example of the maximum value Dmax of distance D is 15 Å. Note that the binding morphology model M C Among them, the non-sulfur-doped model M C Regarding (0), the solvation model MS Lithium ion and unsulfur-doped model M C The carbon atoms in (0) are placed at a distance D from each other. Step S105 is repeated until the distance D is less than the threshold, but in the first step, the distance D is set to the maximum value Dmax.

[0054] In step S106, the solvation model M S and combined form model M C Total energy E SC Calculate (1). Also, increment the variable "i" in the total energy Es(i) to "2".

[0055] In step S107, the structurally optimized solvation model M S , coupling morphology model M C Only the total energy E S ,E C Calculate each of them. In step S108, the solvation model M S Lithium ions in the bond configuration model M C The material is moved a distance d towards the sulfur atom in the mixture. The distance d can be set, for example, as a predetermined ratio to the maximum value Dmax. An example of a distance d is 1 Å.

[0056] In step S109, the solvation model M S and combined form model M C Total energy E SC (2) Calculate the total energy E. SC Store the value of (2). In step S110, lithium ions and the binding morphology model M C This determines whether the distance D to the sulfur atom is less than the threshold Dth. An example of a threshold Dth is 2 Å.

[0057] Lithium ion and binding morphology model M CThe distance D between the lithium ion and the sulfur atom decreases with each step S108. In other words, with each step S108, the lithium ion moves closer to the sulfur atom.

[0058] In step S110, lithium ions and the binding morphology model M C If the distance D to the sulfur atom is less than the threshold Dth (S110: YES), the process proceeds to step S111. Meanwhile, the lithium ion and the bonding morphology model M C If the distance D to the sulfur atom is greater than or equal to the threshold D (S110:NO), return to step S108. That is, lithium ion and bonding morphology model M C Steps S108 to S110 are repeated until the distance D to the sulfur atom is less than the threshold.

[0059] In step S111, the total energy E SC (1), E SC (2), ...E SC (i) The maximum value among them is the total energy maximum E max It will be acquired as follows. In step S112, the activation energy E a Calculate.

[0060] Figure 8 shows the total energy E in relation to the value obtained by integrating the distance d (integrated value Σd). SC An example of a graph is shown. Total energy E S and total energy E C The sum of these values ​​is the maximum value E max The activation energy E is the value obtained by subtracting it from the activation energy. a It can be calculated.

[0061] In step S113, the coupling morphology model M C Calculate the partial charge q. In step S114, the activation energy E calculated in step S112 is used. a The correlation with the partial charge q calculated in step S113 is then obtained. After that, this process is terminated.

[0062] And each bonding morphology model M C Regarding the activation energy E mentioned above, a The correlation between the partial charge q and the following is obtained: (A) sulfide type, (B) sulfoxide type, and (C) sulfonyl type and unsulfur-doped model M. C For each of (0), steps S101 to S114 are performed.

[0063] The graph in Figure 4 above shows each coupling morphology model M C The activation energy E calculated according to the above a This is an example of a graph plotting the correlation between the partial charge q and the bond configuration model M containing a sulfur atom. C In this case, the larger the partial charge q, the greater the activation energy E a A relationship is obtained in which the interaction becomes larger.

[0064] <Operation and Effects of This Embodiment> The operation and effects of this embodiment will now be described. (1) According to this embodiment, the sulfur atom-doped negative electrode carbon material satisfies the above conditions 1 to 3, resulting in lower reaction resistance in the desolvation process compared to an undoped negative electrode carbon material. Furthermore, it is possible to increase the barrier to electron transfer from the negative electrode surface to the electrolyte. This makes it possible to reduce the internal resistance of the lithium-ion secondary battery.

[0065] (Example of change) This embodiment can be implemented with the following modifications. This embodiment and the following modifications can be combined with each other to the extent that they do not contradict each other technically.

[0066] • Combination Model M C Activation energy E a The relationship between the partial charge q and the Boltzmann factor integrated value ΣF of the negative electrode carbon material model is B The calculation can be performed each time, or a pre-calculated value can be used. Combined morphology model M CActivation energy E a If the correlation between the partial charge q and the Boltzmann factor ΣF is stored in the device's memory beforehand, then by using this correlation to perform each process, the Boltzmann factor integrated value ΣF of the negative electrode carbon material model can be calculated. B It can be calculated.

[0067] The method for evaluating the reaction resistance of the desolvation process is not limited to the method described in the above embodiment. The negative electrode carbon material may have bonding configurations other than (A) sulfide type, (B) sulfoxide type, and (C) sulfonyl type with respect to the bonding configuration between carbon atoms and sulfur atoms. The negative electrode carbon material may contain other bonding configurations to the extent that they do not impair the effect of the negative electrode carbon material.

[0068] (Examples) The negative electrode carbon material for lithium-ion secondary batteries will be described in more detail based on the following examples. Note that the negative electrode carbon material for lithium-ion secondary batteries is not limited to the configurations described in the Examples section.

[0069] (A) A proportion of sulfide-type bond configurations, (B) a proportion of sulfoxide-type bond configurations, and (C) a proportion of sulfonyl-type bond configurations were created to create negative electrode carbon material models with different proportions. These negative electrode carbon material models were designated as Conventional Examples 1-4, Examples 1-6, Reference Examples 1-3, and Comparative Examples 1-25. Conventional Example 1 is shown in Table 1, Examples 1-6 and Reference Examples 1-3 are shown in Table 2, and Comparative Examples 1-25 are shown in Table 3. For the Conventional Examples, examples of sulfur-doped carbon structures described in "Japanese Patent Publication No. 6691782" were used as a reference. The sulfur-doped carbon structures in the Conventional Examples do not take into account the reaction resistance of the desolvation process.

[0070] First, the correlation between activation energy and partial charge was obtained according to the procedure of the above embodiment. The steps involving various calculations can be performed using GAMESS, Spartan (manufactured by Wavefunction), etc. Calculation accuracy should be good; for example, conditions using B3LYP / 6-31G(d,p) and B3LYP / 6-31G(d) as functionals / basis functions are preferred. It is preferable to incorporate solvent effects by using the polarization dielectric model method (PCM method) for the calculation. In this case, the dielectric constant of the solvent may be any value; for example, the dielectric constant of ethylene carbonate (EC) may be set to 90.

[0071] Furthermore, each model was evaluated using the first performance index according to the procedure of the embodiment described above. In this evaluation, to reduce the computational load, the functional HF and basis set 3-21G were used in step S2 of Figure 2, and the functional B3LYP and basis set 6-31G(d,p) were used in step S3, and a continuous dielectric model (PCM) with a dielectric constant of 90 was used. In Tables 1 to 3, "Performance Index" shows the value of the first performance index, with the Boltzmann factor normalized by molecular weight.

[0072] [Table 1] In Tables 2 and 3, values ​​below the upper limit of Condition 1 (percentage A is 28.6%) are indicated with "○", values ​​above the lower limit of Condition 2 (percentage B is 50%) are indicated with "○", and values ​​above the lower limit of Condition 3 (S / C ratio is 5.66%) are indicated with "○". Furthermore, among Conventional Examples 1 to 3, the first performance index "1.50E-13 (1.50 × 10)" is used for Conventional Example 3, which has the highest value. -13 The first performance indicator was set to a standard of " )" and those with a higher value than this standard were marked with "○" for good performance, and those with a lower value were marked with "×" for poor performance. In "LUMO", the estimated energy levels of the LUMO for each example are shown.

[0073] In addition, in Table 3, Comparative Examples 6 and 10 have a proportion A of (A) sulfide-type bonding form of "0%", which satisfies the upper limit of Condition 1 but does not meet the lower limit, while Comparative Examples 18, 21, 23-35 satisfy the lower limit of Condition 3 but do not meet the upper limit.

[0074] [Table 2]

[0075] [Table 3] (Conventional examples 1-4) The first performance index for the sulfur-doped carbon structures in Conventional Examples 1-4 was "1.35E-13 to 1.50E-13". As mentioned above, the first performance index for the sulfur-doped carbon structure in Conventional Example 3 was the highest at "1.50E-13".

[0076] (Examples 1-6) Examples 1 to 6 were models that met the above conditions 1 to 3. The first performance index of each sulfur-doped carbon structure model was "1.54E-13 to 1.66E-13", all of which were higher than the maximum value of "1.50E-13" for the conventional example.

[0077] (Reference examples 1~3) Reference examples 1-3 satisfy conditions 1 and 2, but their S / C ratio exceeds the upper limit set by condition 3. The first performance index for each sulfur-doped carbon structure model was "1.64E-13 to 1.74E-13," all of which were higher than the maximum value of "1.50E-13" in the conventional examples.

[0078] (Comparative Examples 1-25) Comparative Examples 1-25 are models that do not satisfy at least one of conditions 1-3. The first performance index for each sulfur-doped carbon structure model ranged from "9.34E-14 to 1.47E-13," all of which were lower than the maximum value of "1.50E-13" for the conventional example.

[0079] (Second performance index) Next, in order to calculate the second performance index, multiple regression analysis was performed on the sulfur-doped carbon structure models of Examples 1-6, Reference Examples 1-3, and Comparative Examples 1-25 according to equation (5).

[0080] The intercept b was "-3.28528", the coefficient for proportion A was "0", the coefficient for proportion B was "-0.58546", the coefficient for proportion C was "-0.59102", and the coefficient for the S / C ratio was "-4.46413".

[0081] Furthermore, by setting coefficients and intercepts in equation (5) and substituting 200 patterns of levels, including Examples 1-6, Reference Examples 1-3, and Comparative Examples 1-25, the LUMO energy level of each model was estimated. Each level differs in its proportions A-C and S / C ratio.

[0082] By estimating the LUMO energy levels, we identified patterns in which electron transfer to the electrolyte is less likely to occur. In this process, we used the LUMO energy level of "-4.071" from Conventional Example 3, which had the highest first performance index among Conventional Examples 1-4, as the baseline.

[0083] As shown in Figure 9, a barrier level higher than the standard was marked with a "○" indicating a good model. Conversely, an energy level below the standard was marked with a "×" indicating an unsatisfactory model. Figure 10 shows the distribution of levels with good and bad barrier heights ΔE for each level, which is partially shown in Figure 9. The horizontal axis represents the proportion B of the (B) sulfoxide type, and the vertical axis represents the S / C ratio. From this distribution, it can be seen that models with high barriers exist in the range where the proportion B is 79% or less and the S / C ratio is 8.7% or less. In other words, models below the upper limits of conditions 2 and 3 have high LUMO energy levels and electron transfer is less likely to occur. [Explanation of symbols]

[0084] 10...Lithium-ion secondary battery, 23...Positive electrode composite layer, 26...Negative electrode composite layer.

Claims

1. It has a structure in which a carbon material is doped with sulfur atoms, When the bonding form between the carbon atoms and sulfur atoms contained in the carbon material is (A) sulfide type, (B) sulfoxide type, and (C) sulfonyl type sulfur atoms, With respect to the total number of sulfur atoms in all of the aforementioned bonding forms, the proportion A of the sulfur atoms in the (A) sulfide-type bonding form is 0% < A ≤ 28.6%. With respect to the total number of sulfur atoms in the aforementioned bonding configuration, the ratio B of the sulfur atoms in the (B) sulfoxide type bonding configuration is 50.0% ≤ B ≤ 79.0%. A negative electrode carbon material for a lithium-ion secondary battery, wherein the ratio S / C of the number of sulfur atoms to the number of carbon atoms in the carbon material is 5.66% ≤ S / C ≤ 8.70%, and when ratio C is the ratio of the sulfur atoms in the (C) sulfonyl type bond to the total number of sulfur atoms in all the bond forms, the sum of ratios A to C is 100%.

2. The carbon material for the negative electrode of a lithium-ion secondary battery according to claim 1, wherein the carbon material has a polycyclic aromatic hydrocarbon skeleton, and the bond configuration is achieved by substituting the carbon atoms at the ends of the skeleton with sulfur atoms.

3. A negative electrode comprising the negative electrode carbon material described in claim 1 or 2, A positive electrode containing a lithium composite metal oxide, A lithium-ion secondary battery containing an electrolyte.

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