Negative electrode for lithium ion secondary battery and lithium ion secondary battery including the same

Abstract

The problem to be solved by the present invention is to provide a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery having the negative electrode for a lithium ion secondary battery, wherein the durability is improved compared to the past. In order to solve the above problems, the present invention is a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery having the negative electrode for a lithium ion secondary battery, wherein the negative electrode for a lithium ion secondary battery comprises a negative electrode active material, wherein the negative electrode active material is chemically bonded with an organic molecule having a dielectric constant greater than that of the electrolyte solvent. Preferred organic molecules include: organic molecules having a relative dielectric constant of 90 or more at a frequency of 10 kHz, organic molecules having a polarized molecular structure within a single molecule or between molecules, zwitterionic compounds having positive and negative charges within a molecule, hydroxy acids, and organic molecules having a molecular weight of 89 to 616.

Description

Technical Field

[0001] The present invention relates to a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery including the negative electrode. Background Art

[0002] A lithium-ion secondary battery has been proposed that uses a positive electrode active material composed of a lithium composite oxide and sintered with a ferroelectric having a relative dielectric constant of 500 or greater (see, for example, Patent Document 1). In the lithium-ion secondary battery of Patent Document 1, the surface of the ferroelectric in contact with the non-aqueous electrolyte is induced to be positively charged, while the surface in contact with the positive electrode active material is induced to be negatively charged. As a result, lithium ions in the positive electrode active material can be smoothly transferred in and out even at low temperatures, resulting in improved output characteristics.

[0003] [Prior technical literature]

[0004] (Patent Document)

[0005] Patent Document 1: Japanese Patent Application Laid-Open No. 2011-210694 Summary of the Invention

[0006] [Problems to be solved by the invention]

[0007] However, in the lithium-ion secondary battery disclosed in Patent Document 1, solvent decomposition progresses near the outermost surface (edge surface) of the graphite crystals serving as the negative electrode active material, forming a solid electrolyte interface (SEI) film. The SEI film is composed of lithium compounds, so the lithium consumed as a material for the SEI film cannot contribute to the charge capacity as lithium ions, resulting in a decrease in capacity retention after endurance testing.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery including the negative electrode for a lithium ion secondary battery, the durability of which is improved compared to conventional ones.

[0009] [Technical means to solve the problem]

[0010] (1) The present invention provides a negative electrode for a lithium ion secondary battery, which comprises a negative electrode active material, wherein the negative electrode active material is chemically bonded with an organic molecule having a dielectric constant greater than that of an electrolyte solvent.

[0011] According to the invention of (1), organic molecules having a dielectric constant greater than that of the electrolyte solvent are chemically bonded to the negative electrode active material, so that the SEI film formed on the lithium insertion surface of the graphite or the like constituting the negative electrode active material is modified to be highly polar and fixed. In particular, organic molecules having a dielectric constant greater than that of the electrolyte solvent and exhibiting dielectric properties can dissolve in the electrolyte and are low molecular weight, so they can enter the outermost surface (edge surface) of the negative electrode active material. Therefore, direct contact of the non-aqueous solvent in the electrolyte with the negative electrode active material can be suppressed, and reductive decomposition can be suppressed. In addition, the desolvation of the lithium salt in the electrolyte is promoted, and the electrolyte is also stabilized, so that the decomposition of the electrolyte can be suppressed. Accordingly, the consumption of lithium is reduced, and the capacity retention rate after endurance can be improved. Therefore, according to the invention of (1), a negative electrode for a lithium ion secondary battery with improved durability than before can be provided.

[0012] (2) In the aforementioned organic molecule of the invention of (1), the relative dielectric constant of the aforementioned organic molecule at a frequency of 10 kHz may be 90 or greater.

[0013] (3) In the invention of (1) or (2), the organic molecule may have a polarized molecular structure within a single molecule or between molecules.

[0014] (4) In the invention according to any one of (1) to (3), the organic molecule may be a zwitterionic compound having positive and negative charges within one molecule.

[0015] (5) In the invention according to any one of (1) to (4), the organic molecule may be a hydroxy acid.

[0016] (6) In the invention according to any one of (1) to (5), the molecular weight of the organic molecule may be 89 to 616.

[0017] According to the inventions of (2) to (6), the effect of the invention of (1) can be more reliably exerted.

[0018] (7) The present invention provides a lithium ion secondary battery comprising: a positive electrode, a negative electrode for a lithium ion secondary battery according to any one of (1) to (6), and a non-aqueous electrolyte comprising a non-aqueous solvent; wherein the non-aqueous electrolyte comprises at least one lithium salt selected from the group consisting of LiPF6, LiBF4, LiCF3BF3, LiBF2(C2O4), LiB(C2O4)2, LiClO4, LiCF3SO3, LiAsF6, LiN(SO2F)2, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2C4F9)(SO2CF3), LiC4F9SO3, LiC(CF3SO2)3, LiCF3CO2 and LiSbF6.

[0019] According to the invention of (7), since the negative electrode for a lithium ion secondary battery according to any one of (1) to (6) is provided, a lithium ion secondary battery having improved durability compared to conventional batteries can be provided.

[0020] (Effects of the Invention)

[0021] According to the present invention, a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery including the negative electrode for a lithium ion secondary battery can be obtained, wherein the durability is improved compared to conventional ones. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Graph showing the results of elemental analysis of a cross section of the negative electrode active material of Example 3 using transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDX).

[0023] Figure 2 1 is a diagram showing the results of TEM-EDX elemental analysis of a cross section of the negative electrode active material of Comparative Example 1. DETAILED DESCRIPTION

[0024] An embodiment of the present invention will be described in detail.

[0025] The lithium-ion secondary battery negative electrode according to this embodiment has improved durability compared to conventional negative electrodes, exhibits a high capacity retention rate after endurance testing, and exhibits a low resistance increase rate. The lithium-ion secondary battery negative electrode according to this embodiment is characterized by comprising a negative electrode active material chemically bonded to organic molecules having a greater dielectric constant than that of the electrolyte solvent.

[0026] The negative electrode for a lithium ion secondary battery according to this embodiment includes a current collector and a negative electrode active material layer formed on at least one surface of the current collector. The negative electrode active material layer contains a negative electrode active material.

[0027] The current collector is not particularly limited, and any conventionally known current collector used in lithium-ion secondary batteries can be used. Examples of the material of the current collector include stainless steel (SUS), nickel (Ni), copper (Cu), titanium (Ti), aluminum (Al), calcined carbon, conductive polymers, conductive glass, and aluminum-cadmium (Al-Cd) alloys.

[0028] Examples of the shape of the current collector include a foil, a plate, and a mesh. The thickness thereof is not particularly limited and can be, for example, 1 to 20 μm.

[0029] The negative electrode active material layer contains a negative electrode active material as an essential component, and may contain conventionally known components such as a conductive auxiliary agent and a binder as optional components.

[0030] The negative electrode active material may be any negative electrode active material that can occlude and release lithium ions and exhibits a lower potential than the positive electrode. In this embodiment, a negative electrode active material chemically bonded with organic molecules having a larger dielectric constant than the electrolyte solvent is used. Specifically, graphite chemically bonded with organic molecules having a larger dielectric constant than the electrolyte solvent can be preferably used.

[0031] Here, the chemical bond between the organic molecule and the negative electrode active material is preferably a chemical bond formed by dehydration condensation of a hydroxyl group with a carboxyl group, a sulfonic acid group, a sulfinic acid group, a phosphoric acid group, or a phosphonic acid group. The type of chemical bond can be selected from various preferred organic molecules described below.

[0032] During the formation of the chemical bond, an acid catalyst may be used to accelerate the dehydration condensation reaction. Specific examples of acid catalysts include inorganic acids such as hydrochloric acid, sulfuric acid, nitric acid, hydrogen peroxide, and hydrogen fluoride; and sulfonic acids such as methanesulfonic acid, benzenesulfonic acid, and p-toluenesulfonic acid. Furthermore, after the dehydration condensation reaction, washing with water, warm water, an inorganic acid, or an organic solvent such as ethanol, acetone, or hexane may be performed to remove byproducts.

[0033] As the organic molecule, an organic molecule having a relative dielectric constant of 90 or greater at a frequency of 10 kHz can be more preferably used. The organic molecule used in this embodiment has a larger dielectric constant than the electrolyte solvent, so as long as it is an organic molecule having a relative dielectric constant of 90 or greater at a frequency of 10 kHz, there are many options for the organic molecule, which is therefore preferred.

[0034] In addition, as organic molecule, more preferably, there is the organic molecule of polarized molecular structure in single molecule or between molecules.As the organic molecule with polarized molecular structure in single molecule or between molecules, for example can be enumerated: the metal salt consisting of tartaric acid and derivatives thereof, lithium tartrate, sodium tartrate, lithium sodium tartrate, lithium potassium tartrate, potassium tartrate, potassium sodium tartrate, barium tartrate, sodium tartrate rubidium, thallium tartrate, gadolinium tartrate, bismuth oxide tartrate. In addition, as the metal possessed by other tartrates, can be enumerated: bismuth, zinc, aluminum, tin, iron, copper, lead, nickel. These metal salts can make more than one metal form ionic bond in molecule, and also can mix different metal salts. Examples include potassium dihydrogen phosphate, imidazole carboxylic acid and its metal salts, benzimidazole carboxylic acid and its metal salts, chlorobenzimidazole carboxylic acid and its metal salts, guanidines (specifically, aluminum guanidine sulfate), and glycines (specifically, triglycine sulfate). These organic molecules may also be used in combination.

[0035] Furthermore, more preferred organic molecules are zwitterionic compounds having positive and negative charges within a molecule and their metal salts. Examples of zwitterionic compounds having positive and negative charges within a molecule include amino acids (specifically, asparagine, glutamine, cysteine, glycine, proline, alanine, and valine), compounds having acidic and basic functional groups (specifically, bicine, tricine, aminosulfonic acid, lysergic acid, and psilocybin), and alkaloids.

[0036] Furthermore, more preferred organic molecules are hydroxy acids and their metal salts. Hydroxy acids are compounds that possess both hydroxyl and carboxylic acid functional groups, and have a polarized molecular structure within the molecule. Examples of hydroxy acids include aliphatic hydroxy acids, specifically lactic acid, hydroxymalonic acid, glyceric acid, and hydroxybutyric acid; and aromatic hydroxy acids, specifically mandelic acid, (1,3-benzodioxol-5-yl)glycolic acid, and DL-3,4-dihydroxymandelic acid.

[0037] The organic molecules are more preferably those with a molecular weight of 89 to 616. Small organic molecules with a molecular weight of 89 to 616 are preferred because they can easily enter the outermost surface (edge surface) of the graphite crystals serving as the negative electrode active material.

[0038] However, in conventional lithium-ion secondary batteries, during the initial charging process, lithium ions are intercalated into the negative electrode material, forming a SEI (solid electrolyte interface) film on the negative electrode surface. This SEI film has the property of blocking electrons while allowing lithium ions to pass through. The presence of the SEI film allows the negative electrode to function. Furthermore, an SEI film of appropriate thickness can function as a protective film, inhibiting the reaction between the negative electrode material and the electrolyte, thereby improving the battery's cycle life.

[0039] During initial charging, desired additives are added to the electrolyte or electrode mixture. After the battery is formed, a so-called aging process is generally performed. This process actively reacts between the electrode and the electrolyte to form a SEI film on the surface of the electrode active material. However, the SEI film is composed of lithium compounds. Therefore, the lithium consumed as a material for the SEI film cannot contribute to the charge capacity as lithium ions. Therefore, during initial charging, the irreversible capacity (i.e., the difference between the charge capacity and the discharge capacity) increases.

[0040] Here, although it is also possible to consider applying the ferroelectric of the above-mentioned patent document 1 to the negative electrode active material instead of the positive electrode active material, the ferroelectric of patent document 1 has a large number of oxide particles such as barium titanate (BaTiO3, BTO), and the oxide particles are larger molecules, so it cannot enter the negative electrode active material particles and enter the outermost surface (edge surface) that accounts for most of the SEI formation.

[0041] In contrast, in the present embodiment, the above-mentioned various organic molecules that exhibit dielectric properties can be dissolved in the electrolyte and are low molecular weight, and therefore can enter the outermost surface (edge surface) of the graphite crystal of the negative electrode active material. Accordingly, organic molecules having a dielectric constant greater than that of the electrolyte solvent are chemically bonded to the negative electrode active material, so that the SEI film formed on the lithium insertion surface of the graphite etc. constituting the negative electrode active material is modified to be highly polar and fixed. Therefore, it is possible to inhibit the direct contact of the non-aqueous solvent in the electrolyte with the negative electrode active material, and to inhibit reductive decomposition. In addition, the desolvation of the lithium salt in the electrolyte is promoted, so that the electrolyte is also stabilized, and thus the decomposition of the electrolyte can be inhibited. Therefore, according to the negative electrode for lithium ion secondary batteries of the present embodiment, the consumption of lithium is reduced, and the capacity retention rate after endurance can be improved.

[0042] The manufacturing method of the negative electrode for lithium ion secondary batteries according to the present embodiment is not particularly limited, and the usual methods in the art can be applied. For example, a negative electrode paste containing a negative electrode active material as an essential component is applied to a current collector, dried, and then rolled to obtain an electrode for lithium ion secondary batteries. In addition, when manufacturing the negative electrode active material, as described above, when forming a chemical bond between an organic molecule and the negative electrode active material, an acid catalyst is used to promote dehydration condensation. After the dehydration condensation reaction, in order to remove by-products, etc., it can be washed with various solvents.

[0043] Next, a lithium ion secondary battery according to this embodiment will be described.

[0044] The lithium ion secondary battery according to this embodiment includes a positive electrode, the negative electrode for the lithium ion secondary battery according to this embodiment, a nonaqueous electrolyte containing a nonaqueous solvent, and a separator.

[0045] The positive electrode includes a current collector and a positive electrode active material layer formed on at least one side of the current collector. The positive electrode active material layer contains a positive electrode active material.

[0046] Examples of the material of the current collector include metal materials such as stainless steel, nickel, chromium (Cr), gold (Au), platinum (Pt), aluminum, iron, titanium, zinc (Zn), and copper.

[0047] Examples of the shape of the current collector include a foil, a plate, and a mesh. The thickness thereof is not particularly limited and can be, for example, 1 to 20 μm.

[0048] Examples of the positive electrode active material include LiCoO2, LiCoO4, LiMn2O4, LiNiO2, LiFePO4, lithium sulfide, sulfur, nickel cobalt manganese (NCM), etc. As the positive electrode active material, any positive electrode active material that exhibits a higher potential than the negative electrode can be selected from materials that can constitute an electrode.

[0049] The positive electrode active material layer contains a positive electrode active material as an essential component, and may contain conventionally known components such as a conductive auxiliary agent and a binder as optional components.

[0050] According to the lithium ion secondary battery of this embodiment, a nonaqueous electrolyte containing a nonaqueous solvent is used as the electrolyte solution. The electrolyte solution is not particularly limited, and a known electrolyte solution can be used as the electrolyte solution of a lithium ion secondary battery.

[0051] As the non-aqueous solvent, any solvent commonly used to form non-aqueous electrolytes can be used. Examples include cyclic solvents such as ethylene carbonate (EC) and propylene carbonate (PC); and chain-like solvents such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). Fluorinated solvents such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC) can also be used.

[0052] Furthermore, the electrolyte solution may contain known additives. Examples of the additives include vinylene carbonate (VC), vinylethylene carbonate (VEC), and 1,3-propane sultone (PS).

[0053] Furthermore, as a highly concentrated electrolyte, an ionic liquid may be used. Examples of the ionic liquid include pyrrolidinium, piperidinium, and imidazolium ions composed of quaternary ammonium cations.

[0054] As the non-aqueous electrolyte, for example, at least one lithium salt selected from the group consisting of LiPF6, LiBF4, LiCF3BF3, LiBF2(C2O4), LiB(C2O4)2, LiClO4, LiCF3SO3, LiAsF6, LiN(SO2F)2, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2C4F9)(SO2CF3), LiC4F9SO3, LiC(CF3SO2)3, LiCF3CO2, and LiSbF6 can be used. Among these, LiPF6, LiBF4, or a mixture of these lithium salts can be preferably used due to their high ion conductivity and high degree of dissociation.

[0055] Here, LiPF6, a lithium salt in the electrolyte, has low chemical stability. If it comes into contact with moisture, it will decompose and generate HF. In contrast, for example, the imide salt has an extremely strong electron attraction due to the alkylsulfonyl group bonded to the central nitrogen, so the negative charge on the nitrogen is greatly delocalized. Therefore, the degree of lithium ion dissociation is very high and it is also extremely stable thermochemically. Therefore, by mixing this imide salt with an electrolyte with improved chemical stability such as lithium bis(fluorosulfonyl)imide (LiFSI) and using it in conjunction with LiPF6, the amount of HF in the electrolyte can be reduced. In addition, based on this, organic molecules are bonded to the negative electrode active material of the present embodiment, and as a result, the decomposition of the bonded organic molecules can be suppressed, and the capacity retention rate after durability is further improved.

[0056] As the separator, a conventionally known separator can be used. For example, a nonwoven fabric formed by laminating three layers of polypropylene / polyethylene / polypropylene can be used.

[0057] The method for producing the lithium ion secondary battery according to the present embodiment is not particularly limited, and a common method in this technical field can be applied.

[0058] According to the lithium ion secondary battery of this embodiment, since it includes the negative electrode for lithium ion secondary batteries according to this embodiment, it is possible to achieve the same effects as those achieved by the negative electrode for lithium ion secondary batteries according to this embodiment described above.

[0059] The present invention is not limited to the above-described embodiment, and modifications and improvements within the scope that can achieve the object of the present invention are included in the present invention.

[0060] [Example]

[0061] Next, examples of the present invention will be described, but the present invention is not limited to these examples.

[0062] <Positive electrode>

[0063] The same positive electrode was used in Examples 1 to 10 and Comparative Examples 1 to 3. The positive electrode was produced according to the following procedure.

[0064] First, acetylene black dispersion as a conductive aid and polyvinylidene fluoride (PVDF) as a binder are weighed and mixed in a specified ratio, and then dispersed with a rotation and revolution mixer. Next, a specified amount of nickel-cobalt-manganese ternary positive electrode active material NCM622 is added and mixed using a planetary mixer. Then, N-methyl-N-pyrrolidone (NMP) is added to make a positive electrode paste. Next, the prepared positive electrode paste is applied to an aluminum current collector and dried, then pressurized with a roller press and dried in a vacuum at 120°C to obtain a positive electrode plate.

[0065] The resulting positive electrode plates were punched into 30 mm x 40 mm sheets and used as positive electrodes. As shown in Table 3, the mass ratio of the positive electrode active material, acetylene black, and PVDF in the positive electrode was 94.0:4.1:1.9. Furthermore, the thickness of each positive electrode was 70 μm.

[0066] Negative electrode active material

[0067] [Example 1]

[0068] Sodium potassium tartrate, an organic molecule, was measured to a mass ratio of 96.2:0.3 to graphite. The mixture was stirred in an aqueous solution for 1 hour and then concentrated. The mixture was then dried under reduced pressure at 150°C for 12 hours and washed with water to obtain a negative electrode active material in which sodium potassium tartrate was chemically bonded to the graphite surface at the aforementioned mass ratio.

[0069] [Example 2]

[0070] Sodium potassium tartrate, an organic molecule, was measured to a mass ratio of 96.0:0.5 to graphite. The mixture was stirred in an aqueous solution for 1 hour and then concentrated. The mixture was then dried under reduced pressure at 150°C for 12 hours and washed with water to obtain a negative electrode active material in which sodium potassium tartrate was chemically bonded to the surface of the graphite at the aforementioned mass ratio.

[0071] [Example 3]

[0072] Sodium potassium tartrate, an organic molecule, was measured to a mass ratio of 95.5:1.0 to graphite. The mixture was stirred in an aqueous solution for 1 hour and then concentrated. The mixture was then dried under reduced pressure at 150°C for 12 hours and washed with water to obtain a negative electrode active material in which sodium potassium tartrate was chemically bonded to the graphite surface at the aforementioned mass ratio.

[0073] [Example 4]

[0074] Benzimidazole carboxylic acid, an organic molecule, was measured to a mass ratio of 96.2:0.3 to graphite. The solution was stirred in an aqueous solution for 1 hour and then concentrated. The solution was then dried under reduced pressure at 150°C for 12 hours to obtain a negative electrode active material in which the benzimidazole carboxylic acid was chemically bonded to the graphite surface at the aforementioned mass ratio.

[0075] [Example 5]

[0076] Benzimidazole carboxylic acid, an organic molecule, was measured to a mass ratio of 96.0:0.5 to graphite. The solution was stirred in an aqueous solution for 1 hour and then concentrated. The solution was then dried under reduced pressure at 150°C for 12 hours to obtain a negative electrode active material in which the benzimidazole carboxylic acid was chemically bonded to the graphite surface at the aforementioned mass ratio.

[0077] [Example 6]

[0078] Chlorobenzimidazole carboxylic acid, an organic molecule, was measured at a mass ratio of 96.2:0.3 to graphite. The solution was stirred in an aqueous solution for one hour and then concentrated. The solution was then dried under reduced pressure at 150°C for 12 hours to obtain a negative electrode active material in which chlorobenzimidazole carboxylic acid was chemically bonded to the graphite surface at the aforementioned mass ratio.

[0079] [Example 7]

[0080] Imidazole carboxylic acid, an organic molecule, was measured to a mass ratio of 96.2:0.3 to graphite. The solution was stirred in an aqueous solution for 1 hour and then concentrated. The solution was then dried under reduced pressure at 150°C for 12 hours to obtain a negative electrode active material in which imidazole carboxylic acid was chemically bonded to the graphite surface at the aforementioned mass ratio.

[0081] [Example 8]

[0082] Bismuth dioxide tartrate hydrate, an organic molecule, was measured to a mass ratio of 96.4:0.1 to graphite. The mixture was stirred in an aqueous solution for one hour and then concentrated. The mixture was then dried under reduced pressure at 150°C for 12 hours and washed with water to obtain a negative electrode active material in which bismuth dioxide tartrate hydrate was chemically bonded to the graphite surface at the aforementioned mass ratio.

[0083] [Example 9]

[0084] Alanine and mandelic acid, as organic molecules, were measured in a mass ratio of 95.5:0.4:0.6 relative to graphite. The mixture was stirred in an aqueous solution for one hour and then concentrated. The mixture was then dried under reduced pressure at 150°C for 12 hours to obtain a negative electrode active material in which alanine and mandelic acid were chemically bonded to the graphite surface at the aforementioned mass ratio.

[0085] [Example 10]

[0086] Sodium potassium tartrate, an organic molecule, was measured to a mass ratio of 95.5:1.0 to graphite. The mixture was stirred in an aqueous solution for 1 hour and then concentrated. The mixture was then dried under reduced pressure at 150°C for 12 hours and washed with water to obtain a negative electrode active material in which sodium potassium tartrate was chemically bonded to the graphite surface at the aforementioned mass ratio.

[0087] [Comparative Example 1]

[0088] Graphite is used as the negative electrode active material.

[0089] [Comparative Example 2]

[0090] Polyacrylic acid was measured to a mass ratio of 95.5:1.0 to graphite, stirred in an aqueous solution for 1 hour, and then concentrated. The solution was then dried under reduced pressure at 150°C for 12 hours to obtain a negative electrode active material in which polyacrylic acid was chemically bonded to the graphite surface at the aforementioned mass ratio.

[0091] [Comparative Example 3]

[0092] Graphite was used as a negative electrode active material, and benzimidazole carboxylic acid was added to the negative electrode at a mass ratio of 94.5 to graphite to form a negative electrode, and the negative electrode was evaluated.

[0093] Negative electrode

[0094] Using the negative electrode active materials obtained in Examples 1 to 10 and Comparative Examples 1 to 3, negative electrodes were produced according to the following procedure.

[0095] First, carboxymethyl cellulose (CMC) as a binder and acetylene black as a conductive aid are mixed and dispersed using a planetary mixer. Next, the negative electrode active materials obtained in Examples 1 to 10 and Comparative Examples 1 to 3 are added and mixed, and dispersed again using a planetary mixer. Then, a dispersion solvent and styrene butadiene rubber (SBR) as a binder are added and dispersed to prepare a negative electrode paste. Next, the negative electrode paste is applied to a copper current collector and dried, and then pressurized by roller pressing and dried in a vacuum at 100°C to obtain a negative electrode plate.

[0096] The resulting negative electrode plate was punched into a 34 mm x 44 mm size and used as the negative electrode. The mass ratios of organic molecules, negative electrode active material, acetylene black, CMC, and SBR in the negative electrode are shown in Table 1. The thickness of each negative electrode was 90 μm.

[0097] <Lithium-ion secondary batteries>

[0098] After stacking the positive electrode, separator, and negative electrode in this order, they were placed in a bag-shaped article obtained by heat-sealing an aluminum laminate for secondary batteries manufactured by Dai Nippon Printing Co., Ltd. Next, an electrolyte was added to the laminate to obtain the lithium-ion secondary batteries of Examples 1 to 10 and Comparative Examples 1 to 3. In addition, as a separator, a non-woven fabric (thickness 20 μm, porosity 45%) obtained by stacking three layers of polypropylene / polyethylene / polypropylene was used. In addition, as an electrolyte, the electrolytes shown in Tables 1 and 4 were used.

[0099] TEM-EDX analysis

[0100] For the negative electrode active material obtained in Example 3 and the negative electrode active material obtained in Comparative Example 1, a cross section of the negative electrode active material was prepared by focused ion beam (FIB) processing, and the cross section was observed by TEM. In addition, at the same time as the TEM cross section observation, elemental analysis of the cross section was performed by TEM-EDX ("ARM200F" manufactured by JEOL Ltd.). The results are shown in FIG. Figure 1 and Figure 2 .

[0101] <Charge and discharge test>

[0102] Charge and discharge tests were performed on the lithium-ion secondary batteries obtained in Examples 1 to 10 and Comparative Examples 1 to 3. For the initial charge and discharge test, the battery was charged to 4.2 V at a charge and discharge rate (C rate) of 0.1 C and discharged to 2.5 V at 0.1 C. Subsequently, the battery was charged to 4.2 V at 0.2 C and discharged to 2.5 V at 0.25 C.

[0103] Based on the results of the charge and discharge tests, the initial cell capacity and the cell capacity after endurance testing were calculated, and the results are shown in Table 1. Furthermore, the ratio of the cell capacity after endurance testing to the initial cell capacity, i.e., the capacity retention rate after endurance testing, was calculated, assuming the discharge current value at 0.25C was 100. The results are shown in Table 1.

[0104] Similarly, based on the results of the charge-discharge tests, the initial cell internal resistance and the post-durability cell internal resistance were calculated, and the results are shown in Table 1. Specifically, voltage values were plotted against current values, and the slope of the resulting approximate straight line was calculated as the internal resistance. Furthermore, the ratio of the post-durability cell internal resistance to the initial cell internal resistance, i.e., the post-durability resistance increase rate, was calculated, and the results are shown in Table 1.

[0105] [Table 1]

[0106]

[0107] [Table 2]

[0108]

[0109] The relative dielectric constants in Table 2 are relative dielectric constants at a frequency of 10 kHz.

[0110] [Table 3]

[0111]

[0112] [Table 4]

[0113]

[0114] <Inspection>

[0115] Figure 1 1 and 2 are diagrams showing the results of TEM-EDX analysis of a cross section of the negative electrode active material of Example 3. Figure 2 The figure shows the TEM-EDX analysis results of the cross section of the negative electrode active material of Comparative Example 1. In more detail, Figure 1 and Figure 2 These graphs are obtained by performing cross-sectional observation and carbon amount distribution measurement using TEM-EDX on each negative electrode active material before the endurance test.

[0116] Depend on Figure 2 It can be seen that the contrast of the outermost surface (edge surface) side of the negative active material of Comparative Example 1 is relatively weak. This indicates that the carbon density on the outermost surface (edge surface) side is low, so it is in a rougher state. In contrast, the negative active material of Example 3, that is, the negative active material with sodium potassium tartrate bonded to the surface of graphite, has a more uniform contrast as a whole, indicating that it is in a denser state as a whole, including the outermost surface (edge surface) side. This is because sodium potassium tartrate of organic molecules is bonded to the outermost surface (edge surface) side. Therefore, according to this result, in Example 3, it is confirmed that the sodium potassium tartrate of the organic molecule is bonded to the outermost surface (edge surface) of the negative active material. The same is true for other embodiments, and it is confirmed that each organic molecule is bonded to the outermost surface (edge surface) of the negative active material.

[0117] It can be seen from the records in Tables 1 to 4 that the lithium ion secondary batteries of Examples 1 to 10 are obtained by using a negative electrode for a lithium ion secondary battery containing a negative electrode active material, and the negative electrode active material is chemically bonded with an organic molecule having a dielectric constant larger than that of the electrolyte solvent. According to the lithium ion secondary batteries of Examples 1 to 10, compared with any of Comparative Example 1 in which no organic molecules are used, Comparative Example 2 in which a negative electrode active material chemically bonded with an organic molecule having a dielectric constant smaller than that of the electrolyte solvent is used, and Comparative Example 3 in which a negative electrode active material is not chemically bonded with an organic molecule, a higher capacity retention rate after endurance can be obtained, and the resistance increase rate after endurance can be suppressed to a lower level.

[0118] Similarly, it was confirmed that the lithium ions of Examples 1 to 10 used organic molecules with a relative dielectric constant of 90 or more at a frequency of 10 kHz, organic molecules with a polarized molecular structure within a single molecule or between molecules, zwitterionic compounds with positive and negative charges within a molecule, hydroxy acids, and organic molecules with a molecular weight of 89 to 616. The lithium ion secondary batteries according to Examples 1 to 10 can obtain a higher capacity retention rate after endurance and can suppress the resistance increase rate after endurance to a lower level, compared with any of Comparative Example 1 in which no organic molecules are used, Comparative Example 2 in which a negative electrode active material chemically bonded with an organic molecule having a dielectric constant smaller than that of the electrolyte solvent is used, and Comparative Example 3 in which a negative electrode active material not chemically bonded with an organic molecule is used.

[0119] In addition, the results of Example 8 confirmed the following. LiPF6, a lithium salt in the electrolyte, has low chemical stability and decomposes to produce HF if it comes into contact with moisture. In contrast, for example, imide salts have extremely strong electron-attracting properties due to the alkylsulfonyl group bonded to the central nitrogen, so the negative charge on the nitrogen is greatly delocalized. Therefore, the degree of lithium ion dissociation is very high, and it is also extremely stable thermochemically. Therefore, Example 8 mixes this imide salt and an electrolyte with improved chemical stability, such as LiFSI, with LiPF6 and uses them together. According to Example 8, the amount of HF in the electrolyte can be reduced, and the negative electrode active material of this example has organic molecules bonded to it, which can suppress the decomposition of the bonded organic molecules and further improve the capacity retention rate after endurance testing.

Claims

1. A negative electrode for a lithium ion secondary battery, comprising a current collector and a negative electrode active material layer formed on at least one side of the current collector. The negative electrode active material layer comprises a negative electrode active material, wherein the negative electrode active material is chemically bonded with organic molecules having a dielectric constant greater than that of the electrolyte solvent. The negative electrode active material is graphite. The organic molecule is one or more selected from the group consisting of benzimidazole carboxylic acid and chlorobenzimidazole carboxylic acid, in, The organic molecule has a relative dielectric constant of 90 or greater at a frequency of 10 kHz.

2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein The molecular weight of the aforementioned organic molecule is 89 to 616.

3. A lithium-ion secondary battery comprising: positive electrode, The negative electrode for lithium ion secondary battery according to claim 1, and a nonaqueous electrolyte comprising a nonaqueous solvent; in, The non-aqueous electrolyte comprises at least one lithium salt selected from the group consisting of LiPF6, LiBF4, LiCF3BF3, LiBF2(C2O4), LiB(C2O4)2, LiClO4, LiCF3SO3, LiAsF6, LiN(SO2F)2, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2C4F9)(SO2CF3), LiC4F9SO3, LiC(CF3SO2)3, LiCF3CO2 and LiSbF6.

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