JP5733184B2 - Negative electrode carbon material for non-aqueous electrolyte secondary battery, non-aqueous electrolyte secondary battery, and method for producing the negative electrode carbon material - Google Patents

Description

  The present invention relates to a negative electrode carbon material for a nonaqueous electrolyte secondary battery, a nonaqueous electrolyte secondary battery using the negative electrode carbon material, and a method for producing the negative electrode carbon material.

  Along with the rapid market expansion of portable electronic devices such as notebook computers and mobile phones, there is an increasing demand for small and large capacity secondary batteries with high energy density and excellent charge / discharge cycle characteristics. ing. In order to meet this demand, a non-aqueous electrolyte secondary battery using an alkali metal ion such as lithium ion as a charge carrier and utilizing an electrochemical reaction accompanying charge transfer by the charged particles has been developed.

  Here, the non-aqueous electrolyte secondary battery applied to an electric vehicle or the like is desired to have a durability of 10 years or more in accordance with the durable life assumed for the vehicle. In these batteries, lithium, which is a charge carrier, is irreversibly consumed, resulting in a problem that the capacity is reduced. For example, it is known that Li is consumed and deactivated along with the solvent reduction reaction that occurs on the negative electrode.

  As a conventional technique for solving the above-mentioned problems, a part or all of the edge portion of the core carbon material crystal is coated with a carbon material for coating formation, is substantially spherical or elliptical, and has a pulverized surface. A fired double-layer carbon material is disclosed (Patent Document 1).

Further, the positive electrode and the negative electrode are composed of a simple substance or a compound capable of occluding and releasing lithium, and at least the simple substance or the compound constituting the negative electrode is contacted with a fluorinating agent, and then a polymerizable organic substance, an organometallic compound, an organic electrolyte, and an inorganic electrolyte. at least by reacting with one, R value is the ratio of the peak intensity of 1350 ^-1 to the peak intensity of 1570～1620Cm ^-1 in an argon laser Raman spectrum (IA) (IB) is selected from (IB / IA) A negative electrode material for a lithium ion battery, characterized in that a grafted product is formed so as to be 0.18 or more and 2.00 or less, and a lithium ion secondary battery using the negative electrode material It is disclosed (Patent Document 2).

  Furthermore, a negative electrode material capable of inserting and extracting lithium that has been immersed in at least one of an acidic solution and an alkaline solution and then heat-treated is disclosed (Patent Document 3).

JP 11-310405 A JP 2001-110407 A JP 2004-200115 A

  When the double-layer carbon material of Patent Document 1 is employed for the negative electrode of a non-aqueous electrolyte secondary battery, there is a problem that the coating peels off due to expansion and contraction associated with charge and discharge, and the active surface of the negative electrode material is newly exposed. When a new active surface is exposed, the reaction with the electrolytic solution newly proceeds and the consumption of Li proceeds. If it is attempted to suppress the area where the active surface is renewed by simply reducing the specific surface area, the acceptability of Li is lowered and sufficient battery performance cannot be exhibited.

  In the lithium ion secondary battery of Patent Document 2, a fluorine compound such as LiF or HF is generated by residual fluorine, and not only gas is generated, but also the surface area is increased, and a basal surface that is inherently unlikely to deactivate Li. However, there was a problem in that durability was lowered due to Li deactivation.

  The negative electrode material of Patent Document 3 is characterized in that an acidic functional group is modified on the surface to improve the periphery of the liquid. There are problems that increase in resistance due to functional group addition and decrease in initial capacity and durability due to reaction between the functional group and the electrolytic solution occur, but there is no mention of this point. That is, the optimum modification amount and modification form have not been studied in consideration of the properties and reactions of the surface functional groups and the electrolytic solution. In addition, oxidation treatment using an acid solution or an alkali solution causes acid and alkali to penetrate into the graphite crystal to form a graphite intercalation compound, and subsequent heat treatment causes degradation such as cracks at the edge, increasing the specific surface area. That is, a new active surface may be generated.

  The present invention has been completed in view of the above circumstances, and a negative electrode carbon material for a non-aqueous electrolyte secondary battery capable of providing a non-aqueous electrolyte secondary battery excellent in durability characteristics and output characteristics, and the negative electrode carbon material It is a problem to be solved to provide a non-aqueous electrolyte secondary battery adopting the above and a method for producing the negative electrode carbon material.

(1) A negative electrode carbon material for a non-aqueous electrolyte secondary battery of the present invention that solves the above-mentioned problems comprises graphite and low-crystal carbon derived from pitch that covers a part or all of the graphite crystals constituting the graphite. It has a carbon material and an electron withdrawing group which is bonded to the surface of the carbon material and contains an oxygen atom.

  By introducing an electron withdrawing group on the surface, it becomes possible to advance the film formation on the negative electrode surface starting from the electron withdrawing group, and a uniform film can be formed. When a uniform film is formed, the conduction of Li ions also proceeds smoothly. Here, the formation of a uniform film is about the carbon material made of graphite, the active surface is renewed due to the stress applied by the expansion and contraction that progresses with the repetition of the battery reaction, and the consumption of Li ions ( However, since the graphite crystal is covered with low crystalline carbon, the stress associated with expansion / contraction is relieved and the renewal of the active surface can be prevented. Low crystalline carbon alone can easily break down during charge and discharge when it is coated around graphite, which can lead to the formation of a new surface due to the trial of destruction, which can lead to a decrease in capacity or resistance to Li ions on the surface. However, since a uniform film can be formed on the surface derived from the electron withdrawing group, it is possible to selectively enjoy only the advantageous effect of suppressing stress applied by expansion / contraction.

About the structure of said (1), it can illustrate as a preferable aspect that the structure of (2)-(5) shown below is additionally employ | adopted individually or in arbitrary combinations.
(2) The non-aqueous electrolyte secondary battery includes an oxalato complex in the electrolyte. Addition of the oxalato complex has been studied because it can suppress side reactions in the battery by being contained in the electrolyte. Here, the oxalato complex is present as a lithium salt in the electrolyte, but has a low reduction potential and delocalizes electrons as anions, which is unstable compared to a commonly used lithium salt. Since the electron withdrawing group introduced on the surface can selectively fix the lithium salt of the oxalato complex, it can be expected to exhibit a high stability improvement effect.
(3) O / C, which is the ratio of the number of oxygen atoms to carbon atoms on the surface, is 0.01 to 0.10, and the value of O / C at a depth of 100 nm from the surface is 0.002 or less. is there. The value of O / C is a value that increases as the amount of introduction of electron withdrawing groups (including oxygen atoms) increases, and by defining the value, the preferred amount of introduction of electron withdrawing groups to the surface is reduced. Can be prescribed.
(4) The total pore volume (pores having a pore diameter of 3 nm or more) per unit surface area (1 cm ² ) is 0.015 mL or less. The smaller the total pore volume is, the smaller the surface area is, and furthermore, the starting point at which crystal cracks progress is reduced, and the stability of the negative electrode carbon material is improved.
(5) I (002) / I (110), which is the X-ray diffraction peak intensity I (002) of the (002) plane with respect to the X-ray diffraction peak intensity I (110) of the (110) plane, is 50 or less. In the crystal grains of graphite crystals, I (002) is related to the spread in the thickness direction, I (110) is a value related to the spread in the plane direction, and the ratio I (002) / I (110) is It is a value that serves as an index of orientation. When I (002) / I (110) is small, the orientation is small, which means that the edge ratio is small. Since the electron withdrawing group is selectively introduced from a portion where the crystallinity of the terminal edge surface such as graphite crystal is low, the control of the edge ratio affects the introduction form of the electron withdrawing group. By controlling I (002) / I (110) within this range, the degree of introduction of the electron withdrawing group can be within a preferable range.
(6) The non-aqueous electrolyte secondary battery of the present invention that solves the above problems includes a negative electrode containing the above-described negative electrode carbon material as an active material, and a positive electrode. By including a high performance negative electrode carbon material as an active material, it is possible to provide a non-aqueous electrolyte secondary battery having high performance.
(7) A method for producing a negative electrode carbon material for a non-aqueous electrolyte secondary battery that solves the above-mentioned problem is a detarring step in which graphite coated with pitch is fired in a non-oxidizing atmosphere in a temperature range of 300 ° C to 500 ° C. And a high temperature baking step of baking in a non-oxidizing atmosphere in the range of 500 ° C. to 1000 ° C., and a precursor of an electron withdrawing group containing oxygen atoms in a gas phase, and a temperature range of 100 ° C. to 500 ° C. And an electron-attracting group introduction step of firing at.

  A low crystalline carbon layer can be formed around the graphite crystal in the detarring step and the subsequent high-temperature firing step. An electron withdrawing group can be introduced by performing a subsequent electron withdrawing group introduction step.

A preferred embodiment of the negative electrode carbon material for a nonaqueous electrolyte secondary battery of the present invention and a nonaqueous electrolyte secondary battery employing the negative electrode carbon material as an active material for a negative electrode will be described in detail based on the embodiments. The nonaqueous electrolyte secondary battery of this embodiment will be described by taking a lithium secondary battery that employs lithium ions as a charge carrier as an example.
(Negative carbon material for non-aqueous electrolyte secondary battery)
The negative electrode carbon material of this embodiment has a carbon material and an electron withdrawing group bonded to the surface of the carbon material. The form of the negative electrode carbon material of the present embodiment is not particularly limited, but is preferably particulate. Moreover, as for the negative electrode carbon material of this embodiment, it is desirable that the total pore volume per unit surface area (3 nm or more pore volume) is 0.15 mL or less. The pore volume is measured by the nitrogen gas adsorption method, and the surface area (specific surface area) is measured by the BET three-point method. In addition, the X-ray diffraction peak intensity I (002) of the (002) plane with respect to the (110) plane X-ray diffraction peak intensity I (110) of the negative electrode carbon material of the present embodiment when X-ray diffraction peak analysis was performed. I (002) / I (110) is preferably 50 or less, and particularly preferably 40 or less.

  The carbon material is composed of graphite and low crystalline carbon existing around the graphite crystal. Low crystalline carbon is formed using pitch as a raw material, and is fired in a non-oxidizing atmosphere in the presence of a particulate graphite crystal and pitch in a coexisting state. A layer of low low crystalline carbon can be formed. Although graphite is not particularly limited, it is desirable to employ natural graphite.

  It is desirable that the graphite crystal and the low crystalline carbon are chemically bonded. The low crystalline carbon is sufficient if it covers at least part of the surface of the graphite crystal particles.

  The abundance ratio between the graphite crystal and the low crystalline carbon is not particularly limited, but the low crystalline carbon is desirably contained in a content of about 0.5% to 10% based on the total mass of the negative electrode carbon material, It is further desirable to make the content about 1.0% to 4.5%.

The electron withdrawing group is introduced on the surface of the carbon material, and is desirably bonded to part or all of the edge portion of the graphite of the carbon material. The term “electron-withdrawing group is introduced on the surface” means that the electron-withdrawing group is introduced with a thickness of 5 nm or more from the surface.
The electron withdrawing group is desirably 100 nm or less in thickness from the surface. An example of the upper limit of the thickness of the electron withdrawing group is 80 nm. Further, the lower limit of the thickness of the electron withdrawing group can be exemplified by 10 nm and 15 nm. As an example of a method for measuring the thickness of the electron withdrawing group, the thickness of the portion where (number of oxygen atoms) / (number of carbon atoms) (O / C) is greater than 0.002 is measured, and the thickness is measured. Is a thickness in which an electron withdrawing group is introduced. The value of O / C can be measured by depth analysis using TEM or XPS.

What functional group is an electron-withdrawing group is determined by the properties when bonded to the surface of the carbon material. Specifically, a carboxyl group, a carbonyl group, a hydroxyl group, a lactone group, a ketone group, etc. can be exemplified, and it can be easily judged whether oxygen is introduced into the surface of the carbon material (if it is introduced, an electron It can be determined that an attractive group has been introduced). Examples of a method for introducing these functional groups include a method of performing a heat treatment (firing) in the presence of a precursor containing oxygen atoms (preferably a gaseous precursor). Examples of the precursor containing an oxygen atom include oxygen, carbon dioxide, carbon monoxide, nitrous oxide, and volatile components of organic compounds such as carbonates.
(Method for producing negative electrode carbon material for nonaqueous electrolyte secondary battery)
The manufacturing method of the negative electrode carbon material of this embodiment has a detarring process, a high temperature baking process, and an electron withdrawing group introduction process.
Detarring step: The detarring step is a step of heating and baking graphite coated with pitch at a temperature within a range of 300 ° C to 500 ° C. By heating in this temperature range, volatile components in the pitch can be removed (firing).

  The heating atmosphere is a non-oxidizing atmosphere. Of course, the non-oxidizing atmosphere can be judged by completely not containing an oxidizing substance, but as a result of analyzing the product, the product is not oxidatively decomposed (particularly pitch-derived carbon). The component remains, low crystalline carbon remains in the high-temperature firing process described later, and the crystal structure of the graphite crystal remains largely undisturbed. good.

  Although it does not specifically limit as heating time, It can carry out within the range of about 3 hours-15 hours. A desirable index for determining the heating temperature and the heating time can be determined as the time until tar can be removed to a desired extent.

The “graphite coated with pitch” in this step includes a state in which pitch and graphite are simply mixed. By changing the degree of coating of graphite by pitch, the degree of coating of low crystalline carbon can be controlled. It should be noted that it is not always necessary for the graphite coating with low crystalline carbon to be entirely covered with no gaps.
High temperature firing step: The high temperature firing step is a step of firing in a non-oxidizing atmosphere in the range of 500 ° C to 1000 ° C. By heating in this temperature range, the pitch-derived carbon component can be firmly bonded on the surface of the graphite. The non-oxidizing atmosphere is as described in the detarring step.

Although it does not specifically limit as heating time, It can carry out within the range of about 3 hours-15 hours. The heating temperature and heating time can be determined by using as an index that low crystalline carbon existing around the graphite crystal is generated.
Electron-withdrawing group introduction step: This is a step in which an electron-withdrawing group precursor containing an oxygen atom is brought into contact in the gas phase and heated in a temperature range of 100 ° C to 500 ° C. Since the precursor described in the column of the negative electrode carbon material can be used as it is as the precursor containing oxygen atoms, further explanation is omitted. Here, the amount of the precursor to be mixed is a value in which an appropriate range changes depending on the heating temperature and the heating time, and it is desirable to set the amount of disorder of the crystal structure in the portion derived from the graphite crystal after this step.

  As the heating temperature, it is desirable that the electron withdrawing group can be introduced while maintaining the crystal structure of graphite.

  Although it does not specifically limit as heating time, It can carry out within the range of about 0.5 hour-5 hours.

As described above, the addition ratio of the precursor, the heating temperature, and the heating time are such that the disorder of the crystal structure of the portion derived from the graphite crystal is reduced, and the electron withdrawing property is obtained with a necessary thickness around the carbon material. It can be determined as an indicator that a group can be introduced.
(Non-aqueous electrolyte secondary battery)
The nonaqueous electrolyte secondary battery of the present embodiment includes a negative electrode containing the above-described negative electrode carbon material as a negative electrode active material, a positive electrode, a nonaqueous electrolyte, and other members that can be used as necessary.

  The positive electrode is not particularly limited in its material configuration as long as it has a positive electrode active material that can release lithium ions during charging and can be occluded during discharging. it can. In particular, it is preferable to use a material in which a mixture obtained by mixing a positive electrode active material, a conductive material, and a binder is applied to a current collector to form an active material layer.

Although it does not specifically limit as a positive electrode active material, A lithium containing transition metal oxide can be illustrated. The lithium-containing transition metal oxide is a material capable of removing and inserting Li ⁺ , and can be exemplified by a lithium-metal composite oxide having a layered structure or a spinel structure. Specifically, Li ₁ -Z FePO ₄ , Li ₁ -Z NiO ₂ , Li ₁ -Z MnO ₂ , Li ₁ -Z Mn ₂ O ₄ , Li ₁ -Z CoO ₂ , Li ₁ -Z Co _x Mn _y Ni _(1-xy) O _{2 and the} like, and one or more of them can be included. In this illustration, Z is a number from 0 to less than 1, and x and y are numbers from 0 to 1. A material obtained by adding or substituting a transition metal such as Li, Mg, Al, or Co, Ti, Nb, or Cr may be used. Moreover, not only these lithium-metal composite oxides are used alone, but also a plurality of them can be mixed and used. In addition, a conductive polymer material, a material having a radical, or the like can be mixed.

As the positive electrode active material, composite oxides of lithium and transition metals such as LiMnPO ₄ , LiFePO ₄ , Li ₂ FeP ₂ O 7, Li ₂ FeSiO ₄ , LiMn ₂ O ₄ , LiCoO ₂ , LiNiO ₂ are more preferable. That is, since it has excellent performance as an active material such as excellent diffusion performance of electrons and lithium ions, a battery having high charge / discharge efficiency and good cycle characteristics can be obtained. In particular, it is desirable to employ olivine-type phosphoric acid (such as LiMnPO ₄ and LiFePO ₄ ). The olivine-type phosphoric acid is often used at an upper limit potential of about 4.0 V, and the possibility that the low LUMO support salt is decomposed and consumed can be reduced even under normal use conditions.

  The binder has an action of holding the active material particles. As the binder, organic binders and inorganic binders can be used. For example, compounds such as polyvinylidene fluoride (PVDF), polyvinylidene chloride, polytetrafluoroethylene (PTFE), carboxymethyl cellulose, and the like. Can give.

  The conductive material has an action of ensuring the electrical conductivity of the positive electrode. Examples of the conductive material include one or a mixture of two or more carbon materials such as carbon black, acetylene black (AB), and graphite.

  Further, as the current collector of the positive electrode, for example, a material obtained by processing a metal such as aluminum or stainless steel, for example, a foil processed into a plate shape, a net, a punched metal, a foam metal, or the like can be used.

  The negative electrode contains the negative electrode carbon material of the present embodiment as an active material. In addition, a lithium ion can be stored as an active material during charging and can be released during discharging. For example, it is metallic lithium, an alloy-based material, a carbon-based material, and the like, and is not particularly limited by the material configuration, and a known material configuration can be used. A composite material obtained by mixing these negative electrode active materials and a binder is applied to a current collector to form an active material layer.

  The binder has an action of holding the active material particles. As the binder, an organic binder or an inorganic binder can be used, and examples thereof include PVDF, polyvinylidene chloride, PTFE, carboxymethylcellulose, and the like.

  As the current collector for the negative electrode, for example, a material obtained by processing copper, nickel, or the like, for example, a foil, net, punched metal, foam metal, or the like processed into a plate shape can be used.

  The nonaqueous electrolyte may be in any form such as liquid or gel. Examples of the liquid non-aqueous electrolyte include those containing a supporting salt and an organic solvent that dissolves the supporting salt, and those containing an ionic liquid. As an organic solvent, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) have a high oxidative decomposition potential of 4.3 V or higher and are non-aqueous electrolytes. By adopting as a solvent, the stability of the non-aqueous electrolyte secondary battery is increased.

  In addition to these solvents, organic solvents that are commonly used in electrolyte solutions for nonaqueous electrolyte secondary batteries can be employed. For example, carbonates other than the carbonates described above, halogenated hydrocarbons, ethers, ketones, nitriles, lactones, oxolane compounds, and the like can be used. In particular, propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, vinyl carbonate (VC), and a mixed solvent thereof can be employed. It is possible to act as an electrolyte by dissolving the supporting salt in these solvents.

The supporting salt is lithium hexafluorophosphate, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiSbF ₆ , LiSCN, LiClO ₄ , Salt compounds such as LiAlCl ₄ , NaClO ₄ , NaBF ₄ , NaI, and derivatives thereof. Among these, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiN (FSO ₂ ) ₂ , LiN (CF ₃ One or more selected from the group consisting of a derivative of SO ₂ ) (C ₄ F ₉ SO ₂ ), a derivative of LiCF ₃ SO _3, a derivative of LiN (CF ₃ SO ₂ ) _{2 and} a derivative of LiC (CF ₃ SO ₂ ) ₃ It is preferable to use a salt from the viewpoint of electrical characteristics.

  And a nonaqueous electrolyte can also be made into a gel form by containing a gelatinizer.

In addition, an ionic liquid that can be used in a nonaqueous electrolyte secondary battery can be employed instead of or in addition to the organic solvent described above. Examples of the cation component of the ionic liquid include N-methyl-N-propylpiperidinium and dimethylethylmethoxyammonium cation. Examples of the anion component include BF ⁴⁻ , N (SO ₂ CF ₃ ) ^{2−, and the} like. Can be mentioned.

  The nonaqueous electrolyte secondary battery of this embodiment can have other members selected as necessary. Examples of such a member include a separator and a case. The separator is interposed between the positive and negative electrodes and is a member that achieves both electrical insulation and ion conduction. When the employed non-aqueous electrolyte is liquid, the separator also plays a role of holding the non-aqueous electrolyte. Examples of the separator include a porous synthetic resin film, particularly a porous film of a polyolefin polymer (polyethylene or polypropylene). Furthermore, it is preferable that the separator adopts a form larger than the area of the positive electrode and the negative electrode for the purpose of ensuring insulation between the positive electrode and the negative electrode. The case is a member that houses members constituting these batteries. Since the nonaqueous electrolyte secondary battery is easily affected by moisture, it is desirable to form the case from a material having low moisture permeability.

The nonaqueous electrolyte secondary battery of the present invention will be described in detail below based on examples.
-Manufacture of the negative electrode carbon material of a test example The negative electrode carbon material which has the structure shown in Table 1 was manufactured. In the negative electrode materials of Test Examples 1-1 to 1-4 and Test Examples 2-3 and 2-4, 98 parts by mass of natural graphite and 2 parts by mass of pitch were mixed and then fired at 400 ° C. for 8 hours in a nitrogen atmosphere ( Detarring step). Then, it baked at 700 degreeC under nitrogen atmosphere for 5 hours (high temperature baking process). As a result, a layer made of low crystalline carbon was formed on the surface. Then, it baked at 200 degreeC in the air containing 0.4 volume% of carbon dioxide for 2 hours (electron withdrawing group introduction | transduction process). The negative electrode carbon material thus produced was used as the negative electrode carbon material of Test Example 1-1.

  In Test Examples 1-2 to 1-4 and Test Examples 2-3 and 2-4, the thickness of the electron withdrawing group was changed by changing the treatment temperature and the treatment time in the electron withdrawing group introduction step. Specifically, Test Example 1-2 was fired at a treatment temperature of 200 ° C. for 3 hours, Test Example 1-3 was fired at a treatment temperature of 200 ° C. for 5 hours, and Test Example 1-4 was fired at a treatment temperature of 500 ° C. for 3 hours. Example 2-3 was fired at a treatment temperature of 200 ° C. for 0.5 hour, and Test Example 2-4 was fired at a treatment temperature of 500 ° C. for 5 hours. As a result, a negative electrode carbon material having an electron withdrawing group having a thickness shown in Table 1 was obtained.

  For Test Example 2-1, the material before the electron withdrawing group introduction step in Test Example 1-1 was used as it was. Accordingly, no electron withdrawing group is introduced on the surface.

  Test Example 2-2 was fired for 5 hours at a treatment temperature of 350 ° C. in air containing 2% by volume of carbon dioxide with respect to natural graphite (electron-withdrawing group introduction step). Therefore, it has no low crystalline carbon.

Depth analysis of O / C values (the O / C value on the surface and the thickness of the layer having an O / C value of 0.002 or more) and the pore volume per unit surface area for the negative electrode carbon materials of these test examples The value of was measured. The results are shown in Table 1.
-Manufacture of test batteries Test batteries 1-1 to 1-4 and 2-1 to 2-4 were prepared, respectively. Each test battery was prepared by combining the components shown in Table 1 (corresponding negative electrode carbon materials, respectively). Although the manufacturing method will be described by taking the test battery of Test Example 1-1 as an example, other test batteries were manufactured in the same manner.

The test battery of Test Example 1-1 is a lithium secondary battery using a lithium composite oxide represented by the composition formula LiFePO ₄ as a positive electrode active material and graphite as a negative electrode active material.

  The positive electrode was manufactured as follows. First, 80 parts by mass of the positive electrode active material, 10 parts by mass of acetylene black (AB) as a conductive material, and 10 parts by mass of PVdF as a binder are mixed, and an appropriate amount of N-methyl-2-pyrrolidone is added. By adding and kneading, a paste-like positive electrode mixture was obtained. This positive electrode mixture was applied to both surfaces of a positive electrode current collector made of aluminum foil having a thickness of 15 μm, dried, and a sheet-like positive electrode was produced through a pressing process. This positive electrode was cut into a strip shape to obtain a positive electrode plate. The positive electrode mixture was scraped from a part of the positive electrode plate, and the positive battery lead was joined.

  The negative electrode is 98 parts by mass of the negative electrode carbon material of Test Example 1-1, 1 part by mass of carboxymethyl cellulose (CMC) as a binder, and 1 part by mass of styrene butadiene rubber (SBR) as a binder. After mixing, an appropriate amount of N-methyl-2-pyrrolidone was added and kneaded to obtain a paste-like negative electrode mixture. This negative electrode mixture was applied to both sides of a copper foil negative electrode current collector having a thickness of 10 μm, dried, and subjected to a pressing step to produce a sheet-like negative electrode. This negative electrode was cut into a strip shape to obtain a negative electrode plate. The negative electrode mixture was scraped from a part of the negative electrode plate, and the battery lead of the negative electrode was joined.

  A separator (made of polyethylene: thickness 15 μm) was interposed between these positive and negative electrode plates, and wound into a flat type to form a wound type electrode body (power generation element: 25 Ah). The outermost periphery of the electrode body was wound with a separator to ensure insulation from the surroundings. Then, electrode terminals having a width of 100 mm were connected to the battery leads of the positive and negative electrodes by welding. As the electrode terminal, one composed of a terminal body made of aluminum for the positive electrode and a copper resin body (polypropylene: 100 μm) covering the surface thereof was adopted for the negative electrode.

The non-aqueous electrolyte is 10% by mass of LiPF ₆ with respect to a mixed solvent in which ethylene carbonate (EC): dimethyl carbonate (DMC): ethyl methyl carbonate (EMC) is mixed at a ratio of 30:30:40, VC In which 2% by mass was dissolved was used.

  The battery case adopted a laminate outer package.

After the battery was manufactured, the conditioned one was used for testing. In the conditioning, after injecting the non-aqueous electrolyte, gas was generated by charging to SOC 10%. After the generated gas was removed, the periphery of the laminate outer package was completely sealed by thermal fusion, and then charging and discharging were repeated twice in the terminal voltage range of 2V to 4V. Then, it hold | maintained at 60 degreeC for 36 hours.
・ Cycle test (endurance characteristics evaluation)
After conditioning each test battery, at an ambient temperature of 60 ° C., charge / discharge 300 cycles with CC-CV charge (1 / 3C, 4.0V) and CC discharge (up to 2.0V) as one cycle. The capacity maintenance ratio (the degree of capacity deterioration: the degree of deterioration of the power generation element) with respect to the charge capacity (initial capacity) of the second time was calculated. The capacity retention rate was calculated as a relative value when the capacity retention rate of the test battery 2-1 was 100, and is shown in Table 1. The higher this value, the better the durability.

  Further, the output characteristics after the durability test were measured. The measurement of the output characteristics was obtained as a resistance value from a voltage gradient after 10 seconds when discharging was performed at 1C, 2C, 3C, 5C, and 10C for 30 seconds in a state of SOC 60%. The relative values when the value in the test battery 2-1 is 100 are shown in Table 1. The smaller this value, the better the output characteristics.

  As is apparent from Table 1, when the test battery 2-1 having the negative electrode carbon material of Test Example 2-1 in which graphite is coated with low crystalline carbon is considered as a reference, an electron withdrawing group is introduced. In the test battery 2-2 which was not coated with a layer made of low crystalline carbon alone, it was found that although the output characteristics were improved, the durability characteristics were lowered.

On the other hand, in the test batteries 1-1 to 1-4 and the test batteries 2-3 and 2-4 in which a layer made of low crystalline carbon is formed and an electron withdrawing group is introduced on the surface, durability characteristics are examined. It turns out that improvements have been made. Furthermore, it has become clear that the test batteries 1-1 to 1-4 have improved both output characteristics and durability characteristics, and the thickness of the functional group (electron withdrawing group) is preferably 100 nm or less (test battery). 2-4 and other test batteries are considered), and the value of the pore volume per unit surface area is preferably 0.015 mL (consideration from the comparison between test battery 2-3 and other test batteries). I understood that.
-Effect of crystallinity of graphite on battery performance Except for using the graphite material shown below, a negative electrode carbon material was produced in the same manner as in Test Example 1-2, and Test Examples 1-5 and 2-5, respectively. Negative electrode carbon material. In Test Example 1-5, graphite, ketjen black, and acetylene black were mixed at a ratio of 92: 5: 3 and then heated under reduced pressure (200 ° C.) for 2 hours. In Test Example 2-5, graphite and Ketjen black were mixed. A mixture of chain black and acetylene black in a ratio of 98: 1: 1 was used.

  Using these negative electrode carbon materials, test batteries were prepared in the same manner as the test battery 1-2, and the above-described durability characteristics and output characteristics were tested. The results are shown in Table 2.

As is clear from Table 2, the test batteries 1-5 were markedly improved in both durability characteristics and output characteristics. Although test battery 2-5 slightly deteriorated in durability characteristics, remarkable completeness in output characteristics was recognized as compared with other test batteries. From this result, it was found that it is desirable to set the value of I (002) / I (110) to 50 or less in order to improve both durability characteristics and output characteristics.
-Examination of non-aqueous electrolyte The same as described above for test batteries 1-6 to 1-9 produced by the same method as test battery 1-2, except that 2% by mass of the oxalato complex shown in Table 3 was added as a supporting salt. A durability characteristic test and an output characteristic test were conducted. The results are shown in Table 3.

  As is clear from Table 3, it was found that by adding the oxalato complex to the non-aqueous electrolyte, both durability characteristics and output characteristics were excellent.

The addition amount of the oxalato complex is 4 mass% (test battery 1-10), 6 mass% (test battery 1-11), 8 mass% (test battery 1-12), 10 mass% with respect to test battery 1-6. A test battery was produced by the same method except that (Test battery 1-13) was used, and the durability characteristics and output characteristics tests described above were performed. As a result, it was confirmed that the capacity of the test battery 1-10 was improved as compared with the other test batteries and the resistance was lowered. It can be inferred that this is because the oxalato complex is in an appropriate amount, so that an oxalato complex that is not reductively decomposed by a functional group on the surface of the negative electrode is not generated, and is not oxidatively decomposed at the positive electrode with charge / discharge. That is, it was found that the oxalato complex is preferably 0.5% by mass to 4% by mass.
-Examination of production conditions A negative electrode carbon material was produced in the same manner as the negative electrode carbon material of Test Example 1-2 except that the temperature in the electron withdrawing group introduction step was set to 700 ° C, and the negative electrode carbon material was tested. Battery 2-6 was produced. The manufactured test battery 2-6 was subjected to the above-described durability characteristic test and output characteristic test. As a result, the test battery 2-6 had a capacity maintenance ratio of 94 and a resistance of 120. The reason for this is thought to be that the crystal structure of graphite was greatly disturbed due to the high temperature causing oxygen to enter and oxidize between the graphite layers. This confirms the graphite intercalation compound from the XRD analysis conducted together, and supports the above inference.

Claims (6)

A carbon material composed of graphite and pitch-derived low crystalline carbon covering a part or all of the graphite crystals constituting the graphite, and an electron withdrawing group bonded to the surface of the carbon material and containing an oxygen atom. Yes, and
O / C, which is the ratio of the number of oxygen atoms to the surface carbon atoms, is 0.01 to 0.10,
The negative electrode carbon material for a non-aqueous electrolyte secondary battery , wherein the O / C value at a depth of 100 nm from the surface is 0.002 or less .   The negative electrode carbon material according to claim 1, wherein the nonaqueous electrolyte secondary battery includes an oxalato complex in the electrolyte. 3. The negative electrode carbon material according to claim 1, wherein the total pore volume (pores having a pore diameter of 3 nm or more) per unit surface area (1 cm ² ) is 0.015 mL or less. (110) plane of the X-ray diffraction peak intensity is I the relative (110) (002) X-ray diffraction peak intensity I (002) I (002) / I (110) is 50 or less claims 1-3 The negative electrode carbon material according to any one of the above. A nonaqueous electrolyte secondary battery comprising: a positive electrode; and a negative electrode containing the negative electrode carbon material according to any one of claims 1 to 4 as an active material. A detarring step of firing the pitch-coated graphite in a non-oxidizing atmosphere in a temperature range of 300 ° C to 500 ° C;
A high-temperature firing step of firing in a non-oxidizing atmosphere in the range of 500 ° C. to 1000 ° C .;
An electron-withdrawing group introduction step of bringing a precursor of an electron-withdrawing group containing an oxygen atom into contact in a gas phase and firing in a temperature range of 100 ° C to 500 ° C;
The manufacturing method of the negative electrode carbon material for nonaqueous electrolyte secondary batteries characterized by having.