JP2014026991A - Secondary battery, and graphite material for secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a lithium ion secondary battery having high capacity and high reliability by suppressing deterioration of the inside of the battery caused by exposure to thermal history.SOLUTION: A ratio RG (=Gs/Gb) of graphitization degree Gs determined from surface-enhanced Raman spectral spectrum to graphitization degree Gb determined from Raman spectral spectrum using argon laser light, which are structural parameters of carbon material, is prescribed to 4.5 or higher, and a nonaqueous electrolyte secondary battery is constituted by using the carbon material having the RG as a negative electrode. In the surface-enhanced Raman spectral spectrum using the argon laser light, the nonaqueous electrolyte secondary battery is constituted by using material having a peak in a range of 1360 cmor more as the negative electrode.

Description

  The present invention relates to a secondary battery and a graphite material for a secondary battery.

  In recent years, as represented by mobile phones and notebook personal computers, electronic devices are rapidly becoming smaller and more portable, and demands for higher energy secondary batteries are increasing.

  Examples of conventional secondary batteries include lead batteries, Ni-Cd batteries, and Ni-MH batteries, but the discharge voltage is low and the energy density is not sufficiently high. On the other hand, lithium secondary batteries in which metallic lithium, lithium alloy, or a carbon material capable of electrochemically absorbing and releasing lithium ions are used as a negative electrode active material and combined with various positive electrodes have been developed and put into practical use. This type of battery has a high battery voltage and is expected as a secondary battery having a higher energy density per weight or volume than the above-described conventional battery.

  This type of secondary battery was originally studied in a system using metallic lithium or a lithium alloy for the negative electrode, but the negative electrode using metallic lithium or a lithium alloy has problems in charge / discharge efficiency, dent light, and the like. Yes, with some exceptions, it has not been put into practical use.

  Therefore, it has now been considered promising and realized to use a carbon material capable of electrochemically occluding and releasing lithium ions for the negative electrode. Compared with negative electrodes using metallic lithium or lithium alloys, negative electrodes using this type of material do not generate metal lithium dendrites or alloy fines during charging / discharging, and have high coulomb efficiency. A lithium secondary battery excellent in performance can be configured.

In addition, in a battery using this type of material as a negative electrode active material, lithium metal is not deposited in the battery, and a highly safe lithium secondary battery can be configured, and is currently made of a lithium-containing composite oxide. Combined with the positive electrode, it has been commercialized. This battery is referred to as a so-called lithium ion battery, and uses a carbon material for the negative electrode, LiCoO _{2 for} the positive electrode, and a non-aqueous electrolyte composed of a non-aqueous solvent for the electrolyte.

  The carbon material used as a negative electrode is roughly classified as follows. In other words, graphite materials that have been produced in ores and are now able to be artificially produced, graphitizable carbon materials that are precursors of artificial graphite materials, and high temperatures at which graphite is artificially produced It is a non-graphitizable carbon material that does not become graphite when exposed to. At present, graphite materials and non-graphitizable carbon materials are used from the viewpoint of negative electrode capacity.

  Due to the development of electronic devices, the lithium ion battery has been attracting attention as a power source thereof, and has been rapidly installed in notebook personal computers by taking advantage of its small size, light weight and high capacity. Notebook PCs are characterized by their portability, so they must be smaller and have higher performance. By the way, for higher performance, it is necessary to increase the operating frequency of the built-in CPU. However, this increases power consumption and increases the amount of heat generated during operation. Also, the smaller the size, the smaller the space inside the PC body. This makes it difficult for heat generated during use to escape and the temperature inside the PC body to rise.

  As described above, when the temperature inside the electronic device rises, the influence on the battery also increases. That is, when this thermal history is received while the battery is charged, a deterioration reaction occurs in the battery, and the decrease in battery capacity due to this reaction cannot be recovered. Accordingly, an object of the present invention is to suppress deterioration inside the battery caused by exposure to thermal history, and to obtain a high capacity and high reliability secondary battery. Is to provide.

  In order to achieve the above-mentioned problems, the inventors have intensively studied, and in the graphite material for the negative electrode, by defining its structural parameters, even when a battery using this material for the negative electrode is stored at high temperature. As a result, it has been found that a secondary battery with high capacity and high reliability can be obtained with little capacity deterioration.

First, the invention according to claim 1 is a surface-enhanced Raman spectroscopy spectrum in the case where silver having a thickness of 10 nm is deposited on a graphite material using an argon laser beam having a wavelength of 514.5 nm and a spectrometer having a wave number resolution of 4 cm ⁻¹ . The peak of the vibration mode (Psa = 1580 cm ^{−1 to} 1620 cm ⁻¹ ) derived from the graphite crystalline structure is in the range of 1581.22 cm ⁻¹ or more, and is obtained from the Raman spectroscopic spectrum using an argon laser beam. A secondary battery is formed by using, as a negative electrode, a graphite material having a ratio RG (= Gs / Gb) of a graphitization degree Gb and a graphitization degree Gs obtained from a surface-enhanced Raman spectroscopy spectrum of 4.16 or more. .

The invention according to claim 10 is a surface-enhanced Raman spectroscopic spectrum obtained by depositing silver at a thickness of 10 nm using an argon laser beam having a wavelength of 514.5 nm and a spectrometer having a wave number resolution of 4 cm ^−1. Graphitization which has a peak of vibration mode (Psa = 1580 cm ^{−1 to} 1620 cm ⁻¹ ) derived from a crystalline structure in a range of 1581.22 cm ⁻¹ or more and is obtained from a Raman spectroscopic spectrum using an argon laser beam. It is a graphite material for a secondary battery in which the ratio RG (= Gs / Gb) between the degree Gb and the degree of graphitization Gs determined from the surface-enhanced Raman spectrum is 4.16 or more.

Next, a method for measuring the structural parameters used in the present invention will be described.
The structural parameters used in the present invention are measured by applying Raman spectroscopy. Conventionally, Raman spectrum of the graphite material, the vicinity of 1580cm ^-1 ~1620cm ^-1 as a vibration mode derived from a graphite crystalline structure (Pba), 1350cm ^-1 ~1400cm as a vibration mode derived from the turbostratic structure of amorphous ^It has a peak in the vicinity of ^-1 (Pbb). When the disorder of the structure of the graphite material proceeds, on the Raman spectrum, the Pba intensity (height Hba) decreases and the Pbb intensity (height Hbb) increases. The ratio of the two peak heights of Pba and Pbb represents the degree of graphitization.

Surface-enhanced Raman spectroscopy (SERS) is a method of measuring a thin metal film such as silver or gold on the surface of a sample, and was invented by Fleischmann et al. In 1974. It is characterized by an increase. The Raman spectrum of the graphite material obtained by this method is different in analysis depth, but the same vibration mode as in normal Raman spectroscopy can be obtained. And around 1580cm ^-1 ~1620cm ^-1 (Psa) as a vibration mode derived from a graphite crystalline structure, peak near 1350cm ^-1 ~1400cm ^-1 (Psb) as a vibration mode derived from the turbostratic non crystalline The ratio of the obtained Psa strength (height Hsa) to Psb strength (height Hsb) represents the degree of graphitization of the outermost surface layer portion of the particle.

  The secondary battery or the secondary battery graphite material of the present invention defines a range having a surface-enhanced Raman spectroscopic spectrum peak for the graphite material, and uses a graphite material that meets this rule for the negative electrode, so it is stored in a high temperature environment. Suppresses battery capacity degradation.

  According to the present invention, by defining structural parameters in the vicinity of the surface of the graphite material, there is provided a secondary battery or a graphite material for a secondary battery that has a high reliability with little deterioration even when stored at high temperatures. It becomes possible.

1 is a cross-sectional view of a cylindrical secondary battery using a spiral electrode according to the present invention. It is a figure which shows the relationship between a capacity | capacitance recovery rate and RG. It is a figure which shows the relationship between a capacity | capacitance recovery rate and Psb.

Next, embodiments of the invention will be described.
Any material can be used as the graphite material used in the present invention as long as the above-described structural parameters are satisfied. Among them, the graphite material is particularly preferable. Graphite materials include natural graphite produced from ores and the like, and artificial graphite obtained by heat treatment at a high temperature of 2000 ° C. or higher using organic materials as raw materials.

  Typical examples of the organic material used as a starting material for producing the artificial graphite include coal and pitch. As pitch, tars obtained by high-temperature pyrolysis of coal tar, ethylene bottom oil, crude oil, etc., asphalt, etc. (vacuum distillation, atmospheric distillation, steam distillation), thermal polycondensation, extraction, chemical polycondensation, etc. There are also those obtained by operation and other pitches generated during dry distillation of wood. Furthermore, examples of the starting material for forming a pitch include polyvinyl chloride resin, polyvinyl acetate, polyvinyl butyrate, and 3,5-dimethylphenol resin. These coals and pitches are present in a liquid state at a maximum of about 400 ° C. during carbonization, and by maintaining at that temperature, the aromatic rings are condensed and polycyclic to form a laminated orientation, and then a temperature of about 500 ° C. or higher. Then, a solid carbon precursor, that is, semi-coke is formed. Such a process is called a liquid-phase carbonization process and is a typical process for producing graphitizable carbon.

  Other condensed polycyclic hydrocarbon compounds such as naphthalene, phenanthrene, anthracene, triphenylene, pyrene, perylene, pentaphen, and pentacene, other derivatives (for example, carboxylic acids, carboxylic acid anhydrides, carboxylic acid imides, etc.) or mixtures, acenaphthylene Indole, isoindole, quinoline, isoquinoline, quinoxaline, phthalazine, carbazole, acridine, phenazine, phenanthridine and other condensed heterocyclic compounds, and derivatives thereof can also be used as raw materials.

  In order to produce the desired artificial graphite using the above organic material as a starting material, for example, the organic material is carbonized at 300 ° C. to 700 ° C. in an inert gas stream such as nitrogen, and then is raised in an inert gas stream. It is calcined under the conditions of a temperature rate of 1 ° C to 100 ° C per minute, an ultimate temperature of 900 ° C to 1500 ° C, a holding time at the ultimate temperature of about 0 hours to 30 hours, and further heat treated at 2000 ° C or higher, preferably 2500 ° C or higher. Can be obtained. Of course, depending on the case, carbonization and calcination operations may be omitted. The carbon material or graphite material heat-treated at high temperature is pulverized and classified to be used as the negative electrode material. This pulverization is preferably performed before carbonization, calcination, and high-temperature heat treatment.

Furthermore, as a material having more practical performance, it is preferable to use a graphite material having a true density of 2.1 g / cm ³ or more and a bulk specific gravity of 0.4 g / cm ³ or more. Since the graphite material has a high true density, when the negative electrode is formed with this, the electrode filling property is improved and the energy density of the battery is improved. In addition, when a graphite material having a bulk specific gravity of 0.4 g / cm ³ or more is used among the graphite materials, the electrode structure is improved and the cycle characteristics are improved. This is because the graphite material having a large bulk specific gravity can be relatively uniformly dispersed in the negative electrode mixture layer. Furthermore, when a material having a low specificity with a bulk specific gravity of 0.4 g / cm ³ or more and an average shape parameter xave of 125 or less is used, the electrode structure is further improved and the cycle characteristics are further improved. The

In order to obtain the above-described graphite material, a method of performing a heat treatment for graphitization in a state where carbon is formed into a molded body is preferable. By crushing this graphitized molded body, the bulk specific gravity is higher, and the average shape parameter A graphite material with a small xave becomes possible. Further, when graphite powder having a bulk specific gravity and an average shape parameter xave within the above ranges and a specific surface area of 9 m ² / g or less is used as the graphite material, there are few submicron fine particles adhering to the graphite particles, and the bulk specific gravity is low. Becomes higher, the electrode structure becomes better, and the cycle characteristics are further improved.

  Further, a graphite powder having a cumulative 10% particle size of 3 μm or more, a cumulative 50% particle size of 10 μm or more, and a cumulative 90% particle size of 70 μm or less is used in the particle size distribution obtained by the laser diffraction method. Thus, a secondary battery having high safety and reliability can be obtained.

  Particles with a small particle size have a large specific surface area, but by restricting this content rate, it is possible to suppress abnormal heat generation during overcharging due to particles with a large specific surface area and to limit the content rate of particles with a large particle size. As a result, an internal short circuit due to particle expansion during initial charging can be suppressed, and a practical secondary battery having high safety and reliability can be obtained.

In addition, by using graphite powder having an average value of the breaking strength of particles of 6.0 kgf / mm ² or more, it is possible to have many pores for containing the electrolyte in the electrode, and the load characteristics are excellent. Secondary battery becomes possible.

  Below, the manufacturing method of the graphite material prescribed | regulated by this invention is demonstrated. First, artificial graphite will be described. As described above, pitches obtained from petroleum or coal can be used as a raw material as a precursor of artificial graphite. This is heat-treated at 400 ° C. to 500 ° C., and the obtained carbon precursor is heat-treated at 800 ° C. to 1000 ° C. in an inert atmosphere. At this point, after pulverization and particle size adjustment, an appropriate temperature is selected using a reactive gas, and the particle surface is slightly oxidized. Then, the material prescribed | regulated to this invention can be obtained by carrying out the high temperature process in an inert atmosphere, and graphitizing.

The reaction gas can be used any compound as long as it reacts with carbon, oxygen among them, ozone, carbon dioxide, monoxide, chlorine, hydrochloric acid, sulfur dioxide, NO _x, etc. are preferable. The reaction temperature can be appropriately selected depending on the gas used, but is preferably room temperature to 500 ° C. Moreover, the material prescribed | regulated to this invention can be obtained by irradiating powerful light, such as a laser, to the once graphitized particle, removing the unevenness | corrugation of the particle | grain surface, and polishing.

  Next, natural graphite will be described. When natural graphite is pulverized under appropriate conditions, a rhombohedral structure appears in the crystal structure. The material prescribed | regulated to this invention can be obtained by heat-processing this at 2000 degreeC or more. The content of the rhombohedral structure can be determined by X-ray diffraction, and the content is preferably 1% to 40%, more preferably 5% to 30%.

On the other hand, the positive electrode material used in combination with the negative electrode made of the negative electrode material is not particularly limited, but preferably contains a sufficient amount of Li. For example, the general formula LiMO ₂ (where M is Co, Ni, Mn, A composite metal oxide composed of lithium and a transition metal represented by (1) represents at least one of Fe, Al, V, and Ti, and an intercalation compound containing Li are suitable.

  As the non-aqueous electrolyte used in the secondary battery of the present invention, a non-aqueous electrolyte obtained by dissolving an electrolyte in a non-aqueous solvent is used. As a non-aqueous solvent for dissolving the electrolyte, it is premised that a solvent having a relatively high dielectric constant such as ethylene carbonate (EC) is used as a main solvent. To complete the present invention, a multi-component low-viscosity solvent is used. Need to be added.

  As the high dielectric constant solvent, propylene carbonate (PC), butylene carbonate, vinylene carbonate, sulfolanes, butyrolactones, valerolactones and the like are suitable.

  As the low-viscosity solvent, a symmetric or asymmetric chain ester carbonate such as diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate or the like is preferable, and two or more kinds of low-viscosity solvents may be mixed and used. Good results are obtained.

  In particular, when a graphite material is used for the negative electrode, EC is first preferred as the main solvent of the nonaqueous solvent, but a compound having a structure in which the hydrogen atom of EC is substituted with a halogen element is also suitable. In addition, although it is reactive with graphite materials such as PC, a part of the EC or the compound having a structure in which the hydrogen atom of EC is substituted with a halogen element is substituted with a second component solvent. As a result, good characteristics can be obtained.

  As the second component solvent, PC, butylene carbonate, 1,2-dimethoxyethane, 1,2-diethoxymethane, γ-butyrolactone, valerolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4- Methyl-1,3-dioxolane, sulfolane, methylsulfolane and the like can be used, and the addition amount is preferably less than 10 Vol%.

  To complete the present invention, a third solvent is added to the main solvent or to the mixed solvent of the main solvent and the second component solvent to improve conductivity, suppress EC decomposition, and improve low-temperature characteristics. In addition to reducing the reactivity with lithium metal, safety may be improved. As the solvent for the third component, first, a chain carbonate such as DEC (diethyl carbonate) or DMC (dimethyl carbonate) is suitable. Further, asymmetric chain carbonates such as MEC (methyl ethyl carbonate) and MPC (methyl propyl carbonate) are suitable.

  The mixing ratio of the chain carbonate ester as the third component to the main solvent or the mixed solvent of the main solvent and the second component solvent (the main solvent or the mixed solvent of the main solvent and the second component solvent: the third component solvent) is the capacity. The ratio is preferably 15:85 to 40:60, more preferably 18:82 to 35:65. Further, the third component solvent may be a mixed solvent of MEC and DMC. The MEC-DMC mixing ratio is preferably in a range represented by 1/9 ≦ d / m ≦ 8/2, where m is the MEC capacity and d is the DMC capacity.

  The mixing ratio of the main solvent or the mixed solvent of the main solvent and the second component solvent and the third component solvent is MEC-DMC, where the MEC capacity is m, the DMC capacity is d, and the total amount of the solvent is T. A range represented by 3/10 ≦ (m + d) / T ≦ 9/10 is preferable, and a range represented by 5/10 ≦ (m + d) / T ≦ 8/10 is more preferable.

As the electrolyte dissolved in such a non-aqueous solvent, any one used in this main battery can be mixed and used. For example, LiPF ₆ is preferable, but LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiB (C ₆ H ₅ ) ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiCl, LiBr or the like can also be used.

  Below, the secondary battery concerning this invention is demonstrated with reference to FIG. 1 about the experimental example 1-experimental example 5 and the comparative example 1-comparative example 4 for comparing with this. The present invention is not limited to these examples.

<Experimental example 1>
The negative electrode 1 was produced as follows. Petroleum pitch was added to coal pitch coke and mixed, and then pressed and shaped. This was heat-treated at 500 ° C. in an inert atmosphere, then pulverized and classified, and then heat-treated at 1000 ° C. in an inert atmosphere to obtain a graphite precursor. The precursor and oxygen gas were sealed and treated at 200 ° C. for 20 hours. Then, this was heat-processed in inert atmosphere at 2950 degreeC for 1 hour, and the sample was obtained.

  90 parts by weight of the graphite material powder thus obtained and 10 parts by weight of polyvinylidene fluoride (PVDF) as a binder were mixed to prepare a negative electrode mixture. This negative electrode mixture was dispersed in N-methylpyrrolidone as a solvent to form a slurry (paste). A strip-shaped copper foil having a thickness of 10 μm was used as a negative electrode current collector. A negative electrode mixture slurry was applied to both surfaces of the current collector, dried, and then compression molded to prepare a strip-shaped negative electrode 1.

The positive electrode 2 was produced as follows.
Lithium carbonate 0.5 mol and cobalt carbonate 1 mol were mixed and calcined in air at 900 ° C. for 5 hours to obtain LiCoO ₂ . 91 parts by weight of LiCoO ₂ as a positive electrode active material, 6 parts by weight of graphite as a conductive agent, and 3 parts by weight of polyvinylidene fluoride as a binder were mixed to obtain a positive electrode mixture. This positive electrode mixture was dispersed in N-methylpyrrolidone to form a slurry (paste). A strip-shaped aluminum foil having a thickness of 20 μm was used as the positive electrode current collector, and the positive electrode mixture slurry was uniformly applied to both surfaces of the current collector, dried, and then compression molded to prepare a strip-shaped positive electrode 2.

  The negative electrode 1 and the positive electrode 2 produced as described above were laminated in the order of the negative electrode 1, the separator 3, the positive electrode 2, and the separator 3 in the order of the negative electrode 1, the separator 3, the positive electrode 2, and the separator 3. Then, the coil 3 was wound many times, and the final end portion of the separator 3 was fixed with a tape to produce a spiral electrode element.

  The electrode element produced in this way is accommodated in the battery can 5, and insulating plates 4 are provided on both upper and lower surfaces of the electrode element. The positive electrode lead 13 with the insulating tape stretched out from the positive electrode current collector 11 and led to the safety valve device 8 which conducts to the battery lid 7, and the negative electrode lead 12 from the negative electrode current collector 10 and welded to the battery can 5. . The battery can 5 is made of iron having an outer diameter of 18 mm (inner diameter: 17.38 mm, thickness: 0.31 mm) and a height of 65 mm.

In the battery can 5, an electrolytic solution in which LiPF ₆ was dissolved at a ratio of 1 mol / liter in an equal volume mixed solvent of ethylene carbonate and dimethyl carbonate was injected. Next, the battery can 5 was caulked through the sealing gasket ₆ to fix the battery lid 7 and to maintain the airtightness in the battery, thereby producing a secondary battery.

<Experimental example 2>
Using acenaphthylene pitch as a raw material, heat treatment was performed at 400 ° C. under a high pressure of 10 kg / cm ² or more to grow a bulk mesoface. This was heat-treated at 500 ° C. in an inert atmosphere, then pulverized and classified, and then heat-treated at 1000 ° C. in an inert atmosphere to obtain a graphite precursor. This graphite precursor was heat-treated at 3050 ° C. for 1 hour in an inert atmosphere to obtain a sample. A secondary battery was prepared in the same manner as in Experimental Example 1 except that this was used as the negative electrode material.

<Experimental example 3>
A small amount of sulfuric acid was added to acenaphthylene pitch, and this was heat-treated at 400 ° C. under a high pressure of 10 kg / cm ² or more to grow a bulk mesophase. This was heat-treated at 500 ° C. in an inert atmosphere, then pulverized and classified, and then heat-treated at 1000 ° C. in an inert atmosphere to obtain a graphite precursor. This precursor was heat-treated at 3050 ° C. for 1 hour in an inert atmosphere to obtain a sample. A secondary battery was prepared in the same manner as in Experimental Example 1 except that this was used as the negative electrode material.

<Experimental example 4>
Natural graphite having a purity of 99% or more was pulverized with a ball mill and then classified. While flowing this, the surface was sufficiently irradiated with laser light in the air, and then heat-treated at 2600 ° C. for 1 hour in an inert atmosphere to obtain a sample. A secondary battery was prepared in the same manner as in Experimental Example 1 except that this was used as the negative electrode material.

<Experimental example 5>
Natural graphite having a purity of 99% or more was subjected to air classification while being pulverized by a jet mill. Measurement by X-ray diffraction revealed that the rhombohedral structure contained 20%. This was heat-treated at 2700 ° C. for 1 hour in an inert atmosphere to obtain a sample. A secondary battery was prepared in the same manner as in Experimental Example 1 except that this was used as the negative electrode material.

<Comparative Example 1>
Petroleum pitch was added to coal pitch coke and mixed, and then pressed and shaped. This was heat-treated at 500 ° C. in an inert atmosphere, then pulverized and classified, and then heat-treated at 1000 ° C. in an inert atmosphere to obtain a graphite precursor. This graphite precursor was heat-treated at 2950 ° C. for 1 hour in an inert atmosphere to obtain a sample. A secondary battery was prepared in the same manner as in Experimental Example 1 except that this was used as the negative electrode material.

<Comparative example 2>
Using acenaphthylene pitch as a raw material, heat treatment was performed at 400 ° C., and when the content of mesophase spherules was 50%, the matrix and the spherules were separated and taken out. This was heat-treated at 500 ° C. in an inert atmosphere, then pulverized and classified, and then heat-treated at 1000 ° C. in an inert atmosphere to obtain a graphite precursor. This precursor was heat-treated at 2900 ° C. for 1 hour in an inert atmosphere to obtain a sample. A secondary battery was prepared in the same manner as in Experimental Example 1 except that this was used as the negative electrode material.

<Comparative Example 3>
Natural graphite having a purity of 99% or more was pulverized with a ball mill and then classified to obtain a sample. A secondary battery was prepared in the same manner as in Experimental Example 1 except that this was used as the negative electrode material.

<Comparative Example 4>
Natural graphite having a purity of 99% or more was subjected to air classification while being pulverized by a jet mill to obtain a sample. A secondary battery was prepared in the same manner as in Experimental Example 1 except that this was used as the negative electrode material.

  The batteries of Experimental Examples 1 to 5 and Comparative Examples 1 to 4 created as described above were charged under the conditions of a maximum voltage of 4.2 V, a constant current of 1 A, and a charging time of 3 hours. And stored in a 45 ° C. atmosphere for 1 month. This 45 ° C. assumes the internal temperature of the electronic device. Then, it discharges, the discharge capacity after a preservation | save is measured, and it charges again on the said conditions. Thereafter, the battery was discharged at 1 A, and the discharge capacity at this time was measured to obtain the recovery capacity. A value obtained by dividing the recovery capacity by the discharge capacity after storage is defined as a capacity recovery rate. This is shown in Table 1.

For each sample of Experimental Examples 1 to 5 and Comparative Examples 1 to 4, using an argon laser with a wavelength of 514 and 5 nm, a Raman spectrum measured with a Raman spectrometer having a wave number resolution of 4 cm ⁻¹ , and similar measurement conditions, RG, Pbb, Pba, Psb, and Psa were determined from the SERS spectrum of a sample in which silver was deposited to a thickness of 10 nm, and these are also shown in Table 1.

  From these data, FIG. 2 shows the relationship between the capacity recovery rate and RG. It can be seen from the figure that the higher the RG, the higher the capacity recovery rate. In particular, when RG = 4.5 or more, the capacity recovery rate reaches 85% or more, and it can be seen that capacity degradation during storage in a high-temperature environment can be kept low.

FIG. 3 shows the relationship between the capacity recovery rate and Psb. From the figure, it can be seen that the higher the Psb, the higher the capacity recovery rate. In particular, when Psb = 1365 cm ⁻¹ or more, the capacity recovery rate reaches 85% or more, and it can be seen that the capacity deterioration during storage in a high temperature environment can be kept low.

As described above in detail, the ratio RG between the graphitization degree Gb obtained from the Raman spectroscopic spectrum using argon laser light and the graphitization degree Gs obtained from the surface enhanced Raman spectroscopic spectrum, which is a structural parameter of the graphite material. By defining (= Gs / Gb) to be 4.5 or more, capacity deterioration during storage in a high temperature environment of a secondary battery using this as a negative electrode is suppressed. Moreover, in the surface-enhanced Raman spectroscopic spectrum using argon laser light, a material having a peak in the range of 1365 cm ⁻¹ or more is used for the negative electrode, so that capacity degradation during storage in a high-temperature environment is similarly suppressed.

DESCRIPTION OF SYMBOLS 1 ... Negative electrode, 2 ... Positive electrode, 3 ... Separator, 4 ... Insulation board, 5 ... Battery can, 6 ... Sealing gasket, 7 ... Battery cover, 8 ... Safety valve apparatus, 10 ... Negative electrode collector, 11 ... Positive electrode collector , 12 ... negative electrode lead, 13 ... positive electrode lead.

Claims (17)

A vibration mode derived from a graphite crystalline structure in a surface-enhanced Raman spectroscopic spectrum when silver is deposited on a graphite material with a thickness of 10 nm using an argon laser beam having a wavelength of 514.5 nm and a spectrometer having a wave number resolution of 4 cm ^−1. Having a peak of (Psa = 1580 cm ^{−1 to} 1620 cm ⁻¹ ) in the range of 1581.22 cm ⁻¹ or more,
Further, the ratio RG (= Gs / Gb) between the degree of graphitization Gb obtained from the Raman spectrum using argon laser light and the degree of graphitization Gs obtained from the surface-enhanced Raman spectrum is 4.16 or more. Graphite material Was used for the negative electrode,
Secondary battery. A lithium composite oxide represented by LiMO ₂ (M is at least one element selected from Co, Ni, Mn, Fe, Al, V, and Ti) was used as the positive electrode material.
The secondary battery according to claim 1. The graphite material was obtained by graphitizing a carbon precursor after heat treatment at 800 ° C. to 1000 ° C. in an inert atmosphere.
The secondary battery according to claim 1 or 2. The graphite material was obtained by polishing the surface with light irradiation,
The secondary battery according to claim 1. The graphite material was obtained by heat-treating natural graphite having a rhombohedral structure at 2000 ° C. or higher.
The secondary battery according to claim 1. The content of rhombohedral structure in the natural graphite is 1% or more and 40% or less.
The secondary battery according to claim 5. The graphite material has a true density of 2.1 g / cm ³ or more and a bulk specific gravity of 0.4 g / cm ³ or more.
The secondary battery according to claim 1. The graphite material has a cumulative 10% particle size of 3 μm or more, a cumulative 50% particle size of 10 μm or more, and a cumulative 90% particle size of 70 μm or less in a particle size distribution determined by a laser diffraction method.
The secondary battery according to claim 1. The average value of the fracture strength of the graphite material is 6.0 kgf / mm ² or more.
The secondary battery according to claim 1. In a surface-enhanced Raman spectroscopic spectrum when silver is deposited with a thickness of 10 nm using an argon laser beam having a wavelength of 514.5 nm and a spectroscope with a wave number resolution of 4 cm ⁻¹ , a vibration mode (Psa = 1580 cm ^{−1 to} 1620 cm ⁻¹ ) in the range of 1581.22 cm ⁻¹ or more,
And the ratio RG (= Gs / Gb) of the graphitization degree Gb calculated | required from the surface-enhanced Raman spectroscopic spectrum Gb calculated | required from the Raman spectral spectrum using argon laser light is 4.16 or more.
Graphite material for secondary batteries. Obtained by graphitizing the carbon precursor after heat treatment at 800 ° C. to 1000 ° C. in an inert atmosphere,
The graphite material for a secondary battery according to claim 10. Obtained by shining light and polishing the surface,
The graphite material for a secondary battery according to claim 10 or 11. Obtained by heat-treating natural graphite having a rhombohedral structure at 2000 ° C. or higher,
The graphite material for a secondary battery according to any one of claims 10 to 12. The content of rhombohedral structure in the natural graphite is 1% or more and 40% or less.
The graphite material for a secondary battery according to claim 13. The true density is 2.1 g / cm ³ or more and the bulk specific gravity is 0.4 g / cm ³ or more.
The graphite material for a secondary battery according to any one of claims 10 to 14. In the particle size distribution determined by the laser diffraction method, the cumulative 10% particle size is 3 μm or more, the cumulative 50% particle size is 10 μm or more, and the cumulative 90% particle size is 70 μm or less.
The graphite material for a secondary battery according to any one of claims 10 to 15. The average value of the breaking strength is 6.0 kgf / mm ² or more,
The graphite material for a secondary battery according to any one of claims 10 to 16.

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