JP2000299132A - Gel electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-performance gel electrolyte secondary battery exhibiting discharge capacity of a negative electrode material alike in a solution system and in a gel electrolyte system and having a high energy density. SOLUTION: This gel electrolyte secondary battery is constructed of a positive electrode, a negative electrode using a carbon material storing/releasing lithium as an active material, and a polymer gel electrolyte. In the carbon material, an oxygen content is not less than 0.05 wt.%, a quinone type oxygen amount is not less than 20 μmol/g, a total acidity is not less than 10 μmol/g and not more than the quinone type oxygen amount, a ratio of a carboxyl group amount to the total acidity is not more than 0.4. Desirably, the gel electrolyte includes a polymer prepared by chemically cross-linking monomers including a terminal acryloyl modified alkylene oxide polymer at least.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gel electrolyte secondary battery comprising a positive electrode, a negative electrode using a carbon material capable of occluding and releasing lithium as an active material, and a polymer gel electrolyte.

[0002]

2. Description of the Related Art A lithium secondary battery using a solution obtained by dissolving a lithium salt in an aprotic organic solvent as an electrolyte has attracted attention as a high energy density battery and has been widely used as a power source for various electronic devices. However, recent advances in electronics technology have been remarkable, and electronic devices are becoming lighter, smaller, thinner, and more multifunctional, and batteries, which are power sources, have been made smaller, lighter, and thinner. There is a demand for higher performance such as improved reliability. In order to meet such a demand, a solid electrolyte battery in which a positive electrode and a negative electrode are stacked via a solid electrolyte layer has been proposed.

[0003] Since the solid electrolyte basically does not contain a volatile solvent, a container caused by evaporation of the solvent due to heat generation at the time of leakage or short circuit generated in an electrolyte type lithium secondary battery containing a volatile solvent. It is possible to provide a lithium secondary battery which can avoid problems such as rupture and is excellent in safety. Further, since a pressure vessel and a gas venting device are not required as a result of avoiding the rupture, the energy density per weight can be greatly improved. Therefore, by using a solid electrolyte, it is possible to provide a lithium secondary battery having characteristics that exactly meet the above demands in terms of size reduction, weight reduction, and thickness reduction.

As a solid electrolyte applicable to a lithium secondary battery, a polymer solid electrolyte in which a lithium salt is dissolved in a polymer having a basic skeleton of ethylene oxide such as polyethylene oxide, Li ₃ N, LiI, LiI-Li ₂ SP ₂
Problem but inorganic materials such as O ₅ have been studied, one of the solid electrolyte also ionic conductivity to lower than the electrolyte solution system, capacity decreases during a short time charging and heavy load discharge was there.

[0005] Regarding the ionic conductivity of the solid electrolyte, a polymer gel formed by incorporating a lithium salt and an aprotic solvent generally used in a lithium secondary battery into a polymer is used as an electrolyte. Improving ionic conductivity with gel electrolyte technology as the core has been actively studied, for example, ionic conductivity close to that of achievable. Gel electrolytes can be broadly classified into physical gels and chemically crosslinked gels. In the former, gelling occurs when the physical entanglement of polymer chains acts as a cross-linking point.For example, a uniform solution is prepared by heating a polymer and an electrolytic solution, and then cooling is performed. Can be gelled. Since the cross-linking points are physically entangled, the physical gel has basically difficult problems such as remelting at a high temperature, separation of the polymer and the solvent by thermal cycling, that is, exudation of the solvent from the gel.

On the other hand, a chemically crosslinked gel is obtained by polymerizing a mixed solution of a monofunctional or polyfunctional monomer represented by an acrylate ester and an electrolytic solution.
By optimizing the monomer structure and network structure, it is possible to suppress ionic conductivity and mechanical strength, as well as the gel structural degradation caused by heat, which has become a problem with physical gels.
It is a very promising gel electrolyte. Furthermore, since the crosslinking reaction can be completed in a relatively short time, such as photopolymerization and electron beam polymerization, from the viewpoint of the production process,
It is an effective material.

[0007] Among the chemically crosslinked gels, polymer gels obtained by chemically crosslinking an acryloyl-modified alkylene oxide polymer are particularly excellent in solvent retention, and
It shows very excellent characteristics due to the problem of gas generation and liquid leakage due to temperature rise (Japanese Patent Application Laid-Open No. H5-114419). In addition, it is possible to obtain a gel electrolyte having both high elastic modulus and appropriate elongation by making the structure a chemically cross-linked body of a trifunctional macromonomer (JP-A-5-198303, JP-A-5-198303). -205779).

However, the overvoltage at the time of charging or discharging as a battery is not determined solely by the quality of the ionic conductivity of the gel electrolyte, but often depends on the resistance generated at the interface between the active material and the gel electrolyte. Make up a large percentage.

From such a viewpoint, studies have been made for the purpose of reducing the interface resistance. For example, by grafting a polymer such as polyether represented by polyethylene oxide onto the surface of an oxide represented by LiCoO ₂ used for the positive electrode active material, the active material and the polymer are grafted. Attempts have been made to reduce the interfacial resistance by strengthening the interaction with the compound (JP-A-8-111233).

On the other hand, as a study on a negative electrode for the purpose of reducing interface resistance, a report has been made regarding optimization of a crystal structure factor of a carbon material used for the negative electrode by X-ray diffraction (Japanese Patent Application Laid-Open No. 7-320724). . It has been concluded that the development of the graphite crystal structure is basically important for reducing the interfacial resistance, but as a result of intensive studies by the present inventors, a chemically crosslinked product of an acrylate monomer of polyethylene glycol was obtained. When used as a gel electrolyte, even if a graphite material obtained by firing natural graphite having high crystallinity or graphitizable carbon such as pitch coke at 2800 ° C or more is used as a negative electrode, an overvoltage can occur even at 10-hour charging. Because of the large size, the reduction in charge capacity was remarkable.

As described above, in the prior art, although the ionic conductivity of the gel electrolyte is improved, the reduction of the interface resistance between the negative electrode and the gel solid electrolyte has been extremely insufficient.

[0012]

SUMMARY OF THE INVENTION An object of the present invention is to provide a high performance gel electrolyte secondary battery having a high energy density by expressing the discharge capacity of a negative electrode material shown in a solution system in a gel electrolyte system as well. To provide.

[0013]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above object, and as a result, the following carbon materials have been used for the negative electrode with respect to the gel electrolyte. It has been found that the interfacial resistance between carbon and a carbon material is reduced, and the present invention has been completed.

Specifically, the present invention provides a gel electrolyte secondary battery comprising a positive electrode, a negative electrode using a carbon material capable of inserting and extracting lithium as an active material, and a polymer gel electrolyte. The material is under the following conditions: (1) Oxygen content ≧ 0.05% by weight (2) Quinone type oxygen content ≧ 20 μmol / g (3) 10 μmol / g ≦ Total acidity ≦ Quinone type oxygen content (4) Carboxyl group content / Total A gel electrolyte secondary battery characterized by satisfying an acidity ≦ 0.4.

It is preferable that the gel electrolyte has a structure containing a polymer obtained by chemically cross-linking at least a monomer containing an acryloyl-modified alkylene oxide polymer.

Here, the above oxygen content is a value obtained by expressing all oxygen contained in the carbon material by weight% based on an elemental analysis value measured based on a combustion method, and the above quinone type oxygen content is NaB
Quinone type oxygen moles per carbon material unit weight present on the carbon material surface to be detected by reaction with H ₄ ([mu] mol
/ g), the total acidity is expressed in terms of the number of acidic functional groups present on the carbon material surface detected by reaction with NaOH in moles per unit carbon material weight (μmol / g),
The amount of carboxyl groups is the amount of strongly acidic functional groups present on the surface of the carbon material detected by the reaction with NaHCO ₃ , expressed in terms of moles per unit weight of the carbon material (μmol / g).

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.

The essential point in the present invention is that the interface resistance between the carbon material and the gel electrolyte is presumed to be based on the following mechanism. The point is that by examining the introduction and optimizing the type of the functional group and the amount to be introduced, the interfacial resistance was significantly reduced.

Since the surface of a carbon material has basically no polarity irrespective of whether it has an amorphous structure or a crystalline structure, even if it is apparently in close contact with a gel electrolyte, its surface is When viewed microscopically, it is presumed that the solvent molecules are mainly bound by the polar portion constituting the polymer and have little interaction with the carbon material surface. Therefore, compared to the interface composed of the solution-based electrolyte and the carbon material, the interface between the gel electrolyte and the carbon material has 1) an average density of the solvent molecules existing on the carbon material surface is small, 2) It is considered that even solvent molecules existing near the surface of the carbon material have a smaller degree of freedom of movement than in the case of the solution system. And the characteristics of these two interfaces obviously have the effect of hindering the movement of lithium ions. That is, it is estimated that the interface resistance increases.

As a solution to this problem, a polar functional group is introduced into the surface of the carbon material so that the surface of the carbon material has the same or higher polarity as that of the polymer. As a result of intensive studies based on the idea of creating an interface state close to a solution-based electrolyte by increasing the average solvent density, oxygen-containing functional groups were introduced on the carbon material surface, and the type and amount of the functional groups were introduced. By optimizing, a large reduction in interface resistance was achieved.

That is, under the following conditions: (1) oxygen content ≧ 0.05% by weight (2) quinone type oxygen content ≧ 20 μmol / g (3) 10 μmol / g ≦ total acidity ≦ quinone type oxygen content (4) carboxyl group content A reduction in interfacial resistance was achieved by using a carbon material satisfying a total acidity of ≦ 0.4 for the negative electrode.

Hereinafter, each of the indices defining the carbon material will be described in detail.

The oxygen content indicates the amount of oxygen contained in the carbon material. In the present invention, the oxygen content is used as a basic index indicating the magnitude of the polarity of the surface of the carbon material. That is, it is essentially important that oxygen contained in the carbon material used in the present invention is concentrated on the surface of the carbon material in contact with the gel electrolyte. And the oxygen content necessary to strengthen the interaction with the gel electrolyte is 0.
It is at least 05% by weight, more preferably at least 0.10% by weight.

If the oxygen content is less than 0.05% by weight, the density of polar sites on the surface of the carbon material based on oxygen is low, and therefore the effect of reducing the interface resistance is not exhibited because the solvent is not sufficiently incorporated into the surface of the carbon material. .

The organic solvent used for the electrolyte-type Li secondary battery basically has ether-type oxygen or carbonyl-type oxygen in its structure. The important point in the present invention is to reduce the interfacial resistance by introducing a functional group that enhances the interaction with the solvent molecule on the carbon material surface,
As a result of diligent studies, it has been found that introducing quinone-type oxygen to the surface of the carbon material is particularly effective in enhancing the interaction with the solvent and, as a result, reducing the interface resistance. The amount of quinone-type oxygen necessary to enhance the interaction between the carbon material surface and the solvent is 20 μmol / g or more, and more preferably, the amount of quinone-type oxygen ≧ 25 μmol / g. When the amount of quinone-type oxygen is less than 20 μmol / g, the effect of reducing the interface resistance is not exhibited because the solvent is not sufficiently incorporated into the surface of the carbon material based on the quinone-type oxygen.

In the present invention, it is considered that the total acidity is utilized as a conduction path of Li ions. That is, the functional group represented by the total acidity is an acidic functional group and a functional group having a structure that easily releases hydrogen, in other words, a functional group containing chemically highly reactive hydrogen. During charging, a high voltage is applied to the interface between the carbon material as the negative electrode active material and the gel electrolyte, and it is considered that the substitution reaction of hydrogen of the acidic functional group with Li ions proceeds. Li in the acidic functional group substituted by hydrogen and Li will be used as a conduction path of Li ions during subsequent charging or discharging. The amount of functional groups required to develop a sufficiently low resistance as a new ion conduction path for reducing interface resistance is total acidity ≧ 10 μmol / g, more preferably, total acidity ≧ 15
μmol / g.

The second important point in the present invention is that a gel composed of a polymer formed by chemically cross-linking a monomer solution containing a terminal acryloyl-modified alkylene oxide polymer is subjected to the above-mentioned carbonization by quinone-type oxygen. By strengthening the interaction between the material surface and the solvent and forming a Li-ion conduction path by the acidic functional group, further optimizing the detailed distribution of the acidic functional group and quinone-type oxygen present on the carbon material surface It is the first finding that the interfacial resistance can be reduced.

That is, oxygen-containing functional groups generally formed on the surface of a carbon material include carboxyl groups, lactone groups, hydroxyl groups,
The quinone-type oxygen and the cyclic ether oxygen are classified, and as a result of the inventor's intensive studies, in particular, the quantitative magnitude relationship between the total acidity and the quinone-type oxygen, the ratio of the carboxyl group, which is a strongly acidic group, to the total acidity, It has been found that it is particularly effective in reducing the interface resistance.

The carboxyl group content, the total acidity, and the quinone-type oxygen content are specifically described below.

The alkylene oxide polymer is a system having a strong fluid replacement power, that is, a strong interaction between the solvent and the polymer, as represented by an ethylene oxide polymer (polyethylene oxide). Therefore, it is necessary to make the amount of quinone-type oxygen contributing to strengthening the interaction with the solvent relatively higher than that of other oxygen-containing functional groups. A specific index for that purpose is “total acidity ≦ quinone-type oxygen amount”. More preferably, “total acidity ≦ (quinone-type oxygen amount × 0.7)”, in which the relative ratio of the quinone-type oxygen amount to the total acidity is higher. By increasing the relative concentration of the quinone-type oxygen amount with respect to the total acidity, not only capacity development due to reduction in interface resistance but also charge / discharge cycle stability is improved. This is because the physical contact between the gel electrolyte and the carbon material surface is improved by increasing the relative concentration of the quinone-type oxygen amount. It is considered that the peeling off is suppressed.

The proton-donating functional group constituting the total acidity is basically composed of Li
While contributing to the construction of ion conduction pathways, on the other hand, carboxyl groups, in particular, have a high probability of simultaneously contributing to side reactions such as decomposition of the solvent during charging (Li doping reaction). Since the film formed by the side reaction involving carboxyl groups basically acts as a resistance to the conduction of Li ions, the amount of carboxyl groups is relatively small among the functional groups that contribute to the total acidity. By reducing the amount, formation of only the Li ion conduction path can be selectively advanced. This is quantitatively expressed as “amount of carboxyl group / total acidity ≦ 0.4”, and more preferably “amount of carboxyl group / total acidity ≦ 0.35”. When the amount of carboxyl groups / total acidity> 0.4, the side reaction during charging becomes intense and a resistive film is formed, so that the theoretical capacity of the carbon material cannot be sufficiently exhibited.

The surface structure of the carbon material used in the present invention is essentially important as described above, and if the carbon material is capable of electrochemically occluding and releasing Li, particularly the carbon material inside thereof is used. Although there is no particular limitation on the structure, the following materials can be specifically exemplified. Organic polymers (phenolic resin, furan resin, polyacrylonitrile, cellulose, etc.), those obtained by firing coke or pitch in an inert atmosphere or under reduced pressure at a temperature of 500 ° C or more and 3000 ° C or less, Kish graphite And natural graphite can be suitably used. Further, from the viewpoint of high energy density, a material having a developed graphite structure is more preferable, and the plane spacing d _{002 of} graphite by X-ray diffraction is d
Graphitic materials satisfying ₀₀₂ ≦ 0.337 nm can be suitably used.

In the present invention, the combination of the surface structure of the carbon material and the gel electrolyte having a polymer structure formed by chemically cross-linking a monomer solution containing a terminal acryloyl-modified alkylene oxide polymer has an essential feature. Therefore, the structures of the solvent and the lithium salt constituting the gel electrolyte are not limited, but the following structures can be exemplified.

The polymer gel electrolyte is preferably composed of an electrolyte salt composed of lithium ions and a strong acid anion, a polymer compound containing oxygen or nitrogen, and an aprotic organic solvent. Examples of electrolyte salts composed of lithium ions and anions of strong acids include lithium perchlorate (LiClO ₄ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), lithium hexafluorophosphate (LiPF ₆ ), and tetrafluoroboric acid. Lithium (LiBF ₄ ), lithium hexafluoroarsenate (Li
AsF _6), lithium hexafluoro antimonate (LiSbF _6), lithium trifluoromethanesulfonate imide (LiN (CF ₃ SO _2)
2 ) and the like.

As aprotic organic solvents, ethylene carbonate (EC), propylene carbonate (P
C), butylene carbonate (BC), γ-butyrolactone (γ
-BL), sulfolane (SL), 1,2-dimethoxyethane (DME),
1,2-diethoxyethane (DEE), ethoxymethoxyethane
(EME), tetrahydrofuran (THF), 2-methyltetrahydrofuran (2M-THF), 1,3-dioxolane (DOX), 4-methyl-
1,3-dioxolane (4M-DOX) can be exemplified.

As the acryloyl-terminal-modified alkylene oxide polymer, an alkylene oxide polymer having an oxyethylene structure as a main component is preferred from the viewpoints of suppressing crystallinity and increasing the ability to hold a solution. The content of the ethylene oxide structure in the alkylene oxide structure is preferably at least 50 mol%, more preferably at least 60 mol%. As an example, a terminal acryloyl modified product of a random copolymer of oxyethylene and oxypropylene can be suitably used.

Although the positive electrode used in the present invention is not particularly limited, examples thereof include lithium-containing manganese oxide, lithium-containing cobalt oxide, lithium-containing nickel oxide, and manganese, cobalt and nickel. A composite oxide containing at least two selected metals, vanadium pentoxide containing lithium, titanium disulfide, sulfides such as molybdenum disulfide, polyaniline,
It is preferable to use a conductive polymer such as polyacetylene or polypyrrole.

By introducing a polar oxygen-containing functional group to the surface of the carbon material, a part of the solvent constituting the polymer gel electrolyte is captured on the surface of the carbon material, and lithium ions are gelated through the solvent molecules. It is presumed that the interface resistance is reduced by the mechanism of forming a new introduction path for moving from the electrolyte to the carbon material. For this reason, a decrease in the utilization rate of the negative electrode during charging at a large current density is suppressed, and the discharge capacity is not easily reduced.

[0039]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and the present invention can be practiced by appropriately changing the gist of the invention. Is possible.

Example 1 [Preparation of negative electrode] About 30 g of artificial graphite (SFG15, manufactured by TIMCAL Ltd.)
Was mixed with 500 mL of a 30% concentration of hydrogen peroxide solution, and reacted at room temperature for 15 hours with stirring. Then, suction filtration was repeated using distilled water until the filtrate became neutral. The washed graphite powder was dried with hot air at 90 ° C., and further dried in vacuum at 90 ° C. The dried graphite powder and N-polyvinylidene fluoride (PVdF) resin
A slurry was prepared by mixing with a methylpyrrolidone (NMP) solution so that the weight ratio of PVdF resin to graphite was 9: 1. After adjusting the viscosity of the slurry by appropriately adding NMP, the slurry was applied on a copper foil, and the solvent was removed in a dryer at 80 ° C. to prepare an electrode to be used as a negative electrode. The negative electrode was further roll-pressed to adjust the bulk density to 1.0 to 1.1 g / mL and the areal density to 1.2 to 1.5 mg / cm ² , and provided the test electrode.

[Preparation of Gel Electrolyte] In a glove box under an argon atmosphere, LiClO was added to a solvent in which ethylene carbonate (EC) and propylene carbonate (PC) were mixed at a volume ratio of 2: 1.
₄ dissolved at a concentration of 1 mol / L (Toyama Pharmaceutical Co., Ltd.)
And acryloyl-modified macromonomer TA14 at the end of a copolymer of ethylene oxide and propylene oxide.
And 0K (manufactured by Pionix) in a weight ratio of 5: 1.Additionally, 0.10% by weight of 2,2-dimethoxy-2-phenylacetophenone was added as a photopolymerization initiator to obtain a transparent and uniform mixture. A solution was obtained. The solution was thinly coated on the negative electrode, after vacuum impregnated for 30 minutes in order to instill a negative electrode molded inside until sufficient electrolyte, the ultraviolet long wavelength of about 1 mW / cm ² intensity (365 nm)
Irradiation for 20 minutes allowed the photocrosslinking reaction to proceed.

[Assembly of Gel Electrolyte Battery (Coin-Type Battery)] After laminating lithium metal on the negative electrode / gel electrolyte molded body prepared above, cut out into a circle having a diameter of 12 mm to produce a coin-type cell (2032 type). did. The processes from the application of the monomer solution to the electrodes to the cell fabrication were all performed in a glove box in an argon atmosphere.

[Capacity development rate in initial charge / discharge and capacity retention rate in charge / discharge cycle] The coin battery composed of “graphite negative electrode / gel electrolyte / lithium metal” prepared above was subjected to room temperature (constant temperature adjustment to about 25 ° C.). ), Charge (doping reaction of graphite with lithium) and discharge (dedoping reaction of lithium from graphite) with constant current (current density of 0.1 mA / cm ² ) and discharge (de-doping reaction of lithium from graphite) 30 times with 0 V lower limit and 1.0 V upper limit electric potential regulation The charge capacity and the discharge capacity were reduced by cycling. Then, the ratio (100 fraction) of the discharge capacity of the first cycle to the theoretical capacity (372 mAh / g) of graphite represented by the chemical composition formula of C ₆ Li is defined as the capacity development rate, while the maximum discharge rate is defined. The ratio of the discharge capacity at the 10th cycle to the capacity (percentage) was defined as the capacity retention rate, and was used as an index of cycle stability.

Here, in the present invention, the index used for defining the carbon material is defined as follows.

[Oxygen content] was measured using FISONS Instrument
ts 1EA Elemental Analyzer was used.

Oxygen is measured according to the "Unterzaucher Modified" method. That is, oxygen in the carbon material is completely converted to CO by high-temperature pyrolysis under a catalyst, and is detected as a CO gas concentration by a thermal conductivity detector. The calibration curve of oxygen is measured using benzoic acid, and the calibration curve is obtained by changing the charged amount. Samples used for elemental analysis are dried in advance at 90 ° C. for 2 hours or more in order to remove the influence of moisture adsorbed on the carbon material surface, and then subjected to the test.

[Quinone-type oxygen amount] The quinone-type oxygen is quantified by utilizing a reaction in which quinone-type oxygen present on the surface of the carbon material is converted into a hydroxyl group by a reaction with NaBH ₄ as a hydrogenating agent. The reaction follows the formula; 8 (> C = O) + NaBH ₄ + 2H ₂ O → 8 (> C-OH) + NaBO ₂ Excess NaBH ₄ with respect to quinone-type oxygen was added and remained in the above reaction. Unreacted NaBH ₄ is decomposed by the following formula, and hydrogen gas generated at that time is quantified to determine quinone-type oxygen.

NaBH ₄ + 3H ₂ O → NaBO ₃ + 5H _{2 The} actual measurement procedure is as follows: weigh 0.064 g of NaBH ₄ ,
Dissolve in 100 mL of 0.1N NaOH aqueous solution to prepare NaBH ₄ solution. In a flask, 1 g of a carbon material previously vacuum-dried at 90 ° C. for 30 minutes or more and a stirrer are placed, and a rubber stopper is connected. The flask is connected to a gas burette (maximum 10 mL measurement) system for measuring the amount of generated hydrogen gas. After replacing the inside of the system with nitrogen gas, 8 mL of distilled water and 4 mL of NaBH ₄ solution are injected from a rubber stopper with a syringe, and stirring is continued for 1 hour with a stirrer. After adding 3 mL of 6N H ₂ SO ₄ with a syringe, the mixture is stirred for 5 minutes, and the amount of gas (hydrogen gas) generated during the addition is measured with a burette (V1). Based on V1, the amount of consumed NaBH ₄ , that is, the amount of quinone-type oxygen on the surface of the carbon material is calculated based on the above reaction formula.

[Total acidity] Generally, acidic functional groups such as a carboxyl group, a hydroxyl group and a lactone ring are present on the surface of a carbon material. These acidic functional groups are neutralized by substitution with hydrogen and Na by reaction with NaOH. The total acidity is used to evaluate the amount of acidic functional groups present on the carbon material surface by the amount of NaOH consumed by the carbon material due to this neutralization reaction.The details of the measurement are as follows; 1g of material 1
Put into a 00 mL Erlenmeyer flask, add 50 mL of 0.01N NaOH aqueous solution, set up a cooling tube, and boil for 4 hours while stirring with a stirrer on a hot stirrer. After completion of the reaction, the mixture was filtered with a pressure filter, and the filtrate was subjected to neutralization titration with 0.01N HCl, the amount of NaOH consumed was calculated from the titer, and the total acidity (μmol / g) was calculated as the NaOH consumption per 1 g of the carbon material. ) Is calculated.

[Amount of carboxyl group] A method of quantitatively evaluating the amount of only a carboxyl group, which is a strongly acidic group, by utilizing the difference in the dissociation constant of various acidic functional groups present on the surface of the carbon material. That is, the carbon material is neutralized with NaHCO ₃ which is a weak base, and the carboxyl group is quantitatively evaluated from the consumed NaHCO ₃ ; the specific procedure of the measurement is as follows; the carbon material previously dried in vacuum at 90 ° C. 1 g is placed in a 100 mL Erlenmeyer flask, 50 mL of 0.01 N NaHCO ₃ aqueous solution is added, and the reaction is allowed to proceed while shaking for 3 hours at room temperature with a shaker in a sealed stopper. After completion of the reaction, the mixture was filtered with a pressure filter, and the filtrate was added at 0.01 wt.
N HCl aqueous solution is added in excess, boiled for 20 minutes, then quenched,
Perform a neutralization titration with 0.01 N NaOH. Calculate the amount of NaHCO ₃ consumed in the reaction with the carbon material from the titration amount, and calculate
The amount of carboxyl groups (μmol / g) is calculated as NaHCO ₃ consumption.

Example 2 A saturated solution of (NH ₄ ) ₂ S ₂ O ₈ dissolved in 2N sulfuric acid was prepared.
15 g of the artificial graphite used in Example 1 was mixed and reacted at room temperature for 10 days with stirring. After completion of the oxidation reaction, the graphite powder was subjected to suction filtration with distilled water, and suction filtration washing with distilled water was repeated until finally sulfates disappeared from the filtrate. After this drying step, the electrode performance of the oxidized graphite powder was evaluated in the same manner as in Example 1.

Example 3 The artificial graphite used in Example 1 was oxidized by oxygen plasma. For the plasma treatment, a laboratory plasma treatment device PT500 manufactured by Samco International Co., Ltd. was used. The processing conditions were as follows. Put 5g of graphite powder in the reaction vessel,
When the vacuum level drops below 80 mTorr, 30 to 4
Oxygen of 0 mL was introduced (the reaction system was 300 to 900 mTorr), and plasma was generated by high frequency induction to oxidize the surface of the graphite powder. After the subsequent drying step, in the same manner as in Example 1,
The electrode performance of the oxidized graphite powder was evaluated.

Example 4 In Example 3, artificial gas which was subjected to air plasma treatment in the same manner as in Example 3 except that the gas introduced into the plasma processing apparatus was changed to air was prepared. After this drying step, the electrode performance of the oxidized graphite powder was evaluated in the same manner as in Example 1.

Comparative Example 1 The electrode characteristics of the artificial graphite used in Example 1 were evaluated in the same manner as in Example 1 without any treatment.

Comparative Example 2 About 15 g of the artificial graphite used in Example 1 was mixed with 300 cc of fuming nitric acid, and reacted while stirring at a boiling temperature (about 110 ° C.) for 3 hours. The suction filtration was repeated until was neutral. After this drying step, the electrode performance of the oxidized graphite powder was evaluated in the same manner as in Example 1.

Comparative Example 3 About 15 g of the artificial graphite used in Example 1 was mixed in 300 cc of 60% nitric acid and reacted while stirring at 80 ° C. for 3 hours.
Suction filtration was repeated using distilled water until the filtrate became neutral. After the subsequent drying step, in the same manner as in Example 1,
The electrode performance of the oxidized graphite powder was evaluated.

Comparative Example 4 The artificial graphite used in Example 1 was put in a magnetic crucible, and was previously heated at 620 ° C.
Into a muffle furnace, which was kept warm, and air-oxidized for about 1.5 hours. After this drying step, the electrode performance of the oxidized graphite powder was evaluated in the same manner as in Example 1.

The oxygen content, quinone-type oxygen content, total acidity, carboxyl group content, discharge capacity (dedoping amount) of the first cycle, and capacity development of the graphite powders of Examples 1 to 4 and Comparative Examples 1 to 4 described above. The rates and capacity retention rates are summarized in Table 1.

[0059]

[Table 1] It is clear from the results in Table 1 that the carbon material as defined in Examples 1 to 4 has a capacity expression rate of 80% or more, whereas the untreated carbon material having a low oxygen content (comparative) An example
1), and nitric oxide graphite powder having a relatively large amount of carboxyl groups (Comparative Examples 2 and 3), air-oxidized graphite powder having a low oxygen content and a large amount of quinone-type oxygen but a low total acidity (Comparative Example In 4), the overvoltage is large, and the capacity expression rate is 15%.
It was below. In addition, the capacity retention rate was 85% or more in the example, but was 50% or less in the comparative example, and the stability to the charge-discharge cycle of the carbon material defined by the present invention was clearly improved. Is recognized.

[0060]

According to the present invention, the carbon material of the present invention is used for a negative electrode, and in particular, the negative electrode is combined with a polymer gel electrolyte obtained by chemically cross-linking a monomer containing an acryloyl-modified alkylene oxide polymer. The interface resistance between the electrolyte and the gel electrolyte can be reduced, and as a result, the overvoltage at the time of charging is reduced, the discharge capacity close to the theoretical capacity can be exhibited, and the high performance gel electrolyte secondary battery with high energy density Can be provided.

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Claims (2)

[Claims] 1. A gel electrolyte secondary battery comprising a positive electrode, a negative electrode using a carbon material capable of inserting and extracting lithium as an active material, and a polymer gel electrolyte,
The carbon material has the following conditions: (1) oxygen content ≧ 0.05% by weight (2) quinone type oxygen content ≧ 20 μmol / g (3) 10 μmol / g ≦ total acidity ≦ quinone type oxygen content (4) carboxyl group content / A gel electrolyte secondary battery characterized by satisfying a total acidity of ≦ 0.4. 2. The gel electrolyte secondary battery according to claim 1, wherein the polymer gel electrolyte includes a polymer obtained by chemically cross-linking at least a monomer containing an acryloyl-modified alkylene oxide polymer.