JP4170006B2 - Carbon material and negative electrode material for secondary battery using the same - Google Patents

Description

【００２４】
（表の注）
３．測定方法
（１）Ｃ₂Ｈ₆細孔容積：前記方法により実施し、０．４０ｎｍを越える細孔径を有する細孔容積を測定した。
（２）ＣＯ₂細孔容積：前記方法により実施し、０．３３ｎｍ〜０．４０ｎｍの細孔径を有する細孔容積を測定した。
（３）充電容量、放電容量：充電条件は電流２５ｍＡｈ／ｇの定電流で１ｍＶになるまで保持し，放電条件は １．２５ｍＡｈ／ｇ以下に電流が減衰するまでとした。また、放電条件のカットオフ電位は １．５Ｖとした。
（４）充放電効率：（充電容量／放電容量）により算出した。
【００２５】
実施例１及び２はいずれも、特定の細孔径を有する細孔容積が所定の範囲内である本発明の炭素材であり、これを負極材に用いて二次電池としての評価を行ったところ、充放電容量、充放電効率のバランスに優れたものであった。一方、比較例１、２はいずれも、細孔容積が適当でなかったため、いずれも充放電効率が低下したものとなった。本発明の炭素材をこのようにして検証したところ、実施例にて得られた細孔容積が制御された炭素材は、リチウムイオンの挿入・脱離がその細孔容積により制御されることが確認された。
【００２６】
【発明の効果】
本発明は、特定の細孔径を有する細孔容積が所定の範囲にあることを特徴とする炭素材であり、この炭素材を主成分とする非水電解質二次電池用電極材組成物は、充放電容量の理論値に限界点を有する黒鉛系組成物と比較して高エネルギー密度であり、充放電効率にも優れたものである。さらに、リチウムイオンのインターカレーション時に炭素材の膨張が起こらないなど電池特性としての安全性が高く再現性に優れているものであり、特にリチウムイオン二次電池の負極材として好適である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a carbon material and a negative electrode material for a secondary battery using the same.
[0002]
[Prior art]
In recent years, along with the remarkable development of electronic technology, miniaturization and weight reduction of electronic devices have been mentioned as requirements. Along with this, further miniaturization, weight reduction, and higher energy density have been demanded for mobile power supplies. Conventionally used secondary batteries are aqueous secondary batteries such as lead batteries and nickel-cadmium batteries. However, these aqueous secondary batteries exhibit excellent cycle performance, but are not sufficiently satisfactory in terms of battery mass and energy density.
[0003]
Subsequently, lithium metal batteries using lithium or lithium alloys as negative electrode materials were developed as new electrode materials. Although these batteries have a very high energy density as compared with the conventional batteries, lithium metal is very dangerous and has a safety problem, and is difficult to put into practical use. Therefore, a non-aqueous electrolyte type lithium ion secondary battery using a carbon material as a new negative electrode material has been developed. This utilizes the fact that lithium is intercalated / deintercalated between carbon layers, and dendritic lithium is deposited on the negative electrode using the carbon material even if the charge / discharge cycle proceeds. This phenomenon is not seen and high safety is guaranteed. It has a high energy density, is lightweight, and exhibits excellent charge / discharge cycle characteristics.
[0004]
Examples of the above include graphite described in JP-A-5-7457, but the theoretical charge / discharge capacity has a limit of 372 mAh / g. Further, there are graphitizable carbon materials described in JP-A-5-28996 and JP-A-7-73828, but the charge / discharge capacity is determined because graphitization is performed under a condition where the firing temperature exceeds 2000 degrees. In the lower temperature range, the charge / discharge capacity is high, but the cycle characteristics are poor, and the battery characteristics are low. In addition, the carbon negative electrode treated at a low temperature of about 500 ° C. to 700 ° C. has a charge / discharge capacity of 850 mAh / g, exceeding the graphite per capacity, and has good energy merit for low temperature treatment. The disadvantage is that the potential is high and the potential hysteresis at the charge / discharge level is large.
Further, when a liquid in which propylene carbonate is mixed with the electrolytic solution is used, if a graphite-based carbon material is used, a decomposition reaction of the electrolytic solution occurs and a decrease in characteristics is confirmed. However, when a non-graphitic carbon material is used, no decomposition reaction occurs and almost no deterioration in characteristics is confirmed.
[0005]
In addition, examples of the lithium ion negative electrode material other than the carbon material include metal oxides disclosed in JP-A-5-166536. When a metal oxide is used as the negative electrode material, the charge / discharge capacity of 800 mAh / g is excellent. However, since the instantaneous discharge capacity is very high, it is difficult to control and a big problem arises in safety.
[0006]
As a feature of the non-graphite carbon material, its three-dimensional structure has higher three-dimensionality than two-dimensionality. In the case of graphite, there is a theoretical capacity whose upper limit is 372 mAh / g in charge capacity, but this characteristic can be greatly exceeded. However, the irreversible capacity is much larger than that of a normal graphite-based carbon material, and improvement of this point is an essential item.
[0007]
[Problems to be solved by the invention]
The present invention provides a carbon material having excellent charge / discharge efficiency, low irreversible capacity, high energy density, good cycleability, and high safety, and a negative electrode material for a secondary battery using the same. .
[0008]
[Means for solving problems]
Such an object is achieved by the following present inventions (1) to ( 4 ).
(1) A carbon material used for a negative electrode material of a nonaqueous electrolyte secondary battery, wherein pores formed on the surface of the carbon material are
(A) The pore volume having a pore diameter exceeding 0.40 nm is 0.0001 to 0.010 ml / g,
(B) A carbon material characterized in that a ratio (b / a) to a pore volume having a pore diameter of 0.33 nm to 0.40 nm is 1 or more.
( 2 ) The ratio (b / a) of (a) pore volume having a pore diameter exceeding 0.40 nm and (b) pore volume having a pore diameter of 0.33 nm to 0.40 nm is 5 or more. carbon material according to the above (1) is.
( 3 ) The carbon material according to (1) or (2) , wherein the carbon material is obtained by curing and carbonizing a phenol resin.
( 4 ) A negative electrode material for a secondary battery containing the carbon material according to any one of (1) to ( 3 ) above.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Below, the carbon material of this invention and the negative electrode material for secondary batteries using the same are demonstrated.
The carbon material of the present invention is a carbon material used for a negative electrode material of a nonaqueous electrolyte secondary battery, and pores formed on the surface of the carbon material are
(A) The pore volume having a pore diameter exceeding 0.40 nm is 0.0001 to 0.010 ml / g,
(B) A ratio (b / a) to a pore volume having a pore diameter of 0.33 nm to 0.40 nm is 1 or more.
Moreover, the negative electrode material for secondary batteries of this invention uses the said carbon material.
[0010]
First, the carbon material of the present invention will be described in detail.
The pores formed on the surface of the carbon material according to the present invention are mainly formed in the carbonization process when the carbon material is manufactured. The type of carbon material used, the curing conditions of the carbon material, the carbon material It is considered that various modes are formed depending on the carbonization conditions at the time of manufacture. In the carbon material of the present invention, the pore refers to a material formed on the surface of the carbon material and communicates with the outside, and the pore diameter refers to a pore diameter of a portion where the pore communicates with the outside. And pore volume means the sum total of the space volume which the said pore forms in the carbon material inside.
[0011]
Of the pores formed on the surface of the carbon material of the present invention, (a) the pore volume having a pore diameter exceeding 0.40 nm is 0.0001 to 0.010 ml / g. Thereby, when the carbon material of this invention is used for the negative electrode material of a lithium secondary battery, an irreversible capacity | capacitance can be reduced. When the pore volume exceeds 0.010 ml / g, the charge capacity increases, but the irreversible capacity increases and the charge / discharge efficiency decreases. On the other hand, when it is less than 0.0001 ml / g, the irreversible capacity is reduced, but the charge capacity itself is lowered. The pores having the pore diameter of (a) are easily penetrated by the organic solvent in the electrolytic solution, so that the electrolytic solution is decomposed and reacted in the presence of lithium ions and trapped as a lithium salt. It is estimated that this increases the irreversible capacity .
[0012]
Moreover, the carbon material of the present invention has a pore volume (b) having a pore diameter of 0.33 nm to 0.40 nm among the pores formed on the surface, the pore volume having the pore diameter of (a). The ratio (b / a) to the volume is 1 or more. Thereby, charge capacity can be ensured and charging / discharging efficiency can be made favorable. When the volume ratio is less than 1, the charge capacity is reduced, and the charge / discharge efficiency is also lowered accordingly.
The pore having the pore diameter (b) is larger than the pore volume having the pore diameter (a) because the insertion / extraction of lithium ions due to charge / discharge is likely to occur reversibly. It is preferable.
The pore volume ratio (b / a) is more preferably 5 or more. Thereby, still higher charge / discharge efficiency can be maintained.
[0013]
The carbon material of the present invention is not particularly limited, but an average particle diameter of 1 to 50 μm is preferable, and 5 to 30 μm is particularly preferable. When the particle size of the carbon material is within the above range, the handleability during production of the negative electrode material is good, and the negative electrode material application surface after production is smooth. When the particle size of the carbon material is less than the lower limit, powder powder is generated and the workability of preparing the negative electrode material is likely to be deteriorated, and when the upper limit is exceeded, the negative electrode material application surface is likely to be uneven.
[0014]
Although it does not specifically limit as a raw material used for the carbon material of this invention, In addition to thermoplastic resins, such as thermosetting resins, such as a phenol resin, a furan resin, and an epoxy resin, polystyrene, polyamide, petroleum pitch, coal pitch, for spinning Pitch etc. can be used. These raw materials can be used alone or in combination of two or more. Among these, it is preferable to use a phenol resin. Thereby, it can be set as the carbon material which has the structure which the three-dimensional bridge | crosslinking developed. Although it does not specifically limit as a phenol resin, A resol type phenol resin, a novolak type phenol resin, etc. can be used.
When a thermosetting resin is used as the raw material for the carbon material, the curing method is not particularly limited. For example, when a phenol resin is used, thermal curing, thermal oxidation, epoxy curing, isocyanate curing, and the like can be given. Moreover, when an epoxy resin is used, phenol resin curing, acid anhydride curing, amine curing, and the like can be given. Any of these curing methods can be used.
[0015]
The carbon material of the present invention can be obtained by carbonizing the carbon material raw material obtained by the above method. Although it does not specifically limit as carbonization processing conditions for obtaining the carbon material of this invention, Usually, it is carried out at 1000-1500 degreeC for 0.01 to 50 hours. Moreover, as an atmosphere at the time of carbonization, there are a method in which it is performed in air, nitrogen, or an inert gas such as helium gas.
The conditions during the carbonization treatment are important factors in that pore control is performed when obtaining the carbon material of the present invention.
For example, when a phenol resin is used as the raw material for the carbon material, the pore diameter tends to shrink and the pore volume decreases as the processing temperature is increased. In addition, the longer the treatment time, the smaller the pore diameter and the smaller the pore volume. By appropriately selecting these carbonization conditions, a carbon material having a target pore diameter and pore volume can be obtained.
[0016]
In addition, the method for obtaining a carbon material with controlled pores is not particularly limited, in addition to the method based on the carbonization treatment conditions, a method using a blend of a thermosetting resin and a thermoplastic resin as a carbon material raw material, There are a method of controlling the curing reaction by adding an additive to the carbon material, or a method of coating the surface of the carbon material with a resin and performing a curing / carbonization treatment. In addition, it may be modified with a metal or other carbon material that can be used during curing and carbonization, or with other additives such as pigments, lubricants, antistatic agents, and antioxidants. There is no problem.
[0017]
The measuring method of the pore diameter and the pore volume in the carbon material of the present invention is as follows.
The measurement sample was pretreated by vacuum heating at 623 K using a pore distribution measuring device “ASAP2010” manufactured by Shimadzu Corporation, and then measured as CO ₂ (molecular diameter; 0.33 nm), C ₂ H ₆ (molecular diameter; 0.40 nm), the adsorption isotherm at 273.15 K for each was measured, and the pore volume of each adsorbed gas was calculated by the Dubinin-Radushkevich method. Based on this, the pore volume of each was calculated. Calculation was based on the following formula.
W = W ₀ · exp [− (A / E) n], A = RT [ln (Ps / P)]
W: Energy [ml / g] occupied by adsorbed molecules
E: Adsorption characteristic energy [J / mol]
P: Parallel vapor pressure [mmHg]
T: Adsorption temperature [K]
W ₀ : pore volume [ml / g]
Ps: saturated vapor pressure [mmHg]
n: Structural index [−], n = 2.
Next, the negative electrode material for secondary batteries of the present invention will be described.
The negative electrode material for secondary batteries of the present invention contains the carbon material described above. Although it does not specifically limit as a manufacturing method of the negative electrode material for secondary batteries, For example, with respect to 100 weight part of said carbon materials, organic polymer binder (Fluorine polymer containing polyethylene, polypropylene, etc., butyl rubber, butadiene rubber) 1-30 parts by weight of a rubber-like polymer, etc.) and an appropriate amount of a viscosity adjusting solvent (N-methyl-2-pyrrolidone, dimethylformamide, etc.) are added and kneaded, and the mixture made into a paste is compression molded. It can be obtained by forming into a sheet form, a pellet form or the like by roll forming or the like. Moreover, the mixture made into the slurry form with the said solvent for viscosity adjustment can also be obtained by apply-molding on collectors, such as copper foil and nickel foil.
[0019]
【Example】
Hereinafter, the present invention will be specifically described by way of examples. However, the present invention is not limited to the examples. “Parts” and “%” indicate “parts by weight” and “% by weight”, respectively.
[0020]
1. Production of carbon material (Example 1)
After 2 hours curing at a 200 ° C. phenol resin (Sumitomo Bakelite · PR-51464) 100 parts, the temperature was raised at 5 ° C. / min under a nitrogen atmosphere, and held for 10 hours after 1300 ° C. reached Carbonized. Then, it cooled to room temperature and grind | pulverized to 45 micrometers or less using the vibration ball mill, and obtained 50 parts of carbon materials. Using this carbon material, a pore distribution measuring device “ASAP-2010” manufactured by Shimadzu Corporation, the pores having the pore diameter of the above (a) by the ethane gas adsorption measurement and the carbon dioxide adsorption measurement of the above (b) The pore volume was measured for each pore having a pore diameter.
(Example 2 )
A carbon material was obtained in the same manner as in Example 1 except that the temperature was raised at 1 ° C./min in the carbonization treatment step.
[0021]
(Comparative Example 1)
A carbon material was obtained in the same manner as in Example 1 except that the temperature was raised at 20 ° C./min in the carbonization process.
(Comparative Example 2)
A carbon material was obtained in the same manner as in Example 1 except that the carbonization temperature was 2800 ° C.
[0022]
2. Evaluation of secondary battery negative electrode materials (production of bipolar coin cells for secondary battery evaluation)
(1) To the carbon materials obtained in each of the examples and comparative examples, 10% by weight of polyvinylidene fluoride as a binder and an appropriate amount of N-methyl-2-pyrrolidone as a diluting solvent are added to and mixed with these carbon materials. A negative electrode mixture was prepared. The prepared negative electrode slurry-like mixture was applied to both sides of 18 μm copper foil, and then vacuum-dried at 110 ° C. for 1 hour. After vacuum drying, the electrode was pressure-formed by a roll press. This was cut out as a circle having a diameter of 0000 mm to produce a negative electrode.
(2) The positive electrode was evaluated with a bipolar coin cell using lithium metal. As the electrolytic solution, a solution obtained by dissolving 1 mol / liter of lithium perchlorate in a mixed solution of ethylene carbonate and diethylene carbonate having a volume ratio of 1: 1 was used.
[0023]
Characteristic evaluation was performed about the carbon material obtained by the above method and the bipolar coin cell for secondary battery evaluation. The results are shown in Table 1.
[Table 1]

[0024]
(Note to table)
3. Measurement method (1) C ₂ H ₆ pore volume: The pore volume having a pore diameter exceeding 0.40 nm was measured by the method described above.
(2) CO ₂ pore volume: carried out by the method, the pore volume was measured with a pore size of 0.33Nm～0.40Nm.
(3) Charging capacity, discharging capacity: Charging conditions were maintained at a constant current of 25 mAh / g until 1 mV, and discharging conditions were set until the current decayed to 1.25 mAh / g or less. The cut-off potential under discharge conditions was 1.5V.
(4) Charging / discharging efficiency: It was calculated by (charging capacity / discharging capacity).
[0025]
Each of Examples 1 and 2 is a carbon material of the present invention in which the pore volume having a specific pore diameter is within a predetermined range, and when this was used as a negative electrode material, evaluation as a secondary battery was performed. It was excellent in balance between charge / discharge capacity and charge / discharge efficiency. On the other hand, since the comparative examples 1 and 2 were not suitable for the pore volume, the charge / discharge efficiency was lowered. As a result of verifying the carbon material of the present invention in this way, the carbon material with controlled pore volume obtained in the examples is that the insertion / desorption of lithium ions is controlled by the pore volume. confirmed.
[0026]
【The invention's effect】
The present invention is a carbon material characterized in that the pore volume having a specific pore diameter is in a predetermined range, the electrode material composition for a non-aqueous electrolyte secondary battery mainly comprising this carbon material, Compared to a graphite-based composition having a limit point in the theoretical value of charge / discharge capacity, it has a higher energy density and excellent charge / discharge efficiency. Furthermore, since the carbon material does not expand during lithium ion intercalation, it has high battery safety and excellent reproducibility, and is particularly suitable as a negative electrode material for lithium ion secondary batteries.

Claims (4)

A carbon material used for a negative electrode material of a non-aqueous electrolyte secondary battery, wherein pores formed on the surface of the carbon material,
(A) The pore volume having a pore diameter exceeding 0.40 nm is 0.0001 to 0.010 ml / g,
(B) A carbon material characterized in that a ratio (b / a) to a pore volume having a pore diameter of 0.33 nm to 0.40 nm is 1 or more. A ratio (b / a) of (a) a pore volume having a pore diameter exceeding 0.40 nm and (b) a pore volume having a pore diameter of 0.33 nm to 0.40 nm is 5 or more. Item 2. The carbon material according to Item 1 . The carbon material according to claim 1 or 2 , wherein the carbon material is obtained by curing and carbonizing a phenol resin. It claims 1 to negative electrode material for a secondary battery containing carbon material according to any of 3.