JP2009130066A - Lithium ion capacitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a lithium ion capacitor having high energy density, high output density and excellent cycle characteristics. <P>SOLUTION: The lithium ion capacitor includes a positive electrode 1 comprising a polarized electrode containing active carbon, a negative electrode 2 containing a negative electrode active substance capable of occluding/emitting lithium ions and an organic electrolyte containing the lithium ions. The capacitor is characterized by that the negative electrode active substance is easily graphitizable carbon having lattice spacing of a (002) plane of 3.45 Å or larger and an average particle diameter of 100-500 nm. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a lithium ion capacitor having both the characteristics of an electric double layer capacitor and the characteristics of a lithium ion secondary battery.

  In recent years, a positive electrode made of a polarizable electrode using activated carbon, a negative electrode using a carbon material capable of occluding and releasing lithium ions as a negative electrode active material, and an organic electrolysis using a lithium salt as a solute. An energy storage device including a liquid has attracted attention (for example, Patent Document 1). Such an energy storage device is called a lithium ion capacitor, and has the performance of combining the characteristics of a lithium ion secondary battery and an electric double layer capacitor, and has a higher output density and better than a lithium ion secondary battery. In addition to having a long life characteristic, it is characterized by having a higher energy density than an electric double layer capacitor. This lithium ion capacitor is suitable for high output applications where a lithium ion secondary battery is not suitable, and is expected to be used as a power source for hybrid vehicles.

  In Patent Document 2, by using coke, tar, or vinyl chloride resin as a negative electrode active material of a lithium ion capacitor, even if the amount of lithium ions to be doped in advance is small, the capacity and energy density are high, and it is manufactured at low cost. Has proposed a lithium ion capacitor that is easy to use.

  In Patent Document 3, a polyacene material having a 50% volume cumulative diameter (D50) of 0.1 to 2.0 μm is used as a negative electrode active material of a lithium ion capacitor, and energy density and output density at low temperature are used. Lithium ion capacitors are proposed in which the decrease in the capacitance, particularly the decrease in the capacitance is suppressed.

However, the lithium ion capacitor has higher output characteristics and better cycle characteristics than the lithium ion secondary battery, but the negative electrode is accompanied by insertion and extraction of lithium ions. Then, there is a problem that the output characteristics and the cycle characteristics are sacrificed to some extent.
Japanese Patent Laid-Open No. 8-1007048 JP 2006-303118 A JP 2006-303330 A

  An object of the present invention is to provide a lithium ion capacitor having a high energy density, a high output density, and excellent cycle characteristics.

  The present invention relates to a lithium ion capacitor comprising a positive electrode comprising a polarizable electrode containing activated carbon, a negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions, and an organic electrolyte containing lithium ions. The substance is characterized by graphitizable carbon having a (002) plane lattice spacing of 3.45 mm or more and an average particle diameter of 100 to 500 nm.

  In the present invention, graphitizable carbon having a (002) plane lattice spacing of 3.45 mm or more and an average particle diameter of 100 to 500 nm is used as the negative electrode active material. By using such graphitizable carbon as a negative electrode active material, a high energy density lithium ion capacitor excellent in output characteristics and cycle characteristics can be obtained. A carbon material having a lattice spacing of (002) planes of 3.45 mm or more hardly has graphitization and has a random structure. Therefore, the insertion and release of lithium ions are performed smoothly, and the accompanying expansion and contraction are small. For this reason, high output density and excellent cycle characteristics can be obtained. Further, since the average particle diameter is 100 to 500 nm and is very small, the diffusion of solvated lithium ions is very fast, and from this point, a high power density and excellent cycle characteristics can be obtained.

  The upper limit value of the lattice spacing of the (002) plane of the negative electrode active material used in the present invention is not particularly limited, but is generally 4.00 mm or less.

The true specific gravity of the negative electrode active material in the present invention is preferably in the range of 1.7 to 2.1 g / cm ³ . By using the negative electrode active material in such a range, higher power density and better cycle characteristics can be obtained.

  The lattice spacing of the (002) plane of the negative electrode active material can be measured by an X-ray diffraction method. Moreover, the average particle diameter of the negative electrode active material can be measured with a laser diffraction particle size distribution measuring apparatus, and the 50% cumulative particle diameter (D50) was taken as the average particle diameter. The true specific gravity can be determined by a butanol immersion method.

The negative electrode active material of the present invention is preferably formed by a thermal decomposition reaction of a hydrocarbon gas. The thermal decomposition temperature is preferably in the range of 1100 to 1300 ° C. The obtained carbon fine particles can be classified into carbon fine particles having a desired average particle diameter. By heat-treating the obtained carbon fine particles in an inert gas, it is possible to obtain graphitizable carbon having a (002) plane lattice spacing of 3.45 mm or more. The heat treatment temperature is preferably in the range of 600 to 2400 ° C, more preferably in the range of 700 to 1500 ° C. When heated to a temperature of 2500 ° C. or higher, the carbon fine particles are graphitized, the lattice spacing of the (002) plane is less than 3.45 mm, and the true specific gravity exceeds 2.1 g / cm ³ . Examples of the inert gas when performing the heat treatment include argon gas, helium gas, nitrogen gas, and the like. In addition, examples of the hydrocarbon gas used as the raw material gas include toluene, methane, ethane, propane, ethylene, and xylene.

  The negative electrode in the present invention can be produced by a conventionally known method. For example, a negative electrode active material, a binder, and, if necessary, a conductive agent are mixed, and this is added to a solvent to prepare a slurry, which is applied to a metal foil such as a copper foil and dried to form. can do. Moreover, you may shape | mold by press molding etc. and you may produce a negative electrode.

  The positive electrode in the present invention is composed of a polarizable electrode containing activated carbon. The polarizable electrode containing activated carbon can be used without particular limitation as long as it can be used as a polarizable electrode such as an electric double layer capacitor and a lithium ion capacitor. For the positive electrode, for example, activated carbon, a binder, and, if necessary, a conductive agent such as carbon black are mixed and added to a solvent to prepare a slurry, and the slurry is made of a metal foil such as an aluminum foil. It can be prepared by applying on an electric body and then drying. Moreover, you may shape | mold by press molding etc. As the activated carbon, steam activated or KOH activated materials such as coconut shell, phenol resin, petroleum coke and the like can be used.

The non-aqueous electrolyte solution (organic electrolyte solution) in the present invention is not particularly limited as long as it is a non-aqueous electrolyte solution that can be used for an electric double layer capacitor or a lithium ion capacitor. For _{example, LiPF 6, LiBF 4, LiClO} 4, LiN (CF 3 SO 2) 2, LiCF 3 SO 3, LiC (CF 3 SO 2) 3, etc. LiAsF ₆ and LiSbF _6, and the like. Examples of the solvent include one or more selected from ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, sulfolane, and dimethoxyethane.

  The concentration of the lithium salt that is a solute is not particularly limited, but is generally about 0.1 to 2.5 mol / liter, for example.

  In the lithium ion capacitor according to the present invention, the positive electrode, the negative electrode, and the organic electrolyte can be stored in a storage case. The storage case is not particularly limited, and a laminate film, a metal case, a resin case, a ceramic case, or the like is used.

  The negative electrode preferably has lithium ions occluded in advance, and can be occluded by a chemical method or an electrochemical method.

  As a chemical method, lithium ions can be occluded by immersing in an electrolytic solution in a state where a negative electrode and a necessary amount of lithium metal are in contact with each other and applying heat. As an electrochemical method, lithium ions can be occluded by making a negative electrode and lithium metal face each other through a separator and charging with constant current in an electrolytic solution.

  According to the present invention, a lithium ion capacitor having a high energy density, a high output density, and excellent cycle characteristics can be obtained.

  Hereinafter, the present invention will be described with reference to specific embodiments and examples. However, the present invention is not limited to the following embodiments and examples, and may be appropriately changed without departing from the scope of the present invention. Can be implemented.

  FIG. 1 is a schematic cross-sectional view showing a capacitor according to an embodiment of the present invention. In the capacitor shown in FIG. 1, a positive electrode 1 and a negative electrode 2 are provided so as to face each other with separators 3a and 3b interposed therebetween. The positive electrode 1 is composed of a polarizable electrode containing activated carbon. The negative electrode 2 is an electrode including a negative electrode active material capable of inserting and extracting lithium ions. The positive electrode 1 is provided with a positive electrode current collector 1 a. A positive electrode tab 1 b is attached to the positive electrode current collector 1 a, and the positive electrode tab 1 b is taken out from the exterior body 5.

  Similarly, the negative electrode 2 is also provided with a negative electrode current collector 2a. A negative electrode tab 2b is attached to the negative electrode current collector 2a, and the negative electrode tab 2b is taken out from the outer package 5 to the outside. The positive electrode current collector 1a is made of, for example, aluminum or an aluminum alloy. The negative electrode current collector 2a is made of, for example, copper, nickel, stainless steel, and alloys thereof.

  In this embodiment, a reference electrode 4 made of metallic lithium is provided between the separator 3a and the separator 3b. An electrode tab 4 a is attached to the reference electrode 4, and the electrode tab 4 a is taken out of the exterior body 5.

  Separator 3a and 3b can be formed from a polyolefin-type separator, a nonwoven fabric, etc. Moreover, the exterior body 5 can be formed from a laminate film, a metal case, a resin case, a ceramic case, or the like.

  Since the reference electrode 4 is provided in the capacitor of the present embodiment, the potentials of the positive electrode 1 and the negative electrode 2 can be measured using the reference electrode 4.

  However, the capacitor of the present invention may not be provided with the reference electrode as described above, and the separator between the positive electrode 1 and the negative electrode 2 may be a single separator.

In the present invention, the negative electrode active material is preferably doped with lithium in advance. The potential of the negative electrode not doped with lithium is in the vicinity of 3.0 V (Vs.Li/Li ⁺ ). The positive electrode potential is also near 3.0 V (Vs. Li ^/ Li ⁺ ) immediately after assembly, and the cell voltage is almost 0 V. Therefore, the positive electrode potential is repeatedly charged and discharged between the vicinity of 3.0 V (Vs. Li ^/ Li ⁺ ) and the decomposition potential of the electrolytic solution. Usually, since the battery does not discharge to 0 V (Vs.Li/Li ⁺ ), the potential change of the positive electrode is actually smaller. On the other hand, when a cell is assembled after lithium is previously doped into the negative electrode active material, a cell voltage corresponding to a decrease in the negative electrode potential is generated. Therefore, the potential of the positive electrode can be changed from the decomposition potential of the electrolytic solution to 3.0 V (Vs. Li ^/ Li ⁺ ) or less. The amount of change in the positive electrode potential represents the size of the discharge capacity of the cell, and a large discharge capacity can be obtained by doping lithium in advance. Further, by doping lithium into the negative electrode active material, the negative electrode potential can be greatly reduced, and a large cell voltage can be obtained. Since the energy density is higher as the voltage is higher, in order to obtain a higher energy density, it is preferable to previously dope lithium into the negative electrode active material.

  With reference to FIG. 2, the movement of the negative electrode potential during charging and discharging of the lithium ion capacitor will be described.

  FIG. 2 shows the behavior of the negative electrode potential when lithium is first doped in a test cell using graphitizable carbon as a negative electrode active material and lithium metal as a counter electrode. Here, assuming that the negative electrode material is previously doped with lithium, the negative electrode potential at the time of assembly is defined as point a. By charging this capacitor, the negative electrode potential moves toward point b. When charged to the rated cell voltage, the negative electrode potential reaches point b. Next, when switching to discharge, the negative electrode potential moves from point b through point a to point c. And it becomes the minimum cell voltage at the point c. Then, by repeating charge and discharge, the point c and the point b are reciprocated.

In the capacitor according to the present invention, in order to achieve both high energy density and high output density, the negative electrode potential at the point b is preferably in the vicinity of 0.2 V (Vs.Li/Li ⁺ ).

In order to adjust the negative electrode potential when charged to the rated cell voltage to be 0.2 V (Vs. Li ^/ Li ⁺ ), it is preferable to dope lithium in advance as follows.

First, as shown in FIG. 2, the potential behavior of the negative electrode is measured with a sufficiently small current by a test cell using lithium metal as a counter electrode. The electric capacity Q (mAh) required for the electrode potential to be 0.2 V (Vs. Li ^/ Li ⁺ ) is obtained.

Next, the capacity A (mAh) required to reach (V ₀ +0.2) V (Vs.Li/Li ⁺ ) from the potential obtained by immersing the positive electrode in the electrolytic solution is obtained. This capacity A is defined as a positive electrode capacity. V ₀ (V) is a rated cell voltage.

As shown in FIG. 2, the negative electrode potential of the capacitor when charged to the rated cell voltage is 0.2 V (Vs. Li ^/ Li ⁺ ) by previously doping the negative electrode with lithium ions corresponding to QA (mAh). be able to. However, when occluding / releasing lithium ions to / from the carbon material, there is an irreversible capacity that cannot be released once occluded, so the potential in the first charge and the second and subsequent charge / discharge may shift, so the irreversible capacity is taken into account. It is preferable to determine the lithium doping amount.

(Preparation of negative electrode active material)
(Examples 1-3 and Comparative Examples 1-2)
Toluene was used as a source gas, and carbon fine particles were produced by a thermal decomposition reaction of toluene. The thermal decomposition temperature was 1200 ° C. After collecting the obtained carbon fine particles, heat treatment was performed at 1100 ° C. for 2 hours. The carbon fine particles after the heat treatment were classified by a classifier so that the average particle diameter (D50) was 1 μm (1000 nm), 500 nm, 300 nm, 100 nm, and 50 nm. These are referred to as Comparative Example 1, Example 1, Example 2, Example 3, and Comparative Example 2, respectively.

(Example 4 and Comparative Example 3)
The carbon fine particles obtained by the pyrolysis reaction of toluene in the same manner as above were subjected to heat treatment at 2200 ° C. and 2600 ° C. for 2 hours. The carbon fine particles after the heat treatment were classified so that the average particle diameter (D50) was 300 nm.

  The fine carbon particles heat-treated at 2200 ° C. had a lattice spacing of 3.45 mm, which was designated as Example 4. The fine carbon particles heat-treated at 2600 ° C. had a lattice spacing of 3.40 mm, and were designated as Comparative Example 3.

(Comparative Example 4)
As the negative electrode active material, a 2800 ° C. fired product of commercially available mesophase carbon microbeads (MCMB) having an average particle diameter of 6 μm was used. The lattice spacing was 3.37 mm. This was designated as Comparative Example 4.

(Production of negative electrode)
A negative electrode was produced using the negative electrode active materials of Examples and Comparative Examples obtained as described above. For the negative electrode, the negative electrode active material, acetylene black, and polyvinylidene fluoride were mixed at a weight ratio of 80:10:10, and this mixture was added to N-methylpyrrolidone as a solvent and mixed by stirring. Got. This slurry was applied onto a copper foil having a thickness of 18 μm by a doctor blade method and temporarily dried, and then cut so that the electrode size was 20 mm × 30 mm. The electrode thickness was about 50 μm. Before assembling the cell, it was dried in vacuum at 120 ° C. for 5 hours.

[Measurement of negative electrode capacity]
Using the negative electrode obtained as described above, a test cell using lithium metal as a counter electrode was assembled, and the negative electrode capacity was measured. The negative electrode capacity was measured by measuring the discharge capacity when it was once charged to 0 V with a constant current of 0.5 mA and then discharged to 1.5 V, and this discharge capacity was defined as the negative electrode capacity.

  Table 1 shows the average particle diameter (D50), lattice spacing, firing (heat treatment) temperature, negative electrode capacity, and true specific gravity of Examples 1 to 4 and Comparative Examples 1 to 4. The true specific gravity of the negative electrode active material was measured by a butanol immersion method.

[Production of positive electrode]
As the positive electrode active material, activated carbon having a specific surface area of about 2200 m ² / g obtained by an alkali activation method was used. Activated carbon powder, acetylene black, and polyvinylidene fluoride are mixed at a weight ratio of 80:10:10, and this mixture is added to N-methylpyrrolidone, which is a solvent, and mixed by stirring. Got. This slurry was applied onto an aluminum foil having a thickness of 30 μm by a doctor blade method, temporarily dried, and then cut so that the electrode size was 20 mm × 30 mm. The electrode thickness was about 50 μm. Before assembling the cell, it was dried in vacuum at 120 ° C. for 10 hours.

[Doping of lithium into the negative electrode]
The negative electrode obtained as described above was doped with lithium as follows. The negative electrode and lithium metal foil were sandwiched between separators (polypropylene nonwoven fabric) and set in a beaker cell, and a predetermined amount of lithium ions was occluded in the negative electrode over about 10 hours. The amount of lithium doped was about 75% of the negative electrode capacity.

(Preparation of electrolyte)
An electrolytic solution was prepared by dissolving lithium hexafluorophosphate (LiPF ₆ ) in a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 3: 7 so as to have a concentration of 1 mol / liter.

[Assembly of capacitor cell]
A separator (polypropylene nonwoven fabric) was inserted between the positive electrode and the negative electrode obtained as described above, impregnated with an electrolytic solution, and sealed in a storage case made of a laminate film. The completed cell was left as it was for about 1 day until measurement.

  The following electrochemical evaluation measurement was performed by sandwiching the laminate cell between two structure holding plates and fixing with a clip.

[Electrochemical evaluation of capacitor cells]
The laminate cell was subjected to electrochemical evaluation.

  The discharge capacity was the discharge capacity at the fifth cycle when a constant current was charged to 4.0 V at a predetermined current and a constant current was discharged to 2.0 V at the same current as the charge. The charge / discharge current was set to 1C and 100C with a current (1C) that can discharge the cell capacity in one hour as a reference (1C). In Table 2, the discharge capacity at the fifth cycle measured with a charge / discharge current of 1 C is shown as “discharge capacity”. Further, “discharge capacity maintenance ratio at 100 C relative to 1 C” was calculated by the following formula, and the value is shown in Table 2.

Discharge capacity maintenance ratio (%) at 100 C with respect to 1 C = (discharge capacity at the fifth cycle at 100 C) ÷ (discharge capacity at the fifth cycle at 1 C) × 100
Further, a charge / discharge cycle test was conducted at 10C. In the charge / discharge cycle test, 10C was charged at a constant current up to 4.0V, 10C was discharged at a constant current up to 2.0V, and this was taken as one cycle, and 10,000 cycles were charged and discharged. The discharge capacity after 10,000 cycles with respect to the initial discharge capacity is shown in Table 2 as the capacity retention rate.

  All measurements were performed at 25 ° C.

  As shown in Table 2, in Examples 1 to 3, the discharge capacity maintenance rate at 100 C is high, and the capacity maintenance rate after 10,000 cycles is also high. On the other hand, in Comparative Example 1, the deterioration of the discharge capacity after 10,000 cycles was large. Although the detailed cause for this is not clear, when the average particle diameter is larger than 500 nm, the charge transfer distance becomes large, the balance between the occlusion and release of solvated lithium ions is lost, the negative electrode potential is increased, and the cycle characteristics are increased. It seems to have deteriorated.

  In Comparative Example 2, the discharge capacity is slightly reduced, and the deterioration of the discharge capacity after 10,000 cycles is also increased. Although the detailed cause is not clear, when the average particle diameter is smaller than 100 nm, the bulk density is reduced, the discharge capacity per volume is reduced, and furthermore, the adhesion by the binder becomes difficult and the adhesion becomes insufficient. It seems to have deteriorated.

  In Example 4, both the discharge capacity maintenance rate at 100 C and the capacity maintenance rate after 10,000 cycles are smaller than those in Examples 1-4. This seems to be because graphitization of carbon fine particles progressed because the firing temperature was raised to 2200 ° C. In Comparative Example 3, graphitization further progresses, and the discharge capacity retention rate at 100 C and the capacity retention rate after 10,000 cycles are particularly small.

  In Comparative Example 4, the decrease in the discharge capacity retention rate at 100 C is large. This is because MCMB with a calcination temperature of 2800 ° C. is used, and this MCMB is almost graphitized, and the average particle size is also large, so that the insertion and release of solvated lithium ions is very slow. It is believed that there is.

1 is a schematic cross-sectional view showing a lithium ion capacitor of one embodiment according to the present invention. The figure which shows the behavior of the negative electrode potential in the test cell which used graphitizable carbon as a negative electrode and used lithium metal as a counter electrode.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Positive electrode 1a ... Positive electrode collector 1b ... Positive electrode tab 2 ... Negative electrode 2a Negative electrode collector 2b ... Negative electrode tab 3a, 3b ... Separator 4 ... Reference electrode 4a ... Electrode tab 5 ... Exterior body

Claims (3)

A lithium ion capacitor comprising a positive electrode comprising a polarizable electrode containing activated carbon, a negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions, and an organic electrolyte containing lithium ions,
The lithium ion capacitor, wherein the negative electrode active material is graphitizable carbon having a (002) plane lattice spacing of 3.45 mm or more and an average particle diameter of 100 to 500 nm. 2. The lithium ion capacitor according to claim 1, wherein the negative electrode active material has a true specific gravity of 1.7 to 2.1 g / cm ³ .   3. The lithium ion capacitor according to claim 1, wherein the negative electrode active material is formed by a thermal decomposition reaction of a hydrocarbon-based gas.

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