Negative-electrode active material for lithium ion capacitor

Abstract

A negative electrode active material for lithium ion capacitor, which reduces the thickness of a negative-electrode active material layer while maintaining the conventional level of energy density. The negative-electrode active material is a composite carbon material manufactured by kneading a carbon black having an average particle diameter of 12 to 300 nm with a carbon precursor such as pitch, the resulting mixture is baked or graphitized baking between 800 C. to 3200 C., and then pulverized such that the average particle diameter (D.sub.50) thereof is 1 to 20 m and the BET specific surface area is between 100-350 m.sup.2/g. An initial charging capacity is at least 700 mAh/g, and the cell volume is reduced as the thickness of the negative electrode active material layer becomes thinner than the conventional one.

Claims

1. A method of manufacturing a negative electrode active material for a lithium ion capacitor comprising the steps of kneading 100 parts by weight of carbon black having an average diameter of 12-300 nm, as measured by an electron microscope, and BET specific surface area of 254-1500 m.sup.2/g with a 30-200 parts by weight of a carbon precursor to form a kneaded mixture, baking or graphitizing the kneaded mixture at a temperature of 800 C.-3200 C., followed by pulverizing to form a product having an average diameter (D.sub.50) of 1-20 m, and BET specific surface area of above 110 m.sup.2/g and less than 350 m.sup.2/g.

Description

PREFERRED EMBODIMENTS OF THE INVENTION

(1) More detailed explanation of the invention will be described hereinafter for a better understanding of this invention showing embodiments and comparative examples. Needless to say, the scope of the invention is not limited to the following embodiments.

(2) Characteristics of the obtained negative electrode active material, BET specific surface area, initial charging capacity, amount of nitrogen gas adsorption at a relative pressure (P/P.sub.0)=0.9 and volume of micro pores/whole pores are indicated in Table 1.

Embodiment 1

(3) 100 parts by weight of carbon black having an average diameter of 30 nm and BET specific surface area of 254 m.sup.2/g and 50 parts by weight of an optically isotropic pitch having a softening point of 110 C. and metaphase (QI) content of 13% are kneaded in a heating kneader, and the resulting mixture is baked under a non-oxidative atmosphere at a temperature of 1000 C. and then pulverized such that the average particle diameter (D.sub.50) is 2 m to obtain a negative electrode active material for a lithium ion capacitor.

Embodiment 2

(4) 100 parts by weight of carbon black having an average diameter of 30 nm and BET specific surface area of 1000 m.sup.2/g and 100 parts by weight of an optically isotropic pitch having a softening point of 110 C. and metaphase (QI) content of 13% are kneaded in a heating kneader, and the resulting mixture is baked under a non-oxidative atmosphere at a temperature of 1000 C. and then pulverized such that the average particle diameter (D.sub.50) is 2 m to obtain a negative electrode active material for a lithium ion capacitor.

Embodiment 3

(5) 100 parts by weight of carbon black having an average diameter of 30 nm and BET specific surface area of 1000 m.sup.2/g and 30 parts by weight of an optically isotropic pitch having a softening point of 110 C. and metaphase (QI) content of 13% are kneaded in a heating kneader, and the resulting mixture is baked under a non-oxidative atmosphere at a temperature of 1000 C. and then pulverized such that the average particle diameter (D.sub.50) is 2 m to obtain a negative electrode active material for a lithium ion capacitor.

Embodiment 4

(6) 100 parts by weight of carbon black having an average diameter of 30 nm and BET specific surface area of 254 m.sup.2/g and 30 parts by weight of an optically isotropic pitch having a softening point of 110 C. and metaphase (QI) content of 13% are kneaded in a heating kneader, and the resulting mixture is baked under a non-oxidative atmosphere at a temperature of 1000 C. and then pulverized such that the average particle diameter (D.sub.50) is 2 m to obtain a negative electrode active material for a lithium ion capacitor.

Embodiment 5

(7) 100 parts by weight of carbon black having an average diameter of 30 nm and BET specific surface area of 1000 m.sup.2/g and 150 parts by weight of an optically isotropic pitch having a softening point of 110 C. and metaphase (QI) content of 13% are kneaded in a heating kneader, and the resulting mixture is baked under a non-oxidative atmosphere at a temperature of 1000 C. and then pulverized such that the average particle diameter (D.sub.50) is 2 m to obtain a negative electrode active material for a lithium ion capacitor.

Embodiment 6

(8) 100 parts by weight of carbon black having an average diameter of 30 nm and BET specific surface area of 1000 m.sup.2/g and 200 parts by weight of an optically isotropic pitch having a softening point of 110 C. and metaphase (QI) content of 13% are kneaded in a heating kneader, and the resulting mixture is baked under a non-oxidative atmosphere at a temperature of 1000 C. and then pulverized to an average particle diameter (D.sub.50) of 2 m to obtain a negative electrode active material for a lithium ion capacitor.

Embodiment 7

(9) The negative electrode active material for a lithium ion capacitor obtained by embodiment 3 is further baked under a non-oxidative atmosphere at a temperature of 2000 C. to obtain a negative electrode active material for a lithium ion capacitor.

Embodiment 8

(10) A process the same as embodiment 2 except that the pulverized to average particle diameter (D.sub.50) is 5 m to obtain a negative electrode active material for a lithium ion capacitor.

Embodiment 9

(11) A process the same as embodiment 2 except that the pulverized to average particle diameter (D.sub.50) is 10 m to obtain a negative electrode active material for a lithium ion capacitor.

Embodiment 10

(12) A process the same as embodiment 1 except that 60 parts by weight of the optically isotropic pitch is used to obtain a negative electrode active material for a lithium ion capacitor.

Embodiment 11

(13) 100 parts by weight of carbon black having an average diameter of 34 nm and BET specific surface area of 1270 m.sup.2/g and 100 parts by weight of an optically isotropic pitch having a softening point of 110 C. and metaphase (QI) content of 13% are kneaded in a heating kneader, and the resulting mixture is baked under a non-oxidative atmosphere at a temperature of 1000 C. and then pulverized to an average particle diameter (D.sub.50) of 2 m, to obtain a negative electrode active material for a lithium ion capacitor.

Comparative Example 1

(14) 100 parts by weight of carbon black having an average diameter of 48 nm and BET specific surface area of 39 m.sup.2/g and 133 parts by weight of an optically isotropic pitch having a softening point of 110 C. and metaphase (QI) content of 13% are kneaded in a heating kneader, and the resulting mixture is baked under a non-oxidative atmosphere at a temperature of 1000 C. and then pulverized to an average particle diameter (D.sub.50) of 2 m so that a negative electrode active material for a lithium ion capacitor is obtained.

Comparative Example 2

(15) 100 parts by weight of carbon black having an average diameter of 48 nm and BET specific surface area of 39 m.sup.2/g and 54 parts by weight of an optically isotropic pitch having a softening point of 110 C. and metaphase (QI) content of 13% are kneaded in a heating kneader, and the resulting mixture is baked under a non-oxidative atmosphere at a temperature of 1000 C. and then pulverized to an average particle diameter (D.sub.50) of 2 m to obtain a negative electrode active material for a lithium ion capacitor.

Comparative Example 3

(16) 100 parts by weight of carbon black having an average diameter of 24 nm and BET specific surface area of 117 m.sup.2/g and 50 parts by weight of an optically isotropic pitch having a softening point of 110 C. and metaphase (QI) content of 13% are kneaded in a heating kneader, and the resulting mixture is baked under a non-oxidative atmosphere at a temperature of 1000 C. and then pulverized to an average particle diameter (D.sub.50) of 12 m to obtain a negative electrode active material for a lithium ion capacitor.

(17) TABLE-US-00001 TABLE 1 Initial Nitrogen adsorption Micro pore volume/ BET specific area charge capacity P/P.sub.0 = 0.99 Whole pore volume m.sup.2/g mAh/g cm.sup.3/g % Embodiment 1 110 730 228 1.52 Embodiment 2 240 1100 484 0.71 Embodiment 3 330 1400 989 0.52 Embodiment 4 123 749 281 1.29 Embodiment 5 180 935 292 1.59 Embodiment 6 117 729 230 1.42 Embodiment 7 180 935 621 0.17 Embodiment 8 246 1130 575 0.33 Embodiment 9 242 1120 520 0.26 Embodiment 10 100 700 180 1.75 Embodiment 11 350 1600 995 0.21 Comparative example 1 24 334 30 2.89 Comparative example 2 31 424 57 2.12 Comparative example 3 53 634 165 1.16

(18) The measurement method of the necessary data in the embodiments of this invention and the comparative examples of the BET specific surface area, fine pore volume, and pore diameter are all measured with a device utilizing the principle of nitrogen adsorption and desorption, which is an automated gas adsorption analyzer, Tristar 300 made by Micromeritics.

(19) The detailed process of determining the BET specific surface area is as follows;

(20) Measuring a nitrogen volume by an isotherm, which is evaluated according to the multi point measurement method assuming the adsorbed nitrogen, is a mono layer.
P/V(P.sub.0P)=(1/VmC)+{(C1)/VmC}(P/P.sub.0)(1)
S=kVm(2)
P.sub.0: Saturated vapor pressure
P: Adsorption equilibrium pressure
V: Volume of adsorption at adsorption equilibrium pressure P
Vm: Quantity of monolayer adsorption
C: Parameter related to adsorption heat
S: Specific surface area
k: Nitrogen monomolecular exclusive possession area; 0.162 nm.sup.2

(21) A whole pore volume is calculated based on a saturated gas volume assuming the equilibrium relative pressure neighborhood of P/P.sub.0=0.99 obtained from the adsorption isotherm.

(22) A micropore of less than diameter of 2 nm is obtained by the t-plot method in which adsorbed nitrogen gas layer thickness t is plotted against the volume of adsorption. The thickness of the adsorption layer is obtained based on an equation of Harkins & Jura when the range of thickness t is between 0.35-0.50 nm.
t=[13.99/{0.034log(P/P.sub.0)}].sup.0.5(3)
P.sub.0: Saturated vapor pressure
P: Adsorption equilibrium pressure

(23) The measurement of the particle diameter is conducted using MT3300EX system made by NIKKISO Co., Ltd., dispersing the particles in water containing a small amount of a surfactant with an aid of a supersonic mixer.

(24) Measurement of an initial charging capacity is as follows; Preparing a water slurry comprising 100 parts by weight of a negative electrode active material and 5 parts each by weight of a SBR and CMC as a binder and applying the water slurry on the copper foil thickness of 300 m, drying the layer at a temperature of 120 C., pressing the layer by a roll, then punching it to 12 mm to obtain an electrode of 50 m thick. The electrodes are assembled as test coin cells inserting separators between the electrodes with a counter electrode of metal lithium and filling an electrolyte of 1M LiPF.sub.6/EC:DEC(3:7) and charging-discharging tests are conducted.

(25) Tests are conducted at a temperature of 25 C., charging with a constant current of 0.5 mA/cm.sup.2 until a voltage reaches 0.01V, followed by a constant voltage charging until the current value reading reaches 0.01 mA/cm.sup.2.

INDUSTRIAL APPLICABILITY

(26) A lithium ion capacitor using the negative electrode active material of the present invention has a property of an initial charge capacity per unit weight of the negative electrode when a lithium metal of the negative active material is used as a counter electrode is more than 700 mA/g and the energy density is higher than that of the conventional negative electrode active material, thereby the volume of the lithium ion capacitor can be reduced.