CARBON NANOTUBE-CONTAINING THIN FILM

Abstract

This carbon nanotube-containing thin film, which is formed on a base material, has a thickness of 10-500 nm. The ratio of coverage of the base material in a thin film forming portion by carbon nanotubes included in the thin film is 20-100%. The carbon nanotube-containing thin film exhibits a high ratio of coverage of the base material, despite having a thin film thickness, is capable of being ultrasonically welded, and, when used as an undercoat layer, is capable of achieving an energy storage device exhibiting low resistance.

Claims

1. A carbon nanotube-containing thin film formed on a substrate, wherein the thin film has a thickness of from 10 to 500 nm and the carbon nanotubes included in the thin film have a coverage with respect to the substrate in areas where the thin film is formed of from 20 to 100%.

2. The carbon nanotube-containing thin film of claim 1, wherein the thickness is from 20 to 300 nm and the coverage is from 40 to 100%.

3. The thin film of claim 1 or 2, further comprising a carbon nanotube dispersant.

4. An undercoat foil for an energy storage device electrode, comprising a current-collecting substrate and a carbon nanotube-containing undercoat layer formed on at least one side of the current-collecting substrate, wherein the undercoat layer has a thickness of from 10 to 500 nm and the carbon nanotubes included in the thin film have a coverage with respect to the substrate in areas where the thin film is formed of from 20 to 100%.

5. The undercoat foil for an energy storage device electrode of claim 4, wherein the current-collecting substrate is aluminum foil or copper foil.

6. The undercoat foil for an energy storage device electrode of claim 4, wherein the thickness is from 20 to 300 nm and the coverage is from 40 to 100%.

7. The undercoat foil for an energy storage device electrode of any one of claims 4 to 6, further comprising a carbon nanotube dispersant.

8. The undercoat foil for an energy storage device electrode of claim 7, wherein the carbon nanotube dispersant is a triarylamine-based highly branched polymer or a pendant oxazoline group-containing vinyl polymer.

9. An energy storage device electrode comprising the undercoat foil for an energy storage device electrode of claim 4 and an active material layer formed on part or all of a surface of the undercoat layer.

10. The energy storage device electrode of claim 9, wherein the active material layer is formed in such a way as to cover all regions of the undercoat layer other than a peripheral edge thereof.

11. An energy storage device comprising the energy storage device electrode of claim 9 or 10.

12. An energy storage device comprising at least one electrode assembly comprised of one or a plurality of the electrodes of claim 10 and a metal tab, wherein at least one of the electrodes is ultrasonically welded to the metal tab at a region of the electrode where the undercoat layer is formed and the active material layer is not formed.

13. A method for manufacturing an energy storage device that uses one or a plurality of the electrodes of claim 10, which method comprises the step of ultrasonically welding at least one of the electrodes to a metal tab at a region of the electrode where the undercoat layer is formed and the active material layer is not formed.

Description

EXAMPLES

[0175] Examples and Comparative Examples are given below to more fully illustrate the invention, although the invention is not limited by these Examples. The apparatuses and instruments used in the Examples were as follows. [0176] (1) Probe-type ultrasonicator (dispersion treatment): [0177] Apparatus: UIP1000 (Hielscher Ultrasonics GmbH) [0178] (2) Wire bar coater (thin-film production): [0179] Apparatus: PM-9050MC (SMT Co., Ltd.) [0180] (3) Ultrasonic welder (ultrasonic welding test) [0181] Apparatus: 2000Xea (40:0.8/40MA-XaeStand), from Emerson Japan, Ltd. [0182] (4) Charge/discharge measurement system (evaluation of secondary battery): [0183] Instrument: HJ1001 SMSA (Hokuto Denko Corporation) [0184] (5) Micrometer (measurement of binder and active material layer thicknesses): [0185] Instrument: IR54 (Mitutoyo Corporation) [0186] (6) Homogenizing disperser (mixing of electrode slurry) [0187] Apparatus: T.K. Robomix (with Homogenizing Disperser model 2.5 (32 mm dia.)), from Primix Corporation [0188] (7) Thin-film spin-type high-speed mixer (mixing of electrode slurry) [0189] Apparatus: Filmix model 40 (Primix Corporation) [0190] (8) Planetary centrifugal mixer (degassing of electrode slurry) [0191] Apparatus: Thinky Mixer ARE-310 (Thinky) [0192] (9) Roll press (compressing of electrode): [0193] Apparatus: HSR-60150H ultra-small desktop hot roll press (Hohsen Corporation) [0194] (10) Scanning electron microscope (SEM) (for measuring film thickness): [0195] Instrument: JSM-7400F (JEOL, Ltd.) [0196] (11) Scanning electron microscope (SEM) (for surface analysis): [0197] Instrument: JSM-7800F PRIME (JEOL, Ltd.)

Production of Undercoat Foil

Example 1-1

[0198] First, 0.50 g of PTPA-PBA-SO.sub.3H having the formula shown below and synthesized by the same method as in Synthesis Example 2 of WO 2014/042080 was dissolved as the dispersant in 43 g of 2-propanol and 6.0 g of water as the dispersion media, and 0.50 g of MWCNTs (NC7000, from Nanocyl; diameter, 10 nm) was added to the resulting solution. This mixture was ultrasonically treated for 30 minutes at room temperature (about 25 C.) using a probe-type ultrasonicator, thereby giving a black MWCNT-containing dispersion in which MWCNTs were uniformly dispersed and which was free of precipitate.

[0199] Next, 3.88 g of the polyacrylic acid (PAA)-containing aqueous solution Aron A-10H (solids concentration, 25.8 wt %; from Toagosei Co., Ltd.) and 46.12 g of 2-propanol were added to 50 g of the resulting MWCNT-containing dispersion and stirring was carried out, giving Undercoat Slurry Al. In addition, Undercoat Slurry Al was diluted two-fold with 2-propanol, giving Undercoat Slurry A2.

[0200] The resulting Undercoat Slurry A2 was uniformly spread with a wire bar coater (OSP 2; wet film thickness, 2 m) onto aluminum foil (thickness, 15 m) as the current-collecting substrate and subsequently dried for 10 minutes at 120 C. to form an undercoat layer, thereby producing Undercoat Foil B1.

[0201] Film thickness measurement was carried out as follows. The undercoat foil fabricated as described above was cut out to a size of 1 cm1 cm, and the center portion was torn by hand. A region where the undercoat layer lies exposed in the cross-section thereof was examined with a scanning electron microscope (JSM-7400F, from JEOL, Ltd.) at a magnification of 10,000 to 60,000, and the film thickness was measured from the captured image. As a result, the thickness of the undercoat layer on the undercoat foil B1 was about 16 nm.

[0202] Measurement of the coverage was carried out as follows. The undercoat foil produced as described above was cut out to a size of 1 cm1 cm, and the surface was examined with a scanning electron microscope (JSM-7800F PRIME, from JEOL, Ltd.) at a magnification of 10,000. Letting A be the surface area of the resulting image and B be the sum of the surface areas of tubular constituents, the percent coverage was calculated as (B/A)100. The coverages at two places on the same undercoat foil were calculated and the average thereof was treated as the ultimate undercoat foil coverage. The coverage of the undercoat foil B1 determined in this way was 26.3%.

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Example 1-2

[0203] Aside from using Undercoat Slurry Al prepared in Example 1-1, Undercoat Foil. B2 was produced in the same way as in Example 1-1. The thickness of the undercoat layer in Undercoat Foil B2 was measured and found to be 23 nm. The coverage was 40.1%.

Example 1-3

[0204] Aside from using a different wire bar coater (OSP3; wet film thickness, 3 m), Undercoat Foil B3 was produced in the same way as in Example 1-2. The thickness of the undercoat layer in Undercoat Foil B3 was measured and found to be 31 nm. The coverage was 71.3%.

Example 1-4

[0205] Aside from using a different wire bar coater (OSP4; wet film thickness, 4 m), Undercoat Foil B4 was produced in the same way as in Example 1-2. The thickness of the undercoat layer in Undercoat Foil B4 was measured and found to be 41 nm. The coverage was 74.3%.

Example 1-5

[0206] Aside from using a different wire bar coater (OSP6; wet film thickness, 6 m), Undercoat Foil B5 was produced in the same way as in Example 1-2. The thickness of the undercoat layer in Undercoat Foil B5 was measured and found to be 60 nm. The coverage was 80.6%.

Example 1-6

[0207] Aside from using a different wire bar coater (OSP8; wet film thickness, 8 m), Undercoat Foil B6 was produced in the same way as in Example 1-2. The thickness of the undercoat layer in Undercoat Foil B6 was measured and found to be 80 nm. The coverage was 82.0%.

Example 1-7

[0208] Aside from using a different wire bar coater (OSP10; wet film thickness, 10 m), Undercoat Foil B7 was produced in the same way as in Example 1-2. The thickness of the undercoat layer in Undercoat Foil B7 was measured and found to be 105 nm. The coverage was 80.6%.

Example 1-8

[0209] Aside from using a different wire bar coater (OSP13; wet film thickness, 13 m), Undercoat Foil B8 was produced in the same way as in Example 1-2. The thickness of the undercoat layer in Undercoat Foil B8 was measured and found to be 130 nm. The coverage was 78.7%.

Example 1-9

[0210] Aside from using a different wire bar coater (OSP22; wet film thickness, 22 m), Undercoat Foil B9 was produced in the same way as in Example 1-2. The thickness of the undercoat layer in Undercoat Foil B9 was measured and found to be 210 nm. The coverage was 79.2%.

Example 1-10

[0211] Aside from using a different wire bar coater (OSP30; wet film thickness, 30 m), Undercoat Foil B10 was produced in the same way as in Example 1-2. The thickness of the undercoat layer in Undercoat Foil B10 was measured and found to be 250 nm. The coverage was 77.1%.

[2] Production of Electrode and Lithium Ion Battery Using LFP as the Active Material

Example 2-1

[0212] The following were mixed together in a homogenizing disperser at 3,500 rpm for 5 minutes: 17.3 g of lithium iron phosphate (LFP, from TATUNG FINE CHEMICALS CO.) as the active material, 12.8 g of an NMP solution of polyvinylidene fluoride (PVdF) (12 wt %; KF Polymer L#1120, from Kuraray Co., Ltd.) as the binder, 0.384 g of acetylene black as the conductive additive and 9.54 g of N-methylpyrrolidone (NMP). Next, using a thin-film spin-type high-speed mixer, mixing treatment was carried out for 60 seconds at a peripheral speed of 20 m/s, in addition to which deaeration was carried out for 30 seconds at 2,200 rpm in a planetary centrifugal mixer, thereby producing an electrode slurry (solids concentration, 48 wt %; LFP:PVdF:AB=90:8:2 (weight ratio).

[0213] The resulting electrode slurry was uniformly spread (wet film thickness, 200 m) onto Undercoat Foil B1 produced in Example 1, following which the slurry was dried at 80 C. for 30 minutes and then at 120 C. for 30 minutes, thereby forming an active material layer on the undercoat layer. The active material layer was then pressed with a roll press, producing an electrode having an active material layer thickness of 50 m.

[0214] The electrode thus obtained was die-cut in the shape of a 10 mm diameter disk and the weight was measured, following which the electrode disk was vacuum dried at 100 C. for 15 hours and then transferred to a glovebox filled with argon.

[0215] A stack of six pieces of lithium foil (Honjo Chemical Corporation; thickness, 0.17 mm) that had been die-cut to a diameter of 14 mm was set on a 2032 coin cell (Hohsen Corporation) cap to which a washer and a spacer had been welded, and one piece of separator (Celgard 2400) die-cut to a diameter of 16 mm that had been impregnated for at least 24 hours with an electrolyte solution (Kishida Chemical Co., Ltd.; an ethylene carbonate:diethyl carbonate=1:1 (volume ratio) solution containing 1 mol/L of lithium hexafluorophosphate as the electrolyte) was laid on the foil. The electrode was then placed on top with the active material-coated side facing down. One drop of electrolyte solution was deposited thereon, after which the coin cell case and gasket were placed on top and sealing was carried out with a coin cell crimper. The cell was then left at rest for 24 hours, giving a secondary battery for testing.

Example 2-2

[0216] Aside from using Undercoat Foil B2 obtained in Example 1-2, a secondary battery for testing was produced in the same way as in Example 2-1.

Example 2-3

[0217] Aside from using Undercoat Foil B3 obtained in Example 1-3, a secondary battery for testing was produced in the same way as in Example 2-1.

Example 2-4

[0218] Aside from using Undercoat Foil B4 obtained in Example 1-4, a secondary battery for testing was produced in the same way as in Example 2-1.

Example 2-5

[0219] Aside from using Undercoat Foil B5 obtained in Example 1-5, a secondary battery for testing was produced in the same way as in Example 2-1.

Example 2-6

[0220] Aside from using Undercoat Foil B6 obtained in Example 1-6, a secondary battery for testing was produced in the same way as in Example 2-1.

Example 2-7

[0221] Aside from using Undercoat Foil B7 obtained in Example 1-7, a secondary battery for testing was produced in the same way as in Example 2-1.

Example 2-8

[0222] Aside from using Undercoat Foil B8 obtained in Example 1-8, a secondary battery for testing was produced in the same way as in Example 2-1.

Example 2-9

[0223] Aside from using Undercoat Foil B9 obtained in Example 1-9, a secondary battery for testing was produced in the same way as in Example 2-1.

Example 2-10

[0224] Aside from using Undercoat Foil B10 obtained in Example 1-10, a secondary battery for testing was produced in the same way as in Example 2-1.

Comparative Example 2-1

[0225] Aside from using pure aluminum foil, a secondary battery for testing was produced in the same way as in Example 2-1.

[0226] Using the charge/discharge measurement system, the physical properties of the electrodes were evaluated under the following conditions for the lithium-ion secondary batteries produced in above Examples 2-1 to 2-10 and Comparative Example 2-1. Table 2 shows the average voltage during 5C discharge. [0227] Current: Constant-current charging at 0.5C, and constant-current discharging at 5C (the capacity of LFP was set to 170 mAh/g) [0228] Cut-off voltage: 4.50 V-2.00 V [0229] Temperature: room temperature

TABLE-US-00001 TABLE 1 Film Average voltage Undercoat thickness Coverage during 5C discharge foil (nm) (%) (V) Example 2-1 B1 16 26.3 2.91 Example 2-2 B2 23 40.1 3.01 Example 2-3 B3 31 71.1 3.05 Example 2-4 B4 41 74.3 3.05 Example 2-5 B5 60 80.6 3.05 Example 2-6 B6 80 82.0 3.06 Example 2-7 B7 105 80.6 3.06 Example 2-8 B8 130 78.7 3.05 Example 2-9 B9 210 79.2 3.05 Example 2-10 B10 250 77.1 3.03 Comparative 2.52 Example 2-1

[0230] In the battery shown in Comparative Example 2-1 that used pure aluminum foil on which an undercoat layer is not formed, the battery resistance was high and so the average voltage during 5C discharged was confirmed to be low. By contrast, as shown in Examples 2-1 to 2-10, when use was made of an undercoat foil in which CNTs were used as the conductive material, the film thickness was set in the range of 10 to 500 nm and the coverage was set to at least 20%, the battery resistance decreased and so the average voltage during 5C discharge was confirmed to rise.

[0231] Also, it was found that when attempts were made to produce similar thin films using conductive materials such as carbon black, ketjen black or acetylene black, the coverage was extremely low and films could not be formed. By contrast, when CNTs were used as the conductive material, it was possible to form films which, although thin, had a high coverage.