Lithium ion capacitor

Abstract

[Problem] To develop a lithium ion capacitor in which carbon nanowall is used for a positive electrode and a negative electrode instead of activated carbon. [Solution] Disclosed is a lithium ion capacitor which is characterized by including: a negative electrode which is formed as a carbon nanowall that is oriented in a direction that is perpendicular to the surface of a conductive substrate; and a positive electrode which is formed as a carbon nanowall that is oriented in a direction that is perpendicular to the surface of the conductive substrate, the carbon nanowall being coated with lithium fine particles. The lithium ion capacitor is also characterized in that an electrolyte solution and a separator which separates the positive electrode and the negative electrode from each other are provided between the electrodes.

Description

lithium-ion capacitor

[0001] The present invention relates to a lithium ion capacitor.

[0002] Lithium-ion capacitors (LICs) are attracting attention as devices that can be rapidly charged and discharged, have little degradation, and have a long lifespan. LICs combine the properties of supercapacitors (also known as superbatteries or ultracapacitors) to provide high energy density and high power density.

[0003] Lithium-ion capacitors can be charged and discharged very quickly, providing high power density. They are suitable for high-power applications such as electric vehicles and elevators, typically lasting hundreds of thousands of charge / discharge cycles with relatively little degradation, making them ideal for long-life devices. Larger lithium-ion capacitors are used for energy storage and high-power applications, such as electric vehicles and automated factories. Smaller lithium-ion capacitors also exist, suitable for use in small devices such as home appliances, mobile phones, and mobile communications. In the future, further technological advances may improve the performance of lithium-ion capacitors and open up new applications.

[0004] JP 2012-136362 JP 2005-286247 Republished 2004 / 084244

[0005] Activated carbon has traditionally been used as an electrode material for LIC, but the manufacturing process requires the use of a conductive adhesive, which has been pointed out as a potential problem that could result in a decline in performance. In particular, there are concerns that the adhesion of adhesives, impurities, and debris to the surface of the activated carbon could shorten the lifespan of the LIC. It has been pointed out that the conventional method for synthesizing the negative electrode of capacitors involves the presence of substances such as adhesives and impurities and debris on the surface of the activated carbon, which can adversely affect the performance of the electrode in a lithium-ion capacitor.

[0006] To solve this problem, graphene, two-dimensional materials, nanostructured materials, and other materials have been researched as potential electrode materials to develop alternatives to activated carbon. These materials have high conductivity, surface area, and chemical stability, and therefore may offer superior performance compared to conventional activated carbon. Furthermore, after much trial and error, a method for synthesizing carbon nanowalls (CNWs) made of pure carbon has been developed.

[0007] According to the inventor's research, carbon nanowalls (CNWs) are a material in which graphene sheets, which are six-membered rings made of carbon, are arranged in a wall-like shape. They have high density, high heat resistance, electrical conductivity, high acid resistance, and chemical stability. Furthermore, because of their wall-like structure, they are electrically conductive, do not require adhesives, have few impurities on the surface, have an extremely large specific surface area, and there are large amounts of "gaps" between the nanowalls, which makes it possible to store large amounts of electrons and protons within them. These properties make carbon nanowalls (CNWs) ideal for lithium ion capacitors, and they are expected to be used in lithium ion capacitors and other applications.

[0008] Therefore, the present invention proposes a lithium ion capacitor that is characterized by comprising a negative electrode formed on carbon nanowalls oriented vertically to the surface of a conductive substrate, and a positive electrode formed on carbon nanowalls oriented vertically to the surface of a conductive substrate, with lithium microparticles coated on the carbon nanowalls, and an electrolyte solution and a separator separating the two electrodes.

[0009] Conductive metal substrates are substrates used to support the conduction of electrons, and various metals such as Cu (copper), aluminum (Al), and titanium (Ti) are used. These metals are widely used to transmit electrical signals and conduct heat, and it is important to select the appropriate conductive metal substrate depending on the specific application and requirements.

[0010] When an electrolytic copper substrate is used as the conductive metal substrate, copper oxide on the surface must be removed in advance by acid treatment or the like.

[0011] For the negative electrode or positive electrode, a plasma CVD apparatus can be used to form carbon nanowalls that are oriented in the direction perpendicular to the conductive metal surface.

[0012] CH4 or H2 is fed into a plasma CVD apparatus, and carbon nanowalls oriented perpendicular to the surface of a conductive metal substrate are formed in a carbon atmosphere at a temperature of 250 to 800°C, preferably 250 to 500°C.

[0013] For the positive electrode, a pulsed laser deposition device is used to coat the carbon nanowalls with lithium particles.

[0014] The pulsed laser deposition system irradiates the surface of a lithium target with a powerful pulsed laser, converting the lithium into a plasma state, which is then used to adsorb lithium particles onto carbon nanowalls (CNWs).

[0015] The carbon nanowalls used in the negative and positive electrodes of the present invention are carbon-based materials created using nanotechnology, and have a very large surface area and a structure suitable for storing electrons. Therefore, the lithium ion capacitor of the present invention can be widely used as a power storage device, contributing to improved performance with high performance and long life.

[0016] Image of a lithium-ion capacitor in operation Perspective view and front view of a laminated lithium-ion capacitor Diagram showing the manufacturing process of a lithium-ion capacitor Charging process diagram of a lithium-ion capacitor Discharging process diagram of a lithium-ion capacitor SEM image of vertically aligned carbon nanowalls (CNWs) Raman spectroscopy spectrum of vertically aligned carbon nanowalls (CNWs)

[0017] A lithium ion capacitor comprising a negative electrode formed on a surface of electrolytic conductive copper with carbon nanowalls oriented in a vertical direction, and a positive electrode formed on a surface of electrolytic conductive copper with carbon nanowalls oriented in a vertical direction and coated with lithium fine particles, and an electrolyte and a separator separating the electrodes are provided between the negative electrode and the electrolytic conductive copper.

[0018] The present invention will now be described with reference to illustrative embodiments. Figure 1 is a schematic diagram of a capacitor operating according to the method of the present invention. The operational access of the present invention is explained with reference to Figure 3. A specific process for the capacitor's operating principle is illustrated. A capacitor primarily consists of a negative electrode (1) and a positive electrode (2). Between these electrodes is an electrolyte (6) and a separator (5) that separates them. The separator separates the electrolyte from the electrodes and prevents short circuits. When a capacitor is charged, positive charges (7) are accumulated from the positive electrode through the electrolyte and separator to the negative electrode. At the same time, electrons (9) are transferred to the negative electrode via the wire and inserted into the carbon nanowalls (8), where they are charged together with the positive charges. When the capacitor is discharged, the positive charges (7) from the negative electrode are discharged through the electrolyte and separator (5) to the negative electrode, releasing the stored energy and providing current from the electric field.

[0019] 2 is an image of the external appearance of the lithium ion capacitor according to the present invention, which has a stacked lithium ion capacitor structure made up of four lithium ion capacitor units.

[0020] The manufacturing process for lithium-ion capacitors is described in Figure 3. <Step shown in Figure 3A> Two electrolytic copper substrates (1) and (2) are cut (pressed). <Step shown in Figure 3B> The two cut electrolytic copper substrates (1) and (2) are placed in a hydrofluoric acid solution for 10 minutes to remove the copper oxide surface layer from the electrolytic copper. <Step shown in Figure 3C> The two electrolytic copper substrates (1) and (2) are placed in a plasma CVD apparatus, where vertically aligned carbon nanowalls (CNWs) (8) are synthesized on the surfaces of the two electrolytic copper substrates (1) and (2) as the positive and negative electrodes of the lithium-ion capacitor. <Step shown in Figure 3D> The electrolytic copper substrate (1) as the positive electrode refers to the copper substrate used as the positive electrode in capacitor manufacturing, and a pulsed laser deposition (PLD) system is used to deposit or grow materials using a pulsed laser. The lithium target is a lithium material irradiated with a pulsed laser. The energy of the pulsed laser transforms the lithium into a plasma state. The plasma is high-temperature, high-energy, and contains a variety of particles (atoms, ions, clusters, electrons, photons, etc.). The fine particles generated from the lithium plasma are deposited on an electrolytic copper substrate, and the lithium adheres to the electrolytic copper substrate, potentially for use in the manufacturing stage of cathode materials. <The process shown in Figure 3E> This energy storage device is used by stacking a cathode and an anode. The cathode is composed of carbon nanowalls containing lithium fine particles and absorbs and stores lithium ions, while the anode is composed of carbon nanowalls with a high surface area and accepts and supplies electrons, resulting in high energy density and high power density, as well as rapid charge and discharge. <The process shown in Figure 3F> Current collector tab welding is a process used to connect electrode materials, such as the positive and negative electrodes, to the capacitor's external circuitry in capacitor packs and energy storage devices. It is part of the capacitor pack assembly process and is performed to ensure a secure connection between the current collector tabs and the electrode materials, ensuring electrical continuity. Current collector tabs are made of thin metal foil (typically copper or aluminum) and are connected to the battery electrodes. The current collector tabs serve as contact points between the electrodes and the battery pack's external circuitry, providing a path for electrical current. Current collector tab welding is performed to safely and firmly connect the current collector tabs to the electrode materials.Typically, current collecting tab welding is performed using various welding techniques, such as electron beam welding, laser welding, resistance welding, and ultrasonic welding, to firmly connect the electrode material and the current collecting tab, optimizing electrical performance. <Step shown in Figure 3G> Laminating and heat welding is a process for joining dissimilar materials or thin films. A laminating and heat welding machine is used to apply heat to the joint, melting the materials and fusing the joining surfaces. Then, pressure is applied to seal the joint. The melted material is evenly distributed, creating a strong bond. The joint is then cooled and solidified. <Step shown in Figure 3H> Vacuum drying is a process to remove moisture from the capacitor and ensure the stability of the insulating material and internal structure. This is an important step in the manufacture and maintenance of high-capacity capacitors, especially those used for high voltages and high frequencies. The capacitor is placed in a dedicated vacuum chamber, inside which a vacuum pump is installed, which evacuates the air around the capacitor, facilitating moisture evaporation and reducing humidity. After drying is complete, the capacitor's insulation and capacitance are inspected and any necessary repairs are made. <Step shown in Figure 3I> Electrolytes typically consist of water and an electrolyte (typically an ammonium salt or aluminum salt). Once the electrolyte is prepared, it is carefully injected into a capacitor through a specific injection point. Once the injection is complete, the injection point is sealed and the capacitor is confirmed to be sealed. <Step shown in Figure 3J> This process is used to seal the electrodes and insulators within the capacitor. Laminating and heat sealing uses high temperature and pressure to seal the electrodes and insulators, and a heated roller or press is used around the laminate to seal the perimeter of the laminate. This provides insulation to insulate the interior of the capacitor from the air and ensure proper operation. <Step shown in Figure 3K> To verify the performance of a capacitor, several important parameters must be examined. (1) Capacitance (unit: farad, F) represents the amount of charge stored in a capacitor. It is measured using a capacitor meter tester or multimeter. (2) Allowable voltage (rated voltage, unit: volts, V) indicates the maximum voltage at which the capacitor can be safely operated. Care must be taken not to exceed the allowable voltage.(3) ESR (equivalent series resistance, unit: ohm, Ω) indicates the internal resistance of a capacitor; the lower the ESR, the better the performance. (4) Dissipation factor represents the ratio of a capacitor's loss to its reactance; a lower dissipation factor indicates higher capacitor performance. (5) Inductance (unit: henry, H) can affect performance at high frequencies; it is desirable for capacitors for high frequencies to have low inductance. (6) It is important to check the temperature characteristics of the capacitor and use it within the appropriate temperature range.

[0021] Figure 4 shows a model of the reaction at the negative electrode of a lithium-ion capacitor, explaining how this model works when the capacitor is being charged. At the beginning of charging, positively charged lithium ions (7) exist in the electrolyte (6), and travel through the electrolyte and the separator (5) to the negative electrode. The lithium ions (7) enter the carbon nanowalls (8) synthesized on the negative electrode. The carbon nanowalls have a special structure that allows them to efficiently absorb lithium ions. The lithium ions are adsorbed by the carbon nanowalls, where they undergo a chemical reaction, forming a lithium-carbon compound, LiC6 (10), and then forming positively charged lithium ion clusters (11) outside the carbon nanowalls. At the same time, electrons - (9) travels through the wire (12) to the synthesized carbon nanowalls on the negative electrode, - The negative electrode of the lithium-ion capacitor can be charged by combining positive charges (lithium ions)⁺ (7) that enter the carbon nanowalls and react with the carbon to charge the negative electrode.

[0022] Figure 5 illustrates the reaction model at the positive electrode of a lithium-ion capacitor. Lithium particles (4) present in the positive electrode (carbon nanowall) receive a positive charge (7) from the negative electrode, and at the same time, electrons (9) enter the synthetic carbon nanowall of the positive electrode via the wire (12). The lithium particles (4) and electrons (9) combine at the positive electrode to form a positive charge (lithium ion) (7). This process results in discharge.

[0023] Figures 6(a) and (b) show typical SEM images of the synthesized high-density, vertically aligned carbon nanowalls (CNWs), where high-density nanowalls with lengths of 0.9 μm are observed.

[0024] Figure 6 shows the G band, one of the characteristic peaks of graphene, which originates from the in-plane motion of carbon atoms and is located at 1580–1600 cm -1 The D band is known as a disorder band due to structural disorder and defects, and is caused by lattice motion away from the center of the Brillouin zone. It appears around 1270-1450 cm -1 The presence of this peak at 2700 cm (depending on the excitation wavelength) indicates defects or edges in the graphene sample. The 2D band, also called G' (G prime), is a second-order two-phonon scattering. The 2D band exhibits a strong frequency dependence on the excitation laser due to a double resonance process that relates the phonon wave vector to the electronic band structure; this feature appears around 2700 cm -1 which can also be used to determine the number of graphene layers.

[0025] The lithium ion capacitor according to the present invention can be widely used as a power storage device, contributing to improved performance with high performance and long life.

[0026] 1………………Electrolytic copper substrate for the positive electrode 2………………Electrolytic copper substrate for the negative electrode 3………………Carbon nanowalls synthesized on the positive electrode 4………………Lithium fine particles 5………………Separator 6………………Electrolyte 7………………Positive charge (lithium ions) 8………………Carbon nanowalls synthesized on the negative electrode 9………………Electrons ‐ 10………………LiC6 11………………Lithium ion cluster 12………………Electric wire

Claims

1. A lithium ion capacitor comprising a negative electrode formed of carbon nanowalls oriented vertically to the surface of a conductive substrate, and a positive electrode formed of carbon nanowalls oriented vertically to the surface of a conductive substrate and coated with lithium microparticles, with an electrolyte between the two electrodes and a separator separating them.

2. A lithium ion capacitor as described in claim 1, in which, when an electrolytic copper substrate is used as the conductive metal substrate, the copper oxide on the surface is removed in advance by acid treatment, and carbon nanowalls are formed on the surface oriented vertically.

3. The lithium ion capacitor according to claim 1, in which a pulsed laser deposition device is used to irradiate the surface of a lithium target with a powerful pulsed laser, thereby generating lithium particles which are then coated onto the carbon nanowalls.

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