KR102057128B1 - Expansion graphite anode material for lithium capacitors impregnated with lithium expansion graphite - Google Patents

Abstract

The present invention relates to a negative electrode material for lithium ion capacitors in which expanded graphite and expanded graphite are impregnated with lithium in a vacuum and further used as an aqueous polyacrylate binder. Specifically, the steps of expanding graphite, atomizing expanded graphite, Impregnating fine expanded graphite with lithium hydroxide and lithium cobalt oxide (Li ₆ CoO ₄ ) in a vacuum, dissolving the expanded graphite and polyacrylate, an aqueous binder, in distilled water as a solvent, solution Slurrying using a binder to form a sheet to form a negative electrode for a lithium-ion capacitor, and to expand the graphite and impregnated the lithium compound between the layers of the expanded graphite and doped the negative electrode material and manufacturing process To provide.
Therefore, the lithium ion capacitor of the present invention is a negative electrode material for simultaneously satisfying the high power and long life characteristics of a supercapacitor and the high energy density characteristics of a lithium ion battery, using low-cost raw materials, doping of a simplified and uniformized lithium source, and an environmentally friendly aqueous binder. It is to invent and provide use.

Description

Expanded graphite anode material for lithium-ion capacitors with lithium impregnated in expanded graphite {Expansion graphite anode material for lithium capacitors impregnated with lithium expansion graphite}

 The present invention is a technique for producing expanded graphite by expanding the graphite in order to be able to use low-cost expanded graphite in place of graphite as a cathode commonly used in the manufacture of lithium-ion capacitor and a technique of pre-doping lithium to the negative electrode in a simple process A lithium source is inserted in a vacuum between the expanded graphite layers to provide a lithium source, and a hazardous organic binder is used as a binder to provide an anode material for a lithium ion capacitor and a manufacturing process by using an eco-friendly water-based polyacrylate. The present invention relates to an expanded graphite cathode material for lithium-ion capacitors in which lithium is impregnated into expanded graphite.

Electric double layer capacitors have high output characteristics and high cycle reliability. Recently, electric double layer capacitors have been increasingly used in power storage devices such as main power supply or auxiliary power supply for transportation equipment and electric vehicles, solar power generation or wind power generation, and uninterruptible power supply for IT. It is a trend. However, the electric double layer capacitor has a disadvantage of low energy density, and thus, a battery complementing this is a lithium ion capacitor.

Li-ion capacitor is a hybrid energy storage device with an asymmetric structure proposed to simultaneously satisfy the high power and long life characteristics of the electric double layer capacitor and the high energy density characteristics of the lithium ion battery. Therefore, the positive electrode combines the structure of the electric double-charged capacitor and the negative electrode of the structure of the lithium ion battery.

Conventional Li-ion capacitors use activated carbon powder of current electric double-charged capacitors in the anode connected with lithium metal electrodes in series, and lithium ion in carbon or graphite (cathode), which is the cathode of lithium ion batteries. Pre-doping (pre-doping) to the negative electrode, the electrolyte containing lithium is used. Therefore, it is necessary to perform an electrode raw material, a lithium metal which is an expensive lithium doping source, and a complex lithium ion doping process in a low cost and simple process. Also, the use of an organic binder in manufacturing a current lithium ion capacitor battery is an environmental hazard. Of course, since it reacts actively with graphite, hard carbon, and the like as the negative electrode active material, if it is replaced by an environmentally friendly aqueous binder to compensate for its disadvantages, the commercialization and diffusion will be made much faster.

Patent 10-2010-0136610, High thermal conductivity expanded graphite sheet composited with high thermal conductivity fine particles and a method of manufacturing the same Patent 10-2012-0023780, Method for manufacturing silicon carbide nanofibers using expanded graphite and amorphous silica

Electric double layer capacitors have excellent output characteristics, have a low cycle deterioration, but have a very low energy density compared to conventional lithium secondary batteries. Therefore, the battery which greatly improved the energy density is a lithium ion capacitor. That is, a lithium ion capacitor is a battery which improves energy density by hybridizing the principle of physical adsorption reaction of a capacitor and the insertion / desorption of ions of a lithium ion battery.

The electrostatic capacity of the lithium ion capacitor including the lithium ion is adsorbed to the positive electrode in the positive electrode during charging and at the same time the lithium ion in the electrolyte is occluded in the negative electrode. It is expressed by lithium ion desorption.

Lithium-ion capacitors absorb lithium ions in the negative electrode in advance to lower the negative electrode potential than the electrolyte potential, thereby improving the withstand voltage and the capacitance of the capacitor itself, compared to general electric double layer capacitors, thereby obtaining high energy density. It can be discharged deeply until it becomes, and wider voltage range is used to realize higher energy density.

The lithium ion capacitor is composed of a positive electrode, a negative electrode, a separator, an electrolyte, a pre-doped lithium, and the like. In general, the positive electrode is activated carbon, which is a positive electrode of a high capacity capacitor, and the negative electrode is graphite, which is a negative electrode of a lithium secondary battery. Lithium metal (Li Metal) is mostly used as a lithium source.

In order to utilize lithium metal as an electrode, graphite is used as a negative electrode, and lithium metal is doped onto the negative electrode through a direct contact method, an external short circuit method, and an electrical method as shown in FIG. 1, but lithium metal itself is expensive, There is a stability problem with use, and the doping process is complicated. In detail, the conventional lithium metal doping uses an external press plate for the close contact between lithium and the electrode, and doping is performed at 60 degrees for 10 days or more. At this time, if the adhesion is not properly made, the contact resistance is high, if too tightly adhered, it is difficult to control the doping level, such as break the electrode.

In order to solve the above conventional problems, a lithium ion capacitor was added to a ruthenium oxide additive to improve energy density, or lithium was doped by electrochemical method using a lithium-based transition metal oxide as a lithium source. In addition, the electrochemical lithium doping using the existing lithium metal has a problem of securing safety and control of the doping level to effectively improve this, without using a separate lithium metal source as shown in Figure 2 Li ₂ which is a lithium-containing transition metal oxide MoO ₃ , Li ₅ FeO _4, Li ₂ CuO _2, Li ₂ Cu _0.9 Zn _0.1 O ₂ and Li ₆ CoO ₄ can be mixed with the negative electrode active material to have a lithium source of its own, simplifying the manufacturing process, and lithium doping Afterwards, it is reported that the side reaction caused by the remaining lithium can be suppressed.

In detail, Shinshu University of Japan uses lithium manganese oxide, ruthenium oxide nanoparticles or nanosheets as an anode, and applies lithium metal to the cathode to produce a lithium ion capacitor having an energy density of 144 to 544 Wh / Kg at a maximum of 4.3 volts (V). Reported development. Japan's AIST developed lithium ion capacitors with activated carbon and LiNiCoMn or nano-sized lithium phosphate as anodes, and hard carbon or nano-sized lithium titanic acid as cathodes. Reported that it was achieved. In addition, the recent trends in the manufacture of lithium-ion capacitors in developed countries show that the anode is mostly used in the form of a mixture of active carbon and metal or metal oxide, and the cathode is a lithium carbon source, which is a lithium source in hard carbon or graphite. It is reported that a doped by a method or a direct contact method, and invented a lithium ion capacitor with an energy density of 20 ~ 115wh / kg using LiPF ₆ / EC: DMC as an electrolyte.

On the other hand, as a patent invented in the Republic of Korea, the positive electrode active material for lithium ion capacitor of Patent 10-2011-0043457 uses a composite positive electrode in which lithium metal oxide is dispersed on the surface of carbon nanotube, and the lithium of patent 10-2011-0043456 Carbon-based negative electrode material for ion capacitor and its manufacturing method ”uses negative electrode material coated with hard carbon on graphite surface, and“ Negative electrode material, lithium ion capacitor having same and manufacturing method thereof ”of Patent 10-2014-0062807 A composite metal oxide was added to the positive electrode, and lithium iron phosphate (LiFePO _4, ) and activated carbon were used as a positive electrode active material in the lithium ion capacitor of Patent 10-2010-0102786. Patent 10-2014-0011740, "Polyde for Lithium-ion Capacitor and Lithium-ion Capacitor Including the Same", manufactured a positive electrode containing graphene on one surface of the current collector, and a current collector having a three-dimensional structure according to Patent 10-2012-0009599. In the electrode structure comprising a lithium ion capacitor comprising the same and "to prevent the flow down the slurry," Patent No. 10-2013-0050069 "LTO (LiTi ₂ O ₅ ) carbon composite, LTO carbon composite manufacturing method, LTO carbon NTO and hybrid supercapacitors using negative electrode active materials using composites reported the use of LTO and carbon composites.

The above existing domestic and foreign inventions are expensive in terms of raw materials, and in the case of predoping with lithium source doping, the process is very complicated and expensive, whereas particle size is added when lithium metal oxide is added and mixed. Is mixed directly with very different carbon, activated carbon, and graphite, resulting in non-uniformity and agglomeration, and there is also a harmful problem by using an organic solvent or a binder.

Therefore, in order to make lithium pre-doping uniform and well-dispersed at low cost by a simple process, a place to support a lithium source must exist in graphite. In addition, it is absolutely necessary to secure a technology capable of doping an inexpensive lithium oxide or hydrate in an efficient, stable and simple process. In addition, the use of organic solvents in the manufacture of lithium ion capacitor batteries not only indicates environmental hazards due to volatilization of the solvent, but also requires that organic solvents be replaced with environmentally friendly water-based solvents because organic solvents actively react with graphite or hard carbon as an anode active material. There is.

In the present invention, in order to manufacture a lithium-ion capacitor, expanded graphite is used to manufacture a negative electrode at low cost, and a lithium source is vacuum-adsorbed to doped graphite, and doped with a binder that is currently harmful to humans as an aqueous binder. It is invented to use.

Specifically referring to the present invention, a method of expanding and using graphite to uniformly disperse and support a lithium source is uniformly dispersed between graphite layers in a form that is not mixed with the expanded graphite used as a cathode. In addition, the replacement of expanded graphite with graphite of the cathode also makes it possible to reduce the cost of raw materials. As a lithium pre-doping method, a low-cost lithium oxide or a hydrate is invented in a vacuum between the layers of expanded graphite in a vacuum as shown in FIG. In addition, the use of polyvinylidene fluoride (PVDF) organic binders in the manufacture of lithium ion capacitor batteries represents environmental hazards. One thing invented.

The present invention has achieved a low cost of raw materials by inventing a technology for expanding expanded graphite to be used in place of graphite cathodes commonly used in the manufacture of lithium ion capacitors and a process for processing them as negative electrode materials for lithium ion capacitors.

By inventing a technique of providing a uniformly well-dispersed lithium source by inserting a lithium source in a vacuum between layers of the expanded graphite, rather than simply mixing the lithium source on the expanded graphite cathode, to provide an effective, stable and lithium doping.

In addition, there is an effect that the use of expanded graphite as a negative electrode material in the manufacture of a lithium ion capacitor by replacing an oil-based binder harmful to humans with an environmentally-friendly water-based binder.

1 is a method of pre-doping a conventional lithium source by an electrochemical method
2 is a principle in which lithium hydrate and oxide vacuum-immersed in expanded graphite operate as a lithium source
Figure 3 is a natural structure (a), expanded graphite (b), microstructure of the expanded graphite loaded with lithium source (c)
4 shows a microstructure (a) and an X-ray diffraction pattern (b) after lithium hydroxide is immersed in a vacuum.
5 is a charge / discharge cycle curve of a negative electrode prepared by using a lithium source as a negative electrode active material immersed in expanded graphite in a vacuum and using a polyacrylate (PAA) and a polyvinylidene fluoride (PVDF) binder.
6 is a graph of expansion rate according to expansion temperature and heat treatment time of natural graphite
7 is a charge / discharge cycle curve of a negative electrode prepared by immersing lithium hydroxide, a lithium source, in swelling graphite in a vacuum for each mole number;
FIG. 8 is a charge / discharge cycle curve of a negative electrode prepared by immersing lithium cobalt oxide in a vacuum as a lithium source in the expanded graphite of FIG.

In the present invention, the negative electrode material for the lithium ion capacitor is expanded graphite having expanded graphite at about 600 to 1000 percent, and lithium doping is performed on the expanded graphite negative electrode by immersion in vacuum, where lithium source is uniformly well dispersed with lithium. Expanded graphite, Ketjenblack conductive material, polyacrylate binder, dispersant and distilled water solvent.

The expanded graphite includes graphite expanded by removing impurities of natural graphite and making it highly purified, including expanded graphite expanded within 1 minute to 5 minutes at an expansion temperature of 600 ° C. to 900 ° C. or less. (B) may include any type of expanded graphite which has been expanded after processing.

The lithium source in the lithium-doped expanded graphite includes, but is not limited to, lithium oxide (Li ₆ CoO ₄ ) or lithium hydrate (LiOH, LiOH.xH ₂ O), but not limited to lithium-containing transition metal oxide (Li ₂ MoO ₃ , Lithium-containing solid powders or liquid solutions, such as Li ₅ FeO _4, Li ₂ CuO _2, Li ₂ Cu _0.9 Zn _0.1 O ₂ ), lithium chloride, lithium nitride, and lithium-containing polymers, can be supported by vacuum in expanded graphite. It may include.

The aqueous binder may include an aqueous binder such as polyacrylate (PAA, Polyacrylic acid) which can be used in distilled water, but is not limited thereto, and all kinds of aqueous binders used for a conventional lithium ion capacitor or a lithium ion secondary battery. It may include.

The conductive material may include a conductive powder such as ketjen black, but is not limited thereto. Super-p, acetylene black, and carbon used for a typical lithium ion capacitor or a lithium ion secondary battery. It may include all kinds of conductive materials such as black.

The dispersant may include a dispersant of isopropyl alcohol (IPA), hydroxypropyl cellulose (HPC), polyethylene glycol (PEG) -200, Dolapix PC-21 (DOLAPIX PC-21) that can be used in the aqueous system, It is not limited to this, and may include all kinds of aqueous dispersants used for ordinary lithium ion capacitors, lithium ion secondary batteries, and electronic ceramic powders.

Expanded graphite for lithium ion capacitor negative electrode according to the present invention, expanded graphite material containing or immersed lithium-containing solid powder or liquid solution as a lithium source in the expanded graphite and the manufacturing process and polyacrylate, distilled water, conductive agent as an aqueous binder It may also include a material and a manufacturing process, such as a slurry and a manufacturing process, such as forming a sheet and then bonding using a conductive adhesive, or bonding the slurry to the current collector.

The negative electrode current collector may use all materials such as copper, stainless steel, nickel, and alloys thereof, which are conventionally used in lithium ion capacitors and lithium ion secondary batteries. The most preferable current collector among them is copper. Its thickness is about 10-100 micrometers, and it is preferable that it is foil which a microporous micropore penetrates the whole surface.

(Example 1)

<Expansion graphite production>

Before preparing expanded graphite, impurities contained in natural graphite (QKG 196, Samjung C & G Co., Ltd.) were removed. In natural graphite, impurities generate lattice defects and dislocations, increase the interlayer distance, increase the activity of the reaction, increase the rate of chemical reactions, and break the interlayer, negatively affecting intercalation. Impurities were removed by treatment with ₂ SO ₄ , H ₂ O ₂ , and the like. At this time, the ratio of graphite, H ₂ SO ₄ and H ₂ O ₂ was 2: 1: 0.12 mol (mol) ratio. The oxidation treatment was stirred for 120 minutes, washed with water until the pH was neutral, and dried.

When the oxidized graphite is rapidly heated to a high temperature, HSO ₄ ^- ions, which are residual compounds remaining in the interlayer compound of the graphite, are vaporized all at once from the inner surface of the graphite, and the graphite layer rapidly expands at the ejection pressure, thereby having plasticity. It becomes expanded graphite. In the case of natural graphite, the edge was angled and the surface was observed to be smooth, and the interlayer was observed to have almost no gap. However, in the expanded graphite of FIG. 3 (b), the gap was completely widened between layers.

After expansion, a jet mill was used to grind the average central particle size (D ₅₀ ) to about 3-7 micrometers (μm) so that the largest particles were about 15 micrometers or less. Preferred particle sizes were on the order of 5 micrometers.

<Production of negative electrode active material by immersing lithium source in expanded graphite>

A lithium source or a lithium hydrate, which is a lithium source, was immersed in a vacuum as in FIG. 3 (c) between the layers of expanded graphite as shown in FIG. 3 (b), and used as a negative electrode active material.

In the immersion process, first, a lithium source to be immersed in distilled water was added in proportion to the weight of expanded graphite, and stirred at 100 ° C. for 1 hour to prepare a solution by dispersing the lithium source. After adding the expanded graphite to the aqueous solution mixed with the prepared lithium source and mixing and dispersing by using an ultrasonic wave for 30 minutes, and put into a vacuum container, while maintaining the vacuum to 0.9MPa while removing the air between the layers of the expanded graphite If the lithium source is filled in the meantime, the filled lithium source is attached due to the plasticity which is inherent in the expanded graphite itself.

When the lithium hydrate was used as a lithium source and immersed in vacuum in expanded graphite, the immersed state was observed by scanning electron micrographs. As a result, lithium hydroxide was mixed and dissolved in the aqueous solution, and the particles were clearly observed. In addition, X-ray diffraction analysis showed that the main crystal phase was graphite, but the LiOH crystal phase as a lithium source was also present. Therefore, it can be determined that the lithium source is immersed and inserted between the expanded graphite interlayers.

<Negative electrode sheet production using an aqueous binder>

A negative electrode sheet was prepared by using a sample prepared by immersing a lithium source in vacuum in expanded graphite as a negative electrode active material, slurrying the following ratio using Ketjenblack as a conductive agent and aqueous polyacrylate as a binder, and then applying a slurry on copper foil. .

The slurry composition was mixed in a weight ratio of expanded graphite: Ketjenblack conductive agent: water-based polyacrylate binder = 87: 5: 8 impregnated with a lithium source as a solid content, and the mixture was used as a weight ratio of 27.5: 72.5 as a solvent. At this time, the polyacrylate, which is an aqueous binder, had a molecular weight of 450,000, and was diluted with distilled water to be 10 percent by weight.

Meanwhile, dispersants were added and dispersed to prevent agglomeration of solid particles during slurry production. As a dispersant, in addition to isopropyl alcohol, one dispersant among hydroxypropyl cellulose (HPC), polyethylene glycol (PEG) -200, and DOLAPIX PC-21 (DOLAPIX PC-21) set was used. 3 percent of was added to allow the solids to disperse well.

The negative electrode sheet was prepared by applying the slurry prepared above by a doctor blade method on a porous copper foil having a thickness of about 13 micrometers, and drying it for 24 hours in a vacuum at 120 degrees after drying. The thickness of the sheet was about 50 micrometers at this time.

<Battery characteristics>

The produced sheet produced a battery cell in a glove box which is an argon gas atmosphere. The electrolyte (LiPF ₆ ) was dropped one drop before placing the electrode on the case to prevent the electrode from moving. The electrolyte was impregnated on the electrode, and the separator was raised when spreading was observed. The gasket was raised, and lithium metal to be used as a positive electrode was punched and attached to a spacer (Sus-spacer), and then placed on an electrode. Then, the spacer was raised and the electrolyte was covered and then assembled. The assembled battery cell was stabilized by aging at room temperature for 12 hours and then used as a battery cell. As the battery characteristics, charge and discharge cycle tests were conducted at 1C (100 cycle).

(Comparative Example 1)

Polyvinylidene fluoride (PVDF) instead of polyacrylate as a binder for the comparative experiments and the solvent is the same as (Example 1) except that 1-methyl-2-pyrrolidone (NMP) was used instead of distilled water It was carried out.

Results of Example 1 Results of FIG. 5A and Comparative Example 1 As shown in FIG. 5B, the polyacrylate binder was used at an initial capacity of 253 mAh / g and a capacity after 100 cycles of 181 mAh / g showed higher capacity and more stable cycle characteristics, but the polyvinylidene fluoride binder (Comparative Example 1) had an initial capacity of 252mAh / g and a capacity of 183mAh / g after 100 cycles. Accordingly showed unstable progression. Therefore, it was found that performance was improved when water-based binders were used rather than oil-based ones.

(Example 2)

When producing natural graphite from expanded graphite in Example 1, the remaining processes were the same, and the expansion temperature was set to 600, 700, 800, 850 and 900, and the heat treatment time was 1, 3 and 5 seconds. It was measured how much expansion occurs when, the results are shown in FIG. The highest expansion rate was 900 degrees and the expansion rate was 950 percent. At this time, the production of expanded graphite was performed by using a rotary kiln at a rotational speed of 10 alpha (rpm) and a tube angle of 5 °.

The expanded graphite used in the <Example> of the present invention was used for the cathode active material inflated for 1 second at 900 degrees.

(Example 3)

Lithium hydroxide (LiOH), which is a lithium source, was changed to 3, 5, 10, and 20 weight percent of the expanded graphite as the negative electrode active material in Example 1, and then charged and discharged after immersion in vacuum. At this time, the process of dipping the expanded graphite and the lithium source in the expanded graphite, and the process of manufacturing the electrode using a polyacrylate binder was the same as in (Example 1).

When the lithium source (LiOH), a lithium source, was changed to 3, 5, 10 and 20% by weight, the initial capacity of the lithium hydroxide was lower than that of pure expanded graphite, but after 100 cycles. The efficiency was confirmed to be higher. In particular, when 5% of lithium hydroxide was immersed, the capacity increased more than the initial capacity after 100 cycles. When the lithium hydroxide used as the lithium source is over a certain amount, the SEI layer is formed during lithium desorption during charging and discharging. It is believed to decrease.

(Example 4)

When the lithium source was immersed in the expanded graphite which is the negative electrode active material in (Example 1), the lithium hydrate in the above (Example 3) was replaced with lithium cobalt oxide (Li ₆ CoO ₄ ) and immersed in vacuum. At this time, the amount of immersion was changed to 3, 5, 10 and 20 percent by weight ratio. The expanded graphite production process, the process in which the lithium source was immersed in the expanded graphite, and the process of manufacturing the electrode using the polyacrylate binder were the same as in (Example 1). Meanwhile, lithium cobalt oxide (Li ₆ CoO ₄ ) was synthesized using Li ₂ O and CoO in 3: 1 at 700 ° C. for 5 hours in a nitrogen atmosphere.

When lithium cobalt oxide (Li ₆ CoO ₄ ) was immersed in a lithium source, the initial capacity was lower than that of pure expanded graphite, as in the case of dipping lithium hydroxide, but 100 cycles when 5% of lithium cobalt oxide was used. The efficiency was shown to be high after. While lithium cobalt oxide contains a lot of lithium source, it is mainly used as a positive electrode material, and when a large amount is included in the negative electrode material, the potential difference of the battery itself can be narrowed. In particular, the lithium source of lithium cobalt oxide becomes It is believed to be the cause of dose reduction due to the decreasing phenomenon. Lithium cobalt oxide also showed the best efficiency after 100 cycles when 5 percent immersion, like lithium hydroxide, and 5 percent immersion amount is appropriate.

Claims (8)

An anode material in which a lithium source is uniformly immersed in vacuum in expanded graphite prepared by thermal expansion of natural graphite for 1 to 5 minutes within a range of 600 ° C. to 900 ° C., and the anode material is conductive with 87% by weight of expanded graphite loaded with lithium source. An expanded graphite negative electrode material for lithium-ion capacitors in which lithium is impregnated with expanded graphite, comprising 5 wt% of a second (ketjen black) and a binder of water-based polyacrylate at a composition ratio of 8 wt%. delete delete The expanded graphite anode material for lithium ion capacitor according to claim 1, wherein lithium hydroxide is used after being immersed in expanded graphite in a weight ratio of 3, 5, 10, and 20 percent by vacuum as a lithium source. The expanded graphite negative electrode material for lithium ion capacitors according to claim 4, which is used after being immersed in expanded graphite using lithium cobalt oxide instead of lithium hydroxide as a lithium source. The expanded graphite negative electrode material for a lithium ion capacitor according to claim 1, wherein the expanded graphite negative electrode material is prepared by using polyacrylate which is water-based as a binder in manufacturing a lithium ion capacitor using expanded graphite. The expanded graphite negative electrode material for lithium ion capacitors according to claim 4 or 5, wherein the lithium source is used after preparing a slurry using an anode material and an aqueous binder immersed in expanded graphite. The expanded graphite negative electrode material of claim 7, wherein the slurry is used after coating or sheeting the slurry on a current collector Cu sheet.

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