JP2017195102A - Lithium ion battery and negative electrode active material for lithium ion capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an excellent lithium ion battery and a negative electrode active material for a lithium ion capacitor.SOLUTION: The negative electrode active material is a mixed system composed of amorphous carbon and amorphous silicic acid, and has a specific amorphous silicic acid content, BET specific surface area, meso/macropore specific surface area, meso/macropore volume. In particular, those produced from rice husk, which is a plant-based organic matter containing naturally occurring silicic acid, by primary carbonization, partial removal of silicic acid, and secondary carbonization, show large lithium ion occluding and releasing capacity, and has excellent rate characteristics and cycle characteristics.SELECTED DRAWING: None

Description

  The present invention relates to a lithium ion battery and a negative electrode active material for a lithium ion capacitor.

In recent years, there has been a rapid increase in social demand for power storage devices that are small and can input and output large amounts of energy. Products that require operation independently from an external power supply, such as electric vehicles, hybrid vehicles, industrial machines, robots, mobile terminals such as smartphones and tablets, have high energy density and input / output (power) density, and have a long service life. A high-performance electrochemical storage device is required.
Electrochemical power storage devices include secondary batteries such as lithium ion batteries that can achieve high energy density and capacitors such as electric double layer capacitors that can achieve high input / output density. In addition, there is a lithium ion capacitor having intermediate performance between a secondary battery and a capacitor. In particular, in lithium ion batteries and lithium ion capacitors, lithium ions are occluded in the negative electrode, and the electrode potential of the negative electrode is lowered to near the standard electrode potential (−3.045 V) of lithium, whereby the potential difference between the positive electrode and the negative electrode (cell Voltage) and high energy density.

The basic constituent elements of the secondary battery and the capacitor are a positive electrode, a negative electrode, an electrolytic solution, and a separator. As the negative electrode and the positive electrode, a material in which an active material responsible for direct charge transfer is coated on a metal foil as a current collector is often used. The active material is usually granular, and a conductive auxiliary agent that maintains electrical contact between the active material particles and between the active material particles and the current collector, and a binder that maintains their mechanical adhesion are generally added. In addition, the separator is used while being sandwiched between the positive and negative electrodes in order to realize ion movement between the positive and negative electrodes while avoiding electrical contact between the positive and negative electrodes. Paper or resin having fine holes is generally used as a separator.
In a current general lithium ion battery, a carbon-based material such as graphite or hard carbon is used as a negative electrode active material, and a lithium transition metal oxide such as lithium cobaltate or lithium manganate is used as a positive electrode active material. Further, a system using a lithium salt such as lithium hexafluorophosphate (LiPF ₆ ) as an electrolyte and a mixed organic solvent such as ethylene carbonate or diethyl carbonate as a solvent is widely used as an electrolyte.

Focusing on the carbon-based negative electrode active material, graphite having a crystal structure can occlude lithium ions by inserting lithium ions between graphene layers. The maximum storage capacity is 372 mAh / g in the form of LiC ₆ . In the case of graphite, since most of the lithium ions are stored and released at a potential close to the standard electrode potential of lithium, the battery cell voltage is easily maintained even when the discharge proceeds, and the lithium ions are stored and released. It is mentioned that the energy loss accompanying this is small. On the other hand, the active material is likely to be distorted, cracked or deformed due to the expansion and contraction of the crystal structure accompanying the insertion and extraction of lithium ions. Therefore, as the number of charge / discharge cycles increases, the structural change of the active material itself, the separation between the active material particles and the separation between the active material particles and the current collector progresses, and the lithium ion storage capacity as an electrode tends to decrease. The characteristics are inferior. Furthermore, since lithium ions need to move in an active material with high crystallinity, lithium ions are likely to be rate-controlled when the charge / discharge current density is high. Therefore, as the charge / discharge current density increases, the lithium ion storage / release capacity decreases. That is, the rate characteristic is generally not excellent.

  On the other hand, hard carbon, also called non-graphitizable carbon, has a structure in which fine crystalline graphene layers are arranged without regularity. Lithium ion occlusion of hard carbon is due to insertion into the graphene layer and lithium aggregation (lithium metallization) into the space formed between the graphene layers. Since the ratio of occlusion and desorption of lithium ions in the crystalline graphene layer is small, the capacity decrease due to repeated charge / discharge and increase in charge / discharge current density is small, and the cycle characteristics and rate characteristics of hard carbon are generally higher than those of graphite Are better

  A lithium ion capacitor mainly uses a carbon-based material as a negative electrode active material and activated carbon as a positive electrode active material. A non-Faraday reaction called adsorption / desorption of ions in the electrolytic solution proceeds at the positive electrode, and a Faraday reaction called storage / release of lithium ions proceeds at the negative electrode. Lithium ion capacitors are not required to have as high energy density as lithium ion batteries, but are required to input and output energy efficiently with a lifetime of tens of thousands of cycles and a large charge / discharge current density. While the cycle life of a typical lithium ion battery is assumed to be several thousand cycles, it is required for the electrode material of a lithium ion capacitor to maintain a high input / output density over tens of thousands of cycles. The Hard carbon or a mixture of graphite and hard carbon is often used as the carbon-based material used for the negative electrode active material.

The specifications of the battery and the capacitor are set so as to meet the use conditions. For example, a deep cycle specification in which lithium ions are occluded up to the maximum capacity of the negative electrode active material and charge / discharge is repeated until it is completely released, a specification in which charge / discharge is repeated at 80% ± 10% of the maximum capacity, A specification in which charging / discharging is repeated at 60% ± 20% of the capacity is assumed, and the type and physical properties of the negative electrode active material optimal for the specification are selected. In particular, lithium ion batteries and lithium ion capacitors for in-vehicle applications, for which great demand is expected, require excellent time response (rate characteristics) and resistance to repeated charge and discharge (cycle characteristics) with respect to the negative electrode active material.
At present, a hard carbon type is widely used as a negative electrode active material corresponding to the application. However, the hard carbon active material gradually increases its potential as lithium is released. As a result, the potential difference from the positive electrode is reduced, and the electromotive force of the battery cell or capacitor cell is gradually reduced. If the charge / discharge current level is the same, the energy that can be stored and released from the battery cell and the capacitor cell is large in a high electromotive force state. Therefore, charging and discharging in a state where lithium is sufficiently occluded in hard carbon stores and releases energy most efficiently. On the other hand, the insertion and extraction of lithium ions near the maximum storage capacity tends to induce lithium metal plating (precipitation) in the negative electrode, as well as structural and chemical changes in the active material, binder and current collector. That is, there is a demerit that accelerates the performance degradation of the battery and the capacitor, such as a short circuit between the positive and negative electrodes separated by the separator, a structural deterioration of the electrode, and a decrease in charge / discharge capacity.

  In hard carbon, insertion of lithium ions into the graphene layer and aggregation of lithium in the space formed between the graphene layers occur. Although the lithium ion storage capacity of hard carbon varies greatly depending on the type of raw material and carbonization conditions, it is generally about 200 to 400 mAh / g. In addition, in the occlusion and release of lithium ions in the space formed between the graphene layers, lithium that is immobilized until the capacity is stabilized is generated, so the initial irreversible amount is larger than that of graphite. In order to cancel out the initial irreversible capacity, and in order to stabilize the capacity, a pre-doping process in which lithium ions are sufficiently occluded in advance in the active material is often employed. In general, as a method of pre-doping lithium ions into a carbon-based negative electrode active material, there is a method of directly contacting lithium metal or electrically connecting to lithium metal in an electrolyte containing lithium ions. It is possible to control the amount of lithium ions to be doped by changing the time of contact or electrical connection with lithium metal, or by interposing a resistor when electrically connected. However, it is difficult to uniformly dope a negative electrode active material with a desired amount of lithium ions. Therefore, it is simple to secure sufficient time and make the lithium metal and the negative electrode directly contact with each other or electrically short-circuit the lithium metal and the negative electrode to dope up to the maximum storage capacity of lithium ions of the negative electrode active material. . In order to adopt this simple method, the active material needs to have characteristics that hardly cause plating and structural change of lithium metal even when lithium ions are doped to the maximum storage capacity. However, hard carbon-based active materials are not excellent in these characteristics, and the cycle characteristics of lithium ion storage / release after simple pre-doping treatment up to the maximum storage capacity are not excellent. Actually, the present inventor performed a pre-doping treatment for occluding lithium ions up to a maximum occlusion capacity on a plurality of commercially available hard carbon-based active materials and graphite-based active materials, and then performed negative electrode actives for lithium ion batteries and lithium ion capacitors. Although performance evaluation was performed as a substance, sufficient performance was not obtained.

On the other hand, as a negative electrode active material of a lithium ion battery, there are silicon-based active materials such as silicon and silicon oxide. The maximum lithium storage capacity of silicon at room temperature is 3590 mAh / g assuming the alloying of silicon and lithium into Li ₁₅ Si ₄ , which is about 10 times the capacity of graphite. Lithium occlusion theoretical capacity of SiO and SiO ₂ is silicon oxide, by assuming the formation of oxidation and reduction of lithium to Si by a lithium ion, respectively estimated at 2287mAh / g and 1678mAh / g. Silicon has a large lithium storage / release capacity, but at the same time causes large expansion and contraction. As a result, distortion, cracking and deformation occur, and separation between the active material particles and separation between the active material particles and the current collector occur. Therefore, the cycle characteristic of the silicon-based active material is extremely lower than that of the carbon-based active material. However, the large lithium ion occlusion / release capacity of silicon itself, and the presence of partial silicon oxide, can cause silicon generation and lithium oxide reduction by reduction of silicon oxide even when lithium ions are excessively doped into the negative electrode active material. Generation of occlusion and release capacity and lithium metal plating can be greatly suppressed.

As a result of intensive studies in view of the situation of the negative electrode active material, the present inventor has shown that a mixture of carbon and silicic acid (silicon oxide) having low crystallinity is promising. I thought that there should be a pore structure. As a material for such a negative electrode active material, a rice husk that contains naturally about 20% by mass of low crystalline silicic acid, and the rest is composed of plant organic matter such as cellulose, hemicellulose, and lignin. It has been found that it can be used as a mixture precursor.
Rice husk is discharged about 2 million tons every year as agricultural waste in the country. Although there are uses for livestock and horticultural materials, there is no clear use for about one-fourth of the emissions. Rice, which is a silicic acid plant, takes water-soluble silicic acid from soil and accumulates it in rice husk in an amorphous form. Rice husk has a silicic acid content of approximately 20% by weight. Silicic acid in rice husk makes it difficult to treat rice husk as an organic fertilizer source, fuel source and carbon source due to its chemical and structural stability. On the other hand, rice husk is often collected intensively and its collection cost is low. Therefore, by using rice husk as a raw material, an inexpensive negative electrode active material can be realized.

The possibility of a carbonized rice husk lithium-ion battery negative electrode active material has been studied for some time (Patent Document 1, Non-Patent Document 1). Further, an amorphous carbon-based active material for a secondary battery electrode in which most of silicic acid is removed from carbonized rice husk using acid or alkali has been studied (Patent Documents 2 and 3). . In particular, Fey et al. Reported the lithium ion storage and release capacity when silicic acid was partially removed from rice husk charcoal produced at 700 ° C. using aqueous sodium hydroxide solutions having different concentrations (Non-patent Document 2). ). An active material is produced based on rice husk charcoal with varying carbonization temperature, the counter electrode is made of lithium metal, and the cell voltages are in the range of 3 to 0 V and 2 to 0 V, respectively, that is, the lithium ion storage and release capacity of the negative electrode active material The capacity per unit mass was measured under the condition that most of the was used. However, Fey et al. Have not studied optimization of the removal amount of silicic acid at all, and have not found a merit of improving characteristics by partial removal of silicic acid. In particular, no mention is made of resistance to pre-doping, as well as rate and cycle characteristics. On the other hand, rice husk-derived activated carbon containing 10-60 mass% silica component was claimed to be promising as an electric double layer capacitor electrode material (Patent Document 4). However, it stores electricity via a non-Faraday reaction caused by the adsorption / desorption phenomenon of ions in the electrolyte into the pores developed in the electrode material, and lithium ions are occluded and released in the active material. Different from the power storage mechanism. There are reports on negative active materials for lithium-ion batteries and lithium-ion capacitors using graphite and silicon, or graphite and silicon monoxide, which do not use plant-based raw materials that naturally contain carbon and silicic acid such as rice husks (patents) Reference 5). However, there is no mention of the optimum content of silicon or silicon monoxide with respect to graphite, and further the optimum pore structure in the active material. On the other hand, it has been reported that the energy density of the negative electrode material of the electricity storage device pre-doped with lithium ions can be improved by defining the mesopore / macropore specific surface area in the range of 11 to 35 m ² / g (Patent Document 6). ). In general, mesopores are pores having a width of more than 2 nm and not more than 50 nm, and macropores are pores having a width of more than 50 nm. Thus, meso-macro pores refer to both meso-pores and macro-pores, ie pores with a width greater than 2 nm. However, studies have not been made on a mixed active material of carbon and silicic acid that can occlude and release lithium ions. Based on the above, there is an optimum amorphous silicic acid content and a finely divided negative active material for mixed lithium ion batteries and lithium ion capacitors of amorphous carbon and amorphous silicic acid. The presence of a pore structure has not been disclosed so far.

JP2008-124034 JP2012-160456 JP 2014-35915 A JP2013-165161 JP2015-156293A JP 2010-135648 A

Hiroki Nakagawa, Kimi Fushimi, Hidetaka Kanno, Preparation of Si / C / O compounds for negative electrodes of lithium ion secondary batteries using rice husk as a precursor, 36th Annual Meeting of the Carbon Materials Society, 112-113, 2009 November 30 George Ting-Kuo Fey, Chung-Lai Chen, High-capacity carbons for lithium-ion batteries, pre-prepared from rice cooker Hus, Journal of Power Sour. 47-51

The objective of this invention is providing the negative electrode active material for lithium ion batteries and lithium ion capacitors excellent in the performance. More specifically, it is to provide a negative electrode active material for lithium ion batteries and lithium ion capacitors that satisfy the following characteristics (A) to (D).
(A) Even if a pre-doping process that occludes lithium ions up to the maximum occlusion capacity is performed, it is difficult to induce lithium metal plating (precipitation) and change in characteristics.
(B) After performing the said pre dope process, the occlusion discharge | release capacity of lithium ion is large.
(C) After performing the above-mentioned pre-doping treatment, the rate characteristics in the lithium ion storage / release near the maximum storage capacity are excellent.
(D) Similarly, it is excellent in cycle characteristics in the storage and release of lithium ions in the vicinity of the maximum storage capacity.

In particular, the present inventor partially removes amorphous silicic acid present in rice husk charcoal, thereby generating appropriate meso macropores, realizing excellent lithium metal plating resistance, It has been found that the structural change of the negative electrode active material due to the expansion and contraction of the reduced silicon can be suppressed, and the present invention has been achieved.
That is, the object of the present invention is achieved by the following. It is a mixed system composed of amorphous carbon and amorphous silicic acid, each having an amorphous carbon content of 60 to 80% by mass, an amorphous silicic acid content of 40 to 20% by mass, A negative electrode having a BET specific surface area of 70 to 120 m ² / g, a meso / macropore specific surface area of 50 to 100 m ² / g, and a meso / macropore volume of 0.10 to 0.18 cm ³ / g. Active material.
2. Said 1 negative electrode active material derived from rice husk.
3. Said 1 or 2 negative electrode active material by which the pre dope process of lithium ion was made | formed.
4). 2. The negative electrode active according to 2 or 3 above, wherein the rice husk is primary carbonized at 800 ° C. or less, amorphous silicic acid is partially removed from the carbide, and then secondary carbonization is performed at 800 to 1200 ° C. Method of manufacturing the substance.
5. The manufacturing method of the negative electrode active material by which the lithium ion pre-dope process was made | formed by short-circuiting the said 1 or 2 negative electrode active material and lithium metal in a lithium ion containing organic electrolyte solution.
6). An electrochemical power storage device in which a negative electrode has the above-described negative electrode active material of 1, 2, or 3, and repeatedly performs charge / discharge by occluding and releasing lithium ions.
7). 6. The electrochemical storage device according to 6, wherein the electrochemical storage device is a lithium ion battery.
8). 6. The electrochemical storage device according to 6 above, wherein the electrochemical storage device is a lithium ion capacitor.

  The negative electrode active material for lithium ion batteries and lithium ion capacitors of the present invention is more resistant to pre-doping treatment that absorbs lithium ions up to the maximum storage capacity compared to existing hard carbon active materials, and has sufficient lithium ion It is excellent in the rate characteristics and cycle characteristics of occlusion and release in the state of occlusion. Moreover, since it can manufacture from biomass-type waste called rice husk which has a very large amount in Japan, there is also an advantage of reducing the manufacturing cost.

It is a figure showing the X-ray-diffraction pattern of a powdery active material.

The present invention will be described in detail below. The negative electrode active material of the present invention is a mixed system composed of amorphous carbon and amorphous silicic acid. The amorphous carbon content is 60-80% by mass, the amorphous silicon content is 40-20% by mass, preferably 65-75% by mass and 35-25% by mass, respectively. More preferably, they are 68-72 mass% and 32-28 mass%, respectively. When the amorphous carbon content is more than 80% by mass and the amorphous silicic acid content is less than 20% by mass, or the amorphous carbon content is less than 60% by mass, When the content of the silicic acid exceeds 40% by mass, the lithium ion occlusion / release capacity, rate characteristics or cycle characteristics of the active material, or a plurality of performances thereof are deteriorated. Moreover, the BET specific surface area of a negative electrode active material is 70-120 m < ² > / g, Preferably it is 80-110 m < ² > / g, More preferably, it is 90-100 m < ² > / g. When the BET specific surface area of the negative electrode active material is less than 70 m ² / g or more than 120 m ² / g, the lithium ion occlusion / release capacity, rate characteristics or cycle characteristics of the active material, or a plurality of performances thereof are deteriorated. The mesopore / macropore specific surface area is 50 to 100 m ² / g, and the mesopore / macropore volume is 0.10 to 0.18 cm ³ / g, preferably 60 to 90 m ² / g, and It is 0.13-0.17 cm < ³ > / g, More preferably, it is respectively 70-80 m < ² > / g and 0.15-0.16 cm < ³ > / g. Macropore specific surface area is less than ^{50 m} 2 / g, and macropore volume of less than 0.10 m ² / g, or macropore specific surface area exceeds 100 m ² / g, and the meso When the macropore volume exceeds 0.18 m ² / g, the lithium ion occlusion / release capacity, rate characteristics or cycle characteristics of the active material, or a plurality of performances thereof are deteriorated.

The negative electrode active material of the present invention can be produced as follows.
Rice husk is preferably used as a raw material. As described above, rice husk naturally contains silicic acid with low crystallinity of about 20% by mass, and the rest is composed of plant organic substances such as cellulose, hemicellulose, and lignin, and the amorphous carbon and amorphous of the present invention. Preferred as a mixture precursor of porous silicon.
First, the rice husk is subjected to primary carbonization at 400 to 800 ° C, preferably 500 to 700 ° C, more preferably 550 to 650 ° C. No pretreatment such as washing is required. Primary carbonization is performed by maintaining the target temperature for 10 minutes to 3 hours, preferably 30 minutes to 2 hours, more preferably 45 minutes to 1 hour 15 minutes after reaching the target temperature. The carbonizing atmosphere may be in an inert gas such as helium gas, nitrogen gas, or argon gas, but it is desirable to use inexpensive nitrogen gas.
The primary carbonized rice husk charcoal is cooled, washed with distilled water as necessary, and then the desired amount of amorphous silicic acid is removed from the primary carbide by adjusting the leaching conditions of silicic acid. . Silicic acid leaching is performed using an alkaline aqueous solution such as an aqueous sodium hydroxide solution or an acidic aqueous solution such as hydrofluoric acid.
The rice husk primary carbide from which the desired amount of silicic acid has been removed is subsequently subjected to secondary carbonization at 800-1200 ° C, preferably 900-1100 ° C, more preferably 950-1050 ° C. Secondary carbonization is carried out by maintaining the target temperature for 10 minutes to 3 hours, preferably 30 minutes to 2 hours, more preferably 45 minutes to 1 hour 15 minutes after reaching the target temperature. The negative electrode active material of the present invention can be obtained through the above steps.
In the present invention, it is desirable that the active material is pre-doped with lithium ions in advance. Lithium ion pre-doping mainly involves direct contact between an active material and lithium metal in an electrolyte containing lithium ions, or electrically connecting a collector electrode coated with an active material and lithium metal. . In the latter method, after assembling a half battery cell including a collector electrode (positive electrode), a lithium metal (negative electrode) coated with an active material, a lithium ion-containing organic electrolyte, and a separator, a pre-doping treatment can be easily performed. Moreover, it is preferable because the control and evaluation of the pre-doping amount are easy. In that case, in order to pre-dope sufficient lithium ions into the active material, the positive electrode and the negative electrode of the half battery cell are short-circuited for 6 to 48 hours, preferably 12 to 36 hours, more preferably 18 to 30 hours.

Hereinafter, the present invention will be described in detail with reference to examples. Various evaluations and assembly of cells for evaluation were performed by the following methods.
[Evaluation of physical properties of active materials and electrodes]
The silicic acid content of the active material was determined by burning about 10 mg of the active material in air using a differential thermal balance (Rigaku Corporation, Thermo plus EVO TG8120). The active material dried at 100 ° C. for 3 hours or more was heated from room temperature to 850 ° C. at a rate of 10 ° C./min in an air flow atmosphere (500 mL / min). All the active materials showed a substantially constant mass in the temperature range of 600 to 850 ° C., and confirmed the release of the total carbon content and the ashing of the active material. The content of silicic acid was calculated from the mass residual ratio of the active material at 850 ° C., assuming that the mass of the active material at 140 ° C. was 100%.
The pore characteristics of the active material were evaluated by a nitrogen gas adsorption method using a gas adsorption amount measuring device (Quantachrome Instruments, Autosorb-3B). A nitrogen adsorption / desorption isotherm showing the relationship between the relative pressure, which is the ratio between the adsorption equilibrium pressure and the saturated vapor pressure at 77 K, and the nitrogen gas adsorption amount is obtained, and the BET (Brunauer / Emmett / Teller) specific surface area is changed from a relative pressure of 0.1 to 0. From the range of .3, the total pore volume was calculated at a relative pressure of 0.98. At the same time, the micropore specific surface area and volume and the meso / macropore specific surface area were calculated using the t-plot method. The meso / macropore volume was calculated by subtracting the micropore volume from the total pore volume.
X-ray diffractometer (Spectris Co., X'pert Pro, Cu K _alpha) was used to assess the crystallinity of the active material. The X-ray output was 45 kV and 40 mA.

[Electrode active material]
Acetylene black (Electrochemical Industry Co., Ltd.) used as a conductive aid, an active material, and air were dried at 120 ° C. for 5 hours or more. Active material: Acetylene black: Polyvinylidene fluoride (Kureha, KF Polymer W # 9100) = 8: 1: 1 (mass ratio), N-methylpyrrolidone (Tokyo Chemical Industry Co., Ltd.) was added in an appropriate amount, A slurry was prepared by stirring for 10 minutes using a rotation / revolution mixer (Sinky Co., Ltd., Ryotaro Awatori AR-100). Polyvinylidene fluoride functions as a binder. This slurry was applied to a copper foil having a thickness of 20 μm using an applicator, dried in air at 100 ° C. for 5 hours or more, and then punched out with a diameter of 15 mm. And it was used as an electrode. The electrode punched out with a diameter of 15 mm was subjected to a drying treatment for 5 hours or more in a degassing atmosphere at 140 ° C. Then, after cooling to room temperature, in the air, the thickness of the electrode was measured with the micrometer, and the mass was measured with the electronic balance. The coating thickness and the active material mass were calculated by subtracting the thickness and mass of the copper foil itself from the thickness and mass of the electrode having a diameter of 15 mm, respectively.
The surface part of the manufactured electrode was observed using a scanning electron microscope (Keyence Corporation, VE-8800). At the same time, the composition of the electrode surface was analyzed using an energy dispersive X-ray analyzer (Oxford, INCA Energy 250) installed in an electron microscope. The composition analysis was performed under the conditions of an electron microscope magnification of 500 times and a focal length of 30 mm.

[Cell assembly]
The lithium ion occlusion / release characteristics of the active material include a half battery cell composed of an electrode containing the active material and lithium metal, or an electrode containing activated carbon for the positive electrode, an electrode containing the active material for the negative electrode, and lithium metal for the reference electrode. This was evaluated by a tripolar lithium ion capacitor cell.
In the evaluation by the half battery cell, the electrode containing the active material is the positive electrode, and lithium metal is the negative electrode. As the lithium metal, a disk-shaped material having a diameter of 15 mm and a thickness of 0.2 mm manufactured by Honjo Metal Co., Ltd. was used. Lithium metal is placed at the bottom of a two-pole stainless steel cell (Hosen Co., Ltd., flat cell), and then a separator and an electrode containing an active material that has been degassed at 140 ° C. for 5 hours or more are placed. did. The separator used was a polypropylene porous separator (Celgard, # 2500) cut out to a diameter of 23 mm. After pouring 1 mL of a solution (Kishida Chemical Co., Ltd.) containing 1 mol / L of lithium hexafluorophosphate (LiPF ₆ ) in a solvent of ethylene carbonate and diethyl carbonate mixed at a volume ratio of 1: 1 as an electrolytic solution, The cell was sealed. The cell assembly was performed in a glove box (Glove Box Japan Co., Ltd., GBJF080R) in which pure argon gas was enclosed. In addition, all the structural members used what was fully dried. In addition, the cell voltage of the half battery after cell assembly was a little over 3V. Thereafter, the positive electrode and the negative electrode were short-circuited for 24 hours, and a sufficient lithium ion pre-doping treatment for the active material was performed.

The assembly of the tripolar lithium ion capacitor cell consists of (i) production of the positive electrode of the lithium ion capacitor, (ii) pre-doping of lithium ions into the negative electrode containing rice husk-derived active material, and (iii) positive electrode, negative electrode and lithium The assembly was performed in the order of assembling the cell using metal as a reference electrode.
(I) Production of positive electrode of lithium ion capacitor In addition to activated carbon (Kuraray Chemicals Co., RP25) having a BET specific surface area of about 2500 m ² / g, acetylene black (Electrochemical Industry Co., Ltd.) as a conductive auxiliary agent, and styrene as a binder -Butadiene rubber (JSR Corporation, TPD2001) and carboxymethyl cellulose sodium (Serogen 7A, Daiichi Kogyo Seiyaku Co., Ltd.) were used as a dispersant. Activated carbon and acetylene black were dried in air at 120 ° C. for 5 hours or more. Activated carbon: Acetylene black: Styrene / butadiene rubber: Sodium carboxymethyl cellulose = 8: 1: 0.5: 0.5 (mass ratio) Mix, add appropriate amount of distilled water, and use the above-mentioned rotation / revolution mixer And stirred for 10 minutes to prepare a slurry. Using an applicator, this slurry was applied onto an aluminum foil having a thickness of 20 μm and dried at 100 ° C. in air for 5 hours or more. Thereafter, punching was performed at a diameter of 15 mm, and a drying treatment was performed for 5 hours or more in a degassing atmosphere at 140 ° C. Then, after cooling to room temperature, in the air, the thickness of the electrode was measured with the micrometer, and the mass was measured with the electronic balance. The coating thickness and the activated carbon mass were calculated by subtracting the thickness and mass of the aluminum foil itself from the thickness and mass of the electrode having a diameter of 15 mm, respectively.
In the positive electrode of the lithium ion capacitor, charge is transferred by a non-Faraday reaction of adsorption / desorption of ions in the electrolytic solution. Therefore, it is necessary to evaluate the capacity of the positive electrode itself including activated carbon. A bipolar cell was assembled by the same method as the above-described half battery cell assembly, and the ion adsorption / desorption characteristics of the positive electrode itself including activated carbon were evaluated. Note that the positive electrode is not pre-doped.

(Ii) Pre-doping of lithium ions to the negative electrode containing the active material Same as the method for assembling the half battery cell described above, but the negative electrode containing the chaff-derived active material at the cell bottom and then made of polypropylene with a diameter of 23 mm The order of arrangement was changed to a porous separator and a disc-shaped lithium metal having a diameter of 15 mm and a thickness of 0.2 mm. The used cell is also made of a bipolar stainless steel. Thereafter, the positive electrode and the negative electrode were short-circuited for 24 hours, and a sufficient lithium ion pre-doping treatment for the active material was performed.

(Iii) Assembly of Lithium Ion Capacitor Cell Using Positive Electrode, Negative Electrode and Lithium Metal as Reference Electrode After opening the shorted positive electrode and negative electrode, the half battery cell was opened. Only the lithium metal immersed in the electrolytic solution was taken out, and the separator and the negative electrode pre-doped with lithium ions were continuously immersed in the electrolytic solution. The positive electrode containing activated carbon was degassed again at 140 ° C. for 5 hours or more and placed in a place where lithium metal was placed. Further, the above-described disc-shaped lithium metal was cut in half and folded so as to be arranged as a reference electrode. The cell was sealed using the cell lid of a 3-pole stainless steel cell (Hosen Co., Ltd., 3-pole cell). In addition, all the structural members were fully dried and the cell assembly was performed in a glove box in which pure argon gas was sealed.

[Charge / discharge test of half battery cell and lithium ion capacitor cell]
(A) Charging / discharging test of electrode containing active material in half battery cell After 24 hours of pre-doping treatment of the half battery cell, a constant current density of 0.1 mA / cm ² (actual current: 0.1767 mA, electrode cross-sectional area) : 1.767 cm ² ), the electrode potential relative to the lithium metal is 0.002 to 3 Vvs. It was changed to Li / Li ⁺ , that is, the cell voltage was changed from 0.002 to 3 V, and the lithium ion release capacity after pre-doping was determined. Further, at the same current density, the electrode potential is changed from 3 to 0.002 Vvs. Li / Li ⁺ in changing the storage capacity of the lithium ion, followed by, 3Vvs 0.002. By changing to Li / Li ⁺ , the lithium ion release capacity was determined.
Thereafter, the potential range of the electrode is changed from 0,002 to 1 Vvs. Lithium ion was occluded and released for 5 cycles at a constant current density of 0.1 mA / cm ² in the range of Li / Li ⁺ . Subsequently, 0.2 mA / cm ² for 5 cycles, 0.5 mA / cm ² for 10 cycles, 1 mA / cm ² for 10 cycles, ² mA / cm ² for 25 cycles, 5 mA / cm ² for 50 cycles, 10 mA / cm 100 cycles were performed at ² and 100 cycles at 20 mA / cm ² , and the current density dependence of the lithium ion storage / release capacity of the electrode was evaluated.

Furthermore, the current density is constant 1 mA / cm ² and the electrode potential is 0.002 Vvs. After reducing to Li / Li ⁺ and allowing the electrode to sufficiently occlude lithium ions, lithium ions corresponding to a capacity of 130 μAh were released. The capacity of 130 μAh is a capacity at a current density of 1 mA / cm ² of a lithium ion capacitor positive electrode described later. Thereafter, lithium ions corresponding to a capacity of 130 μAh were occluded, and the release of lithium ions corresponding to the same capacity was repeated 49 cycles. Again, 0.002 Vvs. After the electrode potential was lowered to Li / Li ⁺ and lithium ions were sufficiently occluded in the electrode, 130 μAh of lithium ions were released. Then, 949 cycles of lithium ion storage / release for 130 μAh were performed. Again, the electrode potential is 0.002 Vvs. After occlusion of lithium ions until it decreased to Li / Li ⁺ , 130 μAh of lithium ions were released, and 999 cycles of occlusion / release were performed. The electrode potential is 0.002 Vvs. Without consuming 130 μAh capacity. When lowered to Li / Li ⁺ , 0.002 Vvs. The current density was reduced to maintain Li / Li ⁺ and 130 μAh was consumed. From the change in cell voltage when the capacity was made constant, the lithium ion storage / release characteristics of the active material were evaluated.
(B) Lithium ion capacitor positive electrode half-cell charge / discharge test In a lithium ion capacitor positive electrode and a lithium metal half-cell, the potential of the positive electrode (the potential of the positive electrode with respect to lithium metal) was 2 to 4 Vvs. Set to Li / Li ⁺ range, ie, cell voltage to 2-4V range, 5 cycles at constant current density of 0.1 mA / cm ² followed by 5 cycles at 0.2 mA / cm ² , 0.5 mA / cm ² at 10 cycles, with 1 mA / cm ² 10 cycles, 25 cycles at ^{2mA / cm 2, 5mA / cm} 2 at 50 cycles, 10 mA / cm ² at 100 cycles, 20 mA / cm ² at 100 cycles Charging / discharging was performed. In the state where no electric field is applied from the outside, the positive electrode potential is about 3 Vvs. Since Li / Li ⁺ , approximately 3 to 4 Vvs. The adsorption of PF ₆ ⁻ in Li / Li ⁺ is 4 to 3 Vvs. Desorption of PF ₆ ⁻ in Li / Li ⁺ is 3 to 2 Vvs. Li ⁺ Li ⁺ adsorption of Li ⁺ is 2 to 3 Vvs. Li ⁺ desorption occurs in Li / Li ⁺ .

(C) Charge / Discharge Test of Lithium Ion Capacitor Cell After assembling the lithium ion capacitor cell, it was allowed to stand for about 1 hour, and the cell voltage (potential difference between positive and negative electrodes), positive electrode potential and negative electrode potential were measured. And charging / discharging was performed 3 cycles at a sweep rate of 100, 10, and 1 mV / s in a cell voltage range of 2 to 4V, respectively. Then, in the same cell voltage range, 5 cycles at a constant current density 0.1 mA / cm ^2, followed by 5 cycles at 0.2 mA / cm ^2, further 0.5 mA / cm ² at 10 cycles, with 1 mA / cm ² 10 cycles were carried out 25 cycles at 2 mA / cm ^2, 50 cycles at 5 mA / cm ^2, 100 cycles at 10 mA / cm ^2, at 20 mA / cm ² for 100 cycles charging and discharging. Since all the positive electrodes themselves are the same, the rate characteristics of the negative electrode of the lithium ion capacitor can be evaluated. Thereafter, in the same cell voltage range, a current density was set to a constant 1 mA / cm ² and a charge / discharge test of 20000 cycles was performed. This cycle test was performed while measuring the waveform of the cell voltage, the positive electrode-reference electrode voltage, and the negative electrode-reference electrode voltage during a specific period. Before and after the waveform measurement period, charging / discharging was once stopped.

Example 1
[Manufacture of active materials]
The rice husks of Akitakomachi rice harvested in Senboku City, Akita Prefecture are used as raw materials. The obtained rice husk was subjected to a heat treatment at 600 ° C. for 1 hour in a nitrogen gas flowing atmosphere at 1 L / min without performing a special treatment such as washing, thereby performing primary carbonization. The temperature was raised from room temperature to 600 ° C. over 1 hour, and naturally cooled to room temperature after the heat treatment for 1 hour. Rice husk charcoal obtained by primary carbonization was washed with distilled water or silicate leaching treatment with an aqueous sodium hydroxide solution.
In washing with distilled water, industrial paper waste (Nippon Paper Crecia Co., Ltd., Kim Towel) was attached to a plastic funnel, and distilled water was poured over rice husk charcoal obtained by primary carbonization at 600 ° C. During the washing, about 50 mL of distilled water appropriately poured was collected and washed until its pH reached about 9. After the washing treatment, rice husk charcoal sufficiently dried in air at 120 ° C. was designated as RHW600.

Silicic acid leaching treatment for rice husk charcoal obtained at 600 ° C. was performed by immersing rice husk charcoal in a 1 mol / L sodium hydroxide aqueous solution in a polyethylene container. The degree of silicic acid leaching in rice husk charcoal was controlled by the solid-liquid ratio (g / L) of rice husk charcoal and sodium hydroxide aqueous solution, the immersion time, and the immersion temperature (25 or 80 ° C.). First, the solid-liquid ratio of the rice husk charcoal and the sodium hydroxide aqueous solution was set to 50 g / L, and immersed at 25 ° C. for 10 hours. After soaking, it was washed with distilled water and dried in the same manner as described above. This rice husk charcoal was RH600A. On the other hand, after immersion in an aqueous sodium hydroxide solution at 25 ° C. for 19 hours, rice husk charcoal obtained by performing washing and drying treatment in the same manner was designated as RH600B. Silica was leached with respect to rice husk charcoal obtained at 600 ° C. with a solid-liquid ratio of 25 g / L, an immersion temperature of 25 ° C., and an immersion time of 30 hours. Washing and drying were performed in the same manner as described above, and the resulting rice husk charcoal was designated as RH600C. Further, silica leaching was performed on rice husk charcoal obtained at 600 ° C. with a solid-liquid ratio of 25 g / L, an immersion temperature of 80 ° C., and an immersion time of 16 hours. Washing and drying were carried out, and the resulting rice husk charcoal was designated as RH600D.
Thereafter, RHW600, RHW600A, RHW600B, RHW600C, and RHW600D were heat-treated at 1000 ° C. for 1 hour in a nitrogen gas flow atmosphere at 1 L / min to perform secondary carbonization. The temperature was raised from room temperature to 1000 ° C. over 1 hour, and naturally cooled to room temperature after the heat treatment for 1 hour. Rice husk charcoal that has undergone secondary carbonization was designated RHW1000, RHW1000A, RHW1000B, RHW1000C, and RHW1000D, respectively. Furthermore, rice husk charcoal was also produced in which RHW600 was heat-treated at 1400 ° C. for 1 hour in a nitrogen gas flow atmosphere at 1 L / min. Let it be RHW1400. At that time, the temperature was raised from room temperature to 1000 ° C. over 1 hour, and further raised to 1400 ° C. over 1 hour. Moreover, after the heat treatment for 1 hour, it naturally cooled to room temperature.

  Rice husk charcoal of RHW600, RHW600A, RHW1000, RHW1000A, RHW1000B, RHW1000C, RHW1000D, and RHW1400 was pulverized using a planetary ball mill (Fritsch Japan KK, P6). All rice husk charcoal was pulverized for 5 minutes at a rotational speed of 400 rpm using SUS304 stainless steel balls and containers. When the particle size of the powdered rice husk charcoal was measured using a particle size distribution measuring apparatus (Shimadzu Corporation, SALD-200V), all median diameters and average particle diameters were 2 to 5 μm. Hereinafter, the powdered rice husk charcoal is treated as an active material. In addition, the hard carbon (AT-Electrode Co., Ltd., LN-0100) derived from a commercially available phenol resin was selected as a comparison object of a manufacturing active material, and the same analysis and test as the manufacturing active material were performed.

Example 2
[Active materials and electrodes and their analysis]
The silicic acid content of the powdered active material produced in Example 1 and commercially available hard carbon (AT-Electrode Co., Ltd., LN-0100) was measured by the above method. The results are shown in Table 1. Comparing RHW600, RHW1000, and RHW1400, it can be seen that the silicic acid content of the rice husk-derived active material increased as the heat treatment temperature increased. Comparing RHW600 and RHW600A, and further RHW1000, RHW1000A, RHW1000B, RHW1000C, and RHW1000D, it can be seen that the content of silicic acid in the active material was decreased by increasing the time and temperature of the sodium hydroxide aqueous solution immersion.

The X-ray diffraction pattern of the powdery active material is shown in FIG. Except for RHW1400, no clear peak due to the crystal structure related to carbon and silicic acid was observed. Two weak peaks were observed at 2θ of 43 to 45 °, which were due to the mixing of stainless steel balls made of SUS304 used in the active material pulverization and wear powder of the container. In the X-ray diffraction pattern of RHW1400, weak peaks caused by cristobalite SiO ₂ , quartz SiO ₂ , α-Si ₃ N ₄ , and Si ₂ N ₂ O are detected, and crystallization of the active material from 1400 ° C. can be confirmed. In other rice husk-derived active materials, the swelling of the X-ray diffraction pattern due to amorphous silicic acid before crystallization into cristobalite SiO ₂ was observed at around 22 °. Further, as can be seen from the comparison with the X-ray diffraction pattern of commercially available hard carbon, a bulge caused by amorphous carbon was also observed at around 23 °. Like RHW1000C and RHW1000D, when the silicic acid content in the active material is low and the amorphous carbon content is high, the swelling around 22 ° due to the amorphous silicic acid is weakened, and the amorphous carbon The resulting swelling around 23 ° became stronger.

  Table 2 shows the pore characteristics of the powdered active material. When RHW600, RHW1000, and RHW1400 were compared, the RHW600 produced at the lowest temperature had the largest BET specific surface area and total pore volume, and the most developed pores. On the other hand, in RHW1000 manufactured at 1000 ° C., pores did not develop, and the smallest BET specific surface area and total pore volume were shown. When RHW600 and RHW600A were compared, a larger BET specific surface area and total pore volume were measured in RHW600A from which silicic acid had been leached, which was due to the development of meso / macropores rather than micropores. Further, the pores developed in RHW1000 were almost composed of meso / macropores, and the micropores were hardly developed. As the silicic acid content decreased in the order of RHW1000A, RHW1000B, RHW1000C, and RHW1000D, the BET specific surface area and the total pore volume gradually increased. In the RHW1000A with a low silicic acid leaching degree, both micropores and meso / macropores developed. However, even if the silicic acid leaching degree was further increased, the development of micropores was weak and meso / macropores were mainly developed.

  Each active material was applied to a copper foil of a current collector together with acetylene black as a conductive additive and polyvinylidene fluoride as a binder to form an electrode. It was punched into a circle with a diameter of 15 mm and used as an electrode. After the electrode containing each active material was sufficiently dried again, the surface portion was observed using an electron microscope. In all the electrodes, it was confirmed that the conductive auxiliary agent and the binder were sufficiently dispersed in the active material grains, and the active material was uniformly adhered on the copper foil. Table 3 shows the composition of the electrode surface determined using an energy dispersive X-ray analyzer. C, O, F, and Si were mainly detected, and these were derived from amorphous silicic acid, amorphous carbon, and polyvinylidene fluoride in the active material. A trace amount of Fe was detected in all the electrodes except the commercially available hard carbon electrode, which was due to the stainless balls mixed in the pulverization process and the abrasion powder of the container. Further, Na was detected at the electrodes of RHW1000A, RHW1000B, RHW1000C, and RHW1000D, and the Na content increased as the degree of silicic acid leaching increased. As the amount of silicic acid leaching increased, the immersion time and temperature in the aqueous sodium hydroxide solution increased, which resulted in Na being difficult to remove.

  Evaluation of silicic acid content, analysis of crystallinity by X-ray diffraction, evaluation of pore characteristics, microscopic observation of electrode surface, and compositional analysis result in a mixed system of amorphous silicic acid and amorphous carbon Yes, active materials with different ratios of amorphous silicic acid and amorphous carbon and different pore characteristics were produced from rice husks as raw materials, and processed into electrodes capable of evaluating the storage and release characteristics of lithium ions. Prove that.

Example 3
[Charge / discharge test of active material-containing electrode half-cell]
The electrode coated with the active material was punched out with a diameter of 15 mm, and a half battery cell composed of it and lithium metal was assembled. Table 4 shows the active material mass and coating thickness of the electrode. The mass of the active material was 3.96 mg ± 5% excluding RHW1400. Since only the mass of RHW1400 was as small as 2.84 mg, the occlusion / release characteristics of lithium ions are considered separately. The coating thickness of all active materials was 30-50 μm.

The half battery cell containing the active material was short-circuited for 24 hours and subjected to a lithium ion pre-doping treatment. Table 5 shows the lithium ion storage / release capacity of the active material after pre-doping. The release capacity of lithium ions of each active material after pre-doping was large for RHW600A and subsequently for RHW1000. The release capacity decreased in the order of RHW1000A, RHW1000B, RHW1000C, and RHW1000D, which almost coincided with the decrease in silicic acid content. In addition, all the active materials produced from rice husk showed larger capacity than commercial hard carbon in releasing lithium ions after pre-doping. On the other hand, the lithium ion release capacity of RHW1400 was as extremely low as 90 mAh / g. Again, the lithium ion was charged with an electrode potential of 0.002 Vvs. The active material was occluded until it decreased to Li / Li ⁺ , and then the electrode potential was 3 V vs. When lithium ions were released until it increased to Li / Li ⁺ , RHW600A exhibited the maximum storage and release capacity. All of the rice husk-derived active materials except RHW1400 exhibited an occlusion / release capacity that exceeded that of commercially available hard carbon. In particular, the release capacities of RHW600, RHW600A, RHW1000, RHW1000A, and RHW1000B exceeded 450 mAh / g, and had a large storage / release capacity.

Subsequently, the electrode potential is 0.002 to 1 Vvs. In a state where lithium ions are sufficiently occluded in the active material of Li / Li ⁺ , occlusion / release of lithium ions at different current densities was performed for each active material. Table 6 shows the occlusion / release capacity of each active material. When the current density was 0.1 mA / cm ² and the lithium ion occlusion / desorption was slow, occlusion / release capacities exceeding 240 mAh / g were measured for the rice husk-derived active materials excluding RHW600 and RHW1400. This exceeds the absorption / release capacity of commercially available hard carbon of 200 mAh / g. When the current density was increased to 1 mA / cm ² and further to 10 mA / cm ² , RHW600A could not maintain a large storage / release capacity. RHW1000 and RHW1000A-D which performed the secondary carbonization at 1000 degreeC maintained the occlusion discharge capacity even if the current density increased, and showed the occlusion release capacity sufficiently larger than commercial hard carbon.

Subsequently, the lithium ion storage / release capacity was fixed at 130 μAh, the current density was kept constant at 1 mA / cm ² , and lithium ion storage / release was repeated. In the first cycle, the 51st cycle, and the 1001st cycle, the lithium ion in the active material has an electrode potential of 0.002 Vvs. Occlusion was performed until Li / Li ⁺ was obtained. Once lithium ions are occluded in the active material, when lithium ions are occluded and released repeatedly with a certain amount of electricity, it is desirable that the electrode potential after repeated occlusion be maintained constant. In general, when the negative electrode potential of a lithium ion battery and a lithium ion capacitor increases after repeated occlusion and release, the positive electrode potential must also increase to obtain the same battery and capacitor electromotive force. When the positive electrode potential is increased, decomposition of the electrolyte solution in the vicinity of the positive electrode and a structural change of the positive electrode itself are easily induced, and the capacity and structural deterioration of the battery and the capacitor may occur. Similarly, it is desirable that the electrode potential after the release of lithium ions be kept low enough that lithium metal plating does not occur at the negative electrode. Furthermore, when the capacity reduction of the active material proceeds due to repeated occlusion and release of lithium ions, the difference between the electrode potential during occlusion and the electrode potential during release increases. Therefore, it is desired that the potential difference is small.

Table 7 shows the electrode potentials of the active material during occlusion and release of 130 μAh of lithium ions. In the 50th cycle, the electrode potential after occlusion of lithium ions was low for RHW1400 and high for RHW600. In the case of RHW1400, the electrode potential does not decrease gradually and continuously, but decreases rapidly and discontinuously, so that 0.002 Vvs. A low electrode potential of Li / Li ⁺ was measured. This is expected to result from the plating of lithium metal. Further, the electrode potential of RHW600 and RHW1400 after the release of lithium ions is 0.5 Vvs. Li / Li ⁺ was exceeded and was large compared to other active materials. In RHW1400, when the occlusion and release of lithium ions was further repeated, discontinuous and rapid changes in the electrode potential occurred frequently, and the electrode potential itself was higher than that of other active materials. Considering that the mass of the active material was smaller than that of other active materials after exceeding 1000 cycles, it was determined that RHW1400 did not have sufficient performance as an active material, and the test was interrupted. Moreover, the electrode potential after lithium ion release of RHW600 was not higher than that in the 50th cycle, but was higher than that of other active materials. Focusing on the electrode potential after occlusion of lithium ions, the value of RHW1000D having the lowest silicic acid content increases according to the number of occlusion / release cycles, and after the 2000th cycle, the highest value of 0.157 Vvs. Li / Li ⁺ was obtained. A high electrode potential after occlusion is very undesirable because it requires a high positive electrode potential during charging of the battery and capacitor, causing a reduction in capacity and structural deterioration. RHW1000D has the lowest silicic acid content, most of which is composed of amorphous carbon and has developed meso-macropores. The increase in the electrode potential after occlusion of lithium ions is initially 0.002 Vvs. It means that lithium ions occluded to Li / Li ⁺ gradually shift to an immobilization state that does not contribute to a decrease in electrode potential in the active material by repeated occlusion and desorption. That is, it is predicted that the meso / macropores excessively developed in the carbon region promote the immobilization by trapping lithium ions. Furthermore, even when RHW600A, RHW1000A, and RHW1000B were repeatedly subjected to occlusion / release of lithium ions, the electrode potential after occlusion and release was kept low. That is, even in the case of occlusion and release in a state where lithium ions are sufficiently occluded, the electrode potential is kept low and the decrease in capacity is kept small.

The capacity and electrode potential range after pre-doping in the charge / discharge test in the half battery cell of the electrode containing the active material is 0.002 to 1 Vvs. From the evaluation of the current density dependence characteristics of the capacity when limited to Li / Li ⁺ and the stability of the electrode potential when the lithium ion storage and release capacity is fixed at 130 μAh, RHW1000, RHW1000A, RHW1000B, and RHW1000C are: In particular, it can be determined that RHW1000A and RHW1000B exhibit performance superior to that of commercially available hard carbon as a negative electrode active material of a lithium ion battery and further satisfy the requirements (A) to (D). Therefore, performance evaluation as a negative electrode active material of a lithium ion capacitor was performed on RHW1000, RHW1000A, RHW1000B, and RHW1000C.

Example 4
[Charge / discharge test of lithium ion capacitor cell with electrode containing active material]
Similarly to the charge / discharge test in the half battery cell, the electrode coated with the active material was punched out with a diameter of 15 mm and used as the negative electrode of the tripolar lithium ion capacitor cell. In addition, as the positive electrode of the cell, activated carbon having a BET specific surface area of about 2500 m ² / g was coated on an aluminum foil and punched out with a diameter of 15 mm. Four types of lithium ion capacitor cells were assembled using RHW1000, RHW1000A, RHW1000B, RHW1000C and commercially available hard carbon as the negative electrode active material. The reference electrode was lithium metal, and the negative electrode active material was pre-doped with lithium ions for 24 hours. On the other hand, a cell using a lithium metal having a sufficient capacity for occlusion and release of lithium ions as compared with these active materials was also assembled. In this case, not a tripolar cell but a bipolar half battery cell. Table 8 shows the details of the assembled lithium ion capacitor cell and the active material used therefor. The mass of the negative electrode active material was 4.00 mg ± 4%, and the mass of the positive electrode activated carbon was also substantially constant at 2.34 mg ± 3%. The coating thickness was about 40 μm for both the negative electrode and the positive electrode. The theoretical capacity of lithium metal used as the negative electrode was 3861 mAh / g, and the capacity was sufficiently larger than the used rice husk-derived negative electrode active material.

The cell voltage range was set to 2-4V, and the charge / discharge capacity of the lithium ion capacitor cell at different current densities was evaluated. The results are shown in Table 9. When the current density was as small as 0.1 mA / cm ² , all the cells showed the highest charge / discharge capacity. When the negative electrode was lithium metal, the discharge capacity of the cell was 141 μAh, which was almost the same as the discharge capacity of the LIC W1000, LIC W1000A, and LIC W1000B cells. The discharge capacities of LIC W1000C and LIC HC were slightly lower. When the current density was increased to 1 mA / cm ² and further to 10 mA / cm ² , the charge / discharge capacity of all cells decreased, but the capacity decrease of the LIC W1000 cell was the smallest and the rate characteristics were excellent. The charge / discharge capacity at 10 mA / cm ² of the LIC W1000A and LIC W1000B cells was slightly lower than that of the LIC HC cell, but was higher than that when the negative electrode was lithium metal. The LIC W1000C cell showed the lowest charge / discharge capacity at all current densities.

Subsequently, the cell voltage range was similarly set to ² to 4 V, the current density was maintained at a constant value of 1 mA / cm ² , and a charge / discharge cycle test of the lithium ion capacitor cell was performed. Table 10 shows the relationship between the number of charge / discharge cycles and the charge / discharge capacity of the lithium ion capacitor cell. A cycle test was not performed on a LIC LiMetal cell using lithium metal as a negative electrode, which can form dendrites on the surface due to repeated occlusion and release of lithium ions. When there were few 10th cycles and charging / discharging cycles, the lithium ion capacitor cell except LIC W1000C cell showed the charging / discharging capacity | capacitance of about 130 microamperes. LIC W1000A and LIC W1000B maintained excellent charge / discharge capacity of about 120 μAh even after 20000 cycles of charge / discharge, and exhibited excellent cycle characteristics. On the other hand, the capacity of LIC HC was almost lost after 3000 cycles of charge and discharge. The LIC W1000 cell allowed a gradual capacity drop from around 3000th cycle, and after 20000 cycle it was below 100 μAh. The charge / discharge capacity of LIC W1000C once showed the same level of capacity as other cells around the 1000th cycle, but thereafter showed a decrease in capacity. After 20000 cycles, it decreased to about 55 μAh.

Table 11 shows the negative electrode potential in the cycle test of the lithium ion capacitor cell. At a cell voltage of 4V, lithium ions are most occluded in the negative electrode active material, and at a cell voltage of 2V, lithium ions are most released. In the early stage of the charge / discharge cycle, the negative electrode potential of LIC HC was kept very low. However, after exceeding 1000 cycles, the negative electrode potential during lithium ion release is 2 V vs. It increased to the vicinity of Li / Li ⁺ . That is, it was confirmed that excellent cycle characteristics could not be obtained even when a commercially available hard carbon was pre-doped with lithium ions by short-circuiting with lithium metal and used as a negative electrode active material of a lithium ion capacitor cell. In the LIC W1000 and LIC W1000C cells, the negative electrode potential at the time of occlusion of lithium ions at a cell voltage of 4 V did not increase greatly, but after 10,000 cycles, the negative electrode potential at the time of lithium ion release was 1.2 Vvs. Li / Li ⁺ was exceeded, and the capacity decrease of the negative electrode caused the capacity decrease of the cell. On the other hand, the LIC W1000A and LIC W1000B cells have a negative electrode potential of 1 Vvs. It was suppressed to about Li / Li ⁺ . This result shows that RHW1000A and RHW1000B are excellent in stability against charge / discharge cycles, and have particularly preferable performance with the negative electrode active material of the lithium ion capacitor cell.

As a result of the physical property analysis of the prepared negative electrode active material in Example 2, and the performance when these negative electrode active materials in Examples 3 and 4 were used in lithium ion batteries and lithium ion capacitors, the following invention was conceived. it can.
By using a mixed active material of amorphous carbon and amorphous silicic acid (RHW1000A and RHW1000B) with appropriate physical properties, lithium can be stored up to the maximum storage capacity compared to hard carbon active materials that are existing technologies. It is possible to realize a lithium ion battery and a negative electrode for a lithium ion capacitor that have strong resistance to a pre-doping treatment that occludes ions and that are excellent in the rate characteristics and cycle characteristics of occlusion and release in a state where lithium ions are sufficiently occluded. When the content of amorphous silicic acid in the active material is reduced by leaching of silicic acid, the large capacity of occlusion and release of silicon obtained by reduction by lithium ions is reduced, so that the capacity of the active material itself is Decrease. At the same time, however, meso-macro pores are mainly formed in the space where the silicic acid is leached. Since silicon allows large expansion and contraction due to the insertion and extraction of lithium ions, the presence of meso / macropores can suppress the separation between active material particles and the separation of active material and current collector due to the expansion and contraction of silicon. . On the other hand, when silicic acid leaching is excessive, the capacity of the active material as a whole decreases due to the decrease in capacity caused by silicon. At the same time, the meso / macropores developed in the carbon region more than necessary for buffering the expansion and contraction of silicon promotes its immobilization by trapping lithium ions. Therefore, when the occlusion and desorption of lithium ions is repeated, lithium that does not contribute to the decrease in electrode potential in the active material increases, and the negative electrode potential during lithium ion occlusion is increased. This is not preferable because it increases the possibility of inducing the decomposition of the electrolytic solution and the structure decomposition of the positive electrode by raising the positive electrode potential. If the silicic acid content in the active material is high (RHW600 and RHW600A) even though the meso / macropores are excessively developed, the immobilization of lithium is reduced due to the reduced silicon, and the negative electrode potential is increased. Is suppressed. However, due to the decrease in lithium ion transportability due to the presence of excessive mesopores / macropores, the occlusion / release capacity is greatly decreased with the increase in current density, and excellent rate characteristics cannot be obtained. Moreover, the carbonization temperature at the time of active material manufacture influences the structural change of silicic acid and a carbon area | region. Since mesopores and macropores in the active material are easily developed by leaching of silicic acid, a carbonization temperature of 1000 ° C. has an effect of blocking pores in silicic acid and carbon regions, and can suppress the development of pores. The carbonization temperature of 1400 ° C. crystallizes and nitrides the amorphous silicic acid, weakens the lithium ion absorption effect due to the reduction of silicic acid, ie, reduces the suppression effect of the lithium metal plating of the active material .

RHW1000A and RHW1000B exhibited excellent performance as negative electrode active materials for lithium ion batteries and lithium ion capacitors that repeatedly charge and discharge by occluding and releasing lithium ions. Considering the composition and physical properties of these active materials, the content of amorphous carbon is 60 to 80% by mass, the content of amorphous silicic acid is 40 to 20% by mass, and the BET specific surface area is 70 to 120 m ² / g. A mixture of amorphous silicic acid and amorphous carbon, having a mesopore / macropore specific surface area of 50 to 100 m ² / g and a mesopore / macropore volume of 0.10 to 0.18 cm ³ / g The active material (A) hardly induces lithium metal plating (precipitation) and characteristic change even when pre-doping treatment is performed to occlude lithium ions up to the maximum storage capacity. (B) The pre-doping treatment is performed. After that, the lithium ion storage capacity is large. (C) After the pre-doping treatment, the lithium ion storage capacity is excellent in the lithium ion storage and release near the maximum storage capacity. Results in the invention of excellent cycle characteristics in a lithium ion occlusion and release in the vicinity of storage capacity. In addition, the active material can be produced from rice husk, a biomass-based waste with an extremely large amount of domestic presence, and the rice husk is first carbonized at 800 ° C. or lower, and the amorphous silicic acid is partially removed from the carbide. , And can be produced by secondary carbonization at 800 to 1200 ° C.

  As described above, the present invention is excellent as a negative electrode active material for lithium ion batteries and lithium ion capacitors.

Claims (8)

It is a mixed system composed of amorphous carbon and amorphous silicic acid, each having an amorphous carbon content of 60 to 80% by mass, an amorphous silicic acid content of 40 to 20% by mass, A negative electrode having a BET specific surface area of 70 to 120 m ² / g, a meso / macropore specific surface area of 50 to 100 m ² / g, and a meso / macropore volume of 0.10 to 0.18 cm ³ / g. Active material.   The negative electrode active material according to claim 1, which is derived from rice husk.   The negative electrode active material according to claim 1 or 2, which has been pre-doped with lithium ions.   4. The negative electrode according to claim 2, wherein the rice husk is primary carbonized at 800 ° C. or lower, amorphous silicic acid is partially removed from the carbide, and then secondary carbonization is performed at 800 to 1200 ° C. Production method of active material.   The manufacturing method of the negative electrode active material by which the lithium ion pre-dope process was made | formed by short-circuiting the negative electrode active material and lithium metal of Claim 1 or 2 in the lithium ion containing organic electrolyte solution.   An electrochemical power storage device in which a negative electrode has the negative electrode active material according to claim 1, 2 or 3 and realizes repeated charge and discharge by performing occlusion and release of lithium ions.   The electrochemical storage device according to claim 6, wherein the electrochemical storage device is a lithium ion battery.   The electrochemical storage device according to claim 6, wherein the electrochemical storage device is a lithium ion capacitor.