A carbonaceous material for use as an anode of a lithium secondary battery and method of preparation

Abstract: A method of preparing a carbonaceous material for use in the anodes of secondary batteries using non-aqueous solvents comprising the calcination at 800°c or more of a solid obtained by heating tar and/or pitch mixed with furfural in the presence of an acid catalyst

Description: The present invention relates to an anode material for non-aqueous lithium secondary batteries having a large capacity and a small irreversible capacity loss and a method for the production thereof. Non-Aqueous Lithium Secondary Batteries using carbonaceous materials as the anode as lithium secondary batteries have already been put into use because of the advantages of high energy density, small size and weight and the ability to hold a charge for a long period of time. However, the new increase in charge capacity of the anode use carbonaceous materials is inadequate to accommodate the trends in miniaturization of electrical devices and the lengthening of their period of operation without recharging. Up until now. as the carbonaceous material used in the anodes of lithium ion secondary batteries, non-graphite type carbonaceous material with a capacity of nearly 500 mAh/g, greatly surpassing graphite materials having a capacity of 370 mAh/g have been reported. For example, materials made by calcining the reaction products of pitch or tar type compounds that have been reacted with nitrocompounds (Japanese Laid Open Patent Application Hei4-258479 [1993]), air oxidation and calcination of the reaction products of pitch or tar type compounds that have been reacted with nitric acid (Japanese Laid Open Patent Application Hei7-320740 [1995]), the calcination of palm shell charcoal at reduced pressures (Japanese Laid Open Patent Application Hei8-64207 [1996]), and materials made by improving pitch obtained by such methods as polymerization of conjugated polycyclic hydrocarbons followed by stabilization and calcination (Japanese Laid Open Patent Application Hei10-083814 [1998]) may be mentioned. However. the materials made to date have had problems in connection with, in particular, charge and discharge efficiencies, economy, and stability of raw materials and the like. Moreover, materials having a high discharge capacity exceeding 600 mAh/g made by calcining coke at low temperatures or a polyacene made by calcining a phenolic resin (Yada et al.: Abstracts of the 34th Battery Symposium in Japan, p 63, 1993) have been discovered, but the excessive size of the irreversible capacity (the difference in charge capacity and discharge capacity in the first cycle) is a problem. In addition, because the discharge potential is high. there is also the problem that when assembled with the cathode material to make a battery, the average voltage of that battery is low. In addition, in EP-035700 (and US-A-5093216) the use of furtural or a copolymer of furfural together with another aldehyde is reported. Although the anode material prepared from that polymer exhibited a high initial battery capacity, the irreversible capacity loss in the first cycle was observed to be unacceptably high, equalling as much as one third of the total capacity. As explained above, when the lithium secondary batteries using a non-aqueous solvent incorporate as the anode material the carbonaceous materials of the prior art, the capacity that can be obtained as a characteristic of those materials is not adequate. The present invention provides an anode use carbonaceous material that overcomes the difficulties of the prior art, possesses a high capacity of 500 mAh/g or more, reduces the irreversible capacity loss of the anode use carbonaceous material occurring in the first cycle, and when assembled with the cathode material to make a secondary battery, results in a battery with a high average voltage. The inventors of the present invention, based upon the results of earnest investigations of high capacity carbonaceous material for anode use using tar or pitch as a raw material. have succeeded in discovering that the carbonaceous material prepared by calcining at 800°C or more the solid obtained by heating pitch and/or tar and furfural in the presence of an acid catalyst differs from that made using tar, pitch or furfural alone and thereby have discovered an invention possessing superior characteristics as an anode material for a secondary battery using a non-aqueous solution. That is to say, the present invention relates to a carbonaceous material for use in the anode of a secondary battery using an aqueous solvent characterized by calcining at a temperature of at 800°C or more of a solid obtained by heating tar and/or pitch with furfural in the presence of an acid catalyst and to a method for making an anode material for use in the anodes of secondary batteries using non-aqueous solvents characterized by calcination at a temperature of at 800°C or more of a solid obtained by heating tar and/or pitch with furfural in the presence of an acid catalyst. The anode material for use in a secondary battery using a non-aqueous solvent of the present invention is prepared by mixing tar and/or pitch and furfural and heating them in the presence of an acid catalyst and then calcining the solid thus obtained at a temperature of at 800°C or more. By heating furfural in the presence of an acid. a ring opening reaction accompanied by three dimensional polymerization are accomplished and a stabilized insoluble solid results. The furfural in the present invention, because it is a superior solvent for aromatic compounds, plays the role of dissolving all or the largest part of the tar or pitch which are mixtures of every type of aromatic compound. As a result. by adding an acid catalyst to this mixture and heating, a solid copolymerized from furfural and aromatic compounds that make up tar and/or pitch is assumed to be obtained. Accordingly, depending upon the type, composition, and mixing ratio and the like of the tar and/or pitch mixed with the furfural, it is possible to bring about the optimization of the properties of the material obtained. This kind of battery material mixed from furfural and pitch like materials, solidified by a catalyst and calcined has not been reported to date and represent an entirely new material and production method. Furfural is polymerized by reaction with phenol and an alkali catalyst and then heat cured (furfural resin) is well known and this heat cured resin is presently on the market, but that is completely different in characteristics from the material and method of the present invention. As the tar of the present invention, such petroleum type tars as the oil residue of the thermal decomposition of naphtha (ethylene bottom oil), petroleum heavy oil and the oil residue of low pressure cracking of light oils; coal type tars such as those produced as a byproduct of the distillation of coal, synthetic tars obtained by polymerizing polycyclic hydrocarbons or substances containing polycyclic hydrocarbons in the presence of a Lewis acid such as hydrogen fluoride/boron tri-fluoride or aluminum chloride: or the low molecular weight fraction of synthetic pitch. In particular, the oil remaining as a by-product of the distillation of naphtha (ethylene bottoms) are used optimally as a raw material because of the high capacity that may be obtained. As the pitch of the present invention, pitches of the type obtained from petroleum heavy oil, the oil residue of low pressure cracking of light oils, pitch produced as a byproduct of thermal decomposition of naphtha, pitches produced from coal tar raw materials, or synthetic pitches obtained by polymerizing polycyclic hydrocarbons or substances containing polycyclic hydrocarbons in the presence of a Lewis acid such as hydrogen fluoride/boron tri-fluoride or aluminum chloride and the like may be used. Because the carbonaceous material of the present invention are classed among the non-graphitic carbons, as the pitch used, optically isotropic pitch is optimal because of the high capacity achievable. As the isotropic pitch of the present invention, if that pitch is placed in epoxy and polished, and the polished face is viewed under a polarizing microscope, virtually the entire surface will he isotropic. However, as stated before, it is possible to control the properties of the carbonaceous material finally obtained by varying the ratio of pitch to furfural in the mixture, and even if anisotropic pitch is used as the raw material, if more furfural is used than in the case of the use of isotropic pitch, a high capacity can be obtained. In addition, pitch obtained by polymerization of polycyclic hydrocarbons or substances containing them in the presence of hydroflouric acid/boron tri-fluoride is preferably used because there are few impurities and product quality is stable. As the polycyclic hydrocarbons of the present invention, polycyclic hydrocarbons such as naphthalene, anthracene, pyrene. coronene and the like and their derivatives; benzofuran. quinoline. thianaphthene, silanapthene or their derivatives, or compounds made by cross-linking any of these compounds with any of the others, various petroleum distillate fractions comprising these compounds; oil residues from petrochemical processes; and coal tar distillate fractions may be given as examples. The method of synthesizing optically isotropic pitch from polycyclic hydrocarbons in the presence of hydrofluoric acid/boron tri-fluoride is not particularly subject to limitation, however, the amount of catalyst usually used for polycyclic hydrocarbons is 0.1 to 10 moles hydrofluoric acid per mole of polycyclic hydrocarbon and 0.01 to 1.0 moles boron tri-fluoride per mole of polycyclic hydrocarbon, and the reaction is conducted at a temperature of between 0 and 300°C, preferably between 40 and 200°C and more preferably between 60 and 170°C. Moreover, when the pitch thus obtained is treated by heating, distillation, solvent extraction, air blowing, addition of nitric acid, or addition of sulfur and the like. various types of pitch can be prepared. When tar is used as a raw material of the present invention, a solid is obtained by heating 10 parts by weight of tar with between 0.1 and 1000 parts by weight of furfural in the presence of an acid catalyst. The ratio of tar to furfural is preferably 10 parts by weight of tar to between 1 and 100 parts by weight of furtural. When isotropic pitch is used as a raw material of the present invention, a solid is obtained by heating 10 parts by weight of isotropic pitch with between 0.1 and 1000 parts by weight of Furfural in the presence of an acid catalyst. The ratio of isotropic pitch to furfural is preferably 10 parts by weight of isotropic pitch to between 5 and 100 parts by weight of furfural. When anisotropic pitch is used as a raw material of the present invention, a solid is obtained by heating 10 parts by weight of anisotropic pitch with between 0.2 and 1000 parts by weight of furfural in the presence of an acid catalyst. The ratio of anisotropic pitch to furfural is preferably 10 parts by weight of anisotropic pitch to between 5 and 100 parts by weight of furfural. Further details are seen in the claims. Next, the tar or pitch is mixed with furfural. and because the furfural is a liquid at room temperature and has the ability to dissolve tar or pitch, the tar or pitch and furfural arc mixed evenly at between room temperature and 150°C. The method for mixing is not particularly limited but a batch type vessel or a continuous kneader/extruder or the like may be used. As the catalyst to be used to obtain a solid from the mixture of tar and/or pitch and furfural, toluenesulfonic acid, xylenesulfonic acid, sulfuric acid, nitric acid, phenolic boron tri-fluoride and amine complexes and the like may be used. The amount of catalyst that may be added is not particularly limited, however, for one hundred parts by weight of the tar or pitch and furfural mixture, between 0.1 and 50 wt% are preferred and for toluenesulfonic acid between 1 to 20 wt% are preferred. The solidification reaction is usually carried out at atmospheric pressure and at a temperature of between room temperature and 200°C. This temperature range is preferably between 30 and 160°C and if the temperature exceeds 200°C the boiling of the furfural becomes violent so that it becomes difficult to control the solidification reaction. However, under elevated pressures, this limitation is not applicable. The method for carrying out hardening on an industnal scale is not particularly limited, but batch style or continuous reaction style equipment may be optimally employed. The hardening reaction together accelerates with a rise in temperature, and a stabilized solid is obtained. When the solid is prepared by batch production methods, it is necessary to have a means for removing the product from the reactor. As an example, first tar or pitch and furfural together with catalyst are put in a metal pot and mixed at low temperature. They are then hardened at a temperature of around 100°C. After that, by completing hardening at 150°C or more and preferably 300°C or more the mixture shrinks into a solid and it is possible to remove this solid from a simple pot. Because the hardening reaction is exothermic, it is possible to be sure that complete hardening has occurred all the way to the center of the solid. Moreover, when a vessel type reactor is used, by adding an appropriate solvent which solubilizes the raw materials such as chloroform or benzene or the like, the solid can be obtained as a slurry. In this case, using an autoclave that is suitable for elevated pressures. the yield of the solid thus obtained can be increased. Moreover, as the equipment for continuous operation of the hardening reaction. kneader-extruder type devices, and piper reactors such as static mixers can be used. By controlling the reaction temperature and the length of reflux time, it is possible to continuously output material just on the point of hardening and where a slight amount of plasticity remains. After that the reaction can be completed by high temperature treatment to obtain a fully solidified product. As stated above, after hardening, the material is heated to between 150 and 800°C, which together with driving off any remaining unreacted furfural, completes the hardening reaction which is desirable for this invention. During this procedure the atmosphere is usually a non-oxidizing gas but depending upon the situation, after heating the solid to a temperature range of on the order of between 200 and 300°C under an atmosphere of an oxidizing gas such as air, the solid may be heat treated at 300°C or more under an atmosphere of non-oxidizing gas. By calcining the organic compound raw material thus obtained under vacuum or a non-oxidizing gas, the carbonaceous material of the present invention can be obtained. The calcination temperature is between 800 and 1800°C and preferably between 1000 and 1300°C and the calcination time is between I and 50 hours and preferably and from these conditions optional conditions should be selected for each case. Moreover, it is also possible to conduct a pre-calcination at temperatures of less than 800°C. As the non-oxidizing gas, nitrogen or argon are preferred. The non-oxidizing gas is supplied in a continuous flow and a method whereby the gas produced by the calcination of the organic compound raw material is carried away together with this gas flow or the exhaust generated by maintaining the vacuum, as the case may be, may be optimally employed. The carbonaceous material for use in the anode of a secondary battery using a non-aqueous solvent of the present invention has excellent characteristics and, in particular, a discharge capacity of 500 mAh/g or more at a voltage of 0 to 1.5 using a metallic lithium counter-electrode being possible simultaneous with the a very small irreversible capacity loss in the first cycle is the most important characteristic. Practice examples of the present invention are given below, and the results thereof present a more concrete and detailed explanation of the present invention. However, the examples given below are given only for the purpose of such concrete explanation and in no way limit the form. scope or application of the present invention. Example 1. Fifteen grams of ethylene bottom oil and 15 grams of furfural (mixing ration 50/50) were placed in a 200 ml glass flask and, after blending evenly by stirring at 60°C for 1 hour, 1.5 grams of paratoluenesulfonic acid were added and mixed thoroughly. The temperature was raised to 80°C and stirring was continued for 5 minutes until the mixture hardened into a gel. Then the temperature was raised to 450°C under a flow of nitrogen gas and unreacted furfural was removed with the waste gas. The processed material thus obtained was ground to an average particle diameter of 15 µm and then while passing a small flow of nitrogen gas over it, the pressure was reduced to 1333 Pa (10 torr)and it was calcined at 1200°C for 2 hours to obtain a carbonaceous material in powder form. To 90 parts by weight of the material thus obtained was added 10 parts by weight of polyfluorinated vinylidene powder (binder) and dimethylformamide as a solvent for blending and mixing and then the resulting paste was coated onto a copper mesh which. after drying was cut into squares 1 cm on a side which served as test electrodes. Then a half cell having LiClO4 dissolved in a solution of ethylene carbonate/dimethylcarbonate/ diethylcarbonate mixed in a ratio of 1/0.5/0.5 (LiClO4 concentration was 1.0 mole per ) as the electrolyte and a porous membrane of polypropylene having a thickness of 50µ m as the separator. As the counter electrode of this half cell, a lithium metal disk having a diameter of 16mm and a 0.5mm thickness was u sed. Moreover, as the reference electrode, a lithium metal disk similar to the counter electrode was used. Charging was conducted at a current density of 2mA/cm 2 until the potential of the test electrode was 1 V with respect to the reference electrode and then continued to charge for 20 hours at a potential of IV. Next, constant current discharging was conducted at a current density of 1 mA/cm 2 until the potential of the test electrode reached 1.5 V with respect to the reference electrode, and the charge capacity, discharge capacity and irreversible capacity loss were determined. The conditions of preparation and the results are represented in Table 1. Examples 2-5 The ethylene bottom oil and furfural raw materials of Example I were mixed in ratios of 20/80, 30/70, 40/60, and 60/40 and then subjected to the identical procedures and conditions as in Example 1. Then, half cells made with the resulting anode material were evaluated in the same way as in Example 1. The conditions of preparation and the results are given in Table 1. Example 6 Into a heat resistant autoclave, 7 moles of naphthalene, 2.45 moles of hydrofluoric acid and 0.77 moles of boron tri-fluoride were added, and after raising the temperature to 100°C, at spontaneously generated pressure, the reaction was conducted at 100°C for 4 hours. Next, by the usual methods. HF and BF 3 were removed and recycled by flushing the autoclave with nitrogen gas and after the low boiling components were removed, and an optically isotropic pitch with a softening point of 82°C was obtained. To 15 grams of the pitch thus obtained and placed in a 200 ml glass flask were added 15 grams of furfural (mixing ration: 50/50) and after 1 hour of blending these components evenly by stirring at 100 1.5 grams of paratoluenesulfonic acid were added and the mixture was stirred for a further 10 minutes at 100°C and a gel like solid was obtained. This solid was heated to 450°C under a flow of nitrogen gas and unreacted furfural and other volatile components were driven off. Then the processed material thus obtained was ground to an average particle diameter of 15µ m and then while under a small flow of nitrogen, the material was calcined for 2 hours at a reduced pressure of 1333 Pa (10 torr) at 1200°C to obtain a powdered carbonaceous material. Test anodes were made of the carbonaceous material thus obtained, and these were evaluated in the same way as in Example 1. The conditions of preparation and the results are given in Table 1. Examples 7-10 Thirty grams of the naphthalene raw material of Example 6 was mixed with furfural in ratios of 20/80, 30/70, 40/60, and 60/40 and then subjected to the identical procedures and conditions as in Example 6. Then, half cells made with the resulting anode material were evaluated in [he same way as in Example 1. The conditions of preparation and the results are given in Table 1. Example 11 To fifteen grams of isotropic coal tar pitch were added 15 grams of furfural in a 200 ml glass flask (mixing ratio: 50/50) and they were mixed evenly by stirring for 1 hour at 100°C after which 1.5 grams of paratoluenesulfonic acid was added and stirring was continued at 100°C to obtain a gel like solid. This solid was then heated to 450°C under a flow of nitrogen and all unreacted furfural and other volatile components were removed. The processed material thus obtained was ground to an average particle diameter of 15µ m and then while under a small flow of nitrogen, the material was calcined for 2 hours at a reduced pressure of 1333 Pa (10 torr)at 1200'c to obtain a powdered carbonaceous material. Test anodes were made and evaluated in the same way as in Example 1. The conditions of preparation and the results arc given in Table 1. Example 12 Into a heat resistant autoclave. 7 moles of naphthalene, 2.45 moles of hydrofluoric acid and 0.77 moles of boron tri-fluoride were added, and after raising the temperature to 260°C, at spontaneously generated pressure, the reaction was conducted at 260°C for 4 hours. Next, by the usual methods. HF and BF 3 were removed and recycled by flushing the autoclave with nitrogen gas and then the low boiling components were removed and a 100% optically anisotropic pitch with a softening point of 230°C was obtained. To 9 grams of the pitch thus obtained were added 21 grams of furfural (mixing ratio: 30/70) in a 200 ml glass flask and after 1 hour of blending these components evenly by stirring at 100°C, 1.5 grams of paratoluenesulfonic acid we re added and the mixture was stirred a further 10 minutes at 100°C and a gel like solid was obtained. This solid was heated to 450°C under a flow of nitrogen gas and unreacted furfural and other volatile components were driven off. Then the processed material thus obtained was ground to an average particle diameter of 15µ m and then while under a small flow of nitrogen, the material was calcined for 2 hours at a reduced pressure of 1333 Pa (10 torr) at 1200°C to obtain a powdered carbonaceous material. Test anodes were made and evaluated in the same way as in Example 1. The conditions of preparation and the results arc given in Table 1. Comparative Experiment 1 Thirty grams of the ethylene bottom oil used in Example 1 were stirred at 100°C and 1.5 grams of paratoluenesulfonic acid were added and stirring was continued for 1 hour at 100°C. Then the temperature was raised to 600°C under a flow of nitrogen and a solid was obtained. Then the processed material thus obtained was ground to an average particle diameter of 15µ m and then while under a small flow of nitrogen, the material was calcined for 2 hours at a reduced pressure of 1333 Pa (10 torr) at 1200°C to obtain a powdered carbonaceous material. Test anodes were made and evaluated in the same way as in Example 1. The conditions of preparation and the results are given in Table 1. Comparative Experiment 2 Thirty grams of the isotropic pitch obtained in Example 6 were stirred at 200°C and 1.5 grams of paratoluenesulfonic acid were added. Stirring of this mixture was continued for I hour at 200°C, after which the temperature was raised to 600°C under a flow of nitrogen gas and a solid was obtained. Then the processed material thus obtained was ground to an average particle diameter of 15µ m and then while under a small flow of nitrogen, the material was calcined for 2 hours at a reduced pressure of 1333 Pa (10 torr)at 1200°C to obtain a powdered carbonaceous material. Test anodes were made and evaluated in the same way as in Example 1. The conditions of preparation and the results are given in Table 1. Comparative Experiment 3 To thirty grams of furfural in a 200 ml glass flask at 100°C were added 1.5 grams of paratoluenesulfonic acid and stirring of this mixture was continued to obtain a solid in a short time. This solid was heated at 450°C under nitrogen to drive off any unreacted furfural or any other volatile components. Then the processed material thus obtained was ground to an average particle diameter of 15µ m and then while under a small flow of nitrogen, the material was calcined for 2 hours at a reduced pressure of 1333 Pa (10 torr)at 1200°C to obtain a powdered carbonaceous material. Test anodes were made and evaluated in the same way as in Example 1. The conditions of preparation and the results are given in Table 1. Below in Table 1 are the preparation conditions and anode properties of the carbonaceous materials shown by Example and Comparative Experiment Numbers. Compared with the lithium secondary batteries of the prior art, the discharge capacity of this carbonaceous material is larger and the irreversible capacity loss in the first cycle is low so that it is possible by this invention to put into practice a secondary battery with large capacity.

Claims: A method of preparing a carbonaceous material for use in the anodes of secondary batteries using non-aqueous solvents comprising the calcination at 800°c or more of a solid obtained by heating tar and/or pitch mixed with furfural in the presence of an acid catalyst. A method according to claim 1 wherein 10 parts by weight of tar are mixed with between 0.1 and 1000 parts by weight of furfural. A method according to claim 2 wherein 10 parts by weight of tar are mixed with between 1 and 100 parts by weight of furfural. A method according to any preceding claim wherein said solid is obtained by heating the oil obtained as a by-product of naphtha cracking (ethylene bottoms oil). A method according to any preceding claim wherein 10 parts by weight of pitch are mixed with between 0.1 and 1000 parts by weight of furfural and the pitch used is an optically isotropic pitch. A method according to any preceding claim wherein 10 parts by weight of pitch are mixed with between 5 and 100 parts by weight of furfural and the pitch used is an optically isotropic pitch. A method according to any of claims I to 4 wherein said solid is obtained by heating 10 parts by weight of pitch mixed with between 0.2 and 1000 parts by weight of furfural and the pitch used is an optically anisotropic pitch. A method according to claim 7 where said solid is obtained by heating 10 parts by weight of pitch mixed with between 10 and 100 parts by weight of furfural and the pitch used is an optically anisotropic pitch. A method according to any preceding claim wherein said solid is obtained by heating pitch mixed with furfural and the pitch used is pitch obtained by reacting polycyclic hydrocarbons or mixtures containing polycyclic hydrocarbons in the presence of hydrofluoric acid and boron trifluoride or a derivative of such pitch.

Inventors: KANNO KOICHI, SAKAMOTO HITOSHI, TAKAHASHI YUZURU, KANNO, KOICHI, SAKAMOTO, HITOSHI, TAKAHASHI, YUZURU, Kanno, Koichi, c/o Mitsubishi Gas Chem.Co.Inc., Sakamoto, Hitoshi, c/o Mitsubishi Gas Chem.Co.Inc., Takahashi, Yuzuru, c/o Mitsubishi Gas Chem.Co.Inc.
Applicants: MITSUBISHI GAS CHEMICAL CO, MITSUBISHI GAS CHEMICAL COMPANY, INC.
Application year: 1998
Main classification: H01M4
IPC codes: C04B35, H01M4, H01M10