Carbonaceous material and a cell using the same

Abstract: The present invention provides a carbonaceous material suitable for an electrode of a secondary cell. The carbonaceous material have a specific surface area by BET method of at least 10 m²/g and a nitrogen/carbon atom weight ratio of 0.15 to 0.01, and is obtained by heat-treating a polyimide polymer having repeating units represented by: wherein R represents a 4 valent aromatic residue having at least one aromatic ring, of which each 2 valences are bonded with adjacent carbon atom in the aromatic ring, R' represents a divalent aromatic residue having 1 to 4 aromatic rings and n is an integer of at least 2. The present invention also provides a secondary cell using the above mentioned carbonaceous material as electrode.

Description: The present invention relates to a carbonaceous material and a secondary cell using the same. Secondary cells have been employed everywhere in the world and in order to provide cells with high energy density and high power, electrodes of secondary cells have been intensely studied. For example, there have been proposed, as anode active materials, transition metal oxides or transition metal chalcogenates (e.g. TiS 2 , MoS 2 , CoO 2 , V 2 O 5 , FeS 2 , NbS 2 , ZrS 2 , NiPS 3 , VSe 2 , MnO 2 etc.), heat polymerizates of organic materials (e.g. one-dimension graphitizate, fluorinated carbon, graphite etc.) and electroconductive polymers (e.g. polyacetylene, polypyrrole, polyaniline, polyazulene etc.). Also, non-hydrogen cells or capacitors using lithium, lithium alloy or graphite as cathode have been proposed, because they provide cells with light weight and high energy density in comparison with conventional cells. However, when lithium metal is used as cathode, the metal lithium dissolves into the electrolyte as lithium ions upon discharging and the lithium ion upon charging in turn deposit on the cathode. When this charge and discharge cycle is repeated many times, the deposited lithium metal grows as dendrite form and may reach the anode to cause a short-circuit. The short-circuit often reduces the charge and discharge cycles. In order to overcome the problem, many solutions have been proposed. For example, Japanese Kokai Publication 62-243247 suggests that a lithium metal plate which is sandwiched between two aluminum plates is used as cathode for the secondary cells. This reduces the formation of dendrites, but the surface area of the cathode is reduced and the charge and discharge capacity also decreases. Japanese Kokai Publication 1-63268 discloses that, in order to increase the surface area of the cathode, porous Li-Al alloy with a pore size of 5 µ or more is used as cathode. The pore size of the porous Li-Al alloy, however, is large enough to have poor mechanical strength and the alloy easily breaks when mounting into the cells. It is also proposed that lithium is doped into a carbonaceous material to form a cathode. For example, Japanese Kokai Publications 62-122066 and 62-90863 suggest a carbonaceous material which is obtained by carbonizing an organic material. The carbonaceous material is doped with lithium to form an electrode which, however, does not have enough dope amount and has poor performance. In order to improve the above problems, Japanese Kokai Publication 3-13710 provides a method for producing a cathode electrode wherein a carbonaceous material which is produced by carbonizing a furan resin to which a phosphorus compound is added, is powdered and dispersed in an organic solvent together with poly(vinylidene fluoride) powder, which is then coated on stainless steel with removing the organic solvent and compression-molded. The resulting electrode, however, does not have sufficient performance. Japanese Kokai Publication 61-277165 discloses a graphite which is obtained by heat-treating an aromatic polyimide at a temperature of 2,000 to 8,500 °C in an inert atmosphere. The graphite is suggested to be used as at least one of electrodes of a secondary cell. The graphite, however, has poor charge-discharge capacity and coulomb efficiency. The present invention is concerned with a carbonaceous material suitable for an electrode of a secondary cell. Accordingly, in the cell of the present invention there is used a carbonaceous material having a specific surface area by BET method of at least 10 m 2 /g and a nitrogen/carbon atom weight ratio of 0.15 to 0.01, obtained by heat-treating a polyimide polymer having repeating units represented by: wherein R represents a tetra valent aromatic residue having at least one aromatic ring, of which each 2 valences are bonded with adjacent carbon atoms in the aromatic ring, R' represents a divalent aromatic residue having 1 to 4 aromatic rings and n is an integer of at least 2. The polyimide polymer has the repeating unit represented by the formula (1), wherein n is an integer of at least 2, preforably 10 to 500, more preferably 30 to 300. In the formula (1), R represents a 4 valent aromatic residue having at least one aromatic ring, of which each 2 valences are bonded with adjacent carbon atoms in the aromatic ring, for example R' represents a divalent aromatic residue having 1 to 4 aromatic rings, for example Preferably, R is and R' is It is preferred that the polyimide polymer is firstly formed into a powder, fiber, film, plate or a composite thereof, and then heat-treated. The carbonaceous material used in the cell of the present invention may be obtained by gradually heating the polyimide polymer to 400 to 1,400 °C, preferably 500 to 1,200 °C in a non-oxidation atmosphere or under vacuum. The non-oxidation atmosphere can be conducted in a non-oxidation gas, such as nitrogen gas, argon gas, helium gas, neon gas, carbon dioxide gas and the like. It is preferred to heat-treat under vacuum. The carbonaceous material has a nitrogen / carbon atom weight ratio of 0.15 to 0.01 and has a specific surface area of at least 10 m 2 /g, preferably at least 100 m 2 /g, more preferably at least 200 m 2 /g, further more preferably at least 300 m 2 /g, most preferably at least 400 m 2 /g. If the material is outside the above range, it has poor performance as electrode. The carbonaceous material powder has an average particle size of 500 µm or less, preferably 100 µm or less. If the particle size is more than 500 µm, the material is liable to have poor performance as electrode. The carbonaceous material of the present invention preferably has a total pore volume of at least 0.20 ml/g, more preferably at least 0.30 ml/g. The total pore volume is determined by BET method. If the total pore volume is less than 0.20 ml/g, the resulting electrode is liable to have poor performance. The carbonaceous material can be mixed with thermoplastic polymers (e.g. polyethylene, polypropylene, Teflon®, polyvinylidene fluoride etc.) and molded into an electrode of a secondary cell. The present invention thus provides a secondary cell which is fundamentally composed of an anode, a cathode and an electrolyte solution. A separator may be present between both electrodes. The electrolyte solution is generally composed of a solvent and an electrolyte, but may be solid. It is possible that one or both electrodes, for example, the cathode, are formed from the carbonaceous material used in the present invention. The anode may contain as anode active materials, transition metal oxides or transition metal chalcogenates (e.g. TiS 2 , MoS 2 , CoO 2 , V 2 O 5 , FeS 2 , NbS 2 , ZrS 2 , NiPS 3 , VSe 2 , MnO 2 etc.), lithium composite oxides (e.g. LiCoO 2 ), heat polymerizates of organic materials (e.g. one-dimension graphitizate, fluorinated carbon, graphite etc.) and electroconductive polymers (e.g. polyacetylene, polypyrrole, polyaniline, polyazulene etc.). The carbonaceous material used in the present invention is contained in the cathode of the secondary cell. The cathode is preferably formed by mixing the carbonaceous material with other electroconductive materials. The other electroconductive materials are those that do not inhibit cathode electrode reaction and are insoluble in the electrolyte solution. It is preferred that the other electroconductive materials have an electroconductivity of at least 10 0 S/cm, preferably at least 10 1 S/cm, for example including metals (e.g. nickel, titanium, copper, platinum and gold), alloys (e.g. stainless steel), inorganic metal oxides (e.g. SnO 2 and Ti 2 O 3 ), carbon black (e.g. acetylene black), carbon materials (e.g. coke, carbon fiber, electroconductive carbon and graphite) and the like. Preferred are copper powder and carbon materials (e.g. electroconductive carbon and graphite) in view of electroconductivity and stability. The carbon material can either be an electrode active material having an electroconductivity of at least 1 S/cm, or simple electroconductive material. It is preferred that the electrode active material has crystal regularity, i.e. d 002 of 3.37 to 3.6 Å, L c of at least 20 Å and L a of 100 Å. The carbonaceous material used in the present invention may exhibit such regularity, if it is baked at a temperature of more than 800 °C. However, as a cathode active material, the carbonaceous material which is prepared by baking at less than 800 °C is preferred because of cycle characteristics. An amount of the other electroconductive material may be 5 to 70 parts by weight, preferably 15 to 70, more preferably 33 to 60 parts by weight based on 100 parts by weight of the carbonaceous material. Amounts of less than 5 parts by weight deteriorate electroconductivity of the cathode and those of more than 70 % by weight reduce discharging capacity of the cathode. In the secondary cell of the present invention, the cathode preferably has an electroconductivity of at least 10 -3 S/cm, preferably at least 10 -2 S/cm. The cell preferably has a rechargeable capacity ratio of anode to cathode of at least 1.2 times. more preferably at least 1.5 times, and limited to upto 2 times. If the cathode has an electroconductivity of more than 10 -3 the electroconductivity is similar to that of the electrolyte solution and therefore the electrode reaction proceeds uniformly around the cathode. Accordingly, an intemal impedance is kept low and charge and discharge can be conducted at a high electric current density. If the rechargeable capacity is within the above range, the anode always has excess electric capacity in usual charge or discharge cycles and therefore even if it is difficult to set suitable charge or discharge end voltage an anodic voltage can be kept within the permitted voltage range. Accordingly no overcharge or no overdischarge would occur at the anode and thus its recycle characteristics are superior. The control of electric capacity can be conducted by adjusting the ion absorption capacity of the anode and the cathode. Particularly, it is obtained as a ratio of carbon energy density at a discharge end 1.0V vs Li/Li + against a charge end 0V vs Li/Li + to anode energy density obtained as lithium being cathode. The electrolyte (dopant) used in the secondary cell of the present invention includes, as anion, a halogenated anion of V B atoms in periodic table (e.g. PF 6 -, SbF 6 -, AsF 6 -, SbCl 6 -), a halogenated anion of III a atoms in the periodic table (e.g. BF 4 - . BR" 4 - wherein R" represents a phenyl group or an alkyl group), a perchlorate anion (e.g. ClO 4 - ), a halogen anion (e.g. Cl-, Br-, l-), trifluoromethanesulfonic acid; as cation, an alkali metal cation (e.g. Li+, Na+, K+), (R''' 4 N)+ wherein R"' is a hydrocarbon group having 1 to 20 carbon atoms); and the like. Examples of the compound providing the above mentioned dopant are LiPF 6 , LiSbF 6 , LiAsF 6 , LiClO 4 , KPF 6 , NaClO 4 , Kl, KSbF 6 , KAsF 6 , KClO 4 , [(n-Bu) 4 N]+AsF 6 - , [(n-Bu) 4 N]+BF 4 - ,LiAlCl 4 , LiBF 4 , LiCF 3 SO 3 and the like. Preferred are LiClO 4 , LiPF 6 and LiBF 4 . The solvents contained in the electrolyte solution are not limited, but preferably it is one having relatively high polarity. Examples of the solvents are propylene carbonate, ethylene carbonate, benzonitrile, acetonitrile, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyl lactone, dioxolane, triethylphosphite, dimethyl formamide, dimethyl acetamide, dimethyl sulfoxide, dioxane, dimethoxyethane, polyethylene glycol, sulfolane, dichloroethane, chlorobenzene, nitrobenzene, mixtures thereof and the like. Preferred are propylene carbonate, ethylene carbonate, dimethoxyethan and mixtures thereof. The separator may be one which does not show any resistance to ion transference in the electrolyte solution and which is stable therein. Examples of the separators are glass fiber filter; polymer pore filters (e.g. polyester, Teflon®, Polyfulon, polypropylene); non-woven fabric of glass fibers and the above mentioned polymer; and the like. Instead of the electrolyte solution and separator, a solid electrolyte may be used for the cell of the present invention. Examples of such solid electrolytes are metal halides (AgCl, AgBr, Agl, Lil); PbAg 4 l 5 , PbAg 4 l 4 CN; a composite of a polymer matrix (e.g. polyethylene oxide, polypropylene oxide, polyvinylidene fluoride, polyacrylamide) with the above mentioned electrolyte salts; gelled articles thereof; polymer solid electrolytes prepared by grafting a polymer backbone with ion releasing groups (e.g. low molecular weight polyethylene oxide, crown ether); and the like. Secondary cells can be classified to a variety of types, such as coin type, sheet type, cylinder type and the like, but the cells of the present invention can be used as all of these cells. Fig. 1 is a sectional view of a sample cell of Example 8. The present invention is illustrated by the following examples. Example 1 An aromatic polyimide film (available from Toray Dupont Company as Captone, obtained by the condensation reaction of pyromellitic anhydride and diaminodiphenyl ether) was put in a vacuum fumace (available from Fuji Denpa Kogyo K.K.), and heated at a heating speed of about 30 °C/time to a predetermined temperature shown in Table 1. It was then heat-treated at the predetermined temperature for 3 hours to obtain carbonaceous materials 1 to 4. Nitrogen / carbon atom weight ratio and specific surface area of the resulting materials were measured and the results are shown in Table 1. Temperature for heat treatment (°C) Nitrogen/carb on atom ratio Specific surface area (m 2 /g) Polyimide film - 0.116 1.30 No. 1 800 0.061 40.5 No. 2 670 0.081 465.8 No. 3 630 0.087 489.0 No. 4 600 0.090 530.3 Example 2 Preparation of electrode active material A one liter three neck flask was charged with 500 g of 5.5 N diluted sulfuric acid, to which 20 g of aniline was dissolved. To the content, a solution of 200 g of water and 20 g of ammonium persulfate was added dropwise at 5 °C for 30 minutes, and then mixed for another 2 hours at 5 °C. The resulting dark green precipitate was filtered, and rinsed with methanol, followed by vacuum-drying at 30 °C for 12 hours to obtain 8 g of polyaniline powder. Preparation of electroconductive polymer electrode Ten liter of water was mixed for 5 minutes with 5 g of the resulting electrode active material, 1 g of glassy carbon (available from Unitika Ltd. as GCP-10H) and 0.5 g of polyester fiber, and then formed into sheet by a sheet machine (available from Kumagai Riki Kogyo K.K.). The resulting sheet was vacuum-dried at 30 °C for 12 hours and then compression-molded at 120 °C and 100 Kg/cm 2 for 60 second to obtain an electroconductive polymer electrode with 0.8 mm thickness, 6 Kg/cm 2 flexural strength, 75 % pore rate and 260 mmH 2 O pressure loss. Preparation of lithium secondary cell A CR-2016 type lithium secondary cell was prepared using as cathode the carbonaceous material obtained in Example 1, as anode the above polymer electrode and as electrolyte solution a propylene carbonate (PC) solution containing one M of LiClO 4 . The performance of the cell was evaluated by charging and discharging at 2.5 to 3.7 volt and 1 mA. The results are shown in Table 2. No. 1 No.2 No.3 No.4 Voc 3.30 3.30 3.30 3.30 Ah capacity (mA) 50 cycles 4.8 5.1 5.9 6.7 100 cycles 4.6 4.8 5.8 6.4 Production Example 1 Fifteen gram of 4,4'-diaminodiphenyl ether was dissolved in 300 ml of DMAc (dimethylacetamide), to which 16.4 g of pyromellitic dianhydride was added and mixed at 30 °C for one hour. The resulting solution was added to 1,000 ml of chloroform to precipitate polyamide acid powder. It was filtered and rinsed with 1,000 ml of methanol three times, and then dried under a reduced pressure at 35 °C to obtain 29.9 g of polyamide acid powder. It was cured at 200 °C for 24 hours to obtain 20 g of polyimide polymer powder. Example 3 The polyimide polymer powder of Production Example 1 was put in a vacuum furnace (available from Fuji Denpa Kogyo K.K.), and heated at a heating speed of about 30 °C/time to a predetermined temperature shown in Table 3. It was then heat-treated at the predetermined temperature for 5 hours to obtain carbonaceous powder 5 to 8. Total pore volume, specific surface area and average particle size of the resulting powder were measured and the results are shown in Table 3. Comparative Example 1 The polymer powder of Production Example 1 was heated as generally described in Example 3 to obtain carbonaceous powder 9 and 10. Temp. of heat treatment (°C) Total pore volume (ml/g) Specific surface area (m 2 /g) Average particle size (µm) No. 5 750 0.296 252.1 30 No. 6 600 0.305 465.8 32 No. 7 550 0.310 473.0 32 No. 8 500 0.320 491.8 31 No. 9 350 0.040 5.1 45 No. 10 1,500 0.012 0.9 25 Total pore volume and specific surface area were determined by Bel Sorp 28 (available from Nippon Bel Company). Average particle size was determined by observing 100 particles from a microscope photograph of powder and calculating average particle size. Example 4 Nine gram of the carbonaceous powder obtained in Example 3 was dry-blended with one g of polypropylene powder and packed in a mold which was then compression-molded at 180 °C and 100 Kg/cm 2 for 10 minutes to obtain a molded article. A CR-2016 type lithium secondary cell was prepared using as cathode the molded article obtained above, as anode the polymer electrode obtained in Example 2 and as electrolyte solution a propylene carbonate (PC) solution containing one M of LiClO 4 . The performance of the cell was evaluated by charging and discharging at 2.5 to 3.7 volt and 1 mA. The results are shown in Table 4. 50 cycles 200 cycles No. 5 4.5 4.3 No. 6 5.2 4.9 No. 7 6.0 5.9 No. 8 5.3 5.0 No. 9 0.08 0.02 No. 10 0.04 0.01 Example 5 The carbonaceous powder No. 7 obtained in Example 3 was mixed with graphite and Teflon® in an amount ratio of carbonaceous powder/graphite/Teflon® of 6/3/1, and molded at a pressure of 2 t/cm 2 to form a cathode. The cathode had a electroconductivity of 4 X 10 -3 S/cm. The cathode was attached to 200 mesh SUS 304 metal net and used as action electrode. Lithium was employed as cathode and reference electrode and 3M LiClO 4 /propylene carbonate solution was used as electrolyte solution. Then, discharge capacity of the cathode was measured. Charge was conducted at 0.1 volt for 10 hours and discharge was conducted until 2.5 volt at 0.2 mA/cm 2 . The discharge capacity per unit weight of the cathode was 290 mAh/g. Example 6 The electrode of Example 5 was attached to 200 mesh SUS 304 metal net and used as cathode. LiCoO 2 , graphite and Teflon® were mixed at an amount ratio of respectively 7:2:1, and then molded at 2 t/cm 2 and then attached to 200 mesh SUS 304 metal net, which was used as anode. An electrolyte solution was 3.5 M LiClO 4 /(propylene carbonate + dimethoxyethane at volume ratio of 7:3) and a separator was microporous polypropylene. Using the above, a 2016 type coin cell was prepared. A discharge capacity was measured by charging and discharging at 2 to 4.1 volt and 0.5 mA. The discharge capacity was 14.8 mAh and the cell showed good cycle properties. Example 7 A 2016 type coin cell was prepared as generally described in Example 6, with the exception that the anode was one that is prepared by mixing polyaniline and 20 % graphite and then attached to 200 mesh SUS 304 metal net at a pressure of 2 t/cm 2 . A discharge capacity was measured by charging and discharging at 2 to 4.1 volt and 0.5 mA. The discharge capacity was 6.2 mAh and the cell showed good cycle properties. Example 8 Ingredients Parts by weight Carbonaceous powder 6 of Example 3 65 Graphite powder 35 Teflon® powder 5 The above ingredients were mixed and molded at 2 t/cm 2 , and then attached to 200 mesh SUS 304 metal net. The resulting molded article was blanked to a circle having a diameter of 16 mm and used as cathode. Separately, polyaniline and 20 % graphite was mixed and molded at 2 t/cm 2 and attached to 200 mesh SUS 304 metal net. It was then blanked to a circle having a diameter of 16 mm and used as anode. The above cathode and anode were formed into a sample cell as shown in Fig. 1, wherein an electrolyte solution was 3.5 M LiClO 4 /(propylene carbonate + dimethoxyethane at 7:3 volume ratio) and a separator was microporous polypropylene. Using the cell, a discharge capacity was measured by charging and discharging at 1.8 to 3.6 volt and 0.5 mA. In Fig. 1, 1 is the anode, 2 is the cathode, 3 Is an anode terminal, 4 is a cathode terminal, 5 is the separator and 6 is fluorine resin.

Claims: A secondary cell using as cathode a carbonaceous material absorbing or releasing a cation, wherein said carbonaceous material having a specific surface area by BET method of at least 10 m 2 /g and a nitrogen/carbon atom weight ratio of 0.15 to 0.01, is obtained by heat-treating a polyimide polymer having repeating units represented by: wherein R represents a 4 valent aromatic residue having at least one aromatic ring, of which each 2 valences are bonded with adjacent carbon atoms in the aromatic ring, R' represents a divalent aromatic residue having 1 to 4 aromatic rings and n is an integer of at least 2. The secondary cell according to Claim 1 wherein a rechargeable electric capacity ratio of anode to cathode is 1.2 times or more. The secondary cell according to Claim 1 or 2 wherein said anode is formed from electroconductive polymer material. The secondary cell according to anyone of Claims 1 to 3 wherein said carbonaceous material has an electroconductivity of 10 -3 S/cm or more. The secondary cell according to Claim 4 wherein said cathode is prepared from 100 parts by weight of the carbonaceous material and 5 to 70 parts by weight of other electroconductive materials. The secondary cell according to Claim 3 wherein said anode is LiCoO 2. The secondary cell according to Claims 1 to 6 wherein said carbonaceous material is obtained by gradually heating the polyimide polymer to 400 to 1,400 °C in a non-oxidation atmosphere or under vacuum.

Inventors: ASAMI KEIICHI, ECHIGO YOSHIAKI, INOUE KATSUO, KABATA TOSHIYUKI, OHSAWA TOSHIYUKI, TAKAHASHI HIDEKI, ASAMI, KEIICHI, ECHIGO, YOSHIAKI, INOUE, KATSUO, KABATA, TOSHIYUKI, OHSAWA, TOSHIYUKI, TAKAHASHI, HIDEKI
Applicants: RICOH KK, UNITIKA LTD, RICOH COMPANY, LTD., UNITIKA LTD.
Application year: 1993
Main classification: H01M4
IPC codes: D01F9, H01M10, H01M4