USE OF A MACROMOLECULAR COMPOUND FOR MAKING A DEPOLARIZING MIXTURE

Abstract: A depolarizing mixture for primary and secondary batteries, which contains a compound which contains heteroatoms in its molecule to constitute a polar molecule and functions as a dispersing agent so that both a positive active material and a conductive material can be dispersed uniformly in the mixture.

Description: This invention relates to the manufacture of cathode composites for use in primary and secondary batteries. Background Known lithium batteries include primary and secondary batteries in which a chalcogen compound is used for the positive active material and metallic lithium is used as the anode. This type of battery generally has a cathode including a cathode composite, an anode including metallic lithium forming the negative active material, and a separator. Manufacture is generally by laying the cathode on the anode with the separator between and housing this laminate in a battery can, or between plates sealed around the edge with a sealing agent. Particles of the positive active material, particles of carbon such as carbon black forming a conductive agent, polytetrafluoroethylene or solid polymer electrolyte serving as a binder and organic solvent serving as a diluent are mixed to a paste and filled in or applied onto a substrate such as stainless steel net or stainless steel plate to form the cathode composite. The anode is formed by sticking a metallic lithium sheet onto a stainless steel plate, or is composed of the metallic lithium sheet alone. However, there are problems. When mixing the paste the positive active material and conductive agent tend to cohere and be poorly wetted by the binder and organic solvent. For this reason uniform mixing is difficult, uniform application on the substrate is also difficult and furthermore the surface of the cathode composite tends to become corrugated. With the positive active material, conductive agent and binder not being uniformly dispersed the cathode composition sometimes cracks or peels off the substrate. In addition, initial capacity is small compared with theoretical capacity in primary batteries while in secondary batteries capacity is less than half of the initial capacity after 100 cycles. Furthermore the cathode composite may have convex surface portions which sometimes break the separator, reach the anode and cause short-circuiting. In the prior art, US-A-4940553/EP-A-343408 disclose the use of soluble salts of polyphosphoric and polyphosphonic acids as dispersant for nickel hydroxide in an aqueous nickel hydroxide paste for filling into a porous electrode substrate. US-A-4865927/EP-A-302206 describe the use of nitrogen-containing heterocycles such as phenanthroline and pyridinecarboxylic acids as complexing agents to avoid the formation of surface films on a nickel-plated current collector which contacts a cathode composite of manganese dioxide, conductive agent (e.g. carbon black) and binder in an alkaline cell. JP-A-1/243372 describes the use of (inter alia) poly-2-vinylpyridine as a binding resin on a positive electrode current collector, in a composition including carbon black. An aim herein is to provide a cathode composite in which cracking, peeling-off from a substrate and short-circuiting can be avoided, a good discharge characteristic is available in a primary battery, and a good charge/discharge characteristic is available in a secondary battery. The invention is the use, in the manufacture of a cathode composite comprising a mixture of (a) a particulate positive active compound; (b) a particulate conductive agent; and (c) a binder for the positive active agent and conductive agent particles, the binder comprising solid polymeric electrolyte material, of (d), a polar macromolecular compound including pyridine ring, as a dispersant to aid dispersal of the positive active compound and conductive agent particles in the binder before solidification thereof. With the nitrogen heteroatom, a bias is produced in the entire electron cloud of the compound so that it is a polar molecule. The function as a dispersant prevents particles of solid from cohering or precipitating in liquid. The compound is adsorbed to surfaces of the positive active material and the conductive agent and sterically stabilized, so as to prevent the positive active material and the conductive agent from cohering and thereby improve wetting of the positive active material and the conductive agent by the organic solvent and the binder. The positive active material, conductive agent and binder are therefore dispersed uniformly and can be applied on the substrate evenly. A cathode composite having a uniform, non-corrugated surface can be obtained. Accordingly cracking, peeling-off from the substrate and short-circuiting can be prevented. A good discharge characteristic is available when the cathode composite is used for a primary battery, and a good charge/discharge characteristic is available when it is used for a secondary battery. For the positive active material, MnO 2 may be mentioned for primary batteries and MoO 3, V 2 O 5 , V 6 O 13 , LiV 3 O 8 , LiMn 2 O 4 , LiCoO 2 , LiCrO 2 and LiNiO 2 for secondary batteries. Embodiments are now described with reference to the drawings, in which: Fig 1 is a schematic vertical sectional view showing a battery structure; Fig 2 is a diagram showing initial discharge characteristics of Embodiment 1 and Comparison Example 1; Fig 3 is a diagram showing energy densities per volume of the batteries of Embodiment 1 and Comparison Example 1; and Fig 4 is a diagram showing charge/discharge cycle characteristics of Embodiment 2 and Comparison Example 2. [Parts are by weight]. Embodiment 1 15 parts of MnO 2 as positive active material for a primary battery, 1.5 parts of acetylene black as conductive agent, 4 parts of electrolyte material as binder, 0.75 parts of poly-2-vinylpyridine (molecular weight: 150,000) shown by the following formula (I) as dispersant, 10 parts of benzene as diluent and 0.08 weight part of azobisisobutyronitrile as reaction initiator were mixed to prepare a paste for a cathode composite. A mixture of monoacrylate of ethylene oxide, diacrylate of ethylene oxide and triacrylate of ethylene oxide, with molecular weights of about 200, was dissolved in propylene carbonate solution containing lithium perchlorate at a concentration of 2 mol/l to prepare the electrolyte material. The prepared paste was cast onto a stainless steel substrate and left for 1 hour at 100°C in an atmosphere of inert gas to cure, giving a sheet-shaped cathode composite on the stainless steel substrate. Thickness of the prepared cathode composite was 50µm average, 53µm maximum and 47µm minimum. Next, 0.05 parts of azobisisobutyronitrile were dissolved in 10 parts of the above electrolyte material and this solution was cast on the cathode composite and cured in the same way as above, so that an electrolyte film was formed on the cathode composite. The thickness of the prepared electrolyte film was 20µm. The composite sheet thus prepared comprising the stainless steel substrate, the cathode composite and the electrolyte film was cut to a square piece 10 x 10mm. Metallic lithium 100µm thick was attached onto the electrolyte film as an anode, so as to make up a battery having a structure shown in Fig 1, that is, a stainless steel substrate 1, a cathode composite 2, an electrolyte film 3, an anode 4 and another stainless steel substrate 5. A load of 1kg/cm 2 was applied to the prepared battery and an initial discharge test done under this condition, at 25°C with a constant current of 0.1 mA/cm 2 . As a result, a cathode utilization factor of 85% was obtained. The filling rate of active material per volume of cathode composite was 35 vol%. A dispersant of the above formula (I) with a molecular weight in the range 10000 to 250000 may be used. Comparison Example 1 A battery was made up in the same way as Embodiment 1 except that the dispersant was not used. Cohering particles of MnO 2 and acetylene black formed on the cathode composite of the prepared battery, and there was a part having a thickness reaching 500µm. The thickness of the cathode composite at parts other than where the cohering particles were formed was 61µm average, 72µm maximum and 51µm minimum. An initial discharge test as above was done on the prepared battery, and a cathode utilization factor of 70% was obtained. In a discharge test after preservation for 40 days at 60°C, a cathode utilization factor of 45% was obtained. The filling rate of active material per volume of cathode composite was 23 vol%. Fig 2 is a diagram showing the initial discharge characteristics of Embodiment 1 and Comparison Example 1. In Fig 2, A 1 shows the initial discharge characteristic of Embodiment 1 and B 1 shows the initial discharge characteristic of Comparison Example 1. As seen from this diagram, the discharge characteristics of Embodiment 1 were better than those of Comparison Example 1. Fig 3 shows energy/volume densities of batteries of Embodiment 1 and Comparison Example 1. The energy/volume density of Embodiment 1 was better than that of the Comparison Example 1. Embodiment 2 15 parts of LiMn 2 O 4 as positive active material for a secondary battery, 1.5 parts of acetylene black as conductive agent, 4 parts of electrolyte material as binder, 0.75 parts of poly-4-vinylpyridine (molecular weight 140000) of the following formula (II) as dispersant, 10 parts of benzene as diluent and 0.08 parts of azobisisobutyronitrile as reaction initiator were mixed to prepare a paste for a cathode composite. A mixture of monoacrylate of ethylene oxide, diacrylate of ethylene oxide and triacrylate of ethylene oxide, with molecular weights of about 200, was dissolved in propylene carbonate solution containing lithium hexafluoro-arsenate at a concentration of 2 mol/l to prepare the electrolyte material. The prepared paste was cast onto a stainless steel substrate and left for 1 hour at 100°C in an atmosphere of inert gas to cure, giving a sheet-shaped cathode composite on the stainless steel substrate. The thickness of the prepared cathode composite was 50µm average, 54µm maximum and 45µm minimum. Next, 0.05 parts of azobisisobutyronitrile were dissolved in 10 parts of the above electrolyte material and this solution was cast on the cathode composite and cured in the same way as above, so that an electrolyte film was formed on the cathode composite. A thickness of the prepared electrolyte film was 20µm. The composite sheet thus prepared comprising the stainless steel substrate, the cathode composite and the electrolyte film was cut to a square piece 10 x 10mm. Metallic lithium 100µm thick was attached onto the electrolyte film as an anode so as to make up a battery having the Fig 1 structure. A load of 1kg/cm 2 was applied to the prepared battery and a charge/discharge test done under this condition, at 25°C with a constant current of 0.1 mA/cm 2 . An initial capacity of more than or equal to 80% was maintained even after 100 cycles of charging and discharging. The filling rate of active material per volume of cathode composite was 35 vol%. A dispersant compound of the above formula (II) with a molecular weight in the range 10000 to 250000 may be used. Comparison Example 2 A battery was made up in the same way as Embodiment 2 except that the dispersant was not used. Cohering particles of LiMn 2 O 4 and acetylene black formed on the cathode composite of the prepared battery, and there was a part having a thickness reaching 500µm. The thickness of the cathode composite at parts other than where the cohering particles were formed was 58µm average, 66µm maximum and 49µm minimum. A charge/discharge test as in Embodiment 2 was done on the prepared battery, and a capacity after 100 cycles of charging and discharging was smaller than or equal to 50% of the initial capacity. Fig 4 is a diagram showing the charge/discharge cycle characteristic of Embodiment 2 and Comparison Example 2. In Fig 4 A 2 shows the charge/discharge cycle characteristic of Embodiment 2 and B 2 shows the charge/discharge cycle characteristic of Comparison Example 2. As seen, the charge/discharge cycle characteristic of Embodiment 2 is better than that of Comparison Example 2. As described above, since the cathode composites used in the primary and secondary batteries of Embodiments 1 and 2 include the specified compounds functioning as dispersant, the following effects are obtainable. In the cathode composite, the positive active material and the conductive agent are dispersed uniformly and the surface of sheet-shaped cathode composite becomes smooth, without corrugation. As a result the cracking, peeling-off from the substrate and occurrence of short-circuiting can be prevented. Furthermore the utilization factor of positive active material is improved so that the battery capacity can be increased and good discharge and charge/discharge characteristics are shown. Moreover, the filling density of active material per volume of cathode composite is improved so that the energy density per volume of battery can be made better.

Claims: The use, in the manufacture of a cathode composite comprising a mixture of (a) a particulate positive active compound; (b) a particulate conductive agent; and (c) a binder for the positive active agent and conductive agent particles, the binder comprising solid polymeric electrolyte material, of (d), a polar macromolecular compound including pyridine ring, as a dispersant to aid dispersal of the positive active compound and conductive agent particles in the binder before solidification thereof. Use according to claim 1 in which the macromolecular compound (d) is poly-2-vinylpyridine or poly-4-vinylpyridine. Use according to claim 1 or claim 2 in which the particulate positive active compound is selected from MnO 2 , MoO 3 , V 2 O 5 , V 6 O 13 , LiV 3 O 8 , LiMn 2 O 4 , LiCoO 2 , LiCrO 2 and LiNiO 2 . Use according to any one of the preceding claims in which the conductive agent is carbon black. Use according to any one of the preceding claims, further comprising using the cathode composite in the manufacture of a primary or secondary battery.

Inventors: FUKUTOME HIROYUKI, IMACHI HIROSHI, IZUCHI SYUICHI, NAKAJIMA YASUMASA, FUKUTOME, HIROYUKI, IMACHI, HIROSHI, IZUCHI, SYUICHI, NAKAJIMA, YASUMASA
Applicants: YUASA BATTERY CO LTD, YUASA CORPORATION
Application year: 1992
Main classification: H01M4
IPC codes: H01M4, H01M6, H01M10