Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-04-28
2002-06-11
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231800, C429S224000, C429S220000, C423S415100, C423S448000
Reexamination Certificate
active
06403261
ABSTRACT:
TECHNICAL FIELD
The present invention relates in general to electrochemical cells and more specifically to a carbon-containing material for electrodes of electrochemical cells and to a method of making porous electrodes from said material.
The herein-proposed invention can find application in establishing novel carbon-containing energy-saturated electrode materials made use of in electrochemical cells, largely in 1.5- and 3-volt lithium cells.
BACKGROUND ART
There exists at present a problem in electrochemical cells using solid-state electrodes how to provide high energy-capacity of electrodes and simultaneously high operating density of discharge current. The problem concerns predominantly cathodes because anodes of electrochemical cells based on active metals, ie., lithium, sodium, zinc, and others are most frequently used in constructions of electrochemical cells in their compact state, that is, with their density which is realized to a maximum extent in anodes and is equal to or approximates their pycnometric density realizable in crystal lattices of said materials.
To realize maximum characteristics of electrochemical cells as to energy capacity thereof, it is necessary to use electrode-active materials having high specific weight and volume characteristics with a maximum possible filling of the electrode construction with an electrode-active material. However, a higher electrode density due to a more complete filling of the electrode volume with an electrode-active material reduces the electrode surface available for a current-generating reaction to proceed, whereby the effective density of the electrode discharge current as a whole is adversely affected.
On the other hand, the effective density of the electrode discharge current can be increased due to a larger effective electrode surface attainable by a finer dividing of the electrode components and adding to its composition such materials that enhance the electrode conductance. This in turn results in higher electrode porosity but also leads to a lower content of the principal electrode component, that is, an electrode-active material.
Therefore resolving the problem of creating a high energy-capacity electrochemical cell featuring a larger discharge current density is inevitably concerned with a search for compromise between reduction in a total energy capacity of the electrode and an increase in its porosity and electrical conductance with the electrode volume remaining invariable.
The electrode of an electrochemical cell is usually comprised of a mixed composition consisting of an electrode-active material, a binder, and an electrode conductance increasing material. To establish porous structure in electrodes use is made of pore-formation materials (expanding agents) in the capacity of which such substances and materials are applied that are liable to dissolve or volatile upon physico-chemical treatment of preformed electrodes. As a result, a porous structure is established in the electrode, required for lodging therein electrolyte and depositing solid products of the current-producing reaction (cf. a textbook “Electrochemical cells with lithium electrode” by I. A. Kedrinski et al., Krasnoyarsk University Publishers, Krasnoyarsk, 1983, pp. 248, 144-147 (in Russian) [1].
Known in prior art are a variety of expanding agents such as expanded granulated graphite (expandate) (cf. “Active mass of positive electrode of a primary cells” by Peter Faber. USSR Inventor's Certificate # 488,432, IPC Holm 13/02, 21/00, published on Oct. 15, 1975 in Bulletin # 38 [2]; “Positive electrode of an electrochemical cell” by B. K. Makarenko et al., USSR Inventor's Certificate # 564,668, IPC Holm 4/98, 6/14, published on Jul. 5, 1977 in Bulletin # 25 [3]); some organic substances, e.g., camphor (cf. French Patent # 2,093,287, IPC Holm 13/00, 1972 [4]) or soluble inorganic compounds such as potassium hexafluorophosphate (cf. “Process for producing porous electrode for an electrochemical cell with nonaqueous electrolyte” by N. S. Lidorenko et al., USSR Inventor's Certificate # b
527
,
775
, IPC Holm 4/62, published on Sep. 5, 1976 in Bulletin # 33 [5]). Used for the same purpose are also insoluble inorganic compounds and materials featuring a porous structure per se, i.e., zeolites (cf. “An electric cell with organic electrolyte” by N. Watanabe et al., Japan Patents 61-264,679, 61-264,680, 61-264,682, 61-264,681, IPC Holm 6/16 of May 20, 1985 [6]), or activated alumina(cf. “Element of the lithium—fluorocarbon system” by Suetsugu Satiko, Japan Patent N 63-334,457, IPC Holm 4/06 of Dec. 28, 1988 [7]). All the materials mentioned above when introduced in the composition are liable to improve discharge characteristics; however, this reduces the electrode density, with the result that the capacitance of the electrochemical cell is reduced due to a lower content of the electrode-active material.
Methods for making porous cathodes for electrochemical cells boils down to a combination of sequences of the steps of preparing, mixing, and treating the parent components, a most frequently used practice is to mix all the prepared ingredients in a single step (cf. references 1 to 7), in the presence of water or organic solvents, whereupon the semifinished items of a cathode material are isolated, dried, and disintegrated. Then the cathodes are formed, provided with current leads (by, e.g., pressing them into cathode casings) are subjected either to washing out the expanding agent with an appropriate solvent or to thermal treatment for removing the sublimating expanding agents from the cathode. As a result, a porous structure necessary for normal operation of an electrochemical cell is established in the cathode (cf. references 2, 4, 5). However, porous cathodes of electrochemical cells, wherein used as expanding agents are such soluble compounds as camphor [4] or potassium hexafluorophosphate [5] are rather hard to be washed out from an expanding agent. This is concerned with the fact that the particles of the spent expanding agent are occluded during the cathode forming process with a binder or an energy carrier, which affects adversely the properties of a cathode and of an electrochemical cell based thereon. For that reason many times repeated procedures are used for completely removing the expanding agent, which involves the use of further amounts of solvents. In addition, use of said materials and methods of making electrodes for electrochemical cells renders impossible realizing higher energy characteristics in a lithium electrochemical cell having a porosity of 35-50% adequate for serviceability of an electrochemical cell, because porous cathodes of electrochemical cells, wherein used as expanding agents is zeolite [6] or activated alumina [7] feature a reduced weight and volume energy capacity, that is, such expanding agents fail to exhibit electrical activity being therefore no more than useless ballast.
Use of a known active cathode mass in electrochemical cell electrodes which contains a metal oxide (such as manganese dioxide, lead monoxide or mixtures thereof) as an electrode-active material, and graphite expandate [2] as an electrical conductance increasing agent, affects adversely the electrode specific energy capacity. Graphite expandate (expanded graphite) serves in cathode material also as an expanding agent due to its low density (0.007-0.05 g/cu.cm), large specific surface [2], and high porosity. Used for making electrode material is a natural graphite expandate of the coral-like structure which makes up to 25% of the weight of energy carrier. The coral-like structure of the graphite expandate is realized in case of a separately conducted thermolysis of various graphite compounds, such as fluorinated graphite [3]. Adding graphite expandate to the electrode composite is an inconvenient procedure due to its material being a badly dusting one.
One prior-art
Afanasiev Vladimir Leonidovich
Alexandrov Alexandr Borisovich
Galitsky Alexandr Anatolievich
Judanov Nikolai Fedorovich
Mitkin Valentin Nikolaevich
Nexsen Pruet Jacobs & Pollard LLC
O'Toole J. Herbert
Weiner Laura
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