Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Patent
1996-08-29
1999-03-09
Nuzzolillo, Maria
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
H01M 460
Patent
active
058798366
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a lithium battery in which carbon fibrils are used as an anode and/or cathode in a lithium secondary battery.
BACKGROUND OF THE INVENTION
A great deal of research has been conducted on lithium secondary batteries in which metallic lithium is used as the negative electrode because high voltage and high energy density are anticipated. A family of batteries, called variously lithium ion or rocking chair or swing systems, has been developed which include a carbon anode into which lithium intercalates. The ability to intercalate lithium is critical. Classically this requires a relatively good graphite structure. The intercalate is a true compound corresponding to a composition of C.sub.6 Li. It thus has safety and other advantages over a metallic lithium anode. However, the problem with batteries in which metallic lithium is used for the negative electrode is that cycle life is decreased because of reactions between the lithium and the solvent during charging and because of dendrite growth.
In order to eliminate this problem, studies have been conducted of Li-Al alloy and of various types of carbon fiber materials as negative electrode materials. However, new problems such as decrease in mechanical strength and deterioration of self-discharging characteristics have arisen with these materials.
As to the "rocking chair" cell, lithium is the only ionic species moving back and forth between the electrodes during charging and discharging. Specifically, during charging, intercalated lithium ions from the cathode e.g., LiMn.sub.2 O.sub.4, move through the electrolyte to the anode where they pick up an electron in the process of intercalating the carbon. During discharge the reverse reaction takes place, i.e., the uncharged lithium in the carbon loses an electron to the external circuit as it ionizes to Li+ which migrates to and enters the cathode concurrent with local reduction of the oxide lattice by an electron from the external circuit. The half cell reaction are shown below doing discharge.
The prior art lithium ion system is characterized by an intercalatable carbon anode and an intercalatable variable valence metal oxide cathode usually also admixed with carbon for conductivity. It is the use of the carbon anode that sets the rocking chair system apart from earlier configurations using lithium metal anodes. These latter suffered from safety problems related to dendrite formation, flaking and spalling of lithium metal, leading, in turn to dangerously high reaction rates and to side reactions with the electrolyte. These safety problems have been largely sidestepped by the absence of a bulk lithium phase.
Development of new more oxidation resistant electrolytes and, longer range, the development of polymer electrolytes also contributes to safer Li cells. Nevertheless, these batteries utilize very reactive electrode materials and systems. Studies of even the Li intercalation electrode suggest it must be properly engineered to avoid runaway reaction under damage or short circuit conditions.
Although both electrodes of the lithium ion cell use carbon, these carbons, like most electrode materials, were selected from those commercially available. That neither anode nor cathode performance was satisfactory is seen from the poor power density and from the poor approach to theoretical energy density. The lithium ion battery as presently available has commercial utility only because of its inherent thermodynamics and rather in spite of less than optimal kinetics and engineering.
A target use for fibrils is electrodes and current collectors. Fibrils are ca. 100 .ANG. diameter, catalytically grown, graphitic fibers, typically several microns long. While fibrils are graphitic, geometric constraints force some differences with pure graphite. Like graphite, fibrils are composed of parallel layers of carbon but in the form of a series of concentric tubes disposed about the longitudinal axis of the fibers rather than as multi-layers of flat graphite sheets. Thus, because of the
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Hausslein Robert
Hoch Robert
Ikeda Hiroharu
Boyd, Esq. John E.
Chaney Carol
Evans, Esq. Barry
Hyperion Catalysis International Inc.
Nuzzolillo Maria
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