Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-12-29
2002-11-19
Gupta, Yogendra N. (Department: 1751)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
Reexamination Certificate
active
06482545
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to the field of nonaqueous electrolytes and electric current producing cells. More particularly, this invention pertains to non-aqueous electrolytes which comprise (a) one or more solvents; (b) one or more ionic salts; and, (c) a multifunctional monomer. When incorporated into a nonaqueous electrolyte, the multifunctional reactive monomer improves the safety of electric current producing cells by rapidly polymerizing at elevated temperatures to increase the viscosity and internal resistivity of the electrolyte. The present invention also pertains to electric current producing cells comprising such non-aqueous electrolytes, methods of making such non-aqueous electrolytes and electric current producing cells, and methods for increasing the safety of an electric current producing cell.
BACKGROUND
Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
Electric current producing cells, and batteries containing such cells, consist of pairs of electrodes of opposite polarity separated by an electrolyte. The charge flow between electrodes is maintained by an ionically conducting electrolyte.
Successful use of batteries depends on their safety during operation under normal conditions and even under abusive usage. An abusive use such as short circuiting or rapid overcharging of the battery may initiate self-heating of the battery, as opposed to merely resistive heating, leading to thermal runaway. Self-heating of the battery is especially problematic when a lithium anode is utilized. Lithium batteries are capable of much higher power storage densities than batteries not based on lithium, but the reactivity of lithium to most materials and its melting point of only about 180° C. promote self-heating and the potential for thermal runaway. This reactivity of lithium to the electrolyte and other materials of the battery is typically increased in secondary or rechargeable lithium cells upon the cycling of the lithium. The main processes causing self-heating of a secondary lithium cell typically involve the chemical reaction between cycled lithium and electrolyte solvent. These self-heating reactions are believed to be initiated at temperatures near 100° C. At temperatures greater than 100° C., additional contributions to cell self-heating are believed to come from exothermic decomposition of the electrolyte as well as from reactions between lithium and the ionic salt in the electrolyte.
One approach to reduce the potential of unsafe explosive conditions with lithium batteries has been to use blends of electrolyte solvents which have a lower reactivity with lithium. For example, U.S. Pat. No. 4,753,859 to Brand et al. describes the use of one or more polyethylene glycol diallyl ethers with ethylene carbonate and propylene carbonate as the electrolyte solvents to improve the safety characteristics of a nonaqueous lithium cell. Also, for example, U.S. Pat. No. 5,219,684 to Wilkinson et al. describes the use of sulfolane and a glyme as the electrolyte solvents for improved safety in electrochemical cells with a lithium-containing anode and a cathode with a lithiated manganese dioxide as the active material. Although it is beneficial to safety to select the electrolyte solvent composition to reduce the reactivity with the lithium, solvent choice alone does not provide strong protection against overheating and thermal runaway. Among the disadvantages of this approach of using less reactive electrolyte solvents are that the battery is not shutdown or reduced in current flow at high temperatures, but is still capable of high energy electrochemical reactions leading to more heat buildup and degradation reactions which may eventually result in an explosive condition. Also, the electrolyte solvent composition may not be compatible with the specific cathode composition used in the cell. During the discharging and charging of electrochemical cells, many electrochemically reduced and oxidized compounds are formed which may not be stable or otherwise compatible with the electrolyte solvents. For example, solid sulfur-based cathodes are very desirable for use with lithium-containing anodes because of the extremely high power density of this combination of electroactive materials, but the organic and inorganic polysulfides typically formed in the discharge or reduction cycle of these nonaqueous lithium-sulfur type batteries may not be compatible with one or more solvents, such as ethylene carbonate, in the electrolyte solvent blend selected for safety.
Another approach to improve the safety of electrochemical cells has been to incorporate a PTC (positive temperature coefficient) device that has increased electrical resistance at high temperatures and thereby suppresses the flow of current through the cell as, for example, described in U.S. Pat. No. 4,971,867 to Watanabe et al. and U.S. Pat. No. 5,008,161 to Johnston. PTC devices may provide useful safety protection against internal short circuits and against overcharge and overdischarge conditions, but they are not adequate to substantially shutdown or reduce the activity of the reactive chemistry in a cell.
To control overheating under abusive usage, it has been suggested that a thermally activated separator be developed for insertion between the cathode and the anode. It has been further suggested that a microporous sheet might function as this thermally activated separator if it exhibited low resistivity at normal operating temperatures but irreversibly transformed into a product having high resistivity at high temperatures, while maintaining its dimensional integrity. For example, U.S. Pat. No. 4,650,730 to Lundquist et al. describes a multi-ply microporous sheet useful as a battery separator having at least two plies with different transformation temperatures for forming a substantially non-porous sheet at elevated temperatures. Microporous polymeric films presently employed as separators in lithium batteries are generally not capable of preventing uncontrolled heating. In general, polymeric separators degrade, to one extent or another, under the influence of heat and thermal reactions, or become dimensionally unstable, and they do not substantially reduce or shutdown the activity of the reactive chemistry of a cell.
It has been suggested by Laman et al.,
J. Electrochem. Soc
., 1993, 140, L51 to L53, that for a separator to function well as an internal safety device in a lithium battery, it should have the following characteristics: a melting point close to 100° C. for the low melting point component, a high dimensional stability temperature preferably above the melting point of lithium, and a high degree and rate of shutdown, giving rise to an impedance increase of at least three orders of magnitude with an increase of a few degrees Celsius in temperature. They note that it is difficult to achieve these properties in a single separator and that obtaining all these characteristics can be more easily achieved by combining different separators. Using a combination of different polymeric separators, especially when the surface area of separators required in the battery is very large, significantly increases the expense of producing the battery, as well as reducing the volume available for electroactive material, thereby reducing the specific capacity of the cell.
To overcome the safety disadvantages of conventional polymeric films as battery separators, one approach has been to add a thermal fuse material which melts at high temperatures to the polymeric film so that the thermal fuse material melts and irreversibly reduces the porosity of the microporous polymeric films, thereby interrupting the electrochemical reaction in the, battery. For example, U.S. Pat. No. 4,973
Gavrilov Alexei B.
Gorkovenko Alexander A.
Kovalev Igor P.
Skotheim Terje A.
Carlson Steven A.
Gupta Yogendra N.
Moltech Corporation
Nicol Jacqueline M.
Petruncio John M
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