Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Include electrolyte chemically specified and method
Reexamination Certificate
1998-10-28
2002-05-28
Brouillette, Gabrielle (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Include electrolyte chemically specified and method
C429S339000
Reexamination Certificate
active
06395431
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electrolytes which function as a source of alkali metal ions for providing ionic mobility and conductivity. The invention more particularly relates to electrolytic cells where such electrolytes function as an ionically conductive path between electrodes.
BACKGROUND OF THE INVENTION
Electrolytes are an essential member of an electrolytic cell or battery. In one arrangement, a battery or cell comprises an intermediate separator element containing an electrolyte solution through which lithium ions from a source electrode material move between cell electrodes during the charge/discharge cycles of the cell. The invention is particularly useful for making such cells in which the ion source electrode is a lithium compound or other material capable of intercalating lithium ions, and where an electrode separator membrane comprises a polymeric matrix made ionically conductive by the incorporation of an organic solution of a dissociable lithium salt which provides ionic mobility.
Early Lithium Metal Cells
Early rechargeable lithium cells utilized lithium metal electrodes as the ion source in conjunction with positive electrodes comprising compounds capable of intercalating the lithium ions within their structure during discharge of the cell. Such cells relied, for the most part, on separator structures or membranes which physically contained a measure of fluid electrolyte, usually in the form of a solution of a lithium compound, and which also provided a means for preventing destructive contact between the electrodes of the cell. Sheets or membranes ranging from glass fiber, filter paper or cloth to microporous polyolefin film or nonwoven organic or inorganic fabric have been saturated with solutions of an inorganic lithium compound, such as LiClO
4
,LiPF
6
, or LiBF
4
, in an organic solvent to form such electrolyte/separator elements. The fluid electrolyte bridge thus established between the electrodes has effectively provided the necessary Li+ ion mobility at conductivities in the range of about 10
−3
S/cm.
Ion, Rocking Chair Cells and Polymer Cells
Lithium metal anodes cause dendrite formation during charging cycles which eventually leads to internal cell short-circuiting. Some success has been achieved in combatting this problem through the use of lithium-ion cells in which both electrodes comprise intercalation materials, such as lithiated metal oxide and carbon (U.S. Pat. No. 5,196,279), thereby eliminating the lithium metal which promotes the deleterious dendrite growth. Another approach to controlling the dendrite problem has been the use of continuous films or bodies of polymeric materials which provide little or no continuous free path of low viscosity fluid in which the lithium dendrite may propagate. These materials may comprise polymers, e.g., poly(alkene oxide), which are enhanced in ionic conductivity by the incorporation of a salt, typically a lithium salt such as LiClO
4
, LiPF
6
, or the like. A range of practical ionic conductivity, i.e., over about 10
−5
to 10
−3
S/cm, was only attainable with these polymer compositions at well above room temperature, however. (U.S. Pat. Nos. 5,009,970 and 5,041,346.)
“Solid” and “Liquid” Batteries of the Prior Art
More specifically, electrolytic cells containing an anode, a cathode, and a solid, solvent-containing electrolyte incorporating an inorganic ion salt were referred to as “solid batteries”. (U.S. Pat. No. 5,411,820). These cells offer a number of advantages over electrolytic cells containing a liquid electrolyte (i.e., “liquid batteries”) including improved safety factors. Despite their advantages, the manufacture of these solid batteries requires careful process control to minimize the formation of impurities. Solid batteries employ a solid electrolyte matrix interposed between a cathode and an anode. The inorganic matrix may be non-polymeric [e.g., &bgr;-alumina, silver oxide, lithium iodide, etc.] or polymeric [e.g., inorganic (polyphosphazene)polymers] whereas the organic matrix is typically polymeric. Suitable organic polymeric matrices are well known in the art and are typically organic polymers obtained by polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283.
Examples of solvents known in the art are propylene carbonate, ethylene carbonate, &ggr;-butyrolactone, tetrahydrofuran, glyme (dimethoxyethane), diglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane, diethoxyethane, and the like. These are examples of aprotic, polar solvents.
Heretofore, the solid, solvent-containing electrolyte has typically been formed by one of two methods. In one method, the solid matrix is first formed and then a requisite amount of this material is dissolved in a volatile solvent. Requisite amounts of the inorganic ion salt and the electrolyte solvent (usually a glyme and the organic carbonate) are then added to the solution. This solution is then placed on the surface of a suitable substrate (e.g., the surface of a cathode) and the volatile solvent is removed to provide for the solid electrolyte. In another method, a monomer or partial polymer of the polymeric matrix to be formed is combined with appropriate amounts of the inorganic ion salt and the solvent. This mixture is then placed on the surface of a suitable substrate (e.g., the surface of the cathode) and the monomer is polymerized or cured (or the partial polymer is then further polymerized or cured) by conventional techniques (heat, ultraviolet radiation, electron beams, etc.) so as to form the solid, solvent-containing electrolyte. When the solid electrolyte is formed on a cathodic surface, an anodic material can then be laminated onto the solid electrolyte to form a solid battery (i.e., an electrolytic cell).
More recently, a highly favored electrolyte/separator film is prepared from a copolymer of vinylidene fluoride and hexafluoropropylene. Methods for making such films for cell electrodes and electrolyte/separator layers are described in U.S. Pat. Nos. 5,418,091; 5,460,904; and 5,456,000 assigned to Bell Communications Research, each of which is incorporated herein by reference in its entirety. A flexible polymeric film useful as an interelectrode separator or electrolyte member in electrolytic devices, such as rechargeable batteries, comprises a copolymer of vinylidene fluoride with 2 to 25% hexafluoropropylene. The film may be cast or formed as a self-supporting layer retaining about 20% to 70% of a high-boiling solvent or solvent mixture comprising such solvents as ethylene carbonate or propylene carbonate. The film may be used in such form or after leaching of the retained solvent with a film-inert low-boiling solvent to provide a separator member into which a solution of electrolytic salt is subsequently imbibed to displace retained solvent or replace solvent previously leached from the polymeric matrix.
Electrolyte Breakdown
Regardless of which technique is used in preparing an electrolyte/separator, a recurring problem has been the loss of effectiveness of the electrolyte. The electrolyte has been observed to change color, evidencing a degradation that has not been well understood. There is presently no effective means to maintain the useful serviceability of the electrolyte.
In view of the above, it can be seen that it is desirable to have a novel, economical means for maintaining electrolyte integrity; and which maintains cell capacity in a variety of electrolyte/separator configurations, including those described above as exemplary.
SUMMARY OF THE INVENTION
The present invention provides an additive for an electrolyte solution of an electrochemical cell. The additive provides an electrolyte solution stabilized against decomposition during storage and during cyclic operation of an electrochemical cell. The additive is a dialkylamide, desirably a N,N-dialkylamide, and preferably is N,N-dimethylacetamide (DMAC). Advantageously, the additive prevents undesired decomposition of cell components, and particularly electrolyte
Barker Jeremy
Gao Feng
Kelley Tracy
Scordilis-Kelley Chariclea
Shi Hang
Brouillette Gabrielle
Harness & Dickey & Pierce P.L.C.
Tsang Susy
Valence Technology Inc.
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