Electrolytes having improved low temperature performance

Details Electrolytes having improved low temperature performance Electrolytes having improved low temperature performance

1998-11-20

2002-09-03

Chaney, Carol (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

Reexamination Certificate

active

06444370

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 (alkylene 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.
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 Performance
Regardless of which technique is used in preparing an electrolyte/separator, problems occur including operability of the electrolyte in a relatively narrow temperature range; loss of effectiveness of the electrolyte; and electrolyte degradation. There is presently no effective means to maintain the serviceability of the electrolyte over a broad temperature range, particularly low temperature.
In view of the above, it can be seen that it is desirable to have an improved electrolyte which is operable over a relatively broad temperature range, including low temperature, 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 a novel electrolyte solvent which is usable with a variety of carbonaceous and metal oxide electrode active materials, providing improved performance over a broad temperature range, and which is stabilized to maintain cell capacity over a number of cycles. The electrolyte includes a specifically selected class of solvents, and solvent combinations using such new solvents. The new solvents, when used as co-solvents, enhance the operable temperature range of the solvent mixture. The solvents of the invention are esters, generally characterized with lower melting points and higher boiling points compared to the range observed for commonly used solvents, such as dimethyl carbonate or diethyl carbonate. The novel, ester solvents of the invention have further lower melting points and higher boiling points than conventional solvents. The solvents are useful as both high and low temperature solvents but are particularly useful for low temperature applications such as start, light, ignition (SLI). The compounds usable as solvents according to the invention are compounds represented by the general formula R′ COOR″ (alkyl aliphatic ester) where R′ and R″ are each independently selected from the group consisting of ethyl and propyl.
In one embodiment, the ester represented by the general formula is included in a solvent mixture which also comprises ethylene carbonate (EC) and propylene carbonate (PC). In one embodiment, the combined amount of the EC and PC is greater, on a weight basis, than the amount of the ester of the formula stated above.
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