|Publication number||USH1576 H|
|Application number||US 08/315,213|
|Publication date||Aug 6, 1996|
|Filing date||Mar 7, 1994|
|Priority date||Mar 7, 1994|
|Publication number||08315213, 315213, US H1576 H, US H1576H, US-H-H1576, USH1576 H, USH1576H|
|Inventors||Charles W. Walker, Jr., Edward J. Plichta, Wishvender K. Behl|
|Original Assignee||The United States Of America As Represented By The Secretary Of The Army|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (8), Referenced by (2), Classifications (13)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to us of any royalties thereon.
The invention relates to solid polymer electrolytes having an increased conductivity for use in solid state polymer electrolyte batteries, and to solid state batteries including the electrolytes.
Solid polymer electrolytes (SPEs) containing dissolved metal salts have been proposed as alternatives to liquid electrolytes in electrochemical systems. There are many advantages to using a solid electrolyte, such as the capability for high speed production of thin cells constructed in a bipolar configuration. Further, the polymer electrolyte can act as a mechanical barrier between the anode and cathode thereby eliminating the need for an inert porous separator as well as acting as a binder/adhesive to move and conform to electrode volume changes during cycling. The polymer electrolytes also allow and facilitate the fabrication of cells in any geometric shape and also provide an inherent safety advantage over liquid electrolytes since there is no liquid component in the cell to leak out if the integrity of the sealed cell is broken.
One of the most commonly used polymer electrolytes is based on high molecular weight polyethylene oxide (PEO). An ionically conducting solid polymer electrolyte can be prepared by dissolving PEO and an appropriate salt such as lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium trifluoromethanesulfonate (LiCF3 SO3) or lithium hexafluoroarsenate (LiAsF6) in a suitable volatile solvent such as acetonitrile (CH3 CN). By solution casting, acetonitrile is removed by evaporation, leaving a free standing solid, flexible film of good mechanical strength that contains only PEO with dissolved salt. Such films are ionic conductors.
However, because of the poor ionic conductivity of these polymers of about 10-7 S/cm, these polymers are not practical as electrolytes for electrochemical cells and particularly, rechargeable cells.
The general object of this invention is to provide solid polymer electrolytes having an increased conductivity. A more particular object of the invention is to improve the ionic conductivity of a typical polymer such as PEO with dissolved salt such as LiClO4, LiBF4, LiCF3 SO3 or LiAsF6 so that it can be used as an electrolyte in solid state electrochemical cells.
It has now been found that the aforementioned objects can be attained by incorporating particles of a solid solution of lithium germanium oxide (Li4 GeO4) and lithium vanadium oxide (Li3 VO4) and having the general formula, Li3+x Gex V1-x O4, where 0.2<x<0.8 in the PEO-lithium salt polymer electrolyte.
A solid solution with the composition, Li3.6 Ge0.6 V0.4 O4 is prepared by firing a 2.3 cm pellet of a mixture of 1.33 gms of lithium carbonate, 0.628 gm of germanium oxide and 0.364 gram of vanadium pentoxide that is pressed to a pressure of 6800 kg and placed on a gold foil in a ceramic crucible. The pellet is fired at 600° C. for 20 hours to evolve carbon-dioxide followed by heating to 900° C. for 20 hours. The fired pellet is quenched in air at room temperature and ground to a fine powder and stored in an argon filled dry box having a moisture content of less than 0.5 ppm.
The polymer electrolyte films are prepared by dissolving PEO having an average molecular weight of 4×106, dried at 50° C. under vacuum overnight and LiCF3 SO3 that has been dried at 50° C. under vacuum in molar ratio of 20:1 respectively in acetonitrile that has been distilled under a stream of dry argon with stirring in an argon filled dry box having a moisture content of less than 5 ppm. Ten weight percent of the lithium ion conducting powdered solid material Li3.6 Ge0.6 V0.4 O4 is then added to this solution with vigorous stirring. Films are cast by pouring the solution into flat Teflon dishes. After the solvent is completely evaporated, free-standing films of 50 to 100 μm in thickness are peeled from the dishes. The conductivity of the film is then measured by placing the film between stainless steel blocking electrodes and measuring the conductivities using the AC impedance technique in the 5 Hz to 100 kHz frequency range with an EG&G PAR Model 388 Electrochemical impedance system.
The conductivity of the PEO-LiCF3 SO3 films prepared with only 10 weight percent lithium ion conducting solid ceramic additive, Li3.6 Ge0.6 V0.4 O4 is found to be 10 times higher than the films prepared without the additive at 40° C. The amount of ceramic material additive contained in the polymer electrolyte may be varied between 0 and 100 weight percent.
When the improved solid polymer electrolyte of the invention is included in an electrochemical cell, either primary, or rechargeable, the anode of such a cell might be lithium metal, lithium alloy, LiC6, lithiated graphite, or lithiated petroleum coke. Similarly, the cathode of such a cell might be LiCoO2, Ag2 CrO4, CuO, Bi2 O3, Bi2 Pb2 O5, Cr2 O5, Cr3 O8, MnO2, Ni3 S2, TiS2, FeS2, VSe2, NiS2, CoS2, V6 O13, V2 O5, LiNiO2, LiMnO2, CuF2,(CF)n, CuCl2, AgCl, Crx V1-x S2 where x has a value from 0 to 1.
A particular solid state electrochemical cell according to the invention includes lithium as the anode, LiCoO2 on an aluminum foil current collector as the cathode, and (PEO)20 (LiCF3 SO3) containing 10 weight percent Li3.6 Ge0.6 V0.4 O4 as the solid polymer electrolyte. The cycling conditions include a temperature of 66° C. and a charge and discharge at 0.01 mA cm-2 rate between 4.2-2.4 V. The capacity for cycle 1 is 85.9 mAhg-1 ; for cycle 2 is 74.5 mAhg-1 ; and for cycle 3 is 53.5 mAhg-1.
We wish it to be understood that we do not des0ire to be limited to the exact details of construction shown and described for obvious modifications will occur to a person skilled in the art.
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|Citing Patent||Filing date||Publication date||Applicant||Title|
|US5695873 *||Jun 5, 1995||Dec 9, 1997||The University Of Dayton||Polymer-ceramic composite electrolytes|
|WO1999039399A1 *||Jan 26, 1999||Aug 5, 1999||Valence Technology, Inc.||Polymer electrolytes containing lithiated fillers|
|U.S. Classification||429/312, 429/306, 429/310|
|International Classification||H01M10/052, H01M10/0565, H01M6/18|
|Cooperative Classification||H01M10/0565, H01M10/052, H01M6/185, Y02E60/122|
|European Classification||H01M10/052, H01M10/0565, H01M6/18D|