Solid polymer electrolytes

Details Solid polymer electrolytes Solid polymer electrolytes

1999-09-02

2003-11-11

Gulakowski, Randy (Department: 1746)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

C429S306000, C429S309000, C429S310000, C429S312000, C429S313000, C429S314000, C429S316000, C429S319000, C429S320000, C429S321000, C429S322000, C429S323000, C429S233000, C429S234000, C429S162000, C029S623100, C029S623500, C252S062200

Reexamination Certificate

active

06645675

ABSTRACT:

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to the manufacture of solid state polymer electrolytes. More particularly, the invention relates to highly conductive thin lithium polymer electrolyte structures and the methods by which they are made. The invention also relates to methods of manufacturing all-solid-state electrochemical cells based on such electrolytes.
2. Description of the Related Art
Throughout the world there are ongoing efforts to develop an all solid state, rechargeable, high energy density battery using a thin polymer film as the electrolyte. Since the concept of such a battery, based on the use of polyethylene oxide/lithium salt complexes, was first discussed in 1979 by Armand et al. in Fast Ion Transport in Solids (eds. P. Vashista et al., North-Holland, Amsterdam, p.131), development has mainly centered around rechargeable systems utilizing intercalation/insertion compounds. Some of the more recent work has focused on designing novel polymers with higher ionic conductivities at ambient temperature. Solid state battery and capacitor technology, including the evolution of polymer electrolytes, is discussed in the Handbook of Solid State Batteries & Capacitors, M. Z. A. Munshi, ed., World Scientific Publishing, Inc., Singapore, which is incorporated herein by reference.
At the present time, the state-of-the-art lithium battery is a lithium ion battery which uses a carbon electrode as the negative electrode or anode and a lithiated metal oxide, such as lithiated cobalt oxide, lithiated nickel oxide, lithiated manganese oxide, or mixtures of these materials as the positive electrode or cathode, a microporous polypropylene or polyethylene separator that separates the two electrodes and prevents them from shorting electrically, and liquid organic solvents containing a lithium salt as the electrolyte. The electrolyte is usually absorbed into the separator material and provides high ionic conductivity (10
−3
to 10
−2
S/cm) and migration of ions between the electrodes of the cell. An offshoot of the lithium ion battery is the lithium ion polymer electrolyte battery. The electrode chemistry of this battery is the same; however in this case the liquid electrolyte (up to 70% by weight of the electrolyte) is absorbed in a polymer membrane instead of the microporous polypropylene separator.
Another type of rechargeable lithium battery system sometimes used today employs a lithium metal anode. Secondary batteries using lithium metal as the negative electrode, intercalation or insertion compounds as the positive electrode, and non-aqueous organic electrolytes were the focus of intense investigation during the 1970's and 1980's. One problem, however, with using lithium in a rechargeable system is that because of the instability of lithium in these solvents, a large excess of lithium is required to off-set the chemical reaction of lithium with the solvent, usually as much as 3 to 5 times more lithium capacity than the cathode capacity. In addition, the liquid solvent electrolyte employed in any of the above-described cell systems is often corrosive and toxic, and presents handling difficulties due to spillage or leakage from the cell. Liquid solvent electrolyte can also release gas, or outgas, during overcharge or overdischarge or at elevated temperatures, leading to safety problems.
In order to overcome the disadvantages inherent in liquid electrolytes and to obtain better long-term storage stability there is interest in developing solid polymeric electrolytes in which ion mobility is achieved through coordination by sites on the polymer chain of electrolyte ions, thus promoting electrolyte dissolution and salt dissociation. An all-solid-state battery using an ionically conductive polymer membrane as the electrolyte would have several attractive features. It could be made into virtually any shape and size, be reasonably rugged and leakproof, and have low self-discharge. It could be made into thin film power cells or thick film energy cells, would have high open-circuit potentials using a lithium anode, and could be produced by automated fabrication techniques. These features represent a unique combination of properties and give rise to the possibility of using such batteries, as either secondary or primary devices across a wide range of applications.
In an attempt to develop all-solid-state polymer electrolyte, one polymer that has been examined extensively is polyethylene oxide (PEO), which is able to form stable complexes with a number of salts. Because of its low ionic conductivity at ambient temperature of approximately 10
−9
to 10
−8
S/cm, batteries examined using this material had to operate at 100° C. and above. A major problem with PEO based electrolytes at temperatures below 60° C. is their high crystallinity and the associated low ion mobility. In recent years a number of radically different approaches have been taken to improve the conductivity of PEO and PEO-based polymers that have also led to the proposal of other polymers. These approaches included polymer modifications and synthesizing new polymers; forming composite polymers with ceramic materials; using plasticizer salts to increase the ion transport and mobility of the cation; using plasticizing solvents in the polymer again to increase the ionic character of the cation; among other approaches. Several review articles describe these approaches in detail, e.g. “Technology Assessment of Lithium Polymer Electrolyte Secondary Batteries” by M. Z. A. Munshi, Chapter 19 in Handbook of Solid State State Batteries and Capacitors, Ed. M. Z. A. Munshi (World Scientific Pub. Singapore) 1995; A. Hooper, M. Gauthier, and A. Belanger, in: “Electrochemical Science and Technology of Polymers—2, Ed. R. G. Linford (Elsevier Applied Science, London), 1987.
Polymer modification and synthesis of new polymers resulted in some improvement in the ionic conductivity but the mechanical property and integrity were poor. Probably, the best known polymer as a result of this synthesis is poly(bis(methoxyethoxyethoxide))-phosphazene, known as MEEP, which has an ionic conductivity of approximately 10
−5
S/cm at room temperature when combined with a lithium salt, but which has glue-like mechanical properties. On the other hand, materials based on blocked copolymers may provide alternatives. For example, PEO-PPO-PEO crosslinked with trifunctional urethane and a lithium salt has an ionic conductivity of approximately 10
−5
S/cm but is too rigid, brittle and difficult to manufacture.
Inorganic conducting and non-conducting fillers have also been used to increase the ionic conductivity and mechanical property of the polymer. Addition of alpha alumina to (PEO)
8
.LiClO
4
in resulted in a negligible effect on the ionic conductivity but dramatically increased the mechanical property at 100° C., while the addition of other ceramic materials such as ionically conductive beta alumina to PEO-NaI and PEO-LiClO
4
complexes improved the ionic conductivity of PEO based electrolytes to approximately 10
−5
S/cm. In another battery technology, inorganic fillers based on high surface area alumina and silica have been used to enhance the ionic conductivity of lithium iodide solid electrolyte from 10
−7
S/cm to 10
−5
-10
−4
S/cm at room temperature (see C. C. Liang,
J. Electrochemical Society
, Vol. 120, page 1289 (1973)). Plasticizer salts based on lithium. bis(trifluoromethane sulfonyl)imide, LiN(CF
3
SO
2
)
2
trademarked as LiTFSI by Hydro-Quebec and distributed by the 3M Company under the product name, HQ-115 when added to PEO yields a conductivity of about 10
−5
S/cm.
None of the previous approaches toward improving polymer conductivity has resulted in adequate conductivity enhancements of the polymer electrolytes to permit room temperature operation of batteries utilizing the electrolyte. Accordingly, an attempt was made to increase the

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