Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Include electrolyte chemically specified and method
Reexamination Certificate
2000-07-28
2003-04-08
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Include electrolyte chemically specified and method
C429S320000, C429S322000, C252S062200
Reexamination Certificate
active
06544690
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to composite polymer-ceramic material for application as solid state battery electrolytes and, more specifically, to solid polymer electrolytes which are formed from the condensation of ceramic precursors in the presence of a polymer material utilizing a condensation agent comprised of a cation amenable to SPE applications.
2. Description of the Related Art
A battery typically comprises one or more electrochemical cells connected in series, parallel, or both, depending on desired output voltage and capacity. Each cell principally comprises an anode, a cathode, and an electrolyte. The electrolyte serves as the ionic conductor and provides the medium for the transfer of ions inside the cell between the anode and the cathode, and typically comprises liquid, solid, or gel materials. Some batteries are intended for single use, and once discharged are discarded (commonly termed as primary batteries). Other batteries are more readily designed to be recharged essentially to their original condition upon discharge (commonly termed as secondary or rechargeable batteries). During discharge, ions from the anode pass through the liquid electrolyte to the electrochemically active material of the cathode where the ions are taken up with the simultaneous release of electrical energy. During charging, the flow of ions is reversed so that ions pass from the electrochemically active cathode material through the electrolyte and are plated back onto the anode.
Solid polymer electrolytes are useful in numerous applications such as solid state batteries, supercapacitors, fuel cells, sensors, electrochromic devices and the like. Solid polymer electrolytes have been proposed in the past for use in place of liquid electrolytes in such equipment because they combine in one material the function of electrolyte, separator, and binder for the electrode materials, thereby reducing the complexity of the ultimate structure. The advantages inherent in the use of a solid polymer electrolyte (SPE) are the elimination of possible leakage and the preclusion of the possibility of dangerous increases in pressure which sometimes occur when volatile liquid electrolytes are present. Further, such SPEs can be fabricated as thin films which permit space efficient batteries to be designed. Also, flexible solid polymer electrolytes can be fabricated which allow for volume changes in the electrochemical cell without physical degradation of the interfacial contacts.
Development of useful all-solid state batteries requires significant improvement of solid polymer electrolyte materials. Foremost, the SPE must be an excellent conductor of ions at ambient temperatures, as high internal resistance is the most pressing problem in SPE batteries today. Current organic SPE systems are poor ion conductors at ambient temperatures and the most common strategy employed to combat this problem is to use small organic molecules as additives. See, for example, Abraham, et al., U.S. Pat. No. 5,219,679. While this strategy does result in increased ion transport, current commercial additives suffer from numerous problems such as flammability, toxicity, and a lack of oxidative stability. However, phosphazenes exhibit many favorable properties including high ion conductivity, oxidative stability, non-flammability and non-toxicity. Recent research has focused on improving the mechanical properties and ion transport abilities of polymeric phosphazenes.
Among the most pressing problems in solid polymer electrolytes for secondary battery applications, including phosphazenes, is that of cation loading, requiring a concominent anion for inclusion into the polymer matrix. This leads to two main problems: 1) anion migration in non-fixed anions; 2) cation stocking in chelating anionic sites that are fixed.
Additional problems with SPEs are low conductivity, low dimensional stability, and the manner in which mobile cations are introduced into the matrix. Current methods for addressing these problems are through the use of fillers and the introduction of ions as low lattice energy salts (e.g. triflates). See, for example, Gao, U.S. Pat. No. 6,020,087. While these approaches do improve performance characteristics, long-term stability and spectator anion problems have remained unsolved until the instant invention.
A number of SPEs have been suggested for use in the prior art such as thin films formed by complexation between lithium salt and linear polyethers. See, for example, Narang, et al., U.S. Pat. No. 5,061,581. Although these SPEs do have some significant properties, such as high electrochemical and chemical stability characteristics, as well as ease of fabrication in the form of thin films, they have not met with any appreciable commercial success because the conductivity of such electrolytes at ambient temperatures is poor. The need to restrict the use of such electrolytes to electrochemical devices at elevated temperatures clearly limits the number of possible useful applications.
Various attempts have been made to improve the ionic conductivity of polymer electrolytes by a selection of new polymeric materials such as cation conductive phosphazene and siloxane polymers. Other suggestions include the use of the addition of plasticizers to polymer electrolytes to form a gel electrolyte. See, for example, Sun, U.S. Pat. No. 5,609,974. While this procedure does improve ambient temperature conductivity, this is done at the expense of mechanical properties.
Attempts have also been made to improve the dimensional stability of phosphazene films (described by Ferrar et al., Polyphosphazene Molecular Composites, 20, 258-267 (1994)). Ferrar produced an anti-static film with improved dimensional stability while maintaining transparency and negative adhesion. Ferrar was not concerned with ionic conductivity, and said anti-static film did not exhibit sufficient ionic conductivity to serve as a commercially useful electrolyte.
To date, no commercially useful SPE has been developed in the form of a thin film that has good mechanical properties and ionic conductivity in the range of 10
−4
S/cm at ambient temperatures, as well as enhanced electrochemical stability for use in, for example, a high energy, rechargeable solid state battery or for other applications in electrochemical units in which high ionic conductivity at ambient temperatures is a requirement.
Research to this end has expanded considerably in the development of solid polymer electrolytes for application in high energy density batteries. The prevailing theory of ionic conduction in polymer electrolytes teaches that ionic conductivity is facilitated by the large-scale segmental motion of the polymer and that ionic conductivity principally occurs in the amorphous regions of the polymer electrolyte. Crystallinity restricts polymer segmental motion and significantly reduces conductivity. Consequently, polymer electrolytes with high conductivity at room temperature have been sought through polymers which are highly flexible and have largely amorphous morphology. Originally, the ionic conductivity of complexes of alkali metal salts with poly(ethyleneoxide) was observed. Li salt complexes of polymers such as poly[bis(methoxyethoxyethoxy)phosphazene] (MEEP) and poly(ethoxy-ethoxy-ethoxy-vinyl ether) (described by Guglielmi et al., Appl. Organometal. Chem. 13, 339-351 (1999)), prepared on the basis of these principles, have shown room temperature conductivities of around 10
−5
S/cm.
While the ionic conductivities of these polymers at ambient temperatures has been shown to fall within acceptable limits for battery applications, they suffer from physical drawbacks making them inappropriate for use as electrolytes. MEEP, for example, suffers from very low dimensional stability that prevents its extensive use in battery construction technology. At ambient temperature, MEEP is in the visco-elastic flow regime, and can therefore flow like a viscous liquid without retaining its form when subjected to an exter
Harrup Mason K.
Stewart Frederick F.
Wertsching Alan K.
Bechtel BWXT Idaho LLC
Pedersen and Co.
Ryan Patrick
Tsang-Foster Susy
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