SOLID POLYMER BATTERY ELECTROLYTE AND REACTIVE METAL- WATER BATTERY
CONTRACTUAL ORIGIN OF THE INVENTION
The United States Government has rights in this invention pursuant to Contract No. DE-AC07-94ID 13223 between the United States Department of Energy and Lockheed Martin Idaho Technologies Company, now Contract No. DE-AC07-99ID13727 with Bechtel BWXT Idaho, LLC.
TECHNICAL FIELD
This invention relates generally to reactive metal-water batteries and to reactive metal-water battery solid polymer electrolytes.
BACKGROUND OF THE INVENTION
A battery is a device that converts the chemical energy contained in its active materials directly into electrical energy by means of an electrochemical oxidation-reduction reaction. This type of reaction involves the transfer of electrons from one material to another through an electric circuit when the battery is placed under a load. 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 anode, or negative electrode, gives up electrons to the external circuit and is oxidized during the electrochemical reaction. The cathode, or positive electrode, accepts electrons from the external circuit and is reduced during the electrochemical reaction. The electrolyte serves as the ionic conductor and provides the medium for 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 batteries).
Periodic table Group 1A and Group 2 A metals, and particularly lithium, are attractive as battery anode materials because of their light weight, high voltage, high electrochemical equivalence, and good conductivity. One type of battery comprises a reactive metal-water battery, such as a Li-water battery. Such batteries today are principally fabricated for low power and long duration, but may find other uses. With such a battery, the reactive metals such as lithium serve as the anode. The cathode is principally comprised of water. The electrolyte can be solid or liquid. For lithium, the principal reactions are:
Anode Li - e" → Li+
Cathode H2O + e~ OH- +
VΆ2
Overall Li + H2O → LiOH + lΔΑ2
However, as apparent from the overall electrochemical reaction,
lithium and water will also react directly with one another, essentially
resulting in parasitic corrosion of the anode when exposed to water.
The corrosion reaction is highly undesirable because it produces no
useful electrical energy and consumes active lithium. The reaction is
highly exothermic and can detrimentally accelerate local corrosion.
Further, the produced lithium hydroxide reacts with water and eventually
precipitates as a monohydrate crystal. Accordingly, a principal challenge
with respect to reactive metal-water batteries is development of
techniques that minimize this parasitic corrosion, thereby extending
battery life and improving efficiency. With liquid electrolytes, this has
typically focused on liquid additives that enhance or result in formation
of a calcareous film upon the outer surface of the lithium metal as a
means of protecting the metal from contact with water. Work has also
been conducted in the past with solid polymer electrolytes, but without
much success due to one or both of low current output and water
permeability which shortens life.
Accordingly, needs remain for improved reactive metal-water batteries
and solid polymer electrolytes therefor. While the invention was
principally motivated in solving problems with respect to this particular
art area, the artisan will appreciate applicability of the invention in other
areas, with the invention only being limited by the accompanying claims
appropriately interpreted in accordance with the Doctrine of Equivalents.
SUMMARY OF INVENTION
In one implementation, a reactive metal-water battery includes an
anode comprising a metal in atomic or alloy form selected from the
group consisting of periodic table Group 1A metals, periodic table
Group 2A metals and mixtures thereof. The battery includes a cathode
comprising water. Such also includes a solid polymer electrolyte
comprising a polyphosphazene comprising ligands bonded with a
phosphazene polymer backbone. The ligands comprise a aromatic ring
containing hydrophobic portion and a metal ion carrier portion. The
metal ion carrier portion is bonded at one location with the polymer
backbone and at another location with the aromatic ring containing
hydrophobic portion. The invention also contemplates such solid
polymer electrolytes use in reactive metal-water batteries, and in any
other battery.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention are described below with
reference to the following accompanying drawings.
Fig. 1 is a diagrammatic schematic of a reactive metal-water battery
in accordance with an aspect of the invention.
Fig. 2 is a diagrammatic representation of chemical formula of a
solid polymer battery electrolyte in accordance with but one aspect of
the invention.
DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENTS
This disclosure of the invention is submitted in furtherance of the
constitutional purposes of the U.S. Patent Laws "to promote the progress
of science and useful arts" (Article 1, Section 8). Referring to Fig. 1, a
reactive metal-water battery system under load is designated with
numeral 10. Such comprises an anode 12, a cathode 14 and a solid
polymer electrolyte 16. The battery is illustrated as being subjected to
some suitable discharge load 18. In one implementation, anode 12
comprises a metal in atomic or alloy form selected from the group
consisting of periodic table Group 1A metals, periodic table Group 2 A
metals and mixtures thereof. Cathode 14 comprises water. An
electrode 20 is received within cathode 14, with the battery being
discharged through load 18 via a conductive line 22 extending from
anode 12 to electrode 20 and correspondingly cathode 14. Electrode 20
preferably comprises porous conductive copper, and effectively serves to
provide an enlarged conductive surface area from line 22 for electrical
communication with cathode 14.
An exemplary commercial version of a battery in accordance with the
invention would comprise components 12, 16 and 20 received within a
suitable housing and essentially void of cathode-water in storage. Upon
preparation for use, cathode-water would be provided relative to the
housing and the battery placed under load. For use in water such as in
oceanic applications, the housing might be provided to be suitably
permeable to water such that immersion in salt water results in seawater
permeating into the housing to function as a cathode material and
effectively "turn on" a pre-wired load 18.
Solid polymer electrolyte 16 comprises a polyphosphazene material
comprising ligands bonded with a phosphazene polymer backbone. The
ligands comprise an aromatic ring containing hydrophobic portion and a
metal ion carrier portion. The metal ion carrier portion is bonded at
one location with the polymer backbone, and at another location with
the aromatic ring containing hydrophobic portion. The metal ion carrier
portion preferably does not include any aromatic ring. One preferred
desired effect from such a construction results in the polymer backbone
functioning as the core of the material which is surrounded by an ionic
conducting channel which is itself surrounded or otherwise protected by
a hydrophobic material which is ideally remote from the backbone core.
Another preferred desired effect of such a construction, when the battery
is not under load, is receipt or alignment of the reactive metal along the
polymer backbone and corresponding displacement of water therefrom by
the hydrophobic portion of the solid electrolyte material. The reactive
metal will preferably associate with exposed electron pairs of nitrogen
atoms on the backbone. Upon application of suitable load, metal ions
of the anode will displace relative to the nitrogen atoms of the polymer
backbone and be received by and transported/conducted along the ion
carrier portion, thus forming a desired electrical circuit of suitable and
desired conductivity. An example is described below with reference to
Fig. 2.
The metal ion carrier portion preferably comprises an ether, such as
an oxyether or a thioether. More preferred are polyethers, alkeneoxy
ethers and alkeneoxy ether polymers. Examples are materials selected
from the group consisting of ethyleneoxy ethers, propyleneoxy ethers and
mixtures thereof. Further preferred are ethylene thioethers, propylene
thioethers, and mixtures thereof.
The hydrophobic portion preferably comprises an aromatic ring
(including heteroaromatic rings) having at least one hydrocarbon group
or halohydrocarbon group bonded therewith. Further, the hydrophobic
portion preferably comprises at least 7 carbon atoms, and further at least
one terminal hydrocarbon or halohydrocarbon group. Substituted phenols
are example preferred materials, such as hydrocarbon and
halohydrocarbon substituted phenols.
Performance enhancing additives can also of course be provided as
part of the battery or electrolyte. By way of example only, such
include ceramic precursors, substituted phosphazene trimers, and cross
linking materials or treatments.
Fig. 2 illustrates but one exemplary implementation of a solid
polymer battery electrolyte in accordance with the invention. Such
depicts a polyphosphazene polymer backbone 25 having associated
ligands 27. Ligands 27 comprise an ion channel or metal ion carrier
portion 32 bonded at one location with polymer backbone section 25.
Ligands 27 also comprise a hydrophobic portion bonded with metal ion
carrier portions 32 at another location thereof. In the illustrated
example, the hydrophobic portion is depicted as 4-(2,2,3,3
tetramethylbutyl) phenol. The metal ion carrier portion is depicted as an
ethylenoxyether polymer, where "n" is an integer preferably falling from
1 to 16. The illustrated example depicts a homopolymer where all the
ligands are the same, but co-polymers are also of course contemplated
with varying ligands, at least an effective amount of which constitutes
the described metal ion carrier and hydrophobic portions.
One example and preferred technique in battery fabrication in
accordance with the invention will result in a coating of the solid
polymer electrolyte material to cover an entirety of the outer surface of
the lithium or other reactive metal anode. Such is preferably achieved
by dissolving the solid polymer electrolyte material in a suitable solvent,
such as tetrahydrofuran (THF). This liquid material is then brushed
onto the anode, or the anode dipped within the solvent/electrolyte
system, to fully coat the anode. The solid electrolyte can be formed by
combining a polyphosphazene, such as polydiclorophosphazene, with an
alkoxide comprising the ligands, such as a sodium alkoxide. The
sodium alkoxide, for example, can be produced by combining the
ligands in alcohol form with sodium. Alternate techniques of fabrication
are also, of course, possible.
Example
One hundred ten (110) ml of Triton-X-114™ was combined in a
vessel with 300 ml of anhydrous tetrahydrofuran. Triton-X-114™ is
available from Aldrich Chemical Company of Milwaukee, WI. Six (6)
grams of sodium was freshly cut, hexane-washed, dried and added to the
mixture. The mixture was stirred and heated at 65 °C under an Ar
atmosphere until the sodium was consumed.
A solution containing 10 grams of dichloropolyphosphazene was
dissolved in 300 ml of a solvent comprising a 1 : 1 by volume mix of
diglyme and THF. This solution was slowly added to the above
solution under an argon atmosphere. Two hundred (200) ml of
additional diglyme/THF solvent was added, and the mixture stirred.
Such was heated under an argon atmosphere for two days. Salts were
precipitated by multiple water addition and polymer washings with
subsequent combinations with THF. The polymer was ultimately
centrifuged and dried in vacuo to remove all water. One gram of the
resultant polymer was dissolved in 10 grams of THF, and provided with
approximately 4.6 weight percent LiBF4 in solution. A lithium metal
anode was immersed in the subject solution to dip coat the lithium
metal anode. Such was subsequently dried in vacuum to remove the
casting solvent, thus forming a lithium anode coated with a solid
polymer electrolyte in accordance with an aspect of the disclosure.
In compliance with the statute, the invention has been described in
language more or less specific as to structural and methodical features.
It is to be understood, however, that the invention is not limited to the
specific features shown and described, since the means herein disclosed
comprise preferred forms of putting the invention into effect. The
invention is, therefore, claimed in any of its forms or modifications
within the proper scope of the appended claims appropriately interpreted
in accordance with the doctrine of equivalents.