Dioxolane as a proctector for lithium electrodes

Details Dioxolane as a proctector for lithium electrodes Dioxolane as a proctector for lithium electrodes

1999-02-05

2001-05-01

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S303000, C429S307000, C429S329000, C029S623500

Reexamination Certificate

active

06225002

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates generally to lithium-sulfur batteries, and in particular to lithium electrodes having protective dioxolane coatings.
The rapid proliferation of portable electronic devices in the international marketplace has led to a corresponding increase in the demand for advanced secondary batteries. The miniaturization of such devices as, for example, cellular phones, laptop computers, etc., has naturally fueled the desire for rechargeable batteries having high specific energies (light weight). At the same time, mounting concerns regarding the environmental impact of throwaway technologies, has caused a discernible shift away from primary batteries and towards rechargeable systems.
In addition, heightened awareness concerning toxic waste has motivated, in part, efforts to replace toxic cadmium electrodes in nickel/cadmium batteries with the more benign hydrogen storage electrodes in nickel/metal hydride cells. For the above reasons, there is a strong market potential for environmentally benign secondary battery technologies.
Secondary batteries are in widespread use in modern society, particularly in applications where large amounts of energy are not required. However, it is desirable to use batteries in applications requiring considerable power, and much effort has been expended in developing batteries suitable for high specific energy, medium power applications, such as, for electric vehicles and load leveling. Of course, such batteries are also suitable for use in lower power applications such as cameras or portable recording devices.
At this time, the most common secondary batteries are probably the lead-acid batteries used in automobiles. These batteries have the advantage of being capable of operating for many charge cycles without significant loss of performance. However, such batteries have a low energy to weight ratio. Similar limitations are found in most other systems, such as Ni—Cd and nickel metal hydride systems.
Among the factors leading to the successful development of high specific energy batteries, is the fundamental need for high cell voltage and low equivalent weight electrode materials. Electrode materials must also fulfill the basic electrochemical requirements of sufficient electronic and ionic conductivity, high reversibility of the oxidation/reduction reaction, as well as excellent thermal and chemical stability within the temperature range for a particular application. Importantly, the electrode materials must be reasonably inexpensive, widely available, non-toxic, and easy to process.
Thus, a smaller, lighter, cheaper, non-toxic battery has been sought for the next generation of batteries. The low equivalent weight of lithium renders it attractive as a battery electrode component for improving weight ratios. Lithium provides also greater energy per volume than do the traditional battery standards, nickel and cadmium.
The low equivalent weight and low cost of sulfur and its nontoxicity renders it also an attractive candidate battery component. Successful lithium/organosulfur battery cells are known. (See, De Jonghe et al., U.S. Pat. Nos. 4,833,048 and 4,917,974; and Visco et al., U.S. Pat. No. 5,162,175.)
Recent developments in ambient-temperature sulfur electrode technology may provide commercially viable rechargeable lithium-sulfur batteries. Chu and colleagues are largely responsible for these developments which are described in U.S. Pat. Nos. 5,582,623 and 5,523,179 (issued to Chu). The patents disclose a sulfur-based positive electrode for a battery cell that has low equivalent weight and high cell voltage and consequently a high specific energy (greater than about 120 Wh/kg). The disclosed positive electrode addresses deficiencies in the prior art to provide a high capacity sulfur-based positive composite electrode suitable for use with metal (such as lithium) negative electrodes in secondary battery cells. These developments allow electrochemical utilization of elemental sulfur at levels of 50% and higher over multiple cycles. Because sulfur has a theoretical maximum capacity of 1675 mAh/g (assuming all sulfur atoms in an electrode are fully reduced during discharge), the utilization of sulfur in lithium-sulfur cells as described in the above Chu patents typically exceeds 800 milliamp-hours per gram (mAh/g) of sulfur.
The lithium-sulfur batteries described in the above Chu patents provide increased capacity (have a high specific energy) relative to previously available battery cells, such as the above-noted lead-acid batteries. However, like previous battery designs, they are susceptible to the reaction of the lithium electrode with the electrolyte and/or dissolved sulfur species. This leads to several problems including low cycling efficiency of lithium and the possibility of “pillowing”. Low cycling efficiency of lithium is problematic in that it necessitates the use of excess lithium in the cell to make up for that is lost on each cycle. This adds weight and volume to the cell that it undesirable. Lithium-sulfur battery cells are typically composed of lithium and sulfur electrodes and an electrolyte (liquid, gel or solid) sealed in a casing. Pillowing occurs when gas is produced as a result of a chemical reaction within the cell. The increased volume occupied by such a gas relative to the solid and/or liquid phase battery cell components from which it is derived increases the pressure within the sealed casing, causing it to bulge, or “pillow.”
An example of this situation is illustrated in
FIGS. 1A and 1B
. The figures show representations of a lithium-sulfur secondary battery cell
100
. The cell's electrodes and electrolyte are sealed within a flexible pouch-like casing
102
. In a normal cell which functions properly, illustrated in
FIG. 1A
, the casing
102
roughly conforms to its contents.
FIG. 1B
shows a representation of the battery cell
100
after it has pillowed. As noted above, the pillowed cell results from a build up of pressure within the casing
102
due to unintended and undesirable gas formation within the casing
102
. The casing of a pillowed cell no longer roughly conforms to its contents, but instead bulges outward.
Pillowing is undesirable in battery cells for a number of reasons. First, pillowing suggests an unintended reaction within the cell that is likely to detrimentally impact its performance. Second, the deformation of the cell due to pillowing, particularly its increase in size, may result in the cell no longer fitting in its allotted compartment in the electronic device which it is intended to power. Thirdly, the increased internal pressure which causes pillowing may cause the casing of the cell to burst, allowing access of contaminants into the cell, and escape of lithium and other potentially dangerous battery cell components from the cell. Pillowing is typically observed in secondary battery cells, however it may also occur in primary battery cells with similar detrimental consequences.
Accordingly, lithium-sulfur battery cell designs which mitigate pillowing would thus be desirable.
SUMMARY OF THE INVENTION
The present invention provides dioxolane-treated lithium electrodes, battery cells containing such dioxolane-treated lithium electrodes, battery cell electrolytes containing dioxolane, and methods of treating lithium electrodes with dioxolane and battery cells containing such dioxolane-treated lithium electrodes. Treating lithium with dioxolane prevents the lithium from reacting with a wide range of substances which can contaminate battery cells, particularly moisture and other protic impurities, that might otherwise react with the lithium to the detriment of its function as a negative electrode in a battery cell. Battery cells containing dioxolane as an electrolyte co-solvent in accordance with the present invention exhibit improved cycling performance over cells not containing dioxolane. Moreover, the dioxolane treatment does not negatively impact sulfur utilization and improves the lithium's electrochemical function as the negative electrode in th

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