Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
1998-11-10
2001-12-18
Gupta, Yogendra N. (Department: 1751)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
C252S506000, C252S511000, C252S519100, C252S519300, C361S502000
Reexamination Certificate
active
06331282
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to compositions useful for energy conversion and storage. More specifically, the present invention relates to the synthesis of manganese oxyiodides useful for high energy density battery and electrochemical capacitor (supercapacitor) applications.
2. Description of Related Art
Miniaturization in electronics and rapid advances in portable devices have created increasing demand for lightweight, compact, high energy density batteries (Scrosati, 1995). Lithium batteries offer higher energy density and longer shelf life compared to other rechargeable systems (Scrosati, 1995; Oyama et al., 1995). Although layered LiCoO
2
may be used as a cathode in commercially available lithium-ion cells (Nagaura and Tazawa, 1990; Scrosati, 1992), Co is costly and toxic.
Manganese oxides are attractive in this regard because Mn is inexpensive and less toxic. Although spinel LiMn
2
O
4
has been pursued intensively as a cathode, its capacity fading on cycling due to Jahn-Telier distortion poses problems (Thackeray et al., 1983; Tarascon et al., 1994; Thackeray et al., 1998). Attempts to develop other crystalline manganese oxides have largely been unsuccessful because nonspinel oxides tend to transform to the more stable spinel phase on cycling (Gummow et al., 1993; Armstrong et al., 1996; Vitins et al., 1995). For instance, layered LiMnO
2
tends to transform to a spinel phase and exhibits unsatisfactory capacity fading.
Recently, hydrated amorphous manganese oxides employing an aqueous medium have been reported (Xu et al., 1998). Although such oxides exhibit a high capacity, the capacity tends to decline to about 78% in 10 cycles. Because cyclability data is only available for 10 cycles, the stability of these water-containing cathodes upon prolonged cycling remains unclear.
Complex metal oxides used for energy storage devices are traditionally made by repeated grinding and firing of raw materials at elevated temperatures in order to overcome diffusional limitations. Such a “brute force”, high-temperature approach often leads to unfavorable characteristics such as larger grain size, lower surface area, and an inaccessibility of metastable phases that may have unusual valences or atomic arrangements. These drawbacks have created interest in recent years in designing low temperature routes to synthesize complex materials (Stein et al., 1993).
It has been shown (Manthiram et al., 1994) that alkali metal borohydrides such as NaBH
4
can be used effectively to reduce metalate ions (MO
4
)
n−
(M=V, Mo and W) in aqueous solutions to obtain binary oxides M
y
O
z
and ternary oxides Na
x
M
y
O
z
. The method gave amorphous or nanocrystalline phases, which were often metastable, and the binary oxides such as VO
2
and MoO
2
obtained by this approach were found to be attractive as electrode materials for lithium batteries (Tsang and Manthiram, 1997; Manthiram and Tsang, 1996). However, these aqueous-based methods often give hydrated products, and complete removal of water while still maintaining an amorphous structure is difficult.
SUMMARY OF THE INVENTION
The present invention involves a low temperature chemical method to synthesize amorphous or nearly amorphous manganese oxyiodides having a general formula of Li
w
Na
x
MnO
y
I
z
. The process typically involves the reduction of an acetonitrile solution of sodium permanganate by lithium iodide at room temperature followed by annealing at elevated temperatures in vacuum. The values of w, x, y and z in Li
w
Na
x
MnO
y
I
z
depend on ratios of reactants and the annealing conditions, and with benefit of this disclosure may be varied by those skilled in the art to achieve compositions having desired properties. In one embodiment, the disclosed composition shows excellent performance as positive electrodes (cathodes) in rechargeable lithium batteries. The electrochemical characteristics of the samples also suggest possible use in electrochemical capacitor (supercapacitor) applications.
The disclosed compositions utilize environmentally benign and typically less-expensive manganese. Advantageously, in one embodiment, the manganese oxyiodide electrodes described herein may exhibit more than 95% of cell capacity below 4 V, which is particularly useful in avoiding electrolyte instability as polymer electrolytes may decompose above 4 V. In this embodiment, samples may exhibit a substantially smooth discharge-charge curve without any abrupt changes, which is an attractive feature to avoid problems associated with over-charge or discharge.
Operating voltage ranges of the presently disclosed compositions may be particularly attractive for lithium-ion batteries, lithium polymer batteries, or supercapacitors. The disclosed compositions may offer an amorphous or nearly amorphous structure with smaller particle size that may provide an easier access to lithium ions from an electrolyte, thereby providing better lithium ion diffusion and rate capability. Although in one embodiment the disclosed manganese oxyiodides may offer a lower voltage, their larger capacity with a mid-discharge voltage of about 2.6 V leads to an energy density that is about 1.4 times higher than those achieved practically with layered LiCoO
2
or spinet LiMn
2
O
4
cathodes.
In a broad aspect, the invention is a method for synthesizing a manganese oxyiodide including combining a permanganate with a reducing agent to form the manganese oxyiodide.
In other aspects, the method may include annealing the manganese oxyiodide. The annealing may include heating the manganese oxyiodide to a temperature of between about 100° C. and about 300° C. The annealing may occur under vacuum conditions and may take place for between about 2 hours and about 3 days. The annealing may include heating the manganese oxyiodide under vacuum conditions for about 10 hours to a temperature of about 250° C. Combining the permanganate with the reducing agent may occur at ambient temperature, and it may occur in a nonaqueous medium. The permanganate may include an alkali metal permanganate salt. It may include at least one of sodium permanganate, lithium permanganate, potassium permanganate, or a mitre thereof It may include sodium permanganate. The reducing agent may include an alkali metal iodide. It may include an alkali metal borohydride. It may include at least one of lithium iodide, sodium iodide, potassium iodide, sodium borohydride, lithium borohydride, potassium borohydride, or a mixture thereof The reducing agent may also include lithium iodide. The permanganate and the reducing agent may be combined in a molar ratio of from about 1:0.5 to about 1:10.
In another aspect, the invention is a method for preparing a composition including combining an alkali metal permanganate salt with an alkali metal iodide in a nonaqueous medium to form a manganese oxyiodide. The alkali metal permanganate salt and the alkali metal iodide may be combined in a molar ratio of from about 1:0.5 to about 1:10. The method also includes heating the manganese oxyiodide under vacuum conditions to a temperature of between about 100° C. and about 300° C. to form the composition.
In other aspects, the manganese oxyiodide may be heated for between about 2 hours and about 3 days. The alkali metal permanganate salt may include sodium permanganate. The alkali metal iodide may include lithium iodide. The nonaqueous medium may include acetonitrile. The alkali metal permanganate salt and the alkali metal iodide may be combined in a molar ratio of about 1:1.5.
In another aspect, the invention is a method for forming an electrode including combining a permanganate with a reducing agent to obtain a manganese oxyiodide, heating the manganese oxyiodide; and grinding the manganese oxyiodide to form the electrode.
In other aspects, the permanganate may include an alkali metal permanganate salt. The permanganate may include sodium permanganate. The reducing agent may include an alkali metal iodide. The reducing agent may include lithium iodide. The combining of the permanganat
Kim Jaekook
Manthiram Arumugam
Board of Regents , The University of Texas System
Fulbright & Jaworski LLP
Gupta Yogendra N.
Hamlin Derrick G.
LandOfFree
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