Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-08-01
2004-02-24
Ruthkosky, Mark (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S199000, C429S323000
Reexamination Certificate
active
06696200
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a lithium battery comprising a positive electrode, a negative electrode, and a nonaqueous electrolyte containing a solute and a solvent, and more particularly, to a lithium battery improved in charge/discharge cycle performance through suppression of reaction between a positive-electrode active material of the positive electrode and the nonaqueous electrolyte.
2. Description of the Related Art
Recently, rechargeable batteries have found applications in various fields such as electronics. As a novel battery of high power and high energy density, in particular, lithium batteries featuring high electromotive force derived from oxidation/reduction of lithium in the nonaqueous electrolyte have come into wide use.
Such lithium batteries have conventionally employed various metal oxides capable of absorbing and desorbing lithium ions as the positive-electrode active material for use in the positive electrode. More recently, studies have been made on the use of manganese oxides, such as manganese dioxide, as the positive-electrode active material of the lithium battery because manganese oxides generally provide high discharge potentials and are inexpensive.
Unfortunately, in charge/discharge processes of the lithium battery including the positive-electrode active material of manganese oxide, the manganese oxide is repeatedly expanded and contracted so that the crystal structure thereof is destroyed. As a result, the battery suffers a degraded charge/discharge cycle performance.
In recent attempts to improve the charge/discharge cycle performance of the lithium battery including the positive-electrode active material of manganese oxide, a variety of positive-electrode active materials have been proposed. For instance, Japanese Unexamined Patent Publication No. 63-114064(1988) discloses a positive-electrode active material comprising a lithium-manganese complex oxide obtained from manganese dioxide and Li
2
MnO
3
. Japanese Unexamined Patent Publication No. 1-235158 (1989) provides a positive-electrode active material comprising a complex oxide of lithium-containing manganese dioxide wherein lithium is incorporated in the crystal lattice of manganese dioxide. Further, Japanese Unexamined Patent Publication Nos. 4-237970(1992) and 9-265984(1997) disclose positive-electrode active materials comprising lithium-manganese complex oxides added with boron.
SUMMARY OF THE INVENTION
The invention is directed to a lithium battery comprising a positive electrode, a negative electrode, and a nonaqueous electrolyte containing a solute and a solvent, the battery adapted to suppress the reaction between a positive-electrode active material of the positive electrode and the nonaqueous electrolyte for achieving an excellent charge/discharge cycle performance.
A lithium battery according to the invention comprises a positive electrode, a negative electrode, and a nonaqueous electrolyte containing a solute and a solvent, wherein the positive electrode comprises a positive-electrode active material of boron-containing lithium-manganese complex oxide prepared using manganese dioxide with a specific surface area of 15 to 50 m
2
/g.
According to the inventive lithium battery wherein the positive electrode comprises the positive-electrode active material of boron-containing lithium-manganese complex oxide prepared using manganese dioxide with the specific surface area of 15 to 50 m
2
/g, boron in the positive-electrode active material suppresses the reaction between the lithium-manganese complex oxide and the nonaqueous electrolyte during charging. Besides, the boron-containing lithium-manganese complex oxide has the specific surface area in such a suitable range as to obviate a problem that the boron-containing lithium-manganese complex oxide has too great a specific surface area or too great a contact area with the nonaqueous electrolyte, tending to react with the nonaqueous electrolyte. Thus, the reaction between the positive-electrode active material and the nonaqueous electrolyte is more positively suppressed. As a result, the positive-electrode active material is prevented from being dissolved in the nonaqueous electrolyte, so that increase in the internal pressure of the lithium battery is suppressed. Hence, the battery is improved in the charge/discharge cycle performance.
If the boron-containing lithium-manganese complex oxide as the positive-electrode active material has a specific surface area of less than 12 m
2
/g, the current density during the charge/discharge process increases thereby to increase polarization of the positive electrode. This results in an increased possibility of side reaction wherein the nonaqueous electrolyte is decomposed. If the boron-containing lithium-manganese complex oxide as the positive-electrode active material has a specific surface area in excess of 45 m
2
/g, the positive-electrode active material is increased in contact area with the nonaqueous electrolyte, thus becoming more prone to react with the nonaqueous electrolyte. Therefore, the positive-electrode active material of boron-containing lithium-manganese complex oxide may preferably have a specific surface area in the range of 12 to 45 m
2
/g. Such a boron-containing lithium-manganese complex oxide more positively suppresses the increase in the internal pressure of the lithium battery, thereby even further improving the charge/discharge cycle performance of the lithium battery.
For more positive suppression of the reaction between the positive-electrode active material and the nonaqueous electrolyte during charging, the solute in the nonaqueous electrolyte of the inventive battery may preferably include at least one substance selected from the group consisting of lithium trifluoromethanesulfonimide, lithium pentafluoroethanesulfonimide, lithium trifluoromethanesulfonmethide, lithium trifluoromethanesulfonate and lithium hexafluorophosphate. More preferably, the solute may include at least one substance selected from the group consisting of lithium trifluoromethanesulfonate and lithium trifluoromethanesulfonimide.
According to the inventive lithium battery, known solvents generally used in the art may be employed as the solvent for the nonaqueous electrolyte. However, for particular purposes of suppressing the reaction between the nonaqueous electrolyte and the positive-electrode active material as well as of increasing the ionic conductivity of the nonaqueous electrolyte, it is preferred to use a solvent mixture containing at least one organic solvent selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, &ggr;-butyrolactone and sulfolane, and at least one organic solvent selected from the group consisting of 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-ethoxymethoxyethane, tetrahydrofuran, dioxolane, dimethyl carbonate, diethyl carbonate and ethylmethyl carbonate. More preferred is a solvent mixture containing at least one organic solvent selected from the group of propylene carbonate and ethylene carbonate, and 1,2-dimethoxyethane.
For proper suppression of the reaction between the positive-electrode active material and the nonaqueous electrolyte during charging, the solvent mixture may preferably contain the two types of organic solvents in respective concentrations of not less than 10 vol%.
According to the inventive lithium battery, the boron-containing lithium-manganese complex oxide as the positive-electrode active material may be obtained by heat-treating a mixture of a boron compound, a lithium compound and manganese dioxide in the presence of oxygen, the mixture containing boron, lithium and manganese in an atomic ratio (B:Li:Mn) of 0.01-0.20:0.2-2.0:1.
Such a composition offers lithium-manganese complex oxide crystals incorporating therein boron or boron compound in a boron-to-manganese atomic ratio (B/Mn) in the range of 0.01 to 0.20.
If the lithium-manganese complex oxide crystals incorporate therein boron or boron compound in an atomic ratio (B/M
Fujitani Shin
Ota Taeko
Yoshimura Seiji
Ruthkosky Mark
Sanyo Electric Co,. Ltd.
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