Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-08-01
2003-01-21
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S331000, C252S062200
Reexamination Certificate
active
06509120
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a lithium battery comprising a positive electrode comprising a positive-electrode active material of boron-containing lithium-manganese complex oxide, 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 the positive-electrode active material 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 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 with boron added thereto.
Although the lithium batteries using the positive-electrode active materials of the official gazettes are improved in the charge/discharge cycle performance to some degree, there still exists a problem that the positive-electrode active material reacts with the nonaqueous electrolyte in the battery, degrading the charge/discharge cycle performance. On the other hand, the recent electronics with higher performances demand a lithium battery further improved in the charge/discharge cycle performance.
SUMMARY OF THE INVENTION
The invention is directed to a lithium battery comprising a positive electrode comprising a positive-electrode active material of boron-containing lithium-manganese complex oxide, a negative electrode, and a nonaqueous electrolyte containing a solute and a solvent, the battery adapted to suppress the reaction between the positive-electrode active material and the nonaqueous electrolyte for achieving excellent charge/discharge cycle performance.
A lithium battery according to the invention comprises a positive electrode comprising a positive-electrode active material of boron-containing lithium-manganese complex oxide, a negative electrode, and a nonaqueous electrolyte containing a solute and a solvent, wherein the positive-electrode active material is a boron-containing lithium-manganese complex oxide having an atomic ratio(B/Mn) of boron B to manganese Mn in the range of 0.01 to 0.20 and a predischarge mean manganese valence of not less than 3.80, and wherein the solute in the nonaqueous electrolyte includes at least one substance selected from the group consisting of lithium trifluoromethanesulfonimide, lithium pentafluoroethanesulfonimide and lithium trifluoromethanesulfonmethide whereas the solvent in the nonaqueous electrolyte is 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.
In the inventive lithium battery wherein the positive electrode comprises the positive-electrode active material of boron-containing lithium-manganese complex oxide while the nonaqueous electrolyte comprises the above solvent mixture with the above solute dissolved therein, boron contained in the positive-electrode active material suppresses the reaction of the lithium-manganese complex oxide with the nonaqueous electrolyte in the charging process. This prevents the dissolution of the positive-electrode active material in the nonaqueous electrolyte thereby to suppress the increase of internal resistance of the lithium battery. It is also believed that the nonaqueous electrolyte is increased in ionic conductivity thereby contributing to the improved charge/discharge cycle performance of the lithium battery. From the standpoint of suppressing the reaction between the positive-electrode active material and the nonaqueous electrolyte in the charging process, it is preferred for the solvent mixture of the nonaqueous electrolyte to contain the two types of organic solvents in respective concentrations of not less than 10 vol %.
It is for the following reasons that the inventive lithium battery employs, as the positive-electrode active material, the boron-containing lithium-manganese complex oxide with the atomic ratio (B/Mn) of boron B to manganese Mn in the range of 0.01 to 0.20. With the B/Mn ratio of less than 0.01, boron is contained in the positive-electrode active material in too small a concentration to accomplish an adequate suppression of the reaction between the lithium-manganese complex oxide and the nonaqueous electrolyte during the charging process. With the B/Mn ratio in excess of 0.20, on the other hand, boron uninvolved in the charge/discharge process accounts for too great a portion, thus failing to be properly incorporated into the lithium-manganese complex oxide solid. As a result, the positive-electrode active material suffers an instable crystal structure, tending to react with the nonaqueous electrolyte. In both of the above cases, the lithium battery is degraded in the charge/discharge cycle performance.
It is for the following reason that the inventive lithium battery employs, as the positive-electrode active material, the boron-containing lithium-manganese complex oxide having the predischarge mean manganese valence of 3.80 or more. If the complex oxide has a predischarge mean manganese valence of less than 3.80, the mean valence of manganese jumps from less than 3.80 to the order of 4 during the discharge process, resulting in great fluctuations of the mean manganese valence. This leads to the instable crystal structure of the positive-electrode active material which, in turn, tends to react with the nonaqueous electrolyte. Hence, the lithium battery is degraded in the charge/discharge cycle performance.
If the nonaqueous electrolyte employs the solute of lithium trifluoromethanesulfonimide or the solvent mixture containing at least one organic solvent selected from the group consisting of propylene carbonate, ethylene carbonate and butylene carbonate, and 1,2-dimethoxyethane, particularly a mixture of propylene carbonate and 1,2-dimethoxyethane, the inventive lithium battery may be further improved in the charge/discharge cycle performance because of enhanced suppression of t
Fujitani Shin
Nishiguchi Nobuhiro
Ota Taeko
Yoshimura Seiji
Armstrong Westerman & Hattori, LLP
Ryan Patrick
Sanyo Electric Co,. Ltd.
Wills M.
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