text: 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 Li2MnO3. 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.
  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, xcex3-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 the reaction between the nonaqueous electrolyte and the positive-electrode active material.
  In the inventive lithium battery, the boron-containing lithium-manganese complex oxide as the positive-electrode active material may be obtained by heat-treatment of a mixture of a boron compound, lithium compound and manganese compound in the presence of oxygen, the mixture containing boron, lithium and manganese in an atomic ratio (B:Li:Mn) of 0.01-0.20:0.1-2.0:1.
  Examples of a usable boron compound in the preparation of the positive-electrode active material include boron oxide B2O3, boric acid H3BO3, metaboric acid HBO2, lithium metaborate LiBO2, quaternary lithium borate Li2B4O7 and the like. Examples of a usable lithium compound include lithium hydroxide LiOH, lithium carbonate Li2CO3, lithium oxide Li2O, lithium nitrate LiNO3 and the like. Examples of a usable manganese compound include manganese dioxide MnO2, manganese oxyhydroxide MnOOH and the like.
  In the heat-treatment of the boron compound, lithium compound and manganese compound for giving the positive-electrode active material, temperatures below 150xc2x0 C. will result in insufficient incorporation of boron or boron compound into the solid of the lithium-manganese complex oxide and also in insufficient removal of water of crystallization of manganese dioxide. The residual water of crystallization reacts with lithium so as to degrade storability of the lithium battery. On the other hand, heat-treatment temperatures in excess of 430xc2x0 C. will result in decomposed manganese dioxide so that the complex oxide presents a predischarge mean manganese valence of less than 3.80. Hence, as mentioned supra, the crystal structure of the positive-electrode active material becomes instable because of the great fluctuations in the mean manganese valence during the charging process. Thus, the positive-electrode active material is prone to react with the nonaqueous electrolyte, degrading the charge/discharge cycle performance of the lithium battery. Therefore, in the preparation of the positive-electrode active material, the boron compound, lithium compound and manganese compound may be heat-treated at temperatures of 150xc2x0 C. to 430xc2x0 C., preferably of 250xc2x0 C. to 430xc2x0 C., or more preferably of 300xc2x0 C. to 430xc2x0 C.
  If the heat-treatment is performed in such a manner, boron or the boron compound is properly incorporated into the solid of the lithium-manganese complex oxide without altering the crystal structure thereof. Thus is maintained the crystal structure combining Li2MnO3 and MnO2 and featuring excellent charge/discharge cycle performance.
  In the inventive lithium battery, examples of a usable negative-electrode active material in the negative electrode include lithium metals generally used in the art; lithium alloys such as lithium-aluminum alloy, lithium-lead alloy, lithium-tin alloy and the like; and carbon materials capable of absorbing and desorbing lithium ions such as graphite, coke and the like. Where the negative-electrode active material is a lithium-aluminum alloy, the nonaqueous electrolyte forms an ion conductive film over a surface of the negative-electrode active material. The film serves to suppress the reaction of the negative-electrode active material with the nonaqueous electrolyte, thereby further improving the charge/discharge cycle performance of the lithium battery.
  These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawing which illustrates a specific embodiment of the invention.