Recently, as being accompanied by the developments of portable electronic devices such as cellular phones and notebook-size personal computers, or as being accompanied by electric automobiles being put into practical use, and the like, small-sized, lightweight and high-capacity secondary batteries have been required. At present, as for high-capacity secondary batteries meeting these demands, non-aqueous secondary batteries have been commercialized, non-aqueous secondary batteries in which lithium cobaltate (e.g., LiCoO2) and the carbon-system materials are used as the positive-electrode material and negative-electrode material, respectively. Since such a non-aqueous secondary battery exhibits a high energy density, and since it is possible to intend to make it downsize and lightweight, its employment as a power source has been attracting attention in a wide variety of fields. However, since LiCoO2 is produced with use of Co, one of rare metals, as the raw material, it has been expected that its scarcity as the resource would grow worse from now on. In addition, since Co is expensive, and since its price fluctuates greatly, it has been desired to develop positive-electrode materials that are inexpensive as well as whose supply is stable.
Hence, it has been regarded promising to employ lithium-manganese-oxide-system composite oxides whose constituent elements are inexpensive in terms of the prices as well as which include stably-supplied manganese (Mn) in their essential compositions. Among them, a substance, namely, Li2MnO3 that includes tetravalent manganese ions alone but does not include any trivalent manganese ions making a cause of the manganese elution upon charging and discharging, has been attracting attention. Although it has been believed so far that it is impossible to charge and discharge Li2MnO3, it has come to find out that it is possible to charge and discharge it by means of charging it up to 4.8 V, according to recent studies. However, it is needed to further improve Li2MnO3 with regard to the charging/discharging characteristics.
In order to improve the charging/discharging characteristics, it has been done actively to develop xLi2MnO3.(1−x)LiMeO2 (where 0<“x”≦1), one of solid solutions between Li2MnO3 and LiMeO2 (where “Me” is a transition metal element). Note that it is feasible to write and express Li2MnO3 by a general formula, Li(Li0.33Mn0.67)O2, as well, and that it is said to belong to the same crystal structure (i.e., a layered rock-salt structure) as that of LiMO2. Consequently, there arises a case where xLi2MnO3.(1−x)LiMeO2 is set forth as Li1.33−yMn0.67−zMey+zO2 (where 0<“y”<0.33, and 0<“z”<0.67), too, but even any of the two methods for writing it down specify a composite oxide that possesses the same sort of crystal structure.
For example, Patent Literature No. 1 discloses a production process for solid solution between LiMO2 and Li2NO3 (where “M” is one or more kinds that are selected from Mn, Ni, Co and Fe, and “N” is one or more kinds that are selected from Mn, Zr and Ti). This solid solution is obtainable as follows: ammonia water is dropped to a mixed solution, in which salts of respective metallic elements that correspond to “M” and “N” are dissolved, until the pH becomes 7; an Na2CO3 solution is further dropped to it in order to deposit “M”-“N”-system composite carbonates; and the resulting “M”-“N”-system composite carbonates are calcined after mixing them with LiOH.H20.
However, upon employing a secondary battery including Li2MnO3 as the positive-electrode active material, it is needed to activate the positive-electrode active material at the time of the first-time charging. Since the activation is accompanied by a large irreversible capacity, ions having moved to the counter electrode do not come back, and so there is such a problem that charging/discharging balance between the positive electrode and the negative electrode becomes imbalanced. With regard to the mechanism of this activation and to an obtainable capacity by means of the activation, it is the present situation that they have not been clearly clarified yet (see Non-patent Literature No. 1).
Moreover, in a case where a particle diameter of Li2MnO3 is large, since only the particles' superficial layer is activated, it is believed that it is necessary to make the particle diameter of Li2MnO3 smaller in order to turn Li2MnO3 to be employed into an active material serving as battery in the total amount virtually. In other words, it is needed to develop convenient processes for synthesizing fine particles as well. For example, in Patent Literature No. 2, a process for synthesizing nano-order oxide particles is disclosed. In Example No. 3 of Patent Literature No. 2, MnO2 and Li2O2 are added to and are then mixed with a mixture, in which LiOH.H2O and LiNO3 are mixed in a molar ratio of 1:1; and they are turned into 300° C. molten salt after letting the mixture go through a drying step, thereby synthesizing lithium manganate (e.g., LiMn2O4) with a spinel structure, whose manganese has an average oxidation number that is equal to a valence number of 3.5.