In recent years, with the rapid spread of IT- and communication-related devices such as personal computers, camcorders and cellular phones, much attention has been focused on the development of batteries which are used as their power sources. Also in the automobile industry, high-power and high-capacity batteries for electric vehicles and hybrid vehicles are under development. Among various kinds of secondary batteries, a lithium secondary battery is drawing attention due to its high energy density and high power output.
A general lithium secondary battery comprises a positive electrode active material layer comprising a positive electrode active material, a negative electrode active material layer comprising a negative electrode active material, and an electrolyte layer present between the positive and negative electrode active material layers. More specifically, there may be mentioned a lithium secondary battery as shown in FIG. 2, for example. In lithium secondary battery 100 shown in FIG. 2, positive electrode active material layer 2 is present inside positive electrode can 1. Negative electrode active material layer 4 is present on positive electrode active material layer 2 via electrolyte layer 3. Negative electrode active material layer 4 is filled inside negative electrode cap 5 and the cap is set in positive electrode can 1, thus forming a battery structure of positive electrode active material layer 2-electrolyte layer 3-negative electrode active material layer 4. The inside of positive electrode can 1 and that of negative electrode cap 5 are kept airtight with gasket 6.
As the electrode active material of a lithium secondary battery, for example, as the positive electrode active material, LiCoO2, LiMnO2, LiMn2O4, LiNiO2, LiCoMnO4 or the like is used, while Li4Ti5O12 or the like is used as the negative electrode active material. These conventionally-used electrode active materials have a low electron conductivity problem. Therefore, in combination with the electrode active material, a material with high electron conductivity, such as acetylene black or graphite, is generally used as an electron conduction assisting material, in order to ensure electron conductivity of an active material layer. Also, a binder component is sometimes used to bind the electrode active material to the electron conduction assisting material. However, the electron conduction assisting material such as a carbonaceous material and the binder component do not contribute to the capacity of a battery, so that they are one factor which decreases the energy density of a battery.
Therefore, various techniques have been proposed to increase electron conductivity of the electrode active material (for example, patent literature 1). Patent literature 1 discloses a method for producing an active material, in which a nitrogen oxide with a resistivity of less than 1×104 Ωcm and represented by the composition formula LixMeOyNz (wherein 0≦x≦2; 0.1<y<2.2; 0<z<1.4; and Me is at least one kind selected from the group consisting of Ti, Co, Ni, Mn, Si, Ge and Sn) is obtained by heating an oxide with a reactivity of 1×104 Ωcm or more in a reducing atmosphere and then reacting the oxide with ammonia gas.
A method for (not producing an electrode active material but) nitriding an oxide with a nitrogen compound such as urea is also known. For example, patent literature 2 discloses a method for producing an inorganic oxynitride with photocatalytic activity by heating a mixture of an oxide with a given specific surface area (e.g., titanium oxide, zinc oxide, tin oxide, iron oxide) and a nitrogen compound which adsorbs to the oxide at ordinary temperature (e.g., urea).