The present application relates to a cathode active material, a method for producing the cathode active material, and a nonaqueous electrolyte secondary battery; for example, a cathode active material containing a composite oxide including lithium Li and cobalt Co, a method of producing the cathode active material, and a nonaqueous electrolyte secondary battery using the cathode active material.
Recently, demands for small-sized secondary batteries having a high capacity have been increased with spread of portable devices such as video cameras and laptop computers. Although nickel-cadmium batteries using an alkaline electrolyte are currently used as the secondary battery, they have low battery voltages such as about 1.2 V, and it is difficult to improve their energy densities. For this reason, lithium metal secondary batteries using lithium metal, which have the lightest specific gravity among solid chemical elements such as 0.534, a very low potential, and the highest current capacity per unit weight among metal anode materials, have been studied.
In secondary batteries using the lithium metal as an anode, however, resinous lithium (dendrite) is precipitated on the surface of the anode while charging, and its growth is promoted by the charge-discharge cycle. The growth of the dendrite deteriorates the cycling characteristics of the secondary battery, and further causes a trouble that the dendrite erupts through a barrier membrane (separator) placed for preventing the cathode from contacting with the anode, thus resulting in an internal short-circuit.
In order to solve this problem, for example, Japanese Patent Application Laid-Open (JP-A) No. 62-90863 has proposed secondary batteries wherein a carbon material such as coke is used as an anode and the discharge and charge is repeated by de-doping and doping an alkali metal ion. It has been found that this kind of the secondary battery can avoid the deterioration of the anode caused by the repeat of discharge and charge as mentioned above.
On the other hand, inorganic compounds such as transition metal oxides and transition metal chalcogens including an alkali metal are known as a cathode active material capable of giving a battery voltage of about 4 V. Of these, lithium composite oxides such as lithium cobaltate and lithium nickelate are promising materials from the viewpoints of high voltage, stability and long duration of life.
In particular, a cathode active material mainly containing lithium cobaltate shows a high voltage, and it is expected that en energy density can be increased by increasing its charge voltage. When the charge voltage is increased, however, its cycling characteristics may be deteriorated. According to the related art, thus, a small amount of LiMn1/3Co1/3Ni1/3O2, or the like is added or another material is surface-coated to modify the cathode active material.
The above-mentioned technique in which the cathode active material is modified by surface-coating aims at obtaining a coating having high coatability, and various methods for solving the problem are proposed. For example, it is confirmed that a method for applying with a metal hydroxide gives good coatability, and examples of such a method include a method described in JP-A-9-265985 in which cobalt Co and manganese Mn are applied to the surface of lithium nickelate LiNiO2 particles through a step of applying hydroxides thereof. In addition, for example, JP-A-11-71114 discloses a method in which non-manganese metal is applied to the surface of a lithium manganese composite oxide through a step of applying a hydroxide thereof.
In addition, for example, JP-A-2001-106534 describes molybdenum, in addition to many metals, as a metal which is applied to a cathode precursor, nickel hydroxide particles for increasing a tap density of the resulting cathode. Furthermore, cathode active materials having a surface layer containing molybdenum and/or tungsten, and lithium are known by, for example, JP-A-2002-75367.
Further, JP-A-2003-123749 describes that an oxygen-absorbing compound powder is mixed with a composite oxide mainly containing lithium nickelate and the resulting mixture is calcined to give a cathode active material, and vanadium compounds are listed as the oxygen-absorbing compound.
However, if a metal hydroxide is applied to composite oxide particles and the resulting particles are then heated, the calcination between the particles easily progresses, and the particles are easily bound to one another. As a result, when a conductive agent is mixed therewith in the production of a cathode, bound parts and particles are destroyed or cracked, and thus a coating layer is peeled off or defective surfaces of the particles are exposed. Such defective surfaces have very high activity as compared with that of the formed surfaces in calcination, and therefore, deterioration reactions of an electrolyte solution and a cathode active material easily occurs.
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