Lithium ion secondary batteries are widely used for portable electronic instruments such as mobile phones or notebook-size personal computers. As a cathode active material for a lithium ion secondary battery, a composite oxide of lithium with a transition metal, etc. (hereinafter sometimes referred to as lithium-containing composite oxide) such as LiCoO2, LiNiO2, LiNi0.8Co0.2 O2 or LiMn2O4, is employed. In recent years, it is desired to further reduce the size and weight as a lithium ion secondary battery for portable electronic instruments or vehicles, and a further improvement in the discharge capacity per unit mass and in the characteristics such that the discharge capacity will not be decreased after charge and discharge cycles are repeatedly carried out (hereinafter sometimes referred to as cycle characteristic), is desired.
To improve the cycle characteristics, it has been known to be effective to form a covering layer on a lithium-containing composite oxide.
Patent Document 1 discloses a process of adding an ammonium fluoride aqueous solution to a dispersion having a lithium-containing composite oxide dispersed in an aluminum nitrate aqueous solution, followed by filtration, washing and heating to form a covering layer of aluminum fluoride on the surface of the lithium-containing composite oxide. However, by this process, a step of carrying out filtration and washing is essential, whereby the process is complicated and in addition, a waste disposal treatment is necessary, and thus the process is inferior in the productivity. Further, when the wet cake after filtration is dried, the cathode active material tends to be agglomerated to form coarse particles.
Patent Document 2 discloses a method of fluorinating a cathode material using a fluorinating agent such as fluorine (F2), nitrogen trifluoride or chlorine trifluoride. However, this method requires special equipment since a highly toxic gas is used.
Patent Document 3 discloses a process of stirring and mixing an aqueous solution containing zirconium and a lithium-containing composite oxide represented by the formula LipNxMyOzFa (0.9≦p≦1.1, 0.965≦x<1.00, 0<y≦0.035, 1.9≦z≦2.1, x+y=1 and 0≦a≦0.02) wherein the molar amount of Li element is from 0.9 to 1.1 molar times the total molar amount of the transition metal element, and firing the mixture at high temperature of at least 450° C. in an oxygen atmosphere to obtain a cathode active material having the surface layer of the lithium-containing composite oxide covered with zirconium oxide. By this method, it is difficult to cover the lithium-containing composite oxide with a compound other than an oxide.