TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The small diameter active material comprises a diffusion portion into which at least a portion of the manganese-containing compound is diffused, where the diffusion portion is formed to a predetermined thickness from the surface of the small diameter active material. In the positive active material: an average particle diameter (D50) ratio of the large diameter active material to the small diameter active material is 2:1 to 100:1, preferably 3:1 to 20:1; an average particle diameter (D50) of the large diameter active material is 1-50 mu m; the average particle diameter (D50) of the small diameter active material is 0.05-10 mu m; and a weight ratio of the large diameter active material to the small diameter active material is 50:50 to 99:1. The manganese-containing compound: has a spinel crystal structure; and is a lithium manganese oxide of formula (Li1+aMn2-bM1bO4-cA1c), where the manganese is present at 0.01-0.1 moles based on 1 mole of the small diameter active material. The nickel-based lithium-transition metal composite oxide is of formula (LixNi1-yM2yO2-zX1z) or (LixNi1-m-n-ComAlnO2), where the nickel is present at least at 0.5 moles based on 1 mole of the nickel-based lithium-transition metal composite oxide. The large diameter active material comprises: at least one of lithium cobalt oxide, lithium nickel oxide, lithium manganese dioxide, lithium dimanganese tetraoxide, lithium(nickel-cobalt-aluminum)oxide of formula (Li(NipCoqAlr)O2), lithium(nickel-cobalt-manganese)oxide of formula (Li(NipCoqMnr)O2), lithium-nickel-cobalt oxide of formula (LiNi1-tCotO2), lithium-cobalt-manganese-oxide of formula (LiCo1-tMntO2), lithium-nickel-manganese-oxide of formula (LiNi1-tMntO2), lithium(nickel-cobalt-manganese)oxide of formula (Li(NiuCovMnw)O4), lithium-manganese-nickel oxide of formula (LiMn2-dNidO4), lithium-manganese-cobalt oxide of formula (LiMn2-dCodO4), lithium cobalt phosphate, lithium nickelate, lithium iron oxide, lithium iron phosphate, vanadium(V) oxide, titanium(II) sulfide or molybdenum disulfide; and a nickel-based lithium-transition metal composite oxide that is identical to or different from that of the small diameter active material.M1 = at least one of Al, Mg, Ni, Co, Zn, Ca, Sr, Cu, Zr, P, Fe, Ga, In, Cr, Ge or Sn;A1, X1 = at least one of O, F, S or P;a = 0;b = 0-0.2;c = 0-0.02;M2 = at least one of Co, Al, Mn, Mg, Cr, Fe, Ti, Zr, Mo or their alloys;x = 0.8-1.2;y = 0-0.5;z = 0-2;m+n = greater than 0 to less than or equal to 0.2;n = greater than 0 to less than or equal to 0.1;p, q, r = greater than 0 to less than 1, where a+b+c is 1;t = 0-1; andu, v, w, d = greater than 0 to less than 2, where u+v+w is 2. EXAMPLE - Large and small particles of a nickel-cobalt-aluminum oxide (LiNi0.8Co0.15Al0.05O2) were prepared, where D50 of the large particles was 20 mu m, and the D50 of the small particles was 5 mu m. The small particles were dispersed in 1 kg of water and stirred to prepare a slurry. Then separately, 5 g of manganese sulfate was dissolved in 100 g of water, and added to the obtained slurry. The resultant mixture was then stirred for 30 minutes and dried in an oven at 120 degrees C for 5 hours. The dried powder was then heat-treated at 600 degrees C for 1 hour to obtain nickel-cobalt-aluminum small particles. The coated small particles and the large particles were mixed at a weight ratio of 3:7 to obtain the positive active material.