36An electrode assembly was prepared by interposing a separator formed of porous polyethylene between the positive electrode and the negative electrode,which are formed as described above, and located in a case, and an electrolyte solution was injected into the case, thereby producing a lithium secondary battery. Here, the electrolyte solution was prepared by dissolving 1.0M lithium hexafluorophosphate 5(LiPF6) in an organic solvent consisting of ethylene carbonate/dimethyl carbonate/ethyl methyl carbonate (mixing volume ratio of EC/DMC/EMC=3/4/3).Charging/discharging experiments were performed on each lithium secondary battery cell (half cell) produced by using each of the positive electrode active materials prepared according to Examples 1 and 2 and Comparative Examples 1 to 5. 10Specifically, charging/discharging was performed at 0.1C/0.1C and 50 °C, and then charging was performed at 0.1 C in a CCCV mode until a voltage of 4.7V, andwas set to be terminated 130 hours later. Meanwhile, a leakage current was measured for 130 hours, and the results are shown in Table 2 and FIG. 1 below.[Table 2]15Charge capacity/discharge capacity (mAh/g)Average leakage current (mAh/hr, 130hr)Example 1232/2070.00Example 2230/2050.06Comparative Example 1229/2030.46Comparative Example 2231/2070.09Comparative Example 3229/2040.25Comparative Example 4230/2050.32Comparative 232/2070.55
37Example 5Referring to Table 2 and FIG. 1, it can be confirmed that, inExamples 1 and 2, both of the core part and the shell part include Ni, Co, and Mn, the ratio of the core part to the total diameter of the particle was 0.5 to 0.85, the Ni concentration differencebetween the start point of the shell part and the end point thereof was30 5mol% or more, excellent charge/discharge capacitywas exhibited, and almost no leakage current was generated.On the other hand, compared with Examples 1 and 2, it can be seen thatComparative Example 1 was slightly decreased in capacity and the 130-hour leakage current was significantly increased. In Comparative Example 1, since there was no 10shell part having the Ni concentration difference of 30 mol%, stability was decreased. Inaddition, although the capacity of Comparative Example 2 was the same as those of Examples 1 and 2, referring to FIG. 1, Comparative Example 2 had a higher leakage current as a whole than those of Examples 1 and 2. This is because, in Comparative Example2, since the Ni concentration difference in the shell part is less 15than 30 mol%, sufficient stability was not ensured. In addition, in Comparative Examples 3 and 4, the ratio of the core part is less than 0.5, the charge/discharge capacity was decreased,and the leakage current for 130 hours was significantly increased, compared with Examples 1 and 2. In addition, in Comparative Example 5,sincethe core part included only Ni and Co, and the ratio of the core part is more 20than 0.85, the charge/discharge capacity was highly increased, but the leakage current for 130 hours was significantly increased.[Experimental Example 3: Evaluation of lifespan characteristic]
38Like Experimental Example 2, each lithium secondary battery cell (half cell) was produced using each of the positive electrode active materials prepared according to Examples 1 and 2 and Comparative Examples 1 to 5, and then 30 cycles of charging/dischargingwere performed to measure capacity retention [%], in which each cycle including charging at45 °C in a CCCV mode until 0.33C and 4.25V, 5cutting off under a condition of 0.005C, and discharging at a constant current of 0.33C until 2.5V. The result is shown in Table 3.[Table 3]Capacity retention (%) (45 °C, 30 cycles)Example 196.0Example 296.8Comparative Example 195.2Comparative Example 294.0Comparative Example 396.5Comparative Example 496.1Comparative Example 589.0Referring to Table 3, Examples 1and 2 in which both of the core part and the 10shell part include Ni, Co, and Mn, the ratio of the core part is 0.5 to 0.85 with respect to the total diameter of the particle, and the Ni concentration difference between the start point of the shell part and the end point thereof is 30 mol% or more exhibited a very excellent high-temperature lifespan characteristic.On the other hand, Comparative Examples 1 and 2 were decreased in a 15lifespan characteristic, compared with Examples 1 and 2. In Comparative Example 1, since a shell part having the Ni concentration difference of 30 mol% was not formed, stability was decreased, and in Comparative Example 2, since the Ni
39concentration difference in the shell part is less than 30 mol%, sufficient stability was not ensured. In addition, in the case of Comparative Example 5, in which the core part includes only Ni and Co, and the ratio of the core part is more than 0.85, the lifespan characteristic was significantly decreased, compared with Examples 1 and 2. This is because, since there was no Mn/Al in the core part, structural stability was 5decreased, and since the shell part had a small thickness, sufficient surface stability and thermal stability could not be ensured.
40[CLAIMS][Claim 1]A positive electrode active material for a secondary battery, comprising: a core part and a shell part formed around the core part, wherein the core part and the shell part include a lithium composite transition 5metal oxide, which includes Ni and Co, and at least one or more selected from the group consisting of Mn and Al, a ratio of the diameter of the core part to the total diameter of a particle of the positive electrode active material is 0.5 to 0.85, and the shell part has a concentration gradient such that a Ni concentration at the 10start point of the shell part near the core part is 30 mol% or higherthan that at the end point of the shell part near the particle surface.[Claim 2]The positive electrode active material according to claim 1, wherein, in the core part, a Ni content is80 mol% or more among the total metal elements contained 15in the lithium composite transition metal oxide.[Claim 3]The positive electrode active material according to claim 1, wherein, in the core part, a Ni content is88 mol% or more among the total metal elements contained in the lithium composite transition metal oxide.20[Claim 4]The positive electrode active material according to claim 1, wherein the Ni concentration in the core part is constant.
41[Claim 5]The positive electrode active material according to claim 1, wherein the shell part has a concentration gradient such that the Ni concentration is gradually decreased from the start point of the shell part to the end point thereof.[Claim 6]5The positive electrode active material according to claim 1, wherein, inthe shell part, a Ni contentis50 to 90 mol% among thetotal metal elements contained in the lithium composite transition metal oxide.[Claim 7]The positive electrode active material according to claim 1, wherein the shell 10part includes lithium composite transition metal oxideparticles with crystal orientationradially grown in a direction from the center to the surface of the particle of the positive electrode active material.[Claim 8]The positive electrode active material according to claim 1, wherein the ratio 15of the thickness of the shell part to the radius of the particle of the positive electrode active material is 0.15 to 0.5.[Claim 9]The positive electrode active material according to claim 1, wherein the core part and the shell part include a lithium composite transition metal oxide represented 20by Formula 1 below:[Formula 1]LipNi1-(x1+y1+z1)Cox1May1Mbz1Mcq1O2
42where Mais at least one or more elements selected from the group consisting of Mn and Al, Mbis at least one or more elements selected from the group consisting of Zr, W, Mg, Al, Ce, Hf, Ta, La, Ti, Sr, Ba, Nb, Mo, and Cr, Mcis at least one or more elements selected from the group consisting of Al, Zr, Ti, Mg, Ta, Nb, Mo and Cr, 0.9≤p≤1.5, 0