Lithium ion secondary battery
Provided is a secondary battery, specifically, a secondary battery having excellent stability and improved output characteristic and low temperature characteristic by including a cathode active material in which at least one of metals forming the cathode active material has a concentration gradient in at least a portion of a region between a central portion and a surface portion; and a conductive material mixture in which carbon nanotube is mixed with carbon black at an appropriate ratio, the carbon black being a spherical nanoparticle.
1. A secondary battery, comprising:
a cathode active material comprising lithium-metal oxide; and
a conductive material mixture in which carbon nanotube is mixed with carbon black,
wherein the carbon black and the carbon nanotube are mixed at a mixing ratio of 4:1 to 2:3,
wherein the cathode active material comprises M1 which has a concentration gradient region in which a concentration is decreased from a central portion up to a surface portion, M2 which has a predetermined concentration from the central portion up to the surface portion, and M3 which has a concentration gradient region in which a concentration is increased from the central portion up to the surface portion,
wherein M1 is Ni, and M2 and M3 are selected from the group consisting of Co and Mn,
wherein a molar ratio of M1 in the cathode active material is X M1 , a molar ratio of M2 in the cathode active material is X M2 , and a molar ratio of M3 in the cathode active material is X M3 , and
wherein X M1 :(X M2 +X M3 ) is 7:3 to 9:1.
2. The secondary battery of claim 1 , wherein the cathode active material further comprises one or more selected from the group consisting of Na, Mg, Ca, Ti, V, Cr, Cu, Zn, Ge, Sr, Ag, Ba, Zr, Nb, Mo, Al, Ga, and B.
3. The secondary battery of claim 1 , wherein the carbon black is a spherical nanoparticle.
4. The secondary battery of claim 1 , wherein the conductive material mixture has a content of 2 to 8 wt % relative to 100 wt % of the cathode active material.
5. The secondary battery of claim 1 , wherein M2 is Mn and M3 is Co.
6. The secondary battery of claim 1 , wherein M2 is Co and M3 is Mn.
7. The secondary battery of claim 1 , wherein the cathode active material is represented by Chemical Formula 1 below:
Li x Ni a M2 b M3 c M4 d M5 e M6 f M7 g O y [Chemical Formula 1]
(In Chemical Formula 1, M2 and M3 are selected from the group consisting of Co and Mn, and M4, M5, M6, and M7 are selected from the group consisting of Na, Mg, Ca, Ti, V, Cr, Cu, Zn, Ge, Sr, Ag, Ba, Zr, Nb, Mo, Al, Ga, and B, and 0<x≤1.1, 2≤y=2.02, 0.6≤a≤0.95, 0.05≤b+c≤0.4, 0≤d≤1, 0≤e≤1, 0≤f≤1, 0≤g≤1, and 0<a+b+c+d+e+f+g≤1).
8. The secondary battery of claim 7 , wherein 0.7≤a≤0.9, and 0.1≤b+c≤0.3 are satisfied.