Positive electrode material for lithium secondary battery and lithium secondary battery
A positive electrode material for lithium secondary battery having high electron conductivity even at very low temperatures and a lithium secondary battery with the use of the positive electrode material are provided. The positive electrode material for lithium secondary battery has secondary particles formed of primary particles made of lithium complex oxides composited with ultrathin carbon fibers having lengths equal to or smaller than the diameters of the secondary particles of the positive electrode active material. The ultrathin carbon fibers have opposite ends from which an electrolytic solution moves in and out.
1. A positive electrode material for a lithium secondary battery, comprising:
a plurality of primary particles comprising lithium complex oxides, wherein an average particle diameter of the primary particles is 0.1 to 3.0 μm; and
a plurality of ultrathin carbon fibers dispersed uniformly among the plurality of primary particles so that the primary particles are linked to one another to form a secondary particle wherein the ultrathin carbon fibers have a diameter of 10 to 50 nm, wherein the length of the ultrathin carbon fibers is from 2 to 8 μm, and the ultrathin carbon fibers are contained in the positive electrode material in an amount of 6 to 10% by weight;
wherein the ultrathin carbon fibers have opposite ends from which an electrolytic solution can move in and out, and have lengths equal to or smaller than a diameter of the secondary particle.
2. The positive electrode material for lithium secondary battery according to claim 1 , wherein a cumulative amount of mercury penetrating into pores having diameters of 10 to 50 nm that is measured by a mercury press-in method is from 0.01 to 0.05 ml/g.
3. The positive electrode material for lithium secondary battery according to claim 1 , wherein the cumulative amount of mercury penetrating into pores having diameters of 0.1 to 1μm that is measured by the mercury press-in method is from 0.1 to 0.3 ml/g.
4. The positive electrode material for lithium secondary battery according to claim 1 , wherein a volume proportion of the carbon fibers composited with the positive electrode active material is equal to or higher than 50% relative to the total amount of carbon fibers added.
5. The positive electrode material for lithium secondary battery according to claim 1 , wherein the positive electrode active material is composed of lamellar complex oxides represented by a chemical formula Li a MO 2 , where 0<a≦1.2; M represents at least one of Co, Ni, and Mn.
6. The positive electrode material for lithium secondary battery according to claim 1 , wherein the positive electrode active material is composed of lamellar complex oxides represented by a chemical formula Li a Mn x Ni y Co z O 2 , where 0<a≦1.2; 0.1≦x≦0.9; 0.1≦y≦0.44; 0.1≦z≦0.6; x+y+z=1, and the diameter of the ultrathin carbon fibers is from 10 to 50 nm.
7. A positive electrode plate for lithium secondary battery, comprising a film of the positive electrode material for lithium secondary battery according to claim 1 formed on a metallic foil.
8. A lithium secondary battery comprising positive and negative electrodes via separator and an electrolytic solution, wherein the positive electrode comprises the positive electrode plate for lithium secondary battery according to claim 7 .
9. The lithium secondary battery according to claim 8 , wherein an output energy density is 240 to 600 W/kg in a state of a depth of discharge capacity of 50% at a temperature of −30 degrees C.