Composite active material for lithium secondary batteries and method for producing same

Abstract

A composite active material for lithium secondary batteries, which is capable of providing a lithium secondary battery that has large charge and discharge capacity, high-rate charge and discharge characteristics and good cycle characteristics at the same time. A method of producing a composite active material for lithium secondary batteries comprises: a mixing step wherein graphite having a specific surface area of 30 m2/g or more and a battery active material that is capable of combining with lithium ions are mixed with each other, thereby obtaining a mixture; and a spheroidizing step wherein the mixture is subjected to a spheroidization treatment, thereby producing a generally spherical composite active material for lithium secondary batteries, said composite active material containing graphite and the battery active material that is capable of combining with lithium ions.

1 . A generally spherical composite active material for lithium secondary batteries containing graphite and a battery active material capable of combining with lithium ions, wherein the area ratio of the graphite exposed to the surface of the composite active material for lithium secondary batteries observed by scanning electron microscope (SEM) observation at an accelerating voltage of 10 kV or less is 95% or more. 
     
     
         2 . The composite active material for lithium secondary batteries according to  claim 1 , wherein the tap density is 0.8 g/cm3 or more. 
     
     
         3 . The composite active material for lithium secondary batteries according to  claim 1 , wherein the specific surface area is 5 to 100 m2/g. 
     
     
         4 . The composite active material for lithium secondary batteries according to  claim 1 , wherein the battery active material capable of combining with lithium ions has an average particle diameter of 1 μm or less. 
     
     
         5 . The composite active material for lithium secondary batteries according to  claim 1 , wherein a part of the battery active material capable of combining with lithium ions is present in the form of secondary particle that is an aggregate of the battery active material capable of combining with lithium ions, and the secondary particle has an average diameter of 5 μm or less. 
     
     
         6 . A lithium secondary battery comprising the composite active material for lithium secondary batteries as defined in  claim 1 . 
     
     
         7 . The lithium secondary battery according to  claim 6 , wherein the lithium secondary battery is used in a state that the battery active material capable of combining with lithium ions is charged 60% or more and the graphite is charged 50% or less, in a fully charged state. 
     
     
         8 . The lithium secondary battery according to  claim 6 , comprising a negative electrode, wherein the lithium secondary battery is used in a range where the potential of the negative electrode does not fall below 0.4 V with respect to a lithium reference electrode. 
     
     
         9 . A lithium secondary battery comprising a current collector having a three-dimensional structure and an electrode containing the composite active material for lithium secondary batteries as defined in  claim 1  disposed thereon. 
     
     
         10 . The composite active material for lithium secondary batteries according to  claim 2 , wherein the battery active material capable of combining with lithium ions has an average particle diameter of 1 μm or less. 
     
     
         11 . The composite active material for lithium secondary batteries according to  claim 3 , wherein the battery active material capable of combining with lithium ions has an average particle diameter of 1 μm or less.