Disclosed is a lithium secondary battery using a cathode containing a lithium-containing transition metal oxide and an anode containing graphitized carbon, characterized in that an ammonium compound capable of providing ammonium ions is added to a non-aqueous electrolyte. Therefore, the present invention provides a lithium secondary battery having improved high-temperature performance by reducing decreases of residual capacity and recovery capacity resulting from high-temperature storage of the battery, via addition of such an ammonium compound.