Secondary battery, and battery module and apparatus including same

According to one embodiment of the present invention, there may be provided a secondary battery, a battery module comprising the secondary battery as a unit battery, and an apparatus comprising the battery module as a power source, the secondary battery comprising an electrode assembly, the electrode assembly has a first electrode group including a first positive electrode containing a first positive electrode active material, and a second electrode group including a second positive electrode containing a first positive electrode active material and a second positive electrode active material. The first positive electrode active material is a lithium-nickel composite oxide having primary particles having a particle diameter of 1 [mu] m or less, and the second positive electrode active material is a lithium-nickel composite oxide having primary particles having a particle diameter of more than 1 [mu] m. According to the present invention, the high-temperature storage performance and the fast charging performan