NOVELTY - The battery has a positive electrode, a negative electrode containing an active material, and an electrolytic solution, where the active material includes a core portion capable of occluding and releasing lithium ions. The active material includes an amorphous or low-crystalline coating portion arranged on a part of a surface of the core portion, and a fibrous carbon portion arranged on a part of the surface of the coating portion. The coating portion contains silicon and oxide as constituent elements, where atomic ratio of oxygen relative to silicon lies in a range of 0.5-1.8.    USE - Lithium ion secondary battery for use in a battery pack to supply electric power to an electronic apparatus e.g. portable electronic apparatus such as video camera, digital still camera, cell phone, notebook-size personal computer, cordless phone, headphone stereo, portable radio, portable TV, personal digital assistant (PDA), portable life instrument e.g. electric shaver, memory device e.g. backup power supply and memory card, and medical electronic apparatus e.g. pacemaker and hearing aid, power tool such as electric drill and electric saw, electricity storage system such as home battery system utilized for an electric apparatus such as electronics appliance e.g. refrigerator, air conditioner, and water heater, and electric vehicle (all claimed) such as electric car, electric motorbike, and hybrid car.    ADVANTAGE - The core portion occludes and releases lithium ions smoothly and easily and is protected by the coating portion such that a fresh surface is not exposed during charge and discharge while smooth occlusion and release of the ions are maintained. The different active materials are electrically connected to each other through the fibrous carbon portions so that electrical resistance of the active material is reduced significantly so as to improve performance, cycle characteristics, initial charge and discharge characteristics, and load characteristics, of the battery. The coating portion is amorphous as flexibility of the coating portion is improved, so that the coating portion follows expansion and shrinkage of the core portion easily without any damage during charge and discharge. The atomic ratio of the core portion lies in the range of 0 to 0.5 so as to reduce irreversible capacity, and obtain high battery capacity. The negative electrode comprises a collector containing carbon and sulfur as constituent elements so as to improve physical strength of the collector, thus preventing the collector from deforming easily even when the active material layer is expanded or shrunk during charge and discharge.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) a battery pack    (2) an electric vehicle    (3) an electricity storage system    (4) a power tool.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of a negative electrode of a lithium ion secondary battery.    Negative electrode collector (1)    Negative electrode active material layer (2)