A high-energy-capacity, flexible lithium-ion battery is fabricated using a new nanofibril-structured silicon anode on a flexible current collector. Silicon is known to be the highest capacity anode material. However, its huge volume changes during the lithium insertion and extraction results in pulverization, which is the cause of the rapid capacity fade that occurs as the charge-discharge cycles progress. Nanostructured silicon can overcome this pulverization problem. A flexible current collector with high electric conductivity is prepared by a RF-magnetron sputtering of a thin copper layer (<1 μm) onto a porous polymeric membrane. This provides not only flexibility of the electrode but also a template for simple fabrication of nanostructured silicon. Cells using the new, flexible current collector and corresponding silicon nanofibril-structured anode exhibit energy capacities over 2000 mAh g−1 during 30 charge-discharge cycles at C/2. In addition, the coulombic efficiency remains over 99% after the third cycle. These results demonstrate the potential of the new anode for use in commercial high-capacity, flexible lithium-ion batteries.