Antimony (Sb) based anodes with high conductivity and capability have shown great promise for applications in lithium ion batteries (LIBs). However, they often suffer from poor cycling stability because of the drastic volume variation and structural degradation on undergoing lithiation-delithiation processes. Here we demonstrate a novel Sb-based anode with a free-standing structure realized by uniformly implanting intermetallic compound breithauptite (nickel antimonide, NiSb) nanocrystals into nitrogen-doped carbon nanofibers (NiSb@NCNFs). The discharge/charge behavior of NiSb@NCNFs was systematically investigated by ex situ characterization, which revealed a special "dealloying-lithiation/delithiation-realloying" cycling mechanism. The NiSb nanocrystals possess high lithium storage capacity, and the interconnected network of NCNFs can accommodate the volume variation of encapsulated NiSb nanoparticles, while also providing smooth pathways for charge transport. Compared to other Sb-based anodes, the NiSb@NCNF anode presents exceptional reversible capacity (720 mA h g-1 at a current density of 100 mA g-1) and greatly enhanced cycling life at high rates (510 mA h g-1 after 2000 cycles at 2000 mA g-1). Furthermore, the free-standing NiSb@NCNF anode is free of binders, conductive additives and metal current collectors, exhibiting high flexibility and remarkable performances for the construction of flexible and bendable soft-packed full Li-ion pouch cells.