Nanosized silicon has attracted considerable attentions as a new-generation anode material for lithium-ion batteries (LIBs) due to its exceptional theoretical capacity and reasonable cyclic stability. However, serious side reactions often take place at the nanosized silicon/electrolyte interface in LIBs, where critical electrochemical properties such as initial Coulombic efficiency (ICE) are compromised. On the basis of this feature, a new method is developed to synthesize nanosilicon-based particles in a facile, scalable way, which are endowed with the function of prelithiation and storage stability in air. A semisolid lithium rechargeable flow battery (SSFB) technology is used for the first time to convert the micrometer-sized silicon raw material into an amorphous-nanosilicon-based material (ANSBM), as a result of the pulverization process induced by the repeated lithiation/delithiation cycles. The particle size is successfully reduced from 1–4 μm to around 30 nm after cycles in the flow battery. Bulk functionalization of the nano silicon is introduced by the unbalanced lithiation/delithiation cyclic process, which endows ANSBM with a unique prelithiation capability universally applicable to different anode systems such as nanosized Si, SiO x , and graphite, as evidenced by the significantly improved ICEs. Superior air stability (10% relative humidity) is exhibited by ANSBM due to surface functionalization by the stable interfacial layer encapsulated by electron-conductive carbon. The outcome of this work provides a promising way to synthesize dual-functionalized nano silicon with good electrochemical performance in terms of improved capacity and increased initial Coulombic efficiency when it is composited with other typical anode materials.