Flexible, portable electronics and wearable devices are on emergence in the market and are anticipated to be the next generation of electronics advancement. To embrace the device flexibility requirement, the demand for high energy density flexible batteries itself are heavily considered. In this work, structurally controlled and highly engineered Silicon-Iron metal alloy (Si Alloy) was designed as the core chemistry materials for high energy and cycle life, and hosted on flexible substrates to support bending without mechanical degradations. This design integration into a pouch cell confirmed excellent flexible anode properties delivering significantly high capacity and stable cycle life. The battery pouch was bend to ~1.7 cm radius position and over, and was still able to to easily light up different Light-Emitting Diodes (LEDs) ranging from 2 V (Red LED) – 3 V (Blue LED) showcasing its flexibility and bendability features. The results spectacle an enormous potential for high energy ultrathin flexible batteries design and project the capability of great technical innovation.