The use of a rigid battery-based energy storage system is among one of the most important limitations to soft robots. First, the non-stretchable parts in rigid secondary batteries severely limit the range of adaptation or flexibility of the robots. Second, to achieve more prolonged operation for soft robots, a higher mass will be required for the rigid battery system—which creates additional problems such as being over-weight. In the current study, we introduce a new multi-functional energy storage system for a jellyfish soft robot. We solve the problem for traditional rigid batteries by inventing a multi-functional redox flow battery (RFB) system to realize energy storage and tendon-fluid scheme simultaneously. The RFB system exhibits excellent flexibility for the minimum rigid components inside. Electricity is produced by ion exchange between two membrane-separated chemical liquids, and the fluid-based energy storage can be multi-purposed for actuation or structure. We use RFBs for the controllable stiffening of tentacles, which works as the tendon for the swimming of the robot. In this thesis, we introduce three design iterations for the ZnI2 flow battery system and demonstrate the untethered swimming for the fluid-tendon of the flow-battery robot jellyfish out of 3D printed parts and polymer films. We also investigate the effect of graphite felt electrode compression on porosity, resistance, and battery performance. The use of ethanol as an organic additive to minimize dendrite formation is also studied. We have successfully validated the use of flow batteries can facilitate increased efficiency, energy density, and multifunctionality for a jellyfish soft robot.