Implementing the use of solid electroactive materials in redox-flow battery (RFB) configuration is an appealing challenge since the resulting battery technologies benefit from the high energy density of solid materials and the independent scalability of energy and power of RFB configuration. In recent years, two different strategies have emerged to achieve this goal: i) the semi-solid flow batteries and ii) the redox-mediated flow batteries, also referred to as redox targeting or solid booster, each battery type having intrinsic advantages and disadvantages. In this perspective review, recent progress addressing critical factors for each technology is revised. In particular, chemical engineering aspects are of vital importance for further improvement and practical deployment of these two technologies.
