Redox flow batteries (RFBs) are ideal for large-scale, long-duration energy storage applications. However, the limited solubility of most ions and compounds in aqueous and non-aqueous solvents (1M–1.5 M) restricts their use in the days-energy storage scenario, which necessitates a large volume of solution in the numerous tanks and the vast floorspace for these tanks, making the RFB systems costly. To resolve the low energy storage density issue, this work presents a novel way in which the reactants and products are stored in both solid and soluble forms and only the liquid with soluble ions is circulated through the batteries. Storing the active ions in solid form can greatly increase the storage energy density of the system. With a solid to liquid storage ratio of 2:1, for example, the energy density of the electrolyte of vanadium sulfate (VOSO4), an active compound used in the all-vanadium RFB, can be increased from 40 Ah l−1 to 163 Ah l−1 (>4X), allowing an existing 6-h RFB system to become a 24-h system with minimal modifications. To show how the concept works, an H2-V flow battery with a solid/liquid storage system is used, and its successful demonstration validates the solid-liquid storage concept.