Physics / mechanism
Redox flow batteries store energy in liquid electrolytes held in external tanks; electrochemical reactions occur at inert electrodes as electrolyte is pumped through a cell stack. Energy capacity scales with tank volume; power scales with stack area — decoupled scaling is the core architectural advantage. Vanadium RFBs (VRFBs) use V²⁺/V³⁺ and V⁴⁺/V⁵⁺ redox couples in sulfuric acid; round-trip efficiency ~65–80%, cycle life >20,000 cycles, energy density ~20–35 Wh/L. Organic RFBs replace vanadium with earth-abundant quinones or nitroxides — lower cost ceiling but shorter demonstrated cycle life and lower voltage. State of the art: Invinity, Sumitomo, CellCube at MW-scale; Harvard/MIT spinouts (Quino Energy) pushing organic.
Competitive landscape
Lithium iron phosphate (LFP) is the dominant competitor — higher energy density, lower upfront $/kWh at current scale, but degradation above ~4,000 cycles and thermal management costs at multi-MWh scale close the gap. Compressed air and pumped hydro compete at >100 MWh duration. Zinc-bromine and iron-air are adjacent chemistries with overlapping target markets.
Companies using
Connected ideas
Sources
Frontier (open questions)
- To be added.