Sodium-ion batteries (SIBs) for grid-scale applications need active materials that combine a high energy density with sustainability. Given the high theoretical specific capacity 501 mAh g−1, and Earth abundance of disodium rhodizonate (Na2C6O6), it is one of the most promising cathodes for SIBs. However, substantially lower reversible capacities have been obtained compared with the theoretical value and the understanding of this discrepancy has been limited. Here, we reveal that irreversible phase transformation of Na2C6O6 during cycling is the origin of the deteriorating redox activity of Na2C6O6. The active-particle size and electrolyte conditions were identified as key factors to decrease the activation barrier of the phase transformation during desodiation. On the basis of this understanding, we achieved four-sodium storage in a Na2C6O6 electrode with a reversible capacity of 484 mAh g−1, an energy density of 726 Wh kg−1cathode, an energy efficiency above 87% and a good cycle retention.