Compensating Sodium Ions and Regulating Interface Chemistry through a High-Capacity Organic Salt for Practical Na-Ion Batteries

Sodium-ion batteries are promising energy storage devices due to abundant sodium, inherent safety, and low cost, but irreversible sodium loss during initial cycles limits their reversible capacity and long-term stability. Cathode sodium-supplemented additives offer a simple and fabrication-compatible strategy. Herein, an organic sodium-supplemented additive, sodium squarate (Na2C4O4, NCO), is introduced into the high-capacity Prussian white (PW) cathodes to achieve efficient sodium compensation. During the first charge, NCO undergoes decomposition, releasing additional Na+ ions to compensate for irreversible sodium consumption associated with SEI formation and other interfacial side reactions. Meanwhile, the generated CO2 participates in interfacial reactions, leading to the formation of a carbonate-rich cathode–electrolyte interphase (CEI) on the PW cathode, while its diffusion toward the anode promotes the construction of a robust SEI enriched with carbonate species and sodium fluoride (NaF) on the hard-carbon anode. As a result, the PW-10% NCO||HC full cell delivers an enhanced discharge capacity of 100 mAh g–1, compared to 53 mAh g–1 for the pristine PW||HC full cell. Moreover, a multilayer pouch cell obtained a capacity retention of 80.1% after 100 cycles. Overall, this work highlights NCO as an effective cathode sodium-supplemented additive and provides insights into the interfacial regulation for sodium-ion full cells.