High‐Efficiency Cathode Sodium Compensation for Sodium‐Ion Batteries

Abstract Sodium‐ion batteries have gained much attention for their potential application in large‐scale stationary energy storage due to the low cost and abundant sodium sources in the earth. However, the electrochemical performance of sodium‐ion full cells (SIFCs) suffers severely from the irreversible consumption of sodium ions of cathode during the solid electrolyte interphase (SEI) formation of hard carbon anode. Here, a high‐efficiency cathode sodiation compensation reagent, sodium oxalate (Na 2 C 2 O 4 ), which possesses both a high theoretical capacity of 400 mA h g −1 and a capacity utilization as high as 99%, is proposed. The implementation of Na 2 C 2 O 4 as sacrificial sodium species is successfully realized by decreasing its oxidation potential from 4.41 to 3.97 V through tuning conductive additives with different physicochemical features, and the corresponding mechanism of oxidation potential manipulation is analyzed. Electrochemical results show that in the full cell based on a hard carbon anode and a P2‐Na 2/3 Ni 1/3 Mn 1/3 Ti 1/3 O 2 cathode with Na 2 C 2 O 4 as a sodium reservoir to compensate for sodium loss during SEI formation, the capacity retention is increased from 63% to 85% after 200 cycles and the energy density is improved from 129.2 to 172.6 W h kg −1 . This work can provide a new avenue for accelerating the development of SIFCs.