High‐Efficiency Cathode Sodium Compensation for Sodium‐Ion Batteries

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 (Na2 C2 O4 ), 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 Na2 C2 O4 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-Na2/3 Ni1/3 Mn1/3 Ti1/3 O2 cathode with Na2 C2 O4 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.