One-Step Surface-to-Bulk Modification of Nacro2 Cathode for Durable Sodium Ion Batteries

NaCrO2 is a promising O3-type cathode material in sodium-ion batteries. However, NaCrO2 is prone to irreversible phase transitions during cycling, leading to rapid capacity fading. This work shows that adding asphalt during the synthesis process has multiple synergistic effects and can effectively stabilize the material structure and improve cyclability. More importantly, the strategy is universal in that various sources of carbon display similar effect. The asphalt compound in the sample regulates the particle morphology by forming a more pronounced stacking structure, thus facilitating sodium transport. Moreover, the burning and evaporation of the asphalt compound removes surface residual sodium and slightly create sodium vacancy in the crystal structure. In addition, NaCrO2 can be in situ coated with carbon to prevent direct electrolyte/electrode contact. As a result, the modified material has a high initial capacity of 129 mAh g-1 at 0.1 C rate with an initial Coulombic efficiency up to 98.17%. In addition, the modified material shows an excellent rate capability and cycling performance with 81.1% after 1000 cycles at 5 C in Na cells. This work demonstrates a modification method on NaCrO2 that has the potential for practical applications.