Copper surface doping to improve the structure and surface properties of manganese-rich cathode materials for sodium ion batteries†
A facile copper surface doping process is proposed to enhance the surface structural stability of manganese-rich layered oxide cathodes. Herein, the surface structure of P2-type Na0.67Mn0.6Ni0.2Co0.2O2 was stabilized via the formation of a copper-rich surface layer. This layer can reduce the dissolution of manganese in the electrolyte and inhibit side reactions at the electrode/electrolyte interface. Expanded surface lattice channels induced by copper doping contributed to the improvement of Na+ mobility. In addition, it can be deduced from scanning transmission electron microscopy (STEM) images that copper surface doping resulted in a surface transition from the P2 phase to P3 phase. Electrochemical impedance spectroscopy (EIS) measurements confirmed that both the irreversible reaction resistance and charge transfer resistance of the copper surface-doped Na0.67Mn0.6Ni0.2Co0.2O2 (Cu-MNC) were significantly reduced. More importantly, the Cu-MNC electrode delivered an initial specific capacity of 122.2 mA h g−1 with retention of up to 83.3% after 150 cycles at 0.2C. After refreshing the sodium metal anode and electrolyte, it showed a high specific capacity of 102.4 mA h g−1 with retention of 84.14% after 200 cycles. These results open up a new method to optimize manganese-rich oxide materials for SIBs.