Rational Design and Ionic Co-Doping Modification of P2-Type Na-Rich Cathode Materials

Abstract

Currently, improving the capacity and stability of sodium-ion batteries is the key to commercialization. P2-type layered cathode materials have low energy density due to low initial Na content. In this research, Na-rich P2-type cathode material Na0.8Ni0.23Mn0.67Cu0.1O2-0.08Li/F (NMC-LF) is synthesized by a simple high-temperature solid-phase method. The strong electronegativity of F allows the local TM-O (TM represents transition metal elements) bond to be transformed into a TM-F bond and the TMO6 octahedron becomes more stable. The doping of Li+ not only maintains the stability of the structure, but also transforms the two-phase reaction in the high-voltage range into a solid-solution reaction. This leads to an effective suppression of the irreversible phase transition of P2-O2 in the high-voltage phase. At 200 mA·g-1, the discharge specific capacity reaches 101.9 mAh·g-1 and remains at 99.31% after 150 cycles. The diffusion coefficient of NMC-LF is as high as 2.60×10-16 cm2·s−1, which has excellent diffusion kinetics. The result of high-temperature X-ray diffraction shows that there are no impurities presence at 900 °C. Under the joint synergy of Li/F/Cu and the Na-rich P2-type structure, the cyclic and rate performance is significantly improved. This provides new ideas for designing the research of cathode materials for sodium-ion batteries.

Keywords: Sodium ion batteries, Cathode materials, Na-rich structure, F-ion doping