Regulating Na Occupation in P2-Type Layered Oxide Cathode for All-Climate Sodium-Ion Batteries

P2-type Na2/3Ni1/3Mn2/3O2 (NNMO) has been investigated as one of the promising cathode materials of sodium-ion batteries (SIBs) due to a low-cost and wide-temperature-range adaptability. However, its application faces a number of obstacles because of the poor cycling stability and bad rate capabilities. Herein, by accommodating more Na-ions at the e-site (Na-e) in P2-type NNMO, which is thermodynamically more stable, P2-type layered oxides (Na-e/Na-f > 1.64) with outstanding electrochemical performance are obtained. Specifically, the Na0.696Ni0.329Mn0.671O2 (NM-2) exhibits a remarkable capacity retention of 71.9% after 1000 cycles at 1C and an excellent rate capability of 54.33 mAh g(-1) at 50C. In addition, NM-2 exhibits a wide temperature working range, even at extreme temperatures for batteries (-30 or 60 degrees C), it still shows a capacity close to room temperature and good cycle stability compared with 25 degrees C. These performances are demonstrated to be attributed to the fast kinetics of the Na ions in the Na-e site, which has a lower energy barrier compared to Na-f (0.8301 eV for edge sites and 1.0664 eV for face sites). This work gives a fundamental understanding of the Na-storage mechanism in Na2/3Ni1/3Mn2/3O2, and also provides a universal strategy to improve the rate and cycling life of P2-type layered oxide cathode materials.