Activating fast and reversible sodium storage in NASICON cationic defect sites through fluorine doping

Cycling positive electrode materials with high voltages and long lifetimes remains a challenge. On the one hand, operating electrodes at high voltages is usually accompanied by severe structural distortions and irreversible reactions. On the other hand, the significant volume variation upon Na⁺ insertion/extraction limits the long cycling life. Here, we report a defective positive electrode material with a chemical formula of Na_3.2□_0.8Co_0.5Fe_0.5V(PO_3.9F_0.1)₃ (□ represents Na vacancy) through fluorine doping with activated reversible and fast Na⁺ intercalation/deintercalation to increase the energy and power densities. Notably, this positive electrode material achieves a high reversible specific capacity of 151 mAh g^-1 and prominent rate performance ranging from 1.5-4.2 V vs. Na⁺/Na, as well as a long lifespan of 6000 cycles under a high rate of 5 A g^-1 with a capacity retention of ~94%, due to the activated cationic defect sites which reduces the transport barrier at the Na(1) site. This approach is expected to be applied to the rational design of polyanionic materials for batteries.