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□CoFeV(POF) (□ 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 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 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.