OLIVINE NaFePO4 CATHODE SYNTHESIZED BY A GREEN AQUEOUS ELECTROCHEMICAL CONVERSION ROUTE FOR SODIUM ION BATTERIES

摘要

Sodium ion batteries have recently been pursued actively as the most attractive alternative to lithium ion batteries for the application of large-scale energy storage, because of their evident advantages, such as low cost, environmental benignity and wide availability of sodium. Olivine NaFePO4 possesses the highest theoretical specific capacity (154 mAh g-1) and decent working voltage for Na ion storage, while it can only be acquired by chemical or electrochemical redox reaction of olivine LiFePO4. Herein, we report a facile synthesis of olivine NaFePO4 by aqueous electrochemical displacement of Li ions from olivine LiFePO4 by Na ions. This displacement process is quick, easy, cost-saving and green compared to that in organic electrolyte, and the thus-obtained NaFePO4 cathode shows high reversible capacity and cycle performance in sodium ion batteries. Moreover, we reveal, for the first time, that the Na ion intercalation/deintercalation can proceed in a reversible two-step phase transition reaction and a clear phase separation is determined by reaction rate.