Studies on the Kinetic Behaviors of Na Ions Insertion/Extraction in Na2FeSiO4/C Cathode Material at Various Desodiation States.

Na2FeSiO4, as one of the promising cathode materials in sodium-ion batteries, has attracted great interests. However, studies on the kinetic behaviors of Na ions insertion/extraction in Na2FeSiO4 composite electrode have been barely considered, until now. Importantly, the specific capacity and rate capability of Na2FeSiO4 cathode materials are greatly correlated with the kinetics of Na+ transfer in the host material. Herein, on the basis of the characterizations of microstructure and morphologies (i.e., Rietveld refinement, FESEM, HRTEM, etc.), the electrochemical kinetics of Na ions extraction in Na2FeSiO4/C electrode are first studied in detail via two electrochemical techniques (EIS and GITT), establishing the rate-controlling steps of Na+ transport in Na2FeSiO4/C, evaluating series of kinetic parameters, as well as calculating the Na+ diffusion coefficient at various state-of-desodiation. Changes of impedance response of Na2FeSiO4/C electrode depending on the different levels of desodiation show that a serial features of electrode process for Na ions migration have tremendous discrepancies, indicating that the kinetics of Na+ extraction from Na2FeSiO4/C electrode are largely influenced by different electrode reaction processes. These results provide useful insight into the inner properties of Na2FeSiO4/C electrode, and it is significant to optimize the electrochemical performance of Na2FeSiO4/C. Moreover, two models of equivalent circuits are also constructed to simulate the electrode processes and describe the behaviors of Na ions transfer in Na2FeSiO4/C.