Studies on the Kinetic Behaviors of Na Ions Insertion/Extraction in Na 2 FeSiO 4 /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 behaviours 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 material are greatly correlated with the kinetics of Na+ transfer in the host material. Herein, based on 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 firstly 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 process and describe the behaviours of Na ions transfer in Na2FeSiO4/C.