Study of Predominant Dynamics on the O3-Type Layered Transition-Metal Oxide Cathode by Electrochemical Impedance Spectroscopy for Sodium-Ion Batteries

The layered transition metal oxides (Na_xTMO₂) are the most attractive cathode options for the rechargeable sodium-ion batteries (SIBs) owing to their high specific capacity, outstanding sodium desorption ability, and high average operating voltage. However, the kinetic behaviors corresponding to complex and prominent phase transitions are still perplexing. Here, we investigate the detailed electrochemical kinetic characteristics of the NaNi_1/3Fe_1/3Mn_1/3O₂ electrode by electrochemical impedance spectroscopy (EIS) in three-electrode configurations assistance with the distribution of relaxation times (DRT) and trusted equivalent circuit models numerical analysis. The complex and prominent phase transformations evolution of O3-P3-O3' in the charge process and O3'-P3'-O3 during the discharge process are reflected at different degrees of frequencies and potentials, and significant contributions for the charge transfer step are established on this basis. As the charge and discharge proceed, the influence on charge transfer process by phase transform will be weak, however, which still has some expression and can be caught by EIS assistance with DRT. Additionally, a diagrammatic model for Na⁺ extraction/insertion is established to illustrate the physicochemical reaction mechanism in the NaNi_1/3Fe_1/3Mn_1/3O₂ electrode. The results definitely provide certain scientific thoughts and guiding principles for the commercialization of Na_xTMO₂ in SIBs.