Comparison of electrochemical performances of olivine NaFePO₄ in sodium-ion batteries and olivine LiFePO₄ in lithium-ion batteries

Carbon-coated olivine NaFePO₄ (C-NaFePO₄) spherical particles with a uniform diameter of 80 nm are obtained by chemical delithiation and subsequent electrochemical sodiation of carbon-coated olivine LiFePO₄ (C-LiFePO₄), which is synthesized by a solvothermal method. The C-NaFePO₄ electrodes are identical (particle size, particle size distribution, surface coating, and active material loading, etc.) to C-LiFePO₄ except that Li ions in C-LiFePO₄ are replaced by Na ions, making them ideal for comparison of thermodynamics and kinetics between C-NaFePO₄ cathode in sodium-ion (Na-ion) batteries and C-LiFePO₄ in lithium-ion (Li-ion) batteries. In this paper, the equilibrium potentials, reaction resistances, and diffusion coefficient of Na in C-NaFePO₄ are systematically investigated by using the galvanostatic intermittent titration technique (GITT), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV), and compared to those of the well-known LiFePO₄ cathodes in Li-ion batteries. Due to the lower diffusion coefficient of Na-ion and higher contact and charge transfer resistances in NaFePO₄ cathodes, the rate performance of C-NaFePO₄ in Na-ion batteries is much worse than that of C-LiFePO₄ in Li-ion batteries. However, the cycling stability of C-NaFePO₄ is almost comparable to C-LiFePO₄ by retaining 90% of its capacity even after 100 charge–discharge cycles at a charge–discharge rate of 0.1 C.