Magnetic Properties and Environmental Temperature Effects on Battery Performance of Na_0.67Mn_0.5Fe_0.5O₂

Herein, a modified solid state synthesis of Na_0.67Mn_0.5Fe_0.5O₂ and the results of a detailed investigation of the structural and magnetic properties via Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) analysis are reported. The magnetic properties of Na_0.67Mn_0.5Fe_0.5O₂ do not fit the Curie–Weiss law and a model regarding the spin configuration of the Mn and Fe ions and a possible ferrimagnetic order is suggested. Electrochemical measurements and ex situ structural analysis of the cathode material confirm the reversible structural transitions for the cells charged up to 4.0 V. Environmental temperature–dependent electrochemical measurements reveal a strong temperature dependence of both, the initial capacity and the capacity retention. Ex situ SEM, FTIR, and XRD studies on the battery membrane verify the formation of a Na₂CO₃ phase on the membrane, which blocks the Na ion diffusion through membrane pores and is responsible for the capacity fade for this compound.