Preparation, transport and Na-storage properties of monoclinic – Na2FeSiO4 for Na-ion batteries

Na2FeSiO4 (NFS) is considered one of the promising cathode materials for rechargeable sodium-ion batteries (SIBs). In this work, the monoclinic phase of NFS is prepared for the first time by a polymer-assisted synthesis technique. The nano-sized NFS particles and pores/voids are characterized by field-emission scanning transmission electron microscopy (FE-SEM) and N2-adsorption/desorption analysis, respectively. The temperature-dependent transport properties of monoclinic (Pn) NFS are also studied by AC electrochemical impedance spectroscopy (EIS). Monoclinic NFS exhibits electrical conductivity of 2.2 (±0.01) × 10−6 S cm−1. The I-V measurement technique further verifies these results. Moreover, the activation energy obtained for this crystallographic phase is found to be 1.57 eV. The initial charge capacity is found to be 225 mAhg−1 at a current rate of 0.1C (13.5 mAg−1) in the voltage range of 1.5–4.2 V, which is closer to the theoretical capacity (∼276 mAhg−1) of NFS. Despite the better-conducting properties along with less activation energy, the monoclinic phase of NFS is not found to be cyclable for long-term cycling. The plausible reason for capacity fading in this phase of the NFS is discussed in detail.