NaFex(Ni0.5Ti0.5)1-XO2 (x = 0.2 and 0.4) : A Smooth Profile and Narrow Voltage Zone Cathode for Sodium Ion Batteries
NaFeO2 has been explored as one of the candidate for the cathode materials. However initial capacity is limited to ~ 80 mAh/g with a flat voltage profile at 3.3 V . Voltage cutoffs show a severe effect on the stability of the electrode material. A solid solution of NaFeO2 with NaNiO2 studied recently has shown good electrochemical properties with the average operating potential at ~ 3.0 V. Capacity falls to 30% of its initial values in the 30 cycles . In the present study we have shown the electrochemical behavior of the NaFex(Ni0.5Ti0.5)1-xO2 (x = 0.2 and 0.4) series of compounds. Samples have been synthesized by conventional solid state reaction and have been characterized by various tools . Desired composition of the electrode material has been checked by performing ICP -OES measurements. X-ray diffraction has shown the formation of the O3 type layered structure with R3-m space group symmetry. Morphology of the synthesized material has been characterized by using scanning electron microscope, which shows the formation of the agglomerated submicron size particles. Electrochemical properties of the material have been checked by performing galvanostatic charge/discharge measurements in the classical CR2032 coin cells. Similar to α-NaFeO2voltage cutoff limits have severe impact on the structural changes and hence the electrochemical properties as shown in Figure 1. Initial discharge capacity of ~120 mAh/g is observed in the narrow voltage window 3.75 - 2.6V as shown in Figure 2. Ex-situ structural analysis shows the dependence of hexagonal to monoclinic structural transformation on the finally observed composition. Most importantly a smooth charge/discharge profile without a sharp kink has been observed with a very low polarization. Effect of the electrolyte on the electrochemical performance of the material will also be demonstrated.
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Figure Captions :
Figure 1. Effect of cutoff voltage on the irreversible capacity and polarization.
Figure 2. Charge/discharge curves of NaFe0.4Ni0.3Ti0.3O2 using 1M NaFSI in PC as electrolyte. Inset shows the cycling stability of the electrode material in the half cell configuration where Na acts as counter electrode for the first 30 cycles.