Electrochemical Performance of Na3Ni1.5Zn0.5SbO6 in Different Electrolytes

Energy storage for the portable electronic devices has been dominated by lithium ion batteries over the past two decades [1].  Even though sodium ion batteries were studied in the early stages of the development of the rocking chair batteries, but they lack behind the lithium ion batteries in terms of energy density of the system. Very recently scientific society has re-focused on the sodium ion battery development and has shown much improved electrochemical performance than the first generation of the sodium ion batteries, thanks to new emerging materials and knowledge acquires during the lithium ion batteries’ development [2]. On the other hand high voltage electrode materials for sodium ion batteries are in high demand to challenge their possible application and to compete with the already established lithium ion batteries. Electrolytes play a critical role in the development of future generation of sodium ion batteries. Cathode materials with energy density as high as 480 Wh/Kg have been developed in the recent past with average operating voltage of ~ 2.75 V [3]. However the cutoff voltage window in such electrodes spans over 4.3 V to 1.5 V. Therefore it becomes highly important to study the electrochemical performance of such materials at high potentials to make a viable choice of the electrolyte. In the present study we have chosen a cathode material with voltage window range of 4.2 V- 2.5 V. We have studied its electrochemical performance in different organic and ionic liquid electrolytes.

Na₃Ni_1.5Zn_0.5SbO₆ was synthesized by conventional solid state synthesis route using Na₂CO₃, NiO, ZnO and Sb₂O₃ as raw precursors. Raw precursors were milled together in appropriate ratio for one hour and compressed in the form of pellet. Finally the pellets were calcined at 1020 ^oC and quenched down to room temperature using liquid nitrogen. Synthesis material was immediately transferred to the glove box to avoid any moisture exposure. Preliminary structural characterizations have been performed using X-ray diffraction. The microstructure of the material has been observed by scanning electron microscopy. To evaluate the electrochemical performance of the electrode material, active material was coated over the aluminum foil by mixing it with Polyvinylidene fluoride (PVDF) and carbon black in the ratio 75 (AM):10 (PVDF):15 (CB). Slurry of the finally obtained composite was made using NMP as solvent. Coated foil was dried under vacuum at 120 ^oC for two hours and then punched in the form of the circular disc and compressed. The half cell batteries were finally assembled in the CR2032 coin cells. Three different electrolytes have been studied for this compound, i.e., 1M NaFSI, 1M NaClO₄ and 1M NaPF₆in PC. The initial voltage cutoff limits were set as 4.2 – 2.5 V.

It has been observed that it is not possible to reach 4.2 V during the first charging cycle using 1M NaFSI in PC. This is due to the corrosion of the aluminum current collector that is expected at this voltage limit of the charging. Therefore for the further cycles the voltage cutoff limit was set at 4.0. Even though the cell was able to cycle, the signature of aluminum corrosion can be clearly observed. Performance of the material using 1M NaClO₄ and 1M NaPF₆ in PC has also been checked. No electrolyte decomposition or corrosion issues have been observed using classical liquid electrolyte with organic solvent (Fig. 1). The detailed electrochemical properties of Na₃Ni_1.5Zn_0.5SbO₆ in various electrolytes will be discussed. Na₃Ni_1.5Zn_0.5SbO₆shows initial discharge capacity of 115 mAh/g with an average operating voltage of ~ 3.3 V and stable capacity over 100 mAh/g for the first 20 cycles.

References

1. J.-M. Tarascon & M. Armand, Nature 414 (2001) 359-367.

2. V. Palomares, P. Serras, I. Villaluenga, K.B. Hueso, J. Carretero-Gonzalez, T. Rojo, Energy Environ. Sci. 5 (2012) 5884-5901.

3. Naoaki Yabuuchi, Masataka Kajiyama, Junichi Iwatate, Heisuke Nishikawa, Shuji Hitomi, Ryoichi Okuyama, Ryo Usui, Yasuhiro Yamada and Shinichi Komaba, Nature Materials 11 (2012) 512-517.

Figure Caption

Fig.1: First 20 charge/discharge curves of Na₃Ni_1.5Zn_0.5SbO₆/NaClO₄ in PC/Na