AUTHOR=Ji Yongzheng , Honma Tsuyoshi , Komatsu Takayuki 

TITLE=Crystallization of the Na2FexNi1−xP2O7 Glass and Ability of Cathode for Sodium-Ion Batteries

JOURNAL=Frontiers in Materials

VOLUME=7

YEAR=2020

URL=https://www.frontiersin.org/journals/materials/articles/10.3389/fmats.2020.00034

DOI=10.3389/fmats.2020.00034

ISSN=2296-8016

ABSTRACT=<p>Although the sodium phosphate cathode active materials based on the Ni<sup>3+</sup>/Ni<sup>2+</sup> redox reaction are expected to develop a high discharge potential, none of the studies aimed at practical application have been reported due to its poor kinetics showed in the sodium phosphate. Herein, we substituted active Fe for a part of Ni, expecting to activate the potential deriving from the Ni<sup>3+/2+</sup> in Na<sub>2</sub>Fe<sub>x</sub>Ni<sub>1−x</sub>P<sub>2</sub>O<sub>7</sub> glass-ceramics. Precursor glasses were prepared by the melt-quenching method and exhibited surface crystallization tendency due to heterogeneous nucleation. In the charge-discharge testing, all the flat potential showed in the discharge process derived from the reduction of Fe<sup>3+/2+</sup>. However, from the dQ/dV plot, there were two weak reduction peaks at 4.3 and 4.4 V in the discharge process of Na<sub>2</sub>Fe<sub>0.25</sub>Ni<sub>0.75</sub>P<sub>2</sub>O<sub>7</sub>. Combining with the oxidation peak at 4.6 V in the second charge process of Na<sub>2</sub>NiP<sub>2</sub>O<sub>7</sub>, we believe the reduction peaks at 4.3 and 4.4 V were derived from the Ni<sup>3+/2+</sup>.</p>