Nano Ni1-xCoxO system: Composition dependent phase evolution and electrochemical behaviour

Abstract

Present study investigates nano-Ni1-xCoxO (0.0 ≤ x ≤ 1.0) system by a scalable salt-templated synthesis and its potential for energy storage application. The system was characterized by XRD, Raman spectroscopy, XPS, TEM and electrochemical studies. An interesting phase relation was observed wherein NiO with rock salt structure yielded biphasic mixture (rock salt + spinel-type) followed by single-phasic spinel phase upon successive Co-substitution. A wider solubility of NiO in Co3O4 and appearance of biphasicity in NiO-rich region suggests that Co prefers +3 oxidation state, leading to relatively narrower rock-salt phase field. The crystallite size of 18–25 nm was estimated and TEM shows more uniform particles with narrower size distribution with decrease of metal-to-salt ratio during synthesis. Subtle transition from normal to inverse spinel upon Ni-substitution in Co3O4 could be deciphered by careful Raman spectroscopic analysis. The best electrochemical performance was observed for Ni0.5Co0.5O with maximum specific capacitance of 146 F g−1 at 1 A g−1 and 97% capacitance retention (1000 cycles). An asymmetric device fabricated with Ni0.5Co0.5O showed specific energy of 15.38 W h/Kg. Highlight of study is enhanced electrochemical performance of Ni0.5Co0.5O, over either of end members, due to richer redox chemistry owing to simultaneous presence of Ni and Co ions.