Abstract
Layered NaNi0.5Mn0.5O2, employed as cathode materials in sodium ion batteries, is attracting interest due to its high working potential and high-capacity values, thanks to the big sodium amount hosted in the lattice. Many issues are, however, related to their use, particularly, the complex phase transitions occurring during sodium intercalation/deintercalation, detrimental for the structure stability, and the possible Mn dissolution into the electrolyte. In this paper, the doping with Ti, V, and Cu ions (10% atoms with respect to Ni/Mn amount) was used to stabilize different polymorphs or mixtures of them with the aim to improve the capacity values and cells cyclability. The phases were identified and quantified by means of X-ray powder diffraction with Rietveld structural refinements. Complex voltammograms with broad peaks, due to multiple structural transitions, were disclosed for most of the samples. Ti-doped sample has, in general, the best performances with the highest capacity values (120 mAh/g at C/10), however, at higher currents (1C), Cu-substituted sample also has stable and comparable capacity values.
Highlights
IntroductionNowadays, one of the most promising category of cathode materials for sodium ion batteries [1,2,3]
Layered oxides are, nowadays, one of the most promising category of cathode materials for sodium ion batteries [1,2,3]
Layered NaNi0.5Mn0.5O2, employed as cathode materials in sodium ion batteries, is attracting interest due to its high working potential and high-capacity values, thanks to the big sodium amount hosted in the lattice
Summary
Nowadays, one of the most promising category of cathode materials for sodium ion batteries [1,2,3]. NaMeO2 (Me = Fe, Ni, Mn, Cr, Co and others) could be crystallized in the most common O3 type structure with AB-CA-BC layers packing and Na ions in octahedral sites Another energetically favored polymorph, as demonstrated by theoretical calculations, is P2 type, with AB-BA layers and prismatic sodium [5]. Compared with sodium-deficient P2-type materials, this compound has higher sodium content to provide more cyclable sodium and high initial Coulombic efficiency, which play a pivotal role in practical sodium-ion full cell systems. It has a theoretical capacity of about 200 mAh g−1 based on Ni2+/4+ redox couple with a maximum potential of ~4.0 V, high electrochemical activity and reduced cost [9]
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