Most P2-type layered
oxides suffer from multiple voltage plateaus,
due to Na+/vacancy-order superstructures caused by strong
interplay between Na–Na electrostatic interactions and charge
ordering in the transition metal layers. Here, Mg ions are successfully
introduced into Na sites in addition to the conventional transition
metal sites in P2-type Na0.7[Mn0.6Ni0.4]O2 as new cathode materials for sodium-ion batteries.
Mg ions in the Na layer serve as “pillars” to stabilize
the layered structure, especially for high-voltage charging, meanwhile
Mg ions in the transition metal layer can destroy charge ordering.
More importantly, Mg ion occupation in both sodium and transition
metal layers will be able to create “Na–O–Mg”
and “Mg–O–Mg” configurations in layered
structures, resulting in ionic O 2p character, which allocates these
O 2p states on top of those interacting with transition metals in
the O-valence band, thus promoting reversible oxygen redox. This innovative
design contributes smooth voltage profiles and high structural stability.
Na0.7Mg0.05[Mn0.6Ni0.2Mg0.15]O2 exhibits superior electrochemical
performance, especially good capacity retention at high current rate
under a high cutoff voltage (4.2 V). A new P2 phase is formed after
charge, rather than an O2 phase for the unsubstituted material. Besides,
multiple intermediate phases are observed during high-rate charging.
Na-ion transport kinetics are mainly affected by elemental-related
redox couples and structural reorganization. These findings will open
new opportunities for designing and optimizing layer-structured cathodes
for sodium-ion batteries.
Graphene nanosheets (GNS) were employed as an air electrode for a sodium-air battery (SAB). High discharge capacity of 9268 mA h g(-1) with low overpotential was achieved, indicating its superiority to a normal carbon film electrode. Our results indicate that GNS as air electrodes could improve the electrochemical performance of rechargeable SABs.
We report a strategy to complement the galvanic replacement reaction between Ag nanocubes and HAuCl4 with co-reduction by ascorbic acid (AA) for the formation of Ag-Au hollow nanostructures with greatly enhanced SERS activity. Specifically, in the early stage of synthesis, the Ag nanocubes are sharpened at corners and edges because of the selective deposition of Au and Ag atoms at these sites. In the following steps, the pure Ag in the nanocubes is constantly converted into Ag(+) ions to generate voids owing to the galvanic reaction with HAuCl4, but these released Ag(+) ions are immediately reduced back to Ag atoms and are co-deposited with Au atoms onto the nanocube templates. We observe distinctive SERS properties for the Ag-Au hollow nanostructures at visible and near-infrared excitation wavelengths. When plasmon damping is eliminated by using an excitation wavelength of 785 nm, the SERS activity of the Ag-Au hollow nanostructures is 15- and 33-fold stronger than those of the original Ag nanocubes and the Ag-Au nanocages prepared by galvanic replacement without co-reduction, respectively. Additionally, Ag-Au hollow nanostructures embrace considerably improved stability in an oxidizing environment such as aqueous H2O2 solution. Collectively, our work suggests that the Ag-Au hollow nanostructures will find applications in SERS detection and imaging.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.