Polyanion-type
sodium superionic conductor (NASICON) Na3V2(PO4)2F (NVOPF) is a promising
cathode material for sodium ion batteries (SIBs). However, NVOPF shows
relatively low specific capacity and poor long-term performance at
high rates. Herein, we report a remarkable improvement of NVOPF cathode
material by introducing a surface coating of reduced graphene oxide
(RGO). The RGO-coated Na3V2O2(PO4)2F cathode (hereafter denoted as NVOPF@RGO) delivers
outstanding high-rate capability (93.6 mAh g–1 at
60 C) and ultralong cycle stability (∼87% retention after 10 000
cycles at 50 C). This surface-enhanced material also exhibits excellent
full-cell performance when coupled with the Fe1–x
S anode, which sustains a 94.3 mAh g–1 specific capacity after 900 cycles at 20 C. The battery performance
and stability of NVOPF@RGO are among the best in the state-of-the-art
NASICON-based SIBs. Electrochemical measurements have shown that the
coated RGO on NVOPF not only enhances its electric conductivity but
also increases the apparent sodium ion diffusivity notably. We confirmed
the structural reversibility and revealed the long/short-range structural
evolutions of NVOPF@RGO upon electrochemical cycling by multinuclear
solid-state nuclear magnetic resonance (ssNMR) combined with X-ray
diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques.
The emergence of coverage holes in wireless sensor networks (WSNs) means that some special events have broken out and the function of WSNs will be seriously influenced. Therefore, the issues of coverage holes have attracted considerable attention. In this paper, we focus on the identification of boundary nodes and coverage holes, which is crucially important to preventing the enlargement of coverage holes and ensuring the transmission of data. We define the problem of coverage holes and propose two novel algorithms to identify the coverage holes in WSNs. The first algorithm, Distributed Sector Cover Scanning (DSCS), can be used to identify the nodes on hole borders and the outer boundary of WSNs. The second scheme, Directional Walk (DW), can locate the coverage holes based on the boundary nodes identified with DSCS. We implement the algorithms in various scenarios and fully evaluate their performance. The simulation results show that the boundary nodes can be accurately detected by DSCS and the holes enclosed by the detected boundary nodes can be identified by DW. The comparisons confirm that the proposed algorithms outperform the existing ones.
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.