RESUMO
It is very promising and challenging to construct a yolk-shell structure with highly efficient microwave absorption (MA) performance through a simple fabrication process. Here, a novel C/MnO@void@C (MCC) yolk-shell structure has been successfully synthesized by one-step calcination without additional processing. The as-obtained MCC composites with tunable crystallinity degrees and hollowness can be obtained by treatment at various temperatures. The MCC composites treated at 700 °C (MCC-700) show an impressive MA performance, and the optimal reflection loss of -53.2 dB and an effective absorption bandwidth of 5.4 GHz can be obtained. This excellent performance results from multiple balance mechanisms. First, the regulated permittivity of MCC-700 due to proper crystallinity and hollowness is beneficial for the balance between dielectric loss (tan δε) and impedance match (Zim). Second, the optimal balance between the increasing polarization range and decreasing polarization intensity can be achieved, which is favorable for the improvement of the MA performance. Third, the multicore yolk-shell structure of MCC-700 is conducive to multiple scattering and continuous energy dissipation. Thus, our new findings provide a rational way for the utilization of yolk-shell structural manganese-based materials.
RESUMO
Achieving strong adsorption and catalytic ability toward polar lithium polysulfide species (LiPSs) of the sulfur host in lithium-sulfur (Li-S) batteries is essential for their electrochemical cyclic stability. Herein, a strategy of "self-termination of ion exchange" is put forward to synthesize the novel yolk-shell sulfur host composed of ZnO nanoparticles confined in Co-doped NiO (CDN) polyhedron (ZCCDN). After sulfur infiltration, the obtained S/ZCCDN cathode achieves excellent performance of 738.56 mAh g-1 after 500 cycles at 0.5 C with a very low capacity decay rate of only 0.048% per cycle. Even at 1 C, 501.05 mAh g-1 could be retained after 500 cycles, suggesting a capacity decay ratio of only 0.076% per cycle. The good cycle performance is attributed to the improved LiPSs' conversion kinetics, which originates from ZCCDN's sturdy chemical affinity and strong catalytic ability to polar LiPSs. For the first time, by electron holography, the local interfacial polarization electric field is clarified to be existed in the material which is conducive to the capture of LiPSs and the migration of electrons and Li+ from the mesopores. This work provides a rational way for the use of zeolitic imidazolate frameworks (ZIFs) and development of cathode materials for Li-S batteries.
RESUMO
Hollow structures have attracted great attention based on the advantage to accommodate volume expansion. However, template removal usually results in structure destruction. Herein, dandelion-like Mn/Ni co-doped CoO/C hollow microspheres (CMNC-10h) are synthesized via an Ostwald ripening process without templates. The high-angle annular dark field mapping images at the atomic level indicate the successful doping of Mn and Ni into CoO. Via an annular bright field image, oxygen vacancies induced by doping can be clearly observed. The residual two electrons in the oxygen vacancy site are highly delocalized, as confirmed by density functional theory calculations, effectively improving electrical conductivity. According to electron holography analysis, the dielectric polarization field in superficial regions of primary nanoparticles can facilitate insertion of Li+ ions into nanoparticles and thus enhance electrochemical kinetics. Combining those advantages, CMNC-10h demonstrates a high capacity of 1126 mAh g-1 at 1 A g-1 after 1000 cycles as anode material for a lithium-ion battery. Additionally, based on the strong adsorption toward polysulfide, the porous structure to accommodate sulfer/polysulfide, and the effects of oxygen vacancies to immobilize and catalyze polysulfide, CMNC-10h-S as cathode material for a lithium-sulfur battery also displays a high capacity of 642 mAh g-1 after 500 cycles at 1 C.
RESUMO
Ternary transition metal oxides (TMOs) are highly potential electrode materials for lithium ion batteries (LIBs) due to abundant defects and synergistic effects with various metal elements in a single structure. However, low electronic/ionic conductivity and severe volume change hamper their practical application for lithium storage. Herein, nanosheet-assembled hollow single-hole Ni-Co-Mn oxide (NHSNCM) spheres with oxygen vacancies can be obtained through a facile hydrothermal reaction, which makes both ends of each nanosheet exposed to sufficient free space for volume variation, electrolyte for extra active surface area, and dual ion diffusion paths compared with airtight hollow structures. Furthermore, oxygen vacancies could improve ion/electronic transport and ion insertion/extraction process of NHSNCM spheres. Thus, oxygen-vacancy-rich NHSNCM spheres embedded into a 3D porous carbon nanotube/graphene network as the anode film ensure efficient electrolyte infiltration into both the exterior and interior of porous and open spheres for a high utilization of the active material, showing an excellent electrochemical performance for LIBs (1595 mAh g-1 over 300 cycles at 2 A g-1 , 441.6 mAh g-1 over 4000 cycles at 10 A g-1 ). Besides, this straightforward synthetic method opens an efficacious avenue for the construction of various nanosheet-assembled hollow single-hole TMO spheres for potential applications.
