Tuning Metallic Co0.85Se Quantum Dots/Carbon Hollow Polyhedrons with Tertiary Hierarchical Structure for High-Performance Potassium Ion Batteries

Potassium-ion batteries(KIBs)are a potential candidate to lithium-ion batteries(LIBs)but possess unsatisfactory capacity and rate properties.Herein,the metallic cobalt selenide quantum dots(Co0.85Se-QDs)encapsulated in mesoporous carbon matrix were designed via a direct hydrothermal method.Specifically,the cobalt selenide/carbon composite(Co0.85Se-QDs/C)possesses tertiary hierarchical structure,which is the primary quantum dots,the secondary petals flake,and the tertiary hollow micropolyhedron framework.Co0.85Se-QDs are homogenously embedded into the carbon petals flake,which constitute the hollow polyhedral framework.This unique structure can take the advantages of both nanoscale and microscale features:Co0.85Se-QDs can expand in a multidimensional and ductile carbon matrix and reduce the K-intercalation stress in particle dimensions;the micropetals can restrain the agglomeration of active materials and promote the transportation of potassium ion and electron.In addition,the hollow carbon framework buffers volume expansion,maintains the structural integrity,and increases the electronic conductivity.Benefiting from this tertiary hierarchical structure,outstanding K-storage performance(402 mAh g?1 after 100 cycles at 50 mA g?1)is obtained when Co0.85Se-QDs/C is used as KIBs anode.More importantly,the selenization process in this work is newly reported and can be generally extended to prepare other quantum dots encapsulated in edge-limited frameworks for excellent energy storage.

Equivalent electromagnetic parameters for microwave metamaterial absorber using a new symmetry model

Transmission line theory uses the complex nature of permeability and permittivity of a conventional magnetic absorber to evaluate its absorption properties and mechanism. However, because there is no method to obtain the electromagnetic parameters of a metamaterial-absorber integrated layer(composed of a medium layer and a periodic metal array), this theory is seldom used to study the absorption properties of the metamaterial absorber. We propose a symmetry model to achieve an equivalent complex permittivity and permeability model for the integrated layer, which can be combined with the transmission line theory to calculate metamaterial absorption properties. The calculation results derived from both the transmission line theory and the high-frequency structure simulator are in good agreement. This method will be beneficial in practical investigations of the absorption mechanism of a metamaterial absorber.

Synthesis and characterization of MoS2/Fe@Fe3O4 nanocomposites exhibiting enhanced microwave absorption performance at normal and oblique incidences

Herein,we attempted to prepare MoS2/Fe@Fe3O4 nanocomposites capable of strongly absorbing broadband incident electromagnetic(EM)radiation and probed the effects of their composition on complex permittivity and permeability at 2-18 GHz.Calculations of normal-incidence reflection losses(RLs)based on EM parameters revealed that the Fe@Fe3O4 to MoS2 mass ratio strongly influenced the absorption peak intensity and bandwidth.Specifically,an RL peak of-31.8 dB@l5.3 GHz and a bandwidth(RL<-lOdB)of4.8 GHz(13.2-18 GHz)were achieved at a thickness of 1.52 mm and a Fe@Fe3O4 to M0S2 mass ratio of 60:40.Further,RL and bandwidth were investigated for oblique incidence,in which case two kinds of EM waves(TE-electric field perpendicular to plane of incidence;TM-electric field in the plane of incidence)were considered.The absorption peaks of TE and TM waves did not exceed-2 0 dB when the incidence angle increased to 3 0,and the bandwidth(R L<-10 dB)reached 4.2 GHz(TE wave)and 4.0 GHz(TM wave)when this angle was further increased to 40.0°and 50.4,respectively.Finally,the mechanism of microwave absorption was discussed in detail.